Boron in forest trees and forest ecosystems

Forest Ecology and Management 260 (2010) 2053–2069

Contents lists available at ScienceDirect

Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco

Review

Boron in forest trees and forest ecosystems Tarja Lehto a,∗ , Teija Ruuhola a , Bernard Dell b a b

School of Forest Sciences, University of Eastern Finland, P.O. Box 111, 80101 Joensuu, Finland Sustainable Ecosystems Research Institute, Murdoch University, Perth 6150, Australia

a r t i c l e

i n f o

Article history: Received 4 August 2010 Received in revised form 15 September 2010 Accepted 16 September 2010 Keywords: Boron Deficiency Fertiliser Forests Fungi Management Micronutrients Mycorrhizas Nutrient uptake Retranslocation Toxicity Trees Wood

a b s t r a c t This review critically examines the role of boron (B) in forests in view of recent findings on B nutrition and the continuing occurrence of B deficiency. Many perceptions about the role of B in plants and its uptake and mobility have been altered since the last review on B in forest trees in 1990. Now there is evidence for a fundamental role of B in the formation of the pectic structure in primary cell walls in plants, and further roles in membrane function are being explored. In plants, channel-mediated B uptake, active B uptake and B uptake by mycorrhizas have been shown, B transporters have been identified, and B retranslocation has been shown. We explore these findings and their consequences on forest trees and on ecosystems that they dominate. Particular emphasis is placed on B retranslocation and B in mycorrhizal symbiosis, given their importance in trees. Following from impaired development of the primary cell wall in B-deficient trees, disorders in the structural development of organs and whole plants are manifested. This has consequences for tree form, affecting wood quality and productivity. At a stand level, at least part of the value of wood production is lost by the time the deficiency symptoms appear. As symptoms identifying deficiency in many tree species are too easily confused with many other effects, greater use should be made of foliar analysis but this requires establishing robust prognostic values for the trees of interest. There is still no explanation as to why root tip and mycorrhiza development are among the first phenomena to be affected as the B supply decreases. Whether B is required by, or whether it is useful for fungi, is still an open question. Boron remobilisation within trees may be a key factor in the occurrence of forests in areas with very low B availability, as most of the B in the whole stand can be in the standing biomass. The ability to remobilise B varies considerably between species, but we suggest that there is a continuum rather than a strict division to B-retranslocating and non-retranslocating species. Boron output from forest ecosystems with potential for leaching is controlled by adsorption in the soil, which is still poorly understood particularly in soils with abundant organic matter. Increased concentrations of phenolic compounds in B-deficient plants and possibly altered lignin concentration can affect plant defence systems to herbivory and pathogens, and nutrient and carbon release through decomposition. Hence, B nutrition and fertilisation of low-B stands can have implications both to the resistance of trees to biotic stress, as well as influence the cycles of other nutrients and carbon in forests. © 2010 Elsevier B.V. All rights reserved.

Contents 1. 2. 3. 4. 5. 6. 7.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron cycle in forests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Physiological roles of boron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron deficiency responses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron and mycorrhizas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron and root nodules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron uptake and translocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1. Boron uptake by nonmycorrhizal roots . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2. Boron mobility in fungal mycelia and boron uptake by mycorrhizas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3. Soil and meteorological factors affecting B uptake . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author. Tel.: +358 50 3419232; fax: +358 13 2514422. E-mail address: tarja.lehto@uef.fi (T. Lehto). 0378-1127/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2010.09.028

2054 2054 2056 2056 2057 2058 2058 2058 2058 2060

2054

8. 9.

10. 11. 12. 13. 14.

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

Retranslocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Boron nutrition and susceptibility to environmental stress . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1. Drought resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.2. Cold tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidative stress, phenolic compounds, and defense reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Wood quality, litter quality and decomposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Correction of B deficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Excess boron . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Boron deficiencies are wide-spread globally in over 80 countries (Shorrocks, 1997), in regions with sandy soils coupled with strong leaching rainfall regimes, as well as soils with alkaline pH (Bell and Dell, 2008). Sillanpää (1990) assessed the micronutrient status of 190 agricultural soils from selected countries worldwide and found 31% to be deficient in B for crop production. Similar studies have not been done for soils under forests or plantations, except at a local scale (e.g. Ryan, 1989; Lambert and Ryan, 1990). Boron deficiency is common in large areas in south and southeast Asia, eastern Australia and New Zealand, Africa, North and South America and northern Europe. In south-east Asia, B deficiency is more widespread than suggested by Shorrocks (1997). This has become evident as plantations of fast-growing eucalypts and acacias have been established on degraded forest lands, for example on the floodplain of the Mekong River, or have replaced primary rainforest such as on volcanic soils in northern Sumatra (Dell et al., 2001, 2008). Deficiencies occur both spontaneously, for example during reforestation of soils derived from sandstones in southwestern China (Fig. 1D; Dell and Malajczuk, 1994), and in nitrogen-fertile boreal Norway spruce stands (Fig. 1G and H; Tamminen and Saarsalmi, 2004). Macronutrient fertilisation of boreal podzol sites (Mälkönen et al., 1990) and drained peatlands (Veijalainen et al., 1984) have triggered B deficiencies. Experimental liming greatly reduced B availability, which is one of the reasons, why liming never became a practical forest management measure in Finland (Lehto and Mälkönen, 1994). Boron toxicity is a severe problem in many parts of the world, but mainly in arid regions not dominated by trees. The window between deficiency and toxicity is said to be usually very narrow, and this is true for external supply. However, after fertilisation, foliar B concentrations in conifers can increase from for example 1–2 mg kg−1 to over 100 mg kg−1 with no sign of toxicity. Few other nutrients show as large concentration shifts without damage (cf. Marschner, 1995), and in the case of most macronutrients, such large relative shifts are obviously impossible. Boron deficiency causes loss of apical dominance in trees (Fig. 1C and H), which leads to loss of saw-timber quality first and subsequently to loss of yield in quantity as well. As reproductive structures have a higher requirement for B than vegetative structures and the movement of B to reproductive structures may be restricted in some plants (Dell and Huang, 1997), the management of B is especially important in seed orchards. Many fundamental perceptions in plant B nutrition have been challenged since Stone (1990) published the review ‘Boron deficiency and excess in forests’. Earlier it was assumed that B was only taken up passively with the transpiration stream, but now it is known that in (nonmycorrhizal) sunflower B is taken up actively if the external concentration is low (Dannel et al., 2002). Earlier B was assumed to be one of the most immobile plant nutrients, but now the phloem mobility of B has been shown at least in some plants (Brown and Shelp, 1997). Furthermore, membrane B transporters have now

2060 2061 2061 2062 2062 2063 2064 2064 2065 2066 2066

been identified (Takano et al., 2002, 2008; Miwa and Fujiwara, 2010). However, most studies into the primary roles of B in plants, and into the physiology of B nutrition have been made on annual plants. These have been reviewed extensively, and are summarised briefly below (Loomis and Durst, 1992; Brown and Shelp, 1997; Cakmak and Römheld, 1997; Hu and Brown, 1997; Dell and Huang, 1997; Matoh, 1997; Blevins and Lukaszewski, 1998; Brown et al., ˜ et al., 2004; Huang et al., 2005; 2002; Dannel et al., 2002; Bolanos Goldbach and Wimmer, 2007; Takano et al., 2008; Tanaka and Fujiwara, 2008). Notwithstanding the quest for the primary physiological roles of B in plants, the secondary responses of boron can have a strong effect on plant and ecosystem function. Structural disorders resulting from impaired formation of the cell wall may affect the performance of the whole plant, particularly under environmental stress. Boron retranslocation can have a significant role in ecosystems through effects on plant performance, by resulting in more closed nutrient cycles, changes in litter quality and potentially reduced nutrient leaching. Accumulation and oxidation of phenolic compounds not only can affect the carbon dynamics and stress resistance of the tree, but through decomposition processes also other organisms in different parts of the nutrient cycle. Interactions of B with biotic stressors have not been studied much, but there are indications of a role of B in plant defence. The objective of this review is to re-assess the role of boron in forests in view of recent findings on B nutrition. It deals with B nutrition and B deficiency in forest and plantation trees. There is particular emphasis on the (ecto)mycorrhizal symbiosis and retranslocation, given their importance in woody plants.

2. Boron cycle in forests The input of B to the biogeochemical cycle through weathering is limited in soils with parent material that is poor in B, such as those formed from the weathering of granite. Also, tourmaline which has a high B content is very resistant to weathering, and the time scale of the B becoming plant available is long. With downward water movement, B is leached from coarse-textured soils, and it eventually accumulates in sea water, while in arid lands, there is accumulation in drainage pans. Some B is precipitated as a result of burning fossil fuels rich in B. In areas with natural deposition from sea sprays, B originating from seawater is a major B input. In a humid climate, it follows that the occurrence of deficiencies is more likely the further away from the sea the site is (Wikner, 1983). This was shown in a survey of the B status of fertile sites with Norway spruce (Picea abies) in Finland (Tamminen and Saarsalmi, 2004). However, correlation with distance from sea becomes less clear when soil type is taken into account in the example from Finland. Soils with high clay content are much more common in parts of Finland that are close to the Baltic sea. In addition to soil type, the topography of coastal areas affects the precipitation; on the inland side of mountain

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

2055

Fig. 1. (A) Yellowing of leaflets in mildly B-deficient Acacia mearnsii (Lao PDR). (B) Top dieback of severely B-deficient Acacia mangium (Vietnam). (C) Loss of apical dominance in B-deficient Eucalyptus urophylla followed by damage from leaf blight fungi (Vietnam). (D) Prostrate Eucalyptus globulus – symptom of severe B deficiency (PR China). (E) Leaf chlorosis in B-deficient Dalbergia odorifera (PR China). (F) Failure of budburst in the apical bud (white circle), multiple lateral shoots in Picea abies. First visible symptoms of B deficiency (Finland, photo Mikko Räisänen). (G) Repeated top dieback in severely B-deficient P. abies (Finland, photo Tenho Hynönen). (H) Recovery after B fertilization in P. abies (Finland, photo Tenho Hynönen).

ranges there may be little B deposition, as the B-containing spray tends to precipitate at high altitudes. Hence, distance from sea does not always indicate the amount of B in the precipitation. Furthermore, areas with heavy rainfall may have B deficiencies because of leaching, even if located near the ocean (Tanaka and Fujiwara, 2008).

In areas with low input from deposition, the major fluxes in the B cycle are within the forest ecosystem. In stands with low B, a major part of B can be in the stand itself (Hingston, 1986; Aphalo et al., 2002; Tamminen and Saarsalmi, 2004). Nutrient uptake, retranslocation, release from litter by decomposition, and retention in the soil (chemically; in soil organic matter; and through immobiliza-

2056

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

tion in fungi and microbes) are the processes that determine if scarce nutrients are used efficiently, or if they remain unavailable. The major outputs are leaching and harvesting. Apart from harvesting tree trunks, cutting residues are increasingly harvested for fuel use, which increases the loss of nutrients considerably compared to harvesting only trunks. However, the B concentrations in wood and bark are relatively high compared to N; in two Pinus sylvestris stands, the proportion of B in stemwood and bark was 30% and 56%, while the corresponding proportions of N were 15% and 28%. In birch, there was 47% of the B in stemwood and bark, compared to 26% of the N, and the respective numbers in spruce were 36% of B and 17% of N (data recalculated from Finér, 1989). In seven-year old plantation-grown Eucalyptus globulus, the wood contained 2.6 times more B than the bark and ca. 65% of the aboveground B reserve was located in the bole (C. Rogers, N. Pampolina and B. Dell, unpubl.). Output through leaching has not been explored in detail in forest soils. Boron concentrations in soil water tend to be low, and therefore they are not often measured in studies on soil water chemistry. However, many Scandinavian forest soils lose annually more B through leaching than is returned in precipitation (Wikner, 1983). Boron adsorption to soil particles reduces the potential for leaching, but the adsorbed B is not necessarily available to plants (Keren et al., 1985). In addition to clay particles and Al and Fe compounds, organic matter can have a very high B retention capacity (Yermiyahu et al., 1988; Yermiyahu, 1995) even at low pH (Lehto, 1995). This is significant in otherwise coarse-textured soils that allow leaching.

3. Physiological roles of boron The finding of the essentiality of boron to plants is generally attributed to Warrington (1923). A large part of plant B is localised in the cell wall pectic fraction, where it forms a cross-linking structure with rhamnogalacturonan-II (RG-II) units (Matoh, 1997). The function of B in the formation and stabilisation of the primary cell wall structure was confirmed by O’Neill et al. (2001). RG-II is present in the primary walls of all higher plants and is evolutionarily conserved (O’Neill et al., 2004). There are indications that B has other roles (Brown et al., 2002; Goldbach and Wimmer, 2007) but ˜ et al. (2004) sugproof of other functions remains elusive. Bolanos gested that B has a general role of stabilising molecules with cis-diol groups, whatever their function. Membrane related functions suggested by Parr and Loughman (1983) continue to be explored and symptoms of B deficiency in animals are indicative of some membrane function (Brown et al., 2002). There are suggestions for a role of B in the formation of the cytoskeleton and in the membrane–cell wall interaction (Bassil et al., 2004). Boron cross-linked glycoproteins associated with membranes are suggested to be involved in plasmalemma function, and the isolation of B-binding proteins in plant membranes supports this hypothesis (Wimmer et al., 2009). Furthermore, B appears to be involved in the expression levels of a number of genes (Camacho-Cristóbal et al., 2008), and it is likely that other roles in addition to plant cell wall structure will be elucidated in the next few years. Given the importance of bacteria and fungi in nutrient cycling in forest ecosystems, the role of B in these groups of organisms are summarised below. A B-containing molecule produced by the bacterium Vibrio harveyi has been shown to function in quorum sensing signalling (Chen et al., 2002). Heterocystous cyanobacteria have a B requirement (Bonilla et al., 1990), and B is taken to be essential for the N2 fixing symbiosis of Rhizobium and Frankia ˜ et al., 2004). By contrast, B is assumed not to be an essen(Bolanos tial element for fungi (Marschner, 1995), however, it is difficult to prove that it is not. A very large B concentration (1500 mg kg−1 ) has

been found in the cell wall fraction of the ubiquitous plant pathogen Botrytis cinerea (Chardonnet et al., 1999), which might suggest that in some fungi, B could have a similar role as in plants. Fungal cell walls do not contain large amounts of rhamnogalacuronan-II, as their major constituents are ␤-glucans and chitin, however, uronic acid is present in small quantities in Botrytis cinerea (Chardonnet et al., 1999; Cantu et al., 2009). In a survey of sporophores in boreal forests, B accumulated in sporophores of some Basidiomycete species, both mycorrhizal and saprotrophic, even in soils with very low B availability. However, other species had consistently very low B concentrations, the range being from below detection limit to 280 mg kg−1 (Lavola et al., 2010). It is likely therefore that there are higher fungi species that accumulate B, and other species that exclude B. It still remains unknown if B has a function in fungi, but if it does, the amount required would be very small in the B-excluding species. One possibility is that B is present in the cell wall structure of some fungi but not in others, or at some specific developmental stages. Measurable B was found in the spores of two fungal species that had high B concentrations in sporophores (T. Lehto, unpubl.). In experiments on growth responses to B, yeast (Saccharomyces cerevisiae) showed a positive reaction to moderate B addition (Bennett et al., 1999) but several ectomycorrhizal species did not show consistent growth stimulation by B in pure culture (Mitchell et al., 1986; Sakya, 2000; A. Lavola and T. Lehto, unpubl.). Both yeast and ectomycorrhizal species grew also in pure cultures where B had been removed by using B-binding resin and preculture in very lowB media (Bennett et al., 1999; Sakya, 2000; A. Lavola and T. Lehto, unpubl.). However, the yeast cells still contained B after growth in the non-B medium, suggesting that this species vigorously maintains its cellular B content (Bennett et al., 1999). Hence further response experiments would need to include improved removal of B by preculturing.

4. Boron deficiency responses Symptoms of B deficiency occur first in the meristematic tissues of roots, restricting root growth and leading to an increased shoot/root-ratio (Dell and Huang, 1997). Root dry weight (Räisänen et al., 2007) or root tip number (Möttönen et al., 2001a,b) was reduced by low B well before macroscopic above-ground effects occurred in Norway spruce seedlings. The primary occurrence of deficiency below ground may provide an explanation to the sudden occurrence of deficiency above ground. Boron deficiency does not always lead to a gradual decline in growth. On the contrary, aboveground growth can be good, until deficiency manifests dramatically as death of the apical bud, which in conifers and eucalypts leads to loss of apical dominance (Sutinen et al., 2006). If repeated for many years, this leads to stunted, bushy appearance of trees (Fig. 1G). In many broadleaf species, changes in leaf colour and development can precede the loss of apical dominance (Fig. 1A and E). However, when B deficiency is severe and prolonged, crown dieback and dwarfing can result (Fig. 1B and D). In less severe cases in conifers, the base of the stem can become thicker, and the ratio of branch and needle mass to stem is reduced (Reinikainen and Silfverberg, 1983). Diameter growth in conifers is little affected, but height growth may be reduced even when there is no loss of apical dominance (White and Krause, 2001; Saarsalmi and Tamminen, 2005). Silvicultural thinnings in stands where some individuals have growth disorders may reduce the competition for B, and hence the remaining dominant trees do not necessarily show symptoms or loss in growth (Möttönen et al., 2003). There can also be alternate recovery and dieback periods, which is seen in stems as crooks and increased branching density (Saarsalmi and Tamminen, 2005). Symptoms can be exacerbated where root damage occurs due to storm events

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

or biotic agents, as found for example in eucalypts (Dell and Xu, 2006). Many coniferous species are sensitive to loss of apical dominance for a number of other reasons as well, such as insect and mammal herbivory (Räisänen et al., 2004). Therefore, the visible symptoms do not correspond to B concentrations well enough to encourage the use of growth disturbance as a key diagnostic tool. For example, in Norway spruce (Picea abies), there was a correlation between needle B concentration and loss of apical dominance (growth disturbance), yet 20% of trees with adequate B had growth disorders in the crown (Tamminen and Saarsalmi, 2004; Räisänen et al., 2004). Moreover, the value of the trunk for timber may be lost at the first incidence of change of leader, and therefore, from a management point of view, it is too late to wait until deficiency symptoms occur. Needle B analysis may predict the forthcoming deficiency symptoms, but it has limitations as well. Rapid fluctuations in B concentrations (e.g. Sutinen et al., 2006) and the fact that B deficiency at a particular point in time during the growing season can cause irreversible damage (Bell, 2000; Sutinen et al., 2007), cause uncertainties in the prediction of B deficiency. It is of interest to determine the first symptoms of B deficiency which might precede the death of apical buds. Many of the most drastic macroscopic symptoms of B deficiency can be understood as following from abnormality in the formation of cell walls. This can also explain to some extent why B deficiency is apparently an on/off phenomenon in conifers, rather than causing a gradual decline in growth such as N deficiency. Malformation at the cellular level can be found in Scots pine and Norway spruce needles in low-B trees before any visible symptoms appear (Raitio and Rantala, 1977; Sutinen et al., 2006, 2007). In these species, the development of the bud for the following year’s new vegetative shoots starts directly after bud burst in the spring, and continues until the autumn, when the bud starts to harden (Sutinen et al., 2006). During bud development, the primary cell walls form, and therefore B supply is critical for the whole of this time. Application of B later in the growing season did not prevent bud damage in Norway spruce, manifesting as increased numbers of collapsed cells in the bud apex (Sutinen et al., 2007). In species like Norway spruce and Scots pine, the B deficiency limit in forestry is usually given as 4 mg kg−1 in current needles (Braekke, 1983, Jukka, 1988), as the occurrence of shoot dieback increases when foliar concentrations during dormancy are less than this. Despite the fact that responses in roots and mycorrhizas may occur at much higher foliar concentrations at least in seedlings (Möttönen et al., 2001b), the 4 mg kg−1 must be taken as the critical concentration before actual macroscopic damage appears above ground in these species. Eucalypt species appear to be more sensitive to B deficiency than conifers, as the critical level was estimated to be 12–16 mg kg−1 in the youngest fully expanded leaf of E. globulus in a B-buffered solution culture, which was similar to the critical level in young trees in the field (Sakya et al., 2002). In this study, the corresponding external B concentration was as low as 1 ␮M. For other broadleaf trees there is less information. Rikala and Vuorinen (2004) studied a silver birch (Betula pendula) stand with different growth disorders, but they found healthy-looking trees with leaf B concentrations about 6 mg kg−1 , and trees with multiple tops with 15 mg B kg−1 . Ferm and Markkola (1985) found that leaf B remained constant at about 10 mg kg−1 in B. pubescens trees with growth disorders. The deficiency limits may be more difficult to determine in birch than in conifers. As closely related species, B. pendula and B. pubescens may have a different degree of phloem mobility of B (Lehto et al., 2004c), generalisations over different species should be made with caution. In annual plants, seed yield may be reduced dramatically even if vegetative growth is not much affected by B deficiency (Dell and Huang, 1997). The critical phase is fertilisation, as B deficiency will

2057

impede the growth of the pollen tube as the deposition of new cell wall material is impeded by lack of external B (Marschner, 1995). Also, pollen fertility can be markedly reduced by B deficiency (Dell et al., 2002). In nut and fruit trees, for example pistachio, pecan and olive, fruiting is known to be particularly affected by B deficiency (Perica et al., 2001; Wells et al., 2008). To our knowledge, there are no studies on B deficiency effects on seed set in forest trees, but it is likely that these occur in regions with low B availability. Studies into B effects on seed production and viability would give insights to forest regeneration, both natural regeneration and seed production in seed orchards.

5. Boron and mycorrhizas Mycorrhizal associations predominate in the surface and nearsurface soil horizons of forests and are essential for tree nutrition (e.g. Plassard and Dell, 2010). In the field, an increase in the number of ectomycorrhizas per unit soil volume was shown in Norway spruce after B fertilisation (Lehto, 1994). This was mostly due to increased numbers of short root tips available for mycorrhizal colonisation, as the short roots in boreal coniferous trees are normally 100% mycorrhizal. The positive influence of increased B availability on proportional mycorrhiza formation was first shown in arbuscular-mycorrhizal plants, alfalfa and red clover (Lambert et al., 1980), and citrus (Dixon et al., 1989). Subsequently, the positive influence of increasing B supply on the formation of ectomycorrhizas was shown in Pinus echinata seedlings (Mitchell et al., 1986). It is surprising that increasing B still increased mycorrhiza colonization, as the needle B levels in the low-B treatment were 140 mg kg−1 , and in the high-B treatment, up to 800 mg kg−1 (Mitchell et al., 1987, 1990). Nevertheless, both number and percentage (of short root tips) of mycorrhizas were higher in Norway spruce seedlings with needle B concentrations of 24 mg kg−1 than in seedlings with 16 mg kg−1 , and still less when the needle B was reduced to 7 mg kg−1 (Möttönen et al., 2001b), although all these needle B levels are in the range considered as optimal or adequate (Braekke, 1983). The enhancement in mycorrhiza formation by increased B availability is puzzling because it is opposite to the effects of N and P, and because fungi are thought not to have a B requirement. At the whole-plant level, N and P deficiencies tend to increase growth allocation to roots, while B deficiency generally decreases root growth. In agreement with this, B fertilisation increased root sugar levels in shortleaf pine (P. echinata) seedlings. This occurred both in nonmycorrhizal and mycorrhizal seedlings; hence the mycorrhizal fungus did not induce any further increase in root carbohydrates (Atalay et al., 1988). Increased carbohydrate allocation is one likely reason for B effects on mycorrhizas. Indole acetic acid (IAA) has been suggested to increase in B-fertilised plants through decreased oxidation, thereby inducing further carbohydrate transport to roots, which would enhance mycorrhizal colonisation (Lambert et al., 1980). However, the hypothesis was not supported by experimental results, as B fertilisation reduced IAA in the roots of inoculated shortleaf pine seedlings but not in uninoculated plants (Mitchell et al., 1986). In these experiments, the control plants were not B deficient. A further possibility for the B enhancement of mycorrhiza colonisation is that fungi may require a small amount of B for growth. Sword and Garrett (1994) suggested that the mechanism for increased mycorrhizal infection by added B was a reduction in phenolics, and this view was also supported by Marschner (1995). However, in the experiment testing this, it remains open whether there actually was a cause and effect relationship between B, phenolics, and the formation of mycorrhiza. Total phenolics were increased in the root systems of the P. echinata seedlings under

2058

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

low B, but this response was measured 24 weeks after mycorrhiza inoculation (Sword and Garrett, 1994). Other studies have shown a reduction in phenolics rather than an increase as a result of mycorrhizal colonisation (Müntzenberger et al., 1995), and Kikuchi et al. (2007) suggested that flavonoids may act as signal molecules for mycorrhiza formation, as several flavonoids stimulated the germination of Suillus bovinus basidiospores. On the whole, the relationship between phenolics and mycorrhizas is poorly understood, and the relationship between phenolics and B is not well understood either (see below). Experimental studies are needed on all these interrelations. An alternative approach to the many hypotheses above would be to consider whether B has a direct role in the fungus-host cell wall interaction at the onset of infection. In ectomycorrhizas, a communal cell wall matrix is formed between the fungus and host plant, which has similar pectic polysaccharide components as the plant cell wall (Nylund, 1987). The structure of this ‘involving layer’ has been noted to lack the integrity of plant cell walls (Duddridge and Read, 1984). It has been suggested that the penetration of the mycorrhizal fungus between host cells is possible only at an early stage of root development, because the cell walls have a higher pectin:cellulose ratio than mature cell walls (Duddridge and Read, 1984; Nylund, 1987). Harley (1985) suggested that the fungus interferes with the processes of deposition of the plant primary wall and middle lamella of the cell wall. As B has a role in the formation of the plant primary cell wall pectic structures (Matoh, 1997; O’Neill et al., 2004), it may have a specific role also in the formation of the ‘involving layer’. There is a paradox here, as B would be needed for disintegration of the normal cell wall structure. It is likely that the ‘involving layer’ is structurally organised, but in a different way than the regular plant cell wall. The formation of mycorrhizas involves also changes in the structural organisation and protein composition of the cytoskeleton in the mycorrhizal fungus and host plant (Timonen and Peterson, 2002). In arbuscular mycorrhizas, where the fungus forms also intracellular structures, the changes in the cytoskeleton of the host plant are more extensive than those in the fungal structures. By contrast, in ectomycorrhizas the plant cytoskeleton is not as much affected except heavily colonised parts where the plant cytoskeletal filaments disappear. The fungal cells in the symbiotic tissue of the Hartig net are more affected by the symbiosis, as there is a novel reticulate organisation, which is not found in the external mycelium (Timonen and Peterson, 2002). As one of the proposed roles of B in plants is related to the formation of the cytoskeleton (Bassil et al., 2004), the role of B in mycorrhiza formation may also be related to the cytoskeleton formation in symbiosis.

6. Boron and root nodules Boron is taken to be essential for the structure and function of ˜ the symbiotic nodules formed by both Rhizobium (Bolanos et al., ˜ 1994) and Frankia (Bolanos et al., 2002). Boron is required in the Rhizobium nodules during the formation of the nodule cell wall, but B is required also for early symbiont-plant signalling and normal ˜ et al., infection thread development and nodule invasion (Bolanos 1996). The interactions of B with glycoproteins have a role in preventing the rhizobia from being trapped into a glycoprotein matrix, which would prevent the interaction of the bacterium with the cell membrane and normal cell invasion. Furthermore, B is needed for the signalling of plant-derived glycoproteins for bacteroid differ˜ entiation into the N2 -fixing form (Bolanos et al., 2001). Recently, Reguera et al. (2010) concluded that B helps to stabilise nodule macromolecules that are important for rhizobial formation and for regulation of oxygen concentration in nodules of Pisum sativum. In Frankia symbiosis, the known role of B is in the stability of the

envelopes that prevent access of oxygen to the N2 -fixing vesicles ˜ et al., 2002). (Bolanos As can be expected, N2 -fixing nodules have particularly high B ˜ concentrations (Bolanos et al., 1994). In low-B stands, their high B requirement would lead to increasing competition for B, while plant growth would be stimulated by additional N. In highly productive, but low-B Norway spruce stands, an alder (Alnus incana) mixture commonly occurs (Tamminen and Saarsalmi, 2004). It would be interesting to study the interspecific competition for B in these conditions. 7. Boron uptake and translocation 7.1. Boron uptake by nonmycorrhizal roots In recent years, the assumption that all B uptake is passive, and driven only by transpiration (Dugger, 1983), is under challenge. For example, Asad et al. (1997) calculated that the transpired water could account for only part of the B accumulated by canola (Brassica napus) plants at low concentrations of B in a B-buffered nutrient solution. One possible uptake mechanism against a concentration gradient of B is B-complex formation in plant tissues, which would allow continued inflow of boric acid (Hu and Brown, 1997). Indeed, B uptake was found to be increased in transgenic tobacco plants with induced sorbitol production, as sorbitol has high affinity for B-complex formation (Bellaloui et al., 1999). Furthermore, active B uptake has now been shown. While B uptake was passive at higher external concentrations, at lower B availability (<1 ␮M) it was active (Pfeffer et al., 1999). Boric acid uptake is facilitated by membrane intrinsic proteins in squash (Cucurbita pepo) roots (Dordas et al., 2000; Dordas and Brown, 2001). Membrane B transporters for xylem loading have been identified in Arabidopsis (Takano et al., 2002, 2008) and some other herbaceous species (Miwa and Fujiwara, 2010), but families of transporters are likely to be wide-spread. The scope and variation of B transporters have yet to be identified, but a probable homologue for BOR1 has been found in silico in Eucalyptus (Domingues et al., 2005a,b). However, in the poplar genome these transporters were not reported (Tuskan et al., 2006). Moreover, most B uptake studies have been done with plants without mycorrhizas, and it remains to be shown if mycorrhizal trees differ in this respect. 7.2. Boron mobility in fungal mycelia and boron uptake by mycorrhizas Ectomycorrhizas are crucial in the uptake of at least P and N for their hosts (Marschner and Dell, 1994; Smith and Read, 2008; Plassard and Dell, 2010), and the uptake of Ca (Melin, 1952) and Mg (Jentschke et al., 2000) have been shown using isotope tracers. As discussed earlier, B is usually assumed not to be an essential element for fungi, and it has been suggested that ectomycorrhizas do not take up B. This is because many fungal carbohydrates, such as mannitol, have high affinity for complex formation with B, whilst sucrose, the most common compound for carbohydrate translocation in plants, has very low affinity for B. Lewis (1980) hypothesised that it was the ability to translocate sucrose that allowed B to become an essential nutrient for plants in the course of evolution, but not for fungi and microbes. According to this hypothesis, B would be sequestered in an immobile form with fungal carbohydrates, and therefore, not translocated from the fungal partner of the symbiosis to the host. The considerable B accumulation in sporophores of many mycorrhizal and saprotrophic Basidiomycete species shows that B is translocated in the mycelia towards the sporophores (Lavola et al., 2010). However, there are great differences in the amounts of

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

B accumulated in sporophores of different species, as the largest individual value was 280 mg kg−1 in Paxillus involutus, and the lowest measured values were 0.01 mg kg−1 in Amanita muscaria, or below the detection limit in Coprinus comatus. Also, Rozites caperata, Dermocybe semisanguinea, Lactarius rufus and L. campohoratus had values consistently <1–2 mg kg−1 , and many species had quite low median B concentrations, 2–3 mg kg−1 . The lowest values were lower than the total-B concentration in the soil organic matrix or in live Norway spruce needles. The differences between fungal species may be attributed to their different water use, or different carbohydrates used in long-distance translocation. Mannitol-B complexes are possible candidates for B transport as mannitol is found in almost all ectomycorrhizal species studied, although its relative proportion to other soluble carbohydrates can vary considerably (Nehls, 2008). In plants, B complexation with polyols such as mannitol and sorbitol is a mechanism for B phloem transport (Brown and Shelp, 1997; Hu et al., 1997). As the increased sorbitol production in transgenic tobacco plants also increased their B uptake (Bellaloui et al., 1999), it is not impossible that mannitol and other polyols in mycorrhizal fungi also have this effect. Boron transporters have so far been studied only in yeast (Saccharomyces cerevisiae), with the focus on B toxicity, in which the B exporter ATR1 was found to be highly efficient (Kaya et al., 2009). BOR1, the transporter for xylem-loading in plants, was expressed in this species as well, and occurs in some Ascomycete fungi (Kaya et al., 2009). There is now evidence from a stable isotope (10 B) labelling study that B can be taken up by external hyphae of the ectomycorrhizal symbiont P. involutus, and that this B can be translocated to the stems and leaves of the host plant, silver birch (B. pendula) (Lehto et al., 2004a). In this study, the B concentration in the boric acid solution applied directly on the external mycelium was very high, 50 mM, yet the amount of B remaining in the roots and mycorrhizas was considerable after 72 h. It remains to be shown whether ectomycorrhizas increase or decrease B uptake under low-B conditions, and what differences may occur between species. The mechanism of mycorrhizal B uptake remains to be studied, but at least passive uptake of boric acid appears to occur. However, the membrane permeability to B in fungal cells may differ considerably from that of plants because of their different composition. The sterol composition of the membrane affects the permeability to boric acid, and this appears to be an important source of the variability between species and genotypes in channel-mediated B uptake (Dordas and Brown, 2000). There is evidence to support channel mediated water uptake in ectomycorrhizas as aquaporin expression in poplar was increased in inoculated as opposed to nonmycorrhizal seedlings (Marjanovic´ et al., 2005). The form of B taken up is usually assumed to be undissociated boric acid (Hu and Brown, 1997). However, the uptake of organic complexes with B is also conceivable. Boron has high affinity for forming complexes with hydroxyl groups, such as organic acids (Wikner, 1983; Dembitsky et al., 2002). Organic acids and other small-molecule compounds abound in many soils rich in organic matter, and it is increasingly being recognised that they have a role in the carbon cycle in soils (Van Hees et al., 2005). Different soluble compounds need to be studied, as the stability of the mannitolB complex increases with increasing pH up to pH 8.0 (Power and Woods, 1997), which is much higher than the pH in most forest soils. Such B complexes might be taken up by mycorrhizas or roots, but so far uptake of B complexed with organic compounds has not been shown either in nonmycorrhizal or mycorrhizal plants. Complex formation with mannitol and other compounds within the mycelium would allow continued uptake of boric acid from soil against a concentration gradient (cf. Hu and Brown, 1997), and Bmannitol complexes could function in long-distance transport in the fungus. However, transport of B as a mannitol complex to the

2059

host plant would be a very highly specialised mode of transport. It would also be a form of ‘reverse’ carbon translocation in the mycorrhizal symbiosis, as is the uptake of amino acids (Smith and Read, 2008). If fungi actually do not have a B requirement, active uptake by mycorrhizal fungi is less likely than in the case of nutrients required by the fungal symbiont. However, the possibility of a requirement of fungi for B remains open. As to mycorrhizal fungi, the question of a requirement of a nutrient is complex. Obligate mycorrhizal fungi cannot complete their life cycle without a host plant, and as they require the host plant for their growth, development and reproduction, in a sense they cannot be considered not to require a nutrient which is required by the host plant. For the fungus, completing the life cycle includes producing sporophores and viable spores. As there are indications of B in spores of B-accumulating species (T. Lehto, unpubl.), this issue could be first approached by measuring B concentrations in spores of those species that have very low B concentrations. Testing effects of B on fruiting on defined media would be another approach. It is not known whether mycorrhizal plants are superior in terms of B uptake than nonmycorrhizal plants or not. Translocation from mycorrhizas to shoots may also be less efficient in mycorrhizal plants if significant amounts of B are retained in the mycelia. The possibility for nutrient immobilisation by mycorrhizal fungi has been largely overlooked in the past. This is somewhat surprising, as there has been considerable interest in the possible beneficial effects of mycorrhizas in precipitating or otherwise reducing excess amounts of heavy metals (Jentschke and Godbold, 2000). Berthelsen et al. (1995) estimated that nearly 100% of the copper of the mor humus could be in mycorrhizas, although it remained unknown whether more Cu was in the fungal or the root portion of the mycorrhiza. It is possible that the symbiotic system has controls for increasing the uptake of micronutrients when there is shortage in the plant, and reducing it when the supply is excess, but so far there is little evidence for this. If a large part of the B taken up from soil is retained in mycorrhizal fungi, it will be significant in areas of low B availability. Taking the estimates by Högberg and Högberg (2002) and Wallander et al. (2001), the range of biomass of ectomycorrhizal mycelium (mycorrhizas with their external mycelium) is 180–900 kg ha−1 . Assuming that the B concentrations of the mycelia are similar to those in sporophores, and assuming a moderate range for the mean B concentration in the mycelia of different species, 1–12 mg kg−1 B in the mycelium (Lavola et al., 2010), the amount of B in the mycelium in soil would be between 0.18 g ha−1 and 10.8 g ha−1 . The upper estimate is larger than the estimate for annual B uptake in a typical mature Norway spruce stand in Finland, the amount in the tree foliage is 50–100 g ha−1 and the annual B uptake for the aboveground parts is 5–10 g ha−1 (Aphalo et al., 2002). Therefore, the retention processes into/onto the mycelium are of consequence to the micronutrient balance and leaching processes in forests. An important aspect of the nutrient retention in mycelia is its longevity, which is poorly known, and likely to depend on environmental factors such as soil moisture and temperature. The longevity of the external mycelium depends partly on the longevity of fine roots, and there are very different estimates of this currently (Trumbore and Gaudinski, 2003). Boron toxicity in forests is not as wide-spread as deficiency, but there are still some species which may be exposed to excess B. In these circumstances it is of interest to find out if mycorrhizal fungi – ectomycorrhizal, arbuscular, or ericoid mycorrhizal – can protect their host plants from B toxicity. Indeed, Sonmez et al. (2009) found that arbuscular mycorrhizas decreased B accumulation and toxicity symptoms in durum wheat, although yield was not affected by mycorrhizas at the high B levels. However, in an experiment on Pinus banksiana, mycorrhizal inoculation did not affect the

2060

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

tolerance of seedlings to high B levels, although ectomycorrhizas increased their salt (NaCl) tolerance (Calvo-Polanco et al., 2008). Populus may be a useful candidate to explore mycorrhiza x high B interactions as some species are being used for the phytomanagement of B-contaminated sites (Robinson et al., 2007) and roots can associate with both ecto and endomycorrhizal fungi. 7.3. Soil and meteorological factors affecting B uptake In addition to mineral composition, the soil factors that influence B availability to plants include clay content, Al and Fe oxides, organic matter content, and texture. Boric acid in soil solution is leached easily from coarse textured soils with downward water movement. Boron is retained in soil through adsorption on soil particles. Adsorption is based on the same property of B that regulates its function in live tissues, namely the high affinity to bind to hydroxyl groups in cis position. Boron is adsorbed on clay minerals, Fe and Al compounds, and organic matter, and the strength of adsorption is increased with increasing pH (Peterson and Newman, 1978; Lehto, 1995; Yermiyahu, 1995; Goldberg, 1997). In acid forest soils with little organic matter, B retention can be very low (Lambert and Ryan, 1990). Soils affected by acid precipitation, e.g. in Central Europe, may be depleted of B as well as Mg and other nutrients, even though there is some B in precipitation originating from fossil fuels. Adsorbed B is not considered to be readily available for plants, yet adsorption is a major process that reduces B leaching from soils. Aside from retaining B, clay minerals also release B into the soil solution through weathering, and organic matter releases B through decomposition. Boron is released rapidly from litter (Lehto et al., 2010a,b), and the adsorption processes reduce leaching, especially when the litter is from plants with sufficient B. In litter from near-deficient trees, B is mostly bound in cell-wall structures, which probably explains the initial accumulation of B from soil in B-poor litter (Lehto et al., 2010a,b). This B may be transported by fungi into the litter, in the same way as other nutrients are known to accumulate in litter during the first years of decomposition (Berg and McClaugherty, 2008). It is of note that 70–80% of the total-B in the organic layer of podzol soils was hot-water extractable B (calculated from Table 8 in Tamminen and Saarsalmi, 2004), which is considered to be well related to plant-available B. Soil drying is usually taken as a reason for reduced B uptake in crops, as B is translocated in the xylem with the transpiration stream (Hu and Brown, 1997). Comparisons of the variability of needle B concentrations in Norway spruce with the rainfall in the preceding week (Sutinen et al., 2006) or over a growing season in lodgepole pine (Pinus contorta), Douglas fir (Pseudotsuga menziesii, Carter and Brockley, 1990) and black spruce (Picea mariana; White and Krause, 2001) lend support to this also in trees. In eucalypts, symptoms of deficiency are more severe during the dry season and mildly affected trees may partially recover during the following wet season (Dell et al., 2001). In Norway spruce seedlings, a relatively mild drought treatment increased root B concentrations in all B treatments, while needle B was not affected (Möttönen et al., 2001a). After repeated drought treatments, B concentrations decreased in needles; suggesting that translocation to needles was more impaired by stomatal closure than uptake to roots in Norway spruce seedlings (Möttönen et al., 2005). In a survey of Norway spruce stands in Finland, the foliar B concentrations were positively correlated with the occurrence of peatland mosses (Tamminen and Saarsalmi, 2004). This suggests that B uptake was less impeded in slightly paludified podzol stands, where water is available even during rainless periods. Hence B uptake is temporarily affected by soil humidity even in a humid climate; in climatic regions with more droughts during the growing period the temporary B deficiency can be profound (Hopmans and Clerehan, 1991). The fact that B concentrations can vary quickly with water avail-

ability causes additional uncertainty in the interpretation of foliar B analysis data. Keren et al. (1985) did not find a difference in the distribution between the solid (adsorbed to soil particles) and liquid phases of B in soil with variable moisture. Although B mobility is still reduced by drought, this would suggest that the reasons for reduced B uptake during drought could rather be sought in reduced transpiration through stomatal closure than in B mobility in soil. However, in the studies on Norway spruce quoted above, the trees most probably did not suffer from drought, as water would still be available for roots deeper in the mineral layers. The mineral layers would not supply sufficient B, if most of the soil B is in the surface organic layer and surface mineral layers (Wikner, 1983; Tamminen and Saarsalmi, 2004). Similar situations have been observed in eucalypt plantations in Asia, where the surface soils may dry quickly. Although the trees still have access to water at depth, they do not obtain sufficient B, as B is limited to surface soil horizons (B. Dell, personal observation). Hence the moisture content of the surface layers may be critical to B uptake. Environmental factors that influence stomatal closure include light and air humidity, and the light level has been found to affect critical B levels for growth in black gram (Vigna mungo) (Noppakoonwong et al., 1993). Studies separating the soil moisture effects on B availability in soil, and translocation in the xylem are necessary to assess the importance of the different processes in the field where trees have roots in different layers. Frozen or cold soil has the potential of reducing B uptake both by reducing water uptake and transpiration in evergreen trees in early spring (e.g. Lahti et al., 2002), and by reducing active B uptake. Channel mediated transport is reduced at higher temperatures than active uptake, which may affect B uptake through water uptake (Huang et al., 2005). In Finland, B fertiliser applied in early September was not taken up by Norway spruce before the following growing season (Rikala and Vuorinen, 2005). Although root growth is known to occur in September, transpiration is limited by low light, low temperature and high air humidity, and probably also winter hardening processes. It appears that boreal trees are not able to ‘stock’ B during autumn conditions even if the soil temperature and moisture were favourable.

8. Retranslocation For a long time, B was considered as one of the least mobile plant nutrients (Dugger, 1983). Immobility would appear to be a logical consequence of the role of B as a cell wall constituent. However, in non-deficient conditions, the amount of B in cell walls is relatively constant within a species (Dannel et al., 1998), and the fraction of B that is not bound in the cell wall, is potentially mobile. Studies on trees with extremely low B levels may have contributed to the suggestion that B is immobile. In extreme conditions, there is no potentially mobile B, or the concentrations are similar or higher in older needles compared to the youngest needles. In these conditions, B fertilisation alters the situation such that B concentrations become higher in new needles in radiata pine (Hopmans and Clerehan, 1991) and Norway spruce (Lehto and Mälkönen, 1994). In the field, calculations based on variations in needle concentrations and specific needle weight showed that 9–17% of the B requirement was satisfied with retranslocation in Scots pine stands. The trees had relatively low, but not deficient needle B concentrations during dormancy, 8–15 mg kg−1 (Helmisaari, 1990, 1992). In B. pubescens, foliar B concentrations increased or remained stable in several stands of low (3–9 mg kg−1 ) or intermediate (20–25 mg kg−1 ) leaf B (Ferm and Markkola, 1985). However, the stand with the highest leaf B (35 mg kg−1 ) showed a clear decreasing trend during senescence, suggesting retranslocation. Moreover, B mobility from

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

leaves to flowers and fruit was found in apple, pear and prune trees (Hanson, 1991) and olive trees (Delgado et al., 1994). Brown and Hu (1996) demonstrated that B is particularly mobile in tree species which use sorbitol as a major carbohydrate, such as Malus and Pyrus species, but in other fruit tree species, B was not freely mobile. These studies suggested that plant species can be divided into categories where B is either not retranslocated at all, or it is retranslocated very efficiently, depending on the occurrence of phloem mobile polyols such as sorbitol, mannitol and dulcitol in the species (Brown and Shelp, 1997), which form soluble, phloem mobile complexes with B. Complexes of B with mannitol, sorbitol and fructose have been shown in the phloem sap of the extrafloral nectar of species with B mobility (Hu et al., 1997). In transgenic rice and tobacco plants, inducing sorbitol production also induced the ability to remobilise B (Brown et al., 1999; Bellaloui et al., 2003). These findings corroborated the conclusion of Brown and coworkers on the polyol complex mobility as a major mechanism of B phloem mobility. Mannitol and sorbitol function in long-distance phloem transport in many plant species (Zimmermann and Ziegler, 1975; Noiraud et al., 2001), but there is a large number of different polyols that occur in different plant species, and their function in long-distance transport and abilities to form B complexes are still to be explored. There were somewhat different degrees of mobility in woody plants in a subsequent study that were not clearly correlated to the occurrence of polyols (Brown and Hu, 1998a). In a survey of deciduous forest tree species, extensive B mobility was found in Sorbus and Prunus seedlings as expected on basis of their high sorbitol content, but also in Ulmus glabra, which contained only trace amounts of Bcomplexing polyols (Lehto et al., 2004c). Furthermore, B mobility was not shown in Alnus glutinosa although the polyol concentrations were almost identical to a closely related species, A. incana, in which B retranslocation occurs. In Scots pine and Norway spruce the mobility also appeared to be less complete than in other species (Lehto et al., 2000, 2004b). Therefore, it appears that the abilities of different species to retranslocate B, rather than falling into two totally distinct groups, mobilising and non-mobilising, could be better described as a continuum. The remobilisation of B in species such as canola (Stangoulis et al., 2001), sunflower (Matoh and Ochiai, 2005) and white lupin (Huang et al., 2008) further suggest that large amounts of polyols are not an exclusive condition for B phloem mobility. Fructose-B complexes were also shown to occur in peach (Hu et al., 1997). As many plants have the potential for fructose translocation in the phloem, this could provide a partial answer to the restricted, but still clear mobility in some species. Moreover, other compounds such as malic acid (Dembitsky et al., 2002) also have the potential for carrying B, although their role in B transport in plants has not been studied. Active B transporters for xylem loading have been shown in plants (Takano et al., 2002). The transporter function for phloem loading is still not well understood, but active B transporters, together with channel-mediated facilitation as in B uptake, may provide an explanation to the mobility of B in different species, and also for B mobility to non-transpiring tissues such as flowers and fruit. In conditions where the internal B status is not extremely low, B mobility has been shown to increase with lower B status of the plant in a few species including coffee (Leite et al., 2007). Also in the olive tree, leaf phloem mannitol concentration was twice as high at relatively low B supply than in a B fertilised treatment. This was associated with continuous B supply from source to sink leaves (Liakopoulos et al., 2009). The processes of retranslocation vary depending on season. In strongly seasonal climates, the ability to redistribute B internally may be particularly important for growth of new shoots in the spring or in the beginning of the rainy season. Shoot extension growth in boreal conifers occurs within a

2061

relatively short period, and the buds burst when both stomatal conductance and nutrient uptake are limited by low soil temperatures (Domisch et al., 2002). In these circumstances, shoot growth may be prevented by a temporary B deficiency, if sufficient B retranslocation does not occur. Furthermore, many boreal and temperate tree species flower before (Alnus, Corylus) or immediately after leaf emergence (Betula). It is yet to be shown if B will be stored in the root systems and stems of deciduous trees over the winter, to be returned with the spring sap to support budburst. The B concentration in the spring sap of B. pendula was 101 ± 2.7 ␮g l−1 (mean of 3 trees ± S.E., unpublished results by M. Räisänen and T. Lehto), and 142 ± 2.9 ␮g l−1 in B. pubescens, demonstrating that there was a remarkable amount of B in the ascending sap of both species, and a lower concentration in B. pendula than in B. pubescens, growing in the same stand. These results can be compared with the nitrogen concentration of the sap, which was 116 ± 32 mg l−1 in B. pendula, and 102 ± 12 mg l−1 in B. pubescens. In Norway spruce, there was evidence for B mobility from other parts to new growth in spring (Lehto et al., 2004b). However, a B withdrawal study on Scots pine and Norway spruce seedlings indicated that the development of the following year’s primordia was not completely normal if B was not supplied early in the growing season (Sutinen et al., 2007). In this study, the initial B levels were very low, probably not allowing adequate B mobility. Large differences were shown in five deciduous trees and Norway spruce, growing in similar soils with non-deficient B: they had a range of foliar B concentrations between 26 and 27 mg kg−1 in ash (Fraxinus excelsior) and Norway spruce, while lime (Tilia cordata) had up to 77 mg kg−1 and oak (Quercus robur) 47 mg kg−1 (HagenThorn and Stjernquist, 2005). These differences were attributed to differences in transpiration or B retranslocation, and indeed, ash and spruce have been found to mobilise B to a greater extent than these lime and oak species (Lehto et al., 2004b,c). Without B retranslocation, B deficiencies might be much more common in forests, as evergreen trees would sequester a major part of the potentially available B in their standing biomass (Aphalo et al., 2002). In further studies, the ecological significance of this phenomenon needs to be explored. The importance of retranslocation is related to the efficacy of external nutrient cycling through litter and soil processes. This may enable retranslocating species to thrive in sites with lower B availability than species which do not retranslocate, as they have access to stored B in the spring before efficient B uptake and translocation can occur with the transpiration stream. Furthermore, prevention of leaching by winter storage is also potentially of importance. If B tends to be retained in an unavailable form rather than released during decomposition, or leached with water particularly after snowmelt, retranslocation is relatively more important than in conditions of efficient ecosystem cycling.

9. Boron nutrition and susceptibility to environmental stress 9.1. Drought resistance Boron may be thought to affect tree drought resistance directly through membrane integrity, possibly affecting stomatal function. However, earlier studies have shown no direct effect on stomata or photosynthesis (Dell and Huang, 1997), but these can be impaired when the deficiency is severe enough to cause structural damage (e.g. Zhao and Oosterhuis, 2002; Han et al., 2008). Indirectly, B can affect drought resistance through morphological changes such as the structural integrity of the vascular bundle, which in turn can affect whole-plant hydraulic conductivity, or through root growth, affecting water uptake.

2062

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

Zhao and Oosterhuis (2002, 2003) exposed cotton plants with different B nutrition to drought, and found that B nutrition did not make any difference in the first two weeks. Only after the B deficiency became severe enough to affect growth, were photosynthesis and water relations affected, and at this stage there may already have been damage in the root systems. In Norway spruce seedlings subjected to drought, the B level did not directly affect photosynthesis and water relations either, but seedlings with low B were shorter at the end of the drought periods (Möttönen et al., 2001a). Similarly, B had little effect on the recovery of Norway spruce seedlings from a drought treatment as indicated by photosynthesis and water relations, but seedlings with high B grew somewhat better after drought. Only seedlings in the most severe drought treatment with low B (6 mg kg−1 in needles) had occurrences of tip dieback (Möttönen et al., 2005). In these cases, the actual damage was apparently caused by acute B deficiency in the growing point, as the drought would reduce water and boron translocation. Hence the effects of drought reducing B uptake seemed to be clearer than B deficiency reducing water uptake. 9.2. Cold tolerance Boron deficiency often occurs in parts of the world where tree crops are exposed to cold stress. Chilling temperatures can range from just above 0 ◦ C to a few degrees below freezing in warm climates, while the cold hardiness is strongly seasonal in cold climates. Huang et al. (2005) examined B × chilling interactions in warm climate annual crops and concluded that root chilling decreases B uptake efficiency and B utilisation in the shoot and increases the root:shoot ratio. They further noted that B deficiency exacerbates chilling injuries in leaves, under high photon flux density. It has been suggested that B deficiency makes trees more susceptible to frost damage, because the dieback of apical buds is often visible after winter (Kaunisto, 1984; Pietiläinen, 1984). The symptoms of frost damage and dieback may be similar in some species, and possibly confused. However, Cooling (1967) described different symptoms in Eucalyptus grandis, and reported a higher incidence of frost damage in plots without B fertiliser, in a very low-B soil. By contrast, Aronsson (1980) did not find a correlation between nitrogen-induced B deficiencies and frost damage. In cold climates, trees normally harden early enough in the autumn to tolerate frost temperatures. Deep supercooling is the mechanism of avoidance of apoplastic freezing in buds, and in some trees in stems as well (Quamme, 1995). Supercooling only functions to about −40 ◦ C for physical reasons. If the ability of deep supercooling is not adequate, the cells collapse, and this is visible in the following spring as dieback of buds. Another mechanism is tolerance to apoplastic freezing and the concomitant intracellular dehydration with consequent concentrating of cryoprotectant substances in cells. This ability is widespread in plants from cold climates, and it allows tolerance to less than −40 ◦ C. One possible mechanism of increased susceptibility to damage in B-deficient trees is a direct consequence of the role of B in the formation of the cell wall (Matoh, 1997), and the fact that ice crystal formation starts in the intercellular spaces. Membranes may also be dysfunctional in B deficient trees, which may affect their tolerance during intercellular ice formation and thawing. Another possibility is that B deficiency affects the development of vascular tissues, which can lead to problems in the translocation of water and carbohydrates within the plant (Dell and Huang, 1997). The transport of carbohydrates might particularly affect the fine roots, which in many cases are the plant parts most susceptible to frost damage (Ryyppö et al., 1998). However, Räisänen et al. (2004, 2007, 2009) did not find effects of low B on the freezing tolerance or winter mortality of Norway spruce roots.

Different plant parts react differently to freezing, and the tolerance mechanisms vary in different parts of the same plant. In cold acclimated Norway spruce seedlings, the freezing tolerance of roots and needles was not affected by B level. However, the tolerance to freezing dehydration was slightly affected in B-deficient seedlings and there was some indication that freezing tolerance of buds and stems was slightly decreased (Räisänen et al., 2007, 2009). Buds, and possibly stems are organs which acclimate through deep supercooling. Therefore, it appears possible that B deficiency affects this mechanism, rather than hardening through tolerance to extracellular freezing. There is a pectin-rich ice-barrier structure with micropores in buds and stems which harden through deep supercooling (Wisniewski, 1995; Jones et al., 2000). The rhamnogalacturonan content in cell walls increases during cold acclimation (Kubacka-Zebalska and Kacperska, 1999), and B can be involved in the formation of the pectic structure in the ice barriers. Small pore size prevents ice penetration through the barrier, but the size of the micropores can be increased by B deficiency (Fleischer et al., 1999), and through this, the ability to deep-supercooling may be disturbed (Räisänen et al., 2007). The main conclusion from the studies on B and freezing tolerance in Norway spruce is that B can slightly affect the freezing tolerance in organs which acclimate by deep supercooling. However, the effects found are small in extent in cases where the deficiency has not caused severe structural damage (Räisänen et al., 2006a, 2009). When the structure of buds was already incomplete because of B deficiency, the buds did not have the ability to deepsupercool (Räisänen et al., 2006b); however, the freezing effect was secondary. The adverse effects of low B found in structurally normal plant parts were small in extent and are not expected to strongly affect the fitness of Norway spruce (Räisänen et al., 2009).

10. Oxidative stress, phenolic compounds, and defense reactions Aerobic metabolism, respiration and photosynthesis, continuously produce reactive oxygen species (ROS) in organisms. In plants, several biotic stressors such as wounding and herbivory (Orozco-Cárdenas et al., 2001; Musetti et al., 2005; Ruuhola and Yang, 2006), pathogen infection (Levine et al., 1994) and also abiotic stressors like acid rain and heavy metal deposition (Koricheva et al., 1997; Ruuhola et al., 2009) cause an enhanced accumulation of ROS such as singlet oxygen, superoxide anion and hydrogen peroxide. Although ROS are harmful for all living organisms including plants, they are also important signals mediating, for example, defence responses against multiple stressors, stomatal closure and plant development (Laloi et al., 2004). Also environmental factors such as light, temperature or water or nutrient availability affect the efficiency of photosynthetic electron transport and the redox state of chloroplasts causing the rapid accumulation of ROS (Laloi et al., 2004). The accumulation of hexoses due to severe B deficiency has been shown to down regulate photosynthesis and the photosystem electron transport chain becomes over-reduced leading to the accumulation of ROS in Citrus grandis (Han et al., 2008). Plants have antioxidant enzymes such as superoxide dismutase (SOD), catalases (CAT), ascorbate (APX), monodehydroascorbate reductase (MDAR), glutathione reductase (GR) and quaiacol peroxidases (POD), and non-enzymatic antioxidants such as ascorbate (AsA) and gluthatione (GSH) that suppress excess ROS. There are only a few studies investigating the antioxidant related responses of plants to B deficiency, especially in trees, and the results are somewhat conflicting depending on the plant species (Han et al., 2009). In C. grandis, B-deficient leaves showed higher SOD, APX, MDAR and GR activities and AsA content than B-sufficient

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

leaves (Han et al., 2008) whereas in Malus domestica B fertilisation enhanced CAT and GR activities (Wojcik et al., 2008). Low B decreased SOD, APX and CAT activities and AsA in herbaceous Glycine max (Liu and Yang, 2000) and decreased contents of AsA and GSH were detected in B-deficient sunflower (Cakmak and Römheld, 1997). Also phenolic compounds have antioxidative properties (Pourcel et al., 2007 and the oxidation of phenols may act synergistically with AsA in detoxification of H2 O2 ; quinones formed by phenoloxidases can be reduced back to parental phenols by AsA and GSH whereas a decrease in reductants stimulates quinone production and formation of ROS (Cakmak and Römheld, 1997; Takahama and Oniki, 1997). Hence the oxidation of phenolics may be both antioxidative and pro-oxidative processes depending on the circumstances. Boron deficiency is often linked to the browning of leaf and meristematic tissues that is probably a consequence of increased accumulation and oxidation of phenolics. Phenols accumulate in response to B deficiency in a number of herbaceous plants (Pfeffer et al., 1998; Ruiz et al., 1998; Camacho-Cristóbal et al., 2002; Stavrianakou et al., 2006a,b) but only a few studies have been conducted with woody plants. The contents of total phenolics were increased in P. echinata (Sword and Garrett, 1994) and several flavonoids and two phenolic acids in turn accumulated in the leaves of Olea europaea in a growth chamber experiment, whereas in a field study the contents of phenolics were not affected by B deficiency (Liakopoulos and Karabourniotis, 2005). Rummukainen et al. (2007) found that in Norway spruce seedlings, the only effect of added B (in low-B, concentrations less than 3 mg kg−1 ) was a decrease in the concentration of condensed tannins, the predominant group of phenolic compounds in this species, from 55 to 45 g kg−1 . Also the tannin precursor, (+)catechin, was slightly increased by B deficiency. As the small-molecule phenolic compounds were not affected, it was suggested that the phenolic pathway was pushed towards the end product, condensed tannins. On the other hand, after a simulated autumn season, the tannin levels increased to 170 g kg−1 , and the B effects disappeared (Rummukainen et al., 2007). Therefore, the increase in tannins was minor compared to the apparently normal seasonal fluctuation in tannin levels. In more severely B-deficient Norway spruce trees in the field, the number of tanniferous cells was increased compared to trees in a nearby stand with higher B levels (Sutinen et al., 2006). In B. pendula, Ruuhola et al. (unpubl. data) found that B had no direct effect on the contents of phenolic compounds but the B supply interacted with an herbivory treatment: herbivory increased the levels of several phenolics but these changes were found either in Bdeficient or B-fertilised seedlings, rarely the increase was detected at both B levels. Under B deficiency, phenols accumulate apparently due to the increased activity of PAL, phenylalanine ammonia lyase (Cakmak and Römheld, 1997; Camacho-Cristóbal et al., 2002) that is a key enzyme in the biosynthesis of phenylpropanoids. In addition, the oxidation of phenols may increase through the induced activity of phenoloxidases (Cakmak and Römheld, 1997; Pfeffer et al., 1998; Camacho-Cristóbal et al., 2002) and decrease in the AsA and GSH contents (Cakmak and Römheld, 1997). As well, enhanced oxidation of phenols generate ROS, increasing the oxidative stress in plants (Cakmak and Römheld, 1997; Dordas and Brown, 2005; Koshiba et al., 2009). By contrast, Cara et al. (2002) found that both PPO and POD activities declined significantly over time with B deficiency in the roots of squash plants. Ruuhola et al. (unpub. data) found that the activities of both PPOs and PODs were increased as a response to herbivory in birch seedlings with regular B supply, but not in Bdeficient seedlings indicating that B deficiency may impair defence signalling in trees. At the same time, the level of chlorogenic acids,

2063

the substrates of phenoloxidase, were increased but again only in B-fertilised seedlings. Similarly, Ruiz et al. (1999) found that joint application of a fungicide (carbendazim) and B increased the biosynthesis and oxidation of phenolics. The accumulation of phenolic compounds, or an increase in their oxidative status, was suggested not to be a primary effect of B deficiency, but a secondary effect of primary structural problems in cell wall differentiation in a cell culture study (Dordas and Brown, 2005). Nevertheless, the accumulation of secondary compounds can have ecological significance, as condensed tannins may constitute 5–20% of the dry weight of conifer needles (Rummukainen et al., 2007). The reported changes in phenolic metabolism due to B deficiency or B fertilisation may affect the plant–herbivory interaction since several phenolic compounds and phenoloxidase activities in parallel with ROS have defensive properties against plant-eating animals (see e.g. Felton et al., 1989; Bi and Felton, 1995; Ruuhola et al., 2007, 2008). Similarly, resistance to plant pathogens can be affected, and correction of B deficiency has been found to reduce diseases by several pathogens in cotton, bean, tomato and wheat (Dordas, 2008). In addition to chemical defenses, the role of B in cell wall structure and possibly membrane stability and function can decrease susceptibility to pathogens in deficient trees (Dordas, 2008). The effects of B nutrition on resistance of plants against herbivores have yet to be published, but T. Ruuhola, T. Leppänen, R. Julkunen-Tiitto, M. Rantala and T. Lehto (unpubl. data) conducted a study wherein it was found that adequate B, as opposed to a lowB treatment of silver birch seedlings (B. pendula) reduced female pupal weights of Epirrita autumnata and this is probably linked to increased phenoloxidase activities and increased levels of their substrates. In addition, the immunity of moths measured as an encapsulation rate, tended to be lower in B-fertilised seedlings than B-deficient ones. However, due to compensation feeding. B-fertilised seedlings suffered from heavier defoliation than unfertilised ones. In conclusion, B deficiency causes oxidative stress in plants but plant responses to this stress is variable depending on the plant taxon and experimental conditions. Both B deficiency and B application affect the levels of small-molecular antioxidants and phenol metabolism. Exhaustion of reductants increases the oxidation of phenolics, turning this towards a pro-oxidative process and produced quinones and ROS have defensive properties against herbivores (Ruuhola et al., unpubl. data). In other words, B may play an important role in plant defence against herbivores.

11. Wood quality, litter quality and decomposition Boron deficiency affects wood quality directly through deformations, which can limit the usage of wood as structural timber. Eucalypt stems can become so badly deformed that their transport is more tedious. The chemical quality may also be affected. Lignin accumulation has been found in B-deficient plants (Dugger, 1983), although on the other hand, poor lignification has also been considered as a symptom of B deficiency (Lewis, 1980). It is not known in which concentration ranges the lignin effects may occur. Using a histological method, Dell and Malajczuk (1994) found increased lignification in eucalypts after B fertilisation in very low-B conditions, but B fertilisation did not affect lignification in Pinus radiata in another study (Turvey et al., 1992). However, B fertiliser increased tracheid cross-sectional area and wall thickness in 8-year-old P. radiata (Skinner et al., 2003) and histological staining suggested an increase in lignification. In a B rates trial undertaken in China (Dell and Xu, unpub. data) mild B deficiency depressed fibre length but did not alter round wood volume or green weight of E. globulus. In the same species, freshly debarked sapwood from B-deficient trees

2064

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

can darken within minutes due to oxidation of excess phenols (Dell and Huang, 1997). The association of blackening of Diospyros wood with high B wood concentration observed by Minato and Morita (2005) appears to be an unrelated phenomenon. As discussed above, B deficiency may affect the concentrations of secondary metabolites and lignin, which in turn affect the quality of litter for decomposition by concentrations of secondary metabolites and lignin. Lignin concentration is a major determinant of the later stages of the decomposition process, as there are relatively few organisms capable of decomposing lignin. Both small-molecule phenolics and tannins tend to disappear quickly from litter through condensation (in the case of small molecules), decomposition, and leaching (Kainulainen and Holopainen, 2002; Lorenz et al., 2000; Lehto et al., 2010a,b). However, in some species and environmental conditions they can accumulate in litter over the years (Preston et al., 2009). Moreover, tannins leached from litter at the top of the organic layer may accumulate in lower parts (Kanerva et al., 2008). Condensed tannins have a role in decomposition both through their direct effects on decomposers and possibly also through their tendency to precipitate proteins (Hättenschwiler and Vitousek, 2000). Boron can also affect concentrations of pectic substances (hemicellulose), which are relatively easily decomposable compared to cellulose and lignin. In a comparison between different non-woody species, uronic acid concentrations in cell walls were shown to be linearly related to the B requirement of the species (Hu et al., 1996). However, within-species relationships of pectic concentrations and B nutrition have still not been adequately studied, particularly in woody plants. In addition to quantities of different components, their organisation relative to each other, such as the localisation of pectins in relation to lignified cell wall parts, could play a role in the B effect on decomposition (Berg and McClaugherty, 2008). Boron may additionally affect litter quality through structural integrity of tissues. So far, the only published studies on B effects on decomposition and element release were done in Finland with growth-chambergrown birch litter and field-grown Norway spruce litter. Boron level during growth slightly increased the initial mass loss in silver birch leaf litter (Lehto et al., 2010a), however, Norway spruce litter was not affected (Lehto et al., 2010b). Lignin concentrations in Norway spruce needle litter were decreased as a result of B fertilisation, but only if P was also applied. In these studies, the effect of B on element release was stronger than effects on the mass loss. Litter from B-fertilised birch retained more P, S, Cu, Cd, Ni and Zn. In spruce, there was a strong interaction of P and B, increasing the retention of Al, Ca, S and Zn in the decomposing litter. The N + B combination tended to have slightly the opposite effect both on lignin and element retention. As the P + B fertiliser combination also reduced lignin in the litter, it appears worth while studying the interaction of these nutrients on leaf and litter chemistry in more detail.

12. Correction of B deficiency The earlier review of Stone (1990) contains much useful information on correction of B deficiency, and therefore this section will focus on more recent issues over the past two decades. In Bdeficient conifers with repeated loss of the apical bud, the response to B fertilisation is very clear (Fig. 1I). Height growth can recover after B fertilisation also in cases with no loss of apical dominance, but diameter growth is not always affected by B fertilisers (White and Krause, 2000; Saarsalmi and Tamminen, 2005). By contrast, both height and diameter growth were dramatically increased by application of B fertiliser to eucalypts in the Philippines (Dell et al., 2001).

Boron deficiencies can be corrected with soluble fertilisers or naturally occurring minerals (Bell and Dell, 2008), and B is either added with the main nutrient fertilisers or applied separately. The longevity of B fertilisation is at least ten years in fertile mineralsoil under Norway spruce stands (Möttönen et al., 2003), and in peatland sites, statistically and physiologically significant increases in Scots pine needle B levels were still found 20 years after B fertilisation; B levels were 5–8 mg kg−1 in unfertilised plots, and 12–16 mg kg−1 in the fertilised plots (A. Rummukainen, S. Kaunisto and T. Lehto, unpubl. data). This is assumed to be the difference between incipient B deficiency and adequate concentrations for above-ground growth (Braekke, 1983; Jukka, 1988). Where B deficiencies occur on soils with abundant organic matter, such as in parts of Scandinavia and Indonesia, the peat may help to retain B, but may also inhibit B uptake by roots. Other factors that can influence the effectiveness of B fertilisers over time include B retranslocation in the tree (Lehto et al., 2000, 2004b) and the return of B through retention of slash on site at thinning and harvesting (Lehto et al., 2010b). In contrast to the experience above with conifers, in parts of SE Asia (Dell et al., 2003) it is proving difficult to apply sufficient B in high rainfall areas, or where soils are acutely low in B, to maintain fast growing eucalypts through the first rotation without causing toxicity. In an unpublished trial undertaken in SW China (Dell, B. and Xu, unpub. data), symptoms of B toxicity were expressed in trees given 2.6–21.3 kg B ha−1 (1.2–9.6 g B tree−1 banded at 0.3–1.0 m) six months after planting. By year 1, the new growth was free of B toxicity symptoms in trees supplied with 2.6 and 5.2 kg B ha−1 . The optimal height response to B fertiliser shifted over time from 1.3 kg B in year 1 to 2.6 g B in year 2 to 10.4 kg B ha−1 in year 4. As B toxicity was expressed in the older leaves, the apical meristem was unaffected by toxicity. New granular slow release B products (Flores et al., 2006) are required to meet this challenge of providing an adequate but not toxic B supply for perennial crops. Until this is achieved, some plantations will require repeat applications of B fertiliser. Ash fertilisation has been shown to be highly efficient in meeting the B requirements of trees. Wood ash contains B in the same proportions to P, K and other minerals as the original plant material, except N and S which are volatilised during burning. Hence wood ash is most useful for sites which contain sufficient N but not P, K and B, particularly drained peatlands (Kaunisto, 1984). Boron release from granulated ash is slower than from borax, and the increase in soil pH caused by ash further reduces leaching. Ash fertilisation has been tested also in boreal podzols, and it has increased the B concentrations of needles (Jacobson, 2003; Saarsalmi et al., 2004). Therefore, even if ash fertilisation does not necessarily give an immediate growth response in podzols, predominantly limited by N (Jacobson, 2003), it can be justified as returning to the forest those nutrients which have been removed with harvesting wood and particularly cutting residues. Moreover, ash fertiliser applied together with N in podzols has also been reported to support increased growth for longer than N alone (Saarsalmi et al., 2006). Apart from B and other micronutrients, ash fertilisation returns also base cations to the soil, hence reducing their long-term depletion in managed forests (Helmisaari et al., 2009; Saarsalmi et al., 2010).

13. Excess boron Boron toxicity in trees can result from root growth in soils that are naturally high in B or which have become contaminated with anthropogenic effluents/residues containing B or from irrigation water sourced from subsoils high in B. In contrast to B deficiency which is more problematic in wet climates, B toxicity in plants is more common in dry climates, especially on soils of marine or vol-

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

canic origin. However, there are few reports of trees growing on naturally high-B soils, such as the California geysers region (Glaubig and Bingham, 1985). Boron toxicity is an important problem in agriculture in some arid regions (Ryan et al., 1998), but these areas are generally not dominated by trees. Boron toxicity is a factor for irrigated tree plantations where ground-water is naturally high in B (Keren and Bingham, 1985; Nable et al., 1997). There are also reports of B toxicity in Eucalyptus camaldulensis, tested for ability to reduce agricultural drainage effluents (Poss et al., 1999, 2000). Some fossil fuels have high levels of B, and this accumulates in the fly ash. Application of a flue-gas desulphurisation by-product to Quercus rubra stands led to B toxicity symptoms at high doses (Crews and Dick, 1998). This occurred despite the fact that the byproduct was based on dolomite lime, and it increased leachate pH from 4 to 7: liming is known to reduce B availability for plants (Lehto, 1995; Goldberg, 1997). Furthermore, there are sites with industrial deposits, such as oil sand mine tailings with very high B levels, in regions that are dominated by woody plants. In these sites, excess B levels are often combined with other factors that are not common in natural or managed forests, such as excess salt (NaCl) and fluoride (F− ) (Apostol and Zwiazek, 2004). Because some poplars (Populus sp.) can take up high amounts of B they are being considered for the phytomanagement of sites contaminated with B (Robinson et al., 2007). Boron toxicity generally results in necrosis of the tissue in which the B accumulates. Reid et al. (2004) suggest that this results from disruption of many cellular processes which are exacerbated in light by oxidative stress. Keles et al. (2004) compared leaves of Citrus sinensis from orchards irrigated with either B-laden channel water or well water low in B and suggested that damage from free radicals in leaves with high B concentration may have been a consequence of reduced ␣-tocopherol levels. In C. grandis seedlings, leaves with high B concentrations (659 mg kg−1 ) were more sensitive to damage by oxidative stress than B-deficient (12 mg kg−1 ) leaves (Han et al., 2009). In trees where B is phloem immobile, such as eucalypts or poorly mobile such as pines, toxicity symptoms first develop at the tips and margins of old transpiring leaves. However, in species with high phloem B mobility, B toxicity symptoms are associated with impaired meristem development of shoots and reproductive structures such as fruit (Brown and Hu, 1998b). Other symptoms of B toxicity in trees include loss of form, crown dieback, bark splitting and gum exudation. Boron concentrations in foliage tend to increase rapidly after fertilisation. In Norway spruce fertilisation experiments, B concentrations were less than 1 mg kg−1 in unfertilised trees, but they rose up to 50–60 mg kg−1 after fertilisation, without signs of toxicity, and high values remained after 10 years (M. Möttönen, M. Räisänen and T. Lehto, unpubl. data). Excess fertilisation may lead to high, possibly toxic levels of B in trees. The highest dose tested by Hopmans and Clerehan (1991) led to an increase of radiata pine needle B concentrations to as high as 110 mg kg−1 after the first year, but no adverse effects on growth or toxicity symptoms were reported. In an experiment lasting one growing season, Scots pine (Pinus sylvestris) seedling growth did not decrease with a dose that increased needle concentrations to 90 mg kg−1 , but slight losses in growth were found when B in needles was 140 mg kg−1 , and clear losses when the B concentration was 200 mg kg−1 (Rikala, 2003). Yet there were no toxicity symptoms (needle browning) or death of seedlings unless the concentration was nearly 400 mg kg−1 . In jack pine seedlings, the B levels were about 500 mg kg−1 even in control plants, and 1000 mg kg−1 in B treated plants but seedling growth was not decreased, and B alone did not cause necrosis (Calvo-Polanco et al., 2008). In another short-term study, high B (up to 2300 mg kg−1 in needles) did not cause mortality or even loss of growth in six weeks, although it caused reduced stomatal conductance and water flow plus needle tip necrosis (Apostol and

2065

Zwiazek, 2004). By contrast to pine species, eucalypts may suffer at much lower internal concentrations. For example, toxicity symptoms were expressed in juvenile foliage of E. globulus and E. grandis in the field when foliar B concentrations exceeded 75 mg kg−1 (B. Dell, unpubl. data).

14. Concluding remarks Since Stone’s review (1990) there has been major progress in understanding B function in plants and the cycling of B in forests. The role of B in the cell wall has been clarified, and significant findings have been made particularly regarding B uptake, translocation and retranslocation. However, there are still major gaps in our knowledge about B uptake by different tree species under varying environmental conditions, by roots at different depths, and particularly the role of mycorrhizas in B uptake and possible immobilisation. The behaviour and availability of B in soil is still described in only chemical terms, although processes such as microbial immobilisation of B, and the occurrence of B-complexing compounds in root exudates and in products of microbial metabolism may have a significant role in reducing or increasing B availability and leaching. Virtually nothing was known about the role of B in symbiotic N2 -fixing, which has now been shown, and continues to be a field of innovative findings. Much more is known about B retranslocation now than 20 years ago. However, it is still not fully understood, why B is retranslocated in some trees more than in others, and what the extent of seasonal variability and withinspecies genetic variability is. The new information about B has yet to be exploited by tree breeders; in addition to the issues above, the influence of B in pollen and seed production may have direct practical applications. In many parts of the world, B fertilisation is widely practiced to prevent actual deficiency symptoms. However, management of B remains a problem for many tree plantations. In some developing countries, the high cost of inorganic fertilisers, especially in remote areas, has restricted the application of optimum levels of B. However, the small amount of B required to correct or prevent the onset of deficiencies, a few kg per hectare, should not constrain the wider application of B in the future particularly where correct diagnosis of B constraints are available (Bell and Dell, 2008). However, standards for foliar B analysis have only been established for a very limited range of tree species and much more needs to be done to ensure that sub-optimal B levels are not reducing either wood quality or productivity. For some of the major plantation species, it may be possible in the future to either select for natural variation in phloem B mobility or to engineer tree crops to increase the internal retranslocation of B. Recently, Leite et al. (2010) have demonstrated the presence of polyols in E. grandis and E. grandis × E. urophylla clones indicating the potential for progress in this area. Harvesting tree trunks with bark causes significant losses of B, as a relatively large proportion of the B in trees is in these parts. Furthermore, the increasing practice of harvesting the cutting residues and lifting stumps with large roots for fuel wood is another factor which leads to significant removal of B, as well as other nutrients from forest sites (Luiro et al., 2010). Stands with B levels slightly higher than minimum requirements may benefit from thinning, as it reduces competition for B, and the decomposing harvest residue can provide the minimum requirement for the remaining trees (cf. Möttönen et al., 2003). However, if the harvest residue is removed, this benefit of thinning is not achieved. Human-induced environmental changes cause challenges for the future also in the field of nutrition and nutrient management in trees and forests. Nitrogen deposition at present rates is likely to increase growth of trees in less affected regions, and therefore through a dilution effect, to lead to B and other micronutrient defi-

2066

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

ciencies. Climatic warming may have similar effects in cold regions in the longer term. On the other hand, climate change is predicted to make some areas wetter and to increase the frequency and severity of droughts in other areas. These two extremes are likely to further exacerbate B supply to trees on soils low in B, either through increased leaching of B or through reduced uptake in the dry season (Dell and Thu, 2009). The chemical composition and the decomposition of the litter in relation to B, and particularly root growth, are related to the question of carbon sequestration into forest soils. The slowly decomposing humus in the boreal forest is a major store of carbon globally, and currently, means are being sought for increasing the carbon storing ability of the boreal forest without causing new environmental problems. Boron fertilisation has a potential for being such a means, as it is a correction of a naturally occurring nutrient imbalance, which may affect carbon accumulation into the soil. However, the role of B additions in the carbon cycle of low-B forests needs to be confirmed, as so far there are only isolated studies on different parts of the C cycle in relation to B and other micronutrients.

Acknowledgements We thank Pedro J. Aphalo for constructive comments on the manuscript. Academy of Finland (current grant decision no. 123637), Maj and Tor Nessling Foundation and the Foundation for Research of Natural Resources in Finland have provided funding for our boron research.

References Aphalo, P.J., Schoettle, A.W., Lehto, T., 2002. Leaf life span and the mobility of “non-mobile” mineral nutrients – the case of boron in conifers. Silva Fenn. 36, 671–680. Apostol, K.G., Zwiazek, J.J., 2004. Boron and water uptake in jack pine (Pinus banksiana) seedlings. Env. Exp. Bot. 51, 145–153. Aronsson, A., 1980. Frost hardiness in Scots pine (Pinus silvestris L.). II. Hardiness during winter and spring in young trees of different mineral nutrient status. Stud. For. Suec. 155, 1–27. Asad, A., Bell, R.W., Dell, B., 1997. External boron requirements for canola (Brassica napus L.). Ann. Bot. (Lond.) 80, 65–73. Atalay, A., Garrett, H.E., Mawhinney, T.P., Mitchell, R.J., 1988. Boron fertilization and carbohydrate relations in mycorrhizal and nonmycorrhizal shortleaf pine. Tree Physiol. 4, 275–280. Bassil, E., Hu, H., Brown, P.H., 2004. Use of phenylboronic acids to investigate boron function in plants. Possible role of boron in transvacuolar cytoplasmic strands and cell-to-wall adhesion. Plant Physiol. 136, 3383–3395. Bell, R.W., 2000. Temporary boron deficiency – a difficult case for diagnosis and prognosis by plant analysis. Comm. Soil Sci. Plant Anal. 31, 1847–1861. Bell, R.W., Dell, B., 2008. Micronutrients for Sustainable Food, Feed, Fibre and Bioenergy Production. International Fertilizer Industry Association, Paris, France, 175 pp. Bellaloui, N., Brown, P.H., Dandekar, A.M., 1999. Manipulation of in vivo sorbitol production alters boron uptake and transport in tobacco. Plant Physiol. 119, 735–741. Bellaloui, N., Yadavc, R.C., Maw-Sheng, C., Hu, H., Gillen, A.M., Greve, C., Dandekar, A.M., Ronald, P.C., Brown, H.H., 2003. Transgenically enhanced sorbitol synthesis facilitates phloem-boron mobility in rice. Physiol. Plant. 117, 79–84. Bennett, A., Rowe, R.I., Soch, N., Eckhert, C.D., 1999. Boron stimulates yeast (Saccharomyces cerevisiae) growth. J. Nutr. 129, 2236–2238. Berg, B., McClaugherty, C., 2008. Plant Litter. Decomposition, Humus Formation, Carbon Sequestration, second ed. Springer. Berthelsen, B.O., Olsen, R.A., Steinnes, E., 1995. Ectomycorrhizal heavy metal accumulation as a contributing factor to heavy metal levels in organic surface soils. Sci. Total Environ. 170, 141–149. Bi, J.L., Felton, G.W., 1995. Foliar oxidative stress and insect herbivory: primary compounds, secondary metabolites, and reactive oxygen species as components of induced resistance. J. Chem. Ecol. 21, 1511–1530. Blevins, D.G., Lukaszewski, K.M., 1998. Boron plant structure and function. Annu. Rev. Plant Physiol. Plant Mol. Biol. 49, 481–500. ˜ Bolanos, L., Brevin, N.J., Bonilla, I., 1996. Effects of boron on Rhizobiumlegume cell–surface interactions and nodule development. Plant Physiol. 110, 1249–1256. ˜ Bolanos, L., Lukaszewski, K., Bonilla, I., Blevins, D., 2004. Why boron? Plant Physiol. Biochem. 42, 907–912.

˜ Bolanos, L., Redondo-Nieto, M., Bonilla, I., Wall, L.G., 2002. Boron requirement in the Discaria trinervis (Rhamnaceae) and Frankia symbiotic relationship. Its essentiality for Frankia BCU110501 growth and nitrogen fixation. Physiol. Plant. 115, 563–570. ˜ Bolanos, L., Cebrián, A., Redondo-Nieto, M., Rivilla, R., Bonilla, I., 2001. Lectin-like glycoprotein PsNLEC-1 is not correctly glycosylated and targeted in boron-deficient pea nodules. Mol. Plant-Microbe Interact. 14, 663–670. ˜ Bolanos, L., Esteban, E., de Lorenzo, C., Fernández-Pasqual, M., de Felipe, M.F., Gárate, A., Bonilla, I., 1994. Essentiality of boron for symbiotic dinitrogen fixation in pea (Pisum sativum) rhizobium nodules. Plant Physiol. 104, 85–90. Bonilla, I., García-González, M., Mateo, P., 1990. Boron requirement in Cyanobacteria. Its possible role in the early evolution of photosynthetic organisms. Plant Physiol. 94, 1554–1560. Braekke, F., 1983. Occurrence of growth disturbance problems in Norwegian and Swedish forestry. Commun. Inst. For. Fenn. 116, 20–24. Brown, P.H., Hu, H., 1996. Phloem mobility of boron is species dependent: evidence for phloem mobility in sorbitol-rich species. Ann. Bot. (Lond.) 77, 497–505. Brown, P.H., Hu, H., 1998a. Phloem boron mobility in diverse plant species. Bot. Acta 111, 331–335. Brown, P.H., Hu, H., 1998b. Boron mobility and consequent management in different crops. Better Crops Plant Food 82, 28–31. Brown, P.H., Shelp, B., 1997. Boron mobility in plants. Plant Soil 193, 85–101. Brown, P.H., Bellaloui, N., Hu, H., Dandekar, A., 1999. Transgenically enhanced sorbitol synthesis facilitates phloem boron transport and increases tolerance of tobacco to boron deficiency. Plant Physiol. 119, 17–20. Brown, P.H., Bellaloui, N., Wimmer, M.A., Bassil, E.S., Ruiz, J., Hu, H., Pfeffer, H., Dannel, F., Römheld, V., 2002. Boron in plant biology. Plant Biol. 4, 205–223. Cakmak, I., Römheld, V., 1997. Boron deficiency-induced impairments of cellular functions in plants. Plant Soil 193, 71–83. Calvo-Polanco, M., Zwiazek, J.J., Jones, M.D., McKinnon, M.D., 2008. Responses of mycorrhizal jack pine (Pinus banksiana) seedlings to NaCl and boron. Trees 22, 825–834. Camacho-Cristóbal, J.J., Anzellotti, D., González-Fontes, A., 2002. Changes in phenolic metabolism of tobacco plants during short-term boron deficiency. Plant Physiol. Biochem. 40, 997–1002. Camacho-Cristóbal, J., Rexach, J., González-Fontes, A., 2008. Boron in plants: deficiency and toxicity. J. Integr. Plant Biol. 50, 1247–1255. Cantu, D., Greve, L.C., Labavitch, J.M., Powell, A.L.T., 2009. Characterization of the cell wall of the ubiquitous plant pathogen Botrytis cinerea. Mycol. Res. 113, 1396–1403. Cara, F.A., Sánchez, E., Ruiz, J.M., Romero, L., 2002. Is phenol oxidation responsible for the short-term effects of boron deficiency on plasma-membrane permeability and function in squash roots? Plant Physiol. Biochem. 40, 853–858. Carter, R.E., Brockley, R.P., 1990. Boron deficiencies in British Columbia: diagnosis and treatment evaluation. For. Ecol. Manage. 37, 83–94. Chardonnet, C.O., Sams, C.E., Conway, W.S., 1999. Calcium effect on the mycelia cell walls of Botrytis cinerea. Phytochemistry 52, 967–973. Chen, X., Schauder, S., Potier, N., Van Dorsselaer, A., Pelczer, I., Bassler, B., Hughson, F.M., 2002. Structural identification of a bacterial quorum-sensing signal containing boron. Nature 415, 545–549. Cooling, E.N., 1967. Frost resistance in Eucalyptus grandis following the application of fertilizer borate. Rhod. Zamb. Mal. J. Agric. Res. 5, 97–100. Crews, J.T., Dick, W.A., 1998. Liming acid forest soils with flue gas desulfurization by-product: growth of Northern red oak and leachate water quality. Environ. Pollut. 103, 55–61. Dannel, F., Pfeffer, H., Römheld, V., 1998. Compartmentation of boron in roots and leaves of sunflower as affected by boron supply. J. Plant Physiol. 153, 615– 622. Dannel, F., Pfeffer, H., Römheld, V., 2002. Update on boron in higher plants – uptake, primary translocation and compartmentation. Plant Biol. 4, 193–204. Delgado, A., Benlloch, M., Fernández-Escobar, R., 1994. Mobilisation of boron in olive trees during flowering and fruit development. HortScience 29, 616–618. Dell, B., Huang, L., 1997. Physiological response of plants to low boron. Plant Soil 193, 103–120. Dell, B., Malajczuk, N., 1994. Boron deficiency in eucalypt plantations in China. Can. J. For. Res. 24, 2409–2416. Dell, B., Thu, P.Q., 2009. Sustainable management of plantation eucalypts and acacias in Asia. In: Diloksumpun, S., Puangchit, L. (Eds.), Tropical Forestry Change in a Changing World, vol. 7. Commercial Plantation Forestry. Kasetsart University Faculty of Forestry, Bangkok, Thailand, pp. 49–63. Dell, B., Xu, D., 2006. Bacterial wilt and boron deficiency stress: a new disorder in eucalypt plantations in south China. Chin. For. Sci. Technol. 5, 45–50. Dell, B., Malajczuk, N., Xu, D., Grove, T.S., 2001. Nutrient Disorders in Plantation Eucalypts, 2nd ed., ACIAR Monograph No. 74. ACIAR, Canberra, Australia, 188 pp. Dell, B., Huang, L., Bell, R.W., 2002. Boron in plant reproduction. In: Goldbach, H.E., Rerkasem, B., Wimmer, M., Brown, P.H., Thellier, M., Bell, R.W. (Eds.), Boron in Plant and Animal Nutrition. Kluwer Academic/Plenum Publishers, New York, USA, pp. 103–118. Dell, B., Xu, D., Rogers, C., Huang, L., 2003. Micronutrient disorders in eucalypt plantations: causes, symptoms, identification, impact and management. In: Wei, R.P., Xu, D. (Eds.), Eucalyptus Plantations – Research, Management and Development. World Scientific Publishing Co., Singapore, pp. 241–252. Dell, B., Hardy, G., Burgess, T., 2008. Health and nutrition of plantation eucalypts in Asia. South. Forests 70, 131–138.

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069 Dembitsky, V.M., Smoum, R., Al-Quntar, A.A., Ali, H.A., Pergament, I., Srebnik, M., 2002. Natural occurrence of boron-containing compounds in plants, algae and microorganisms. Plant Sci. 163, 931–942. Dixon, R.K., Garrett, H.E., Cox, G.S., 1989. Boron fertilization, vesicular–arbuscular mycorrhizal colonization and growth of Citrus jambhiri Lush. J. Plant Nutr. 12, 687–700. Domingues, D.S., Leite, S.M.M., Farro, A.P.C., Coscrato, V.E., Mori, E.S., Furtado, E.L., Wilcken, C.F., Velini, E.D., Guerrini, I.A., Maia, I.G., Marino, C.L., 2005a. Boron transport in Eucalyptus. 2. Identification in silico of a putative boron transporter for xylem loading in eucalypt. Genet. Mol. Biol. 28, 625–629. Domisch, T., Finér, L., Lehto, T., Smolander, A., 2002. Effect of soil temperature on nutrient allocation and mycorrhizas in Scots pine seedlings. Plant Soil 239, 173–185. Dordas, C., 2008. Role of nutrients in controlling plant diseases in sustainable agriculture. A review. Agron. Sustain. Dev. 28, 33–46. Dordas, C., Brown, P.H., 2000. Permeability of boric acid across lipid bilayers and factors affecting it. J. Membr. Biol. 175, 95–105. Dordas, C., Brown, P.H., 2005. Boron deficiency affects cell viability, phenolic leakage and oxidative burst in rose cell cultures. Plant Soil 268, 293–301. Dordas, C., Chrispeels, M.J., Brown, P.H., 2000. Permeability and channel-mediated transport of boric acid across membrane vesicles isolated from squash roots. Plant Physiol. 124, 1349–1361. Dordas, C., Brown, P.H., 2001. Evidence of channel mediated transport of boric acid in squash (Cucurbita pepo). Plant Soil 235, 95–103. Domingues, D.S., Leite, S.M.M., Farro, A.P.C., Coscrato, V.E., Mori, E.S., Furtado, E.L., Wilcken, C.F., Velini, E.D., Guerrini, I.A., Maia, I.G., Marino, C.L., 2005b. Boron transport in Eucalyptus. 2. Identification in silico of a putative boron transporter for xylem loading in eucalypt. Genet. Mol. Biol. 28 (Suppl. 0., 3 Suppl.), 625–629. Duddridge, J., Read, D.J., 1984. Modification of the host–fungus interface in mycorrhizas synthesized between Suillus bovinus (Fr) O. Kuntz and Pinus sylvestris L. New Phytol. 96, 583–588. Dugger, D.M., 1983. Boron in plant metabolism. In: Läuchli, A., Bieleski, R.L. (Eds.), Encyclopedia of Plant Physiology. New Ser. vol 15B, Inorganic Plant Nutrition. Springer, pp. 626–650. Felton, G.W., Donato, K., Del Vecchio, R.J., Duffey, S.S., 1989. Activation of plant foliar oxidases by insect feeding reduces nutritive quality of foliage for noctuid herbivores. J. Chem. Ecol. 15, 2667–2694. Ferm, A., Markkola, A., 1985. Hieskoivun lehtien, oksien ja silmujen ravinnepitoisuuksien kasvukautinen vaihtelu. Summary: nutritional variation of leaves, twigs and buds in Betula pubescens stands during the growing season. Folia For. 613, 1–28. Finér, L., 1989. Biomass and nutrient cycle in fertilized and unfertilized pine, mixed birch and pine and spruce stands on a drained mire. Acta Forestalia Fennica 208, 1–63. Fleischer, A., O’Neill, M.A., Ehwald, R., 1999. The pore size of non-graminaceous plant cell walls is rapidly decreased by borate ester cross-linking of the pectic polysaccharide rhamnogalacturonan II. Plant Physiol. 121, 8829–8838. Flores, H.R., Mattenella, L.E., Kwok, L.H., 2006. Slow release boron micronutrients from pelletized borates of the northwest of Argentina. Miner. Eng. 19, 364–367. Glaubig, B.A., Bingham, F.T., 1985. Boron toxicity characteristics of four northern California endemic tree species. J. Environ. Qual. 14, 72–77. Goldbach, H.E., Wimmer, M.A., 2007. Boron in plants and animals: is there a role beyond cell-wall structure? J. Plant Nutr. Soil Sci. 170, 39–48. Goldberg, S., 1997. Reactions of boron with soils. Plant Soil 193, 35–48. Hagen-Thorn, A., Stjernquist, I., 2005. Micronutrient levels in some temperate European species: a comparative field study. Trees 19, 572–579. Han, S., Chen, L.-S., Jiang, H.-X., Smith, B.R., Yang, L.-T., Xie, C.-Y., 2008. Boron deficiency decreases growth and photosynthesis, and increases starch and hexoses in leaves of citrus seedlings. J. Plant Physiol. 165, 1331–1341. Han, S., Tang, N., Jiang, H.-X., Yang, L.-T., Li, Y., Chen, L.-S., 2009. CO2 assimilation, photosystem II photochemistry, carbohydrate metabolism and antioxidant system of citrus leaves in response to boron. Plant Sci. 176, 143–153. Hanson, E.J., 1991. Movement of boron out of fruit tree leaves. HortScience 26, 271–273. Harley, J.L., 1985. Specificity and penetration of tissues by mycorrhizal fungi. Proc. Indian Acad. Sci. (Plant Sci.) 94, 99–109. Hättenschwiler, S., Vitousek, P.M., 2000. The role of polyphenols in terrestrial ecosystem nutrient cycling. TREE 15, 238–243. Helmisaari, H.-S., 1990. Temporal variations in nutrient concentrations of Pinus sylvestris needles. Scand. J. For. Res. 5, 177–193. Helmisaari, H.-S., 1992. Nutrient retranslocation in three Pinus sylvestris stands. For. Ecol. Manage. 51, 347–367. Helmisaari, H.S., Saarsalmi, A., Kukkola, M., 2009. Effects of wood ash and nitrogen fertilization on fine root biomass and soil and foliage nutrients in a Norway spruce stand in Finland. Plant Soil 314, 121–132. Hingston, F.J., 1986. Biogeochemical cycling of boron in native eucalypt forests of south-western Australia. Aust. For. Res. 16, 73–83. Högberg, M.N., Högberg, P., 2002. Extramatrical ectomycorrhizal mycelium contributes one third of microbial biomass and produces, together with associated roots, half the dissolved organic carbon in a forest soil. New Phytol. 154, 791–795. Hopmans, P., Clerehan, S., 1991. Growth and uptake of N, P, K and B by Pinus radiata D. Don in response to applications of borax. Plant Soil 131, 115–127. Hu, H., Brown, P.H., 1997. Absorption of boron by plant roots. Plant Soil 193, 49–58. Hu, H., Brown, P.H., Labavich, J.M., 1996. Species variability in boron requirement is correlated with cell wall pectin. J. Exp. Bot. 47, 227–232.

2067

Hu, H., Penn, S.G., Lebrilla, C.B., Brown, P.H., 1997. Isolation and characterization of soluble boron complexes in higher plants. The mechanism of phloem mobility of boron. Plant Physiol. 113, 649–655. Huang, L., Bell, R.W., Dell, B., 2008. Evidence of phloem boron transport in response to interrupted boron supply in white lupin (Lupinus albus L. cv. Kiev mutant) at the reproductive stage. J. Exp. Bot. 59, 575–583. Huang, L., Ye, Z., Bell, R.W., Dell, B., 2005. Boron nutrition and chilling tolerance of warm climate crop species. Ann. Bot. (Lond.) 96, 755–767. Jacobson, S., 2003. Addition of stabilized wood ashes to Swedish coniferous stands on mineral soils – effects on stem growth and needle nutrient concentrations. Silva Fenn. 37, 437–450. Jentschke, G., Godbold, D.L., 2000. Metal toxicity and ectomycorrhizas. Physiol. Plant. 109, 107–116. Jentschke, G., Brandes, B., Kuhn, A.J., Schröder, W.H., Becker, J.S., Godbold, D.L., 2000. The mycorrhizal fungus Paxillus involutus transports magnesium to Norway spruce seedlings. Evidence from stable isotope labelling. Plant Soil 220, 243– 246. Jones, K.S., MacKersie, B.D., Paroschy, J., 2000. Prevention of ice propagation by permeability barriers in bud axes of Vitis vinifera. Can. J. Bot. 78, 3–9. Jukka, L. (Ed.), 1988. Metsänterveysopas. Metsän tuhot ja niiden torjunta. Samerka, Helsinki, 168 pp. Kainulainen, P., Holopainen, J.K., 2002. Concentrations of secondary compounds in Scots pine needles at different stages of decomposition. Soil Biol. Biochem. 34, 37–42. Kanerva, S., Kitunen, V., Loponen, J., Smolander, A., 2008. Phenolic compounds and terpenes in soil organic horizon layers under silver birch, Norway spruce and Scots pine. Biol. Fert. Soils 44, 547–556. Kaunisto, S., 1984. Yhteenveto lannoitustutkimuksista metsikön perustamisen ja taimikonhoidon yhteydessä turvemailla. Suo 35, 116–126. Kaya, A., Karakaya, H.C., Fomenko, D.E., Gladyshev, V.N., Koc, A., July 2009. Identification of a novel system for boron transport: Atr1 is a main boron exporter in yeast. Mol. Cell. Biol., 3665–3674. Keles, Y., Öncel, I., Yenice, N., 2004. Relationship between boron content and antioxidant compounds in Citrus leaves taken from fields with different water source. Plant Soil 265, 345–353. Keren, R., Bingham, F.T., 1985. Boron in water, soils, and plants. Adv. Soil Sci. 1, 229–276. Keren, R., Bingham, F.T., Rhoades, J.D., 1985. Plant uptake of boron as affected by boron distribution between liquid and solid phases in soil. Soil Sci. Soc. Am. J. 49, 297–302. Kikuchi, K., Matsushita, N., Suzuki, K., Hogetsu, T., 2007. Flavonoids induce germination of basidiospores of the ectomycorrhizal fungus Suillus bovinus. Mycorrhiza 17, 563–570. Koricheva, J., Roy, S., Vranjic, J.A., Haukioja, E., Hughes, P.R., Hänninen, O., 1997. Antioxidant responses to simulated acid rain and heavy metal deposition in birch seedlings. Environ. Pollut. 95, 249–258. Koshiba, T., Kobayashi, M., Matoh, T., 2009. Boron nutrition of tobacco BY-2 cells. Oxidative damage is the major cause of cell death induced by boron deprivation. Plant Cell Physiol. 50, 26–36. Kubacka-Zebalska, M., Kacperska, A., 1999. Low temperature-induced modifications of cell wall content and polysaccharide composition in leaves of winter oilseed reape (Brassica napus L. var. oleifera). Plant Sci. 148, 59–67. Lahti, M., Aphalo, P.J., Finér, L., Lehto, T., Leinonen, I., Mannerkoski, H., Ryyppö, A., 2002. Soil temperature, gas exchange and nitrogen statuts of 5-year-old Norway spruce seedlings. Tree Physiol. 22, 1311–1316. Laloi, C., Apel, K., Danon, A., 2004. Reactive oxygen signalling: the latest news. Curr. Opin. Plant. Biol. 7, 323–328. Lambert, D.H., Cole, H.C., Baker, D.E., 1980. The role of boron in plant response to mycorrhizal infection. Plant Soil 57, 431–438. Lambert, M.J., Ryan, P.J., 1990. Boron nutrition of Pinus radiata in relation to soil development and management. For. Ecol. Manage. 30, 45–53. Lavola, A., Aphalo, P.J., Lehto, T., 2010. Boron and other elements in sporophores of ectomycorrhizal and saprotrophic fungi. Mycorrhiza, doi:10.1007/s00572-010rr0321-7. Lehto, T., 1994. Effects of liming and boron fertilization on mycorrhizas of Picea abies. Plant Soil 163, 65–68. Lehto, T., 1995. Boron retention in limed forest mor. For. Ecol. Manage. 78, 11–20. Lehto, T., Mälkönen, E., 1994. Effects of liming and boron fertilization on boron uptake of Picea abies. Plant Soil 163, 55–64. Lehto, T., Kallio, E., Aphalo, P.J., 2000. Boron mobility in two coniferous species. Ann. Bot. (Lond.) 86, 547–550. Lehto, T., Smolander, A., Aphalo, P.J., 2010a. Decomposition and element concentrations of silver birch leaf litter as affected by boron status of litter and soil. Plant Soil 329, 195–208. Lehto, T., Lavola, A., Kallio, E., Aphalo, P.J., 2004a. Boron uptake by ectomycorrhizas of silver birch. Mycorrhiza 14, 209–212. Lehto, T., Lavola, A., Julkunen-Tiitto, R., Aphalo, P.J., 2004b. Boron retranslocation in Scots pine and Norway spruce. Tree Physiol. 24, 1011–1017. Lehto, T., Aphalo, P.J., Saranpää, P., Laakso, T., Smolander, A., 2010b. Decomposition and element concentrations of Norway spruce needle litter with differing B, N, or P status. Plant Soil 330, 225–238. Lehto, T., Räisänen, M., Lavola, A., Julkunen-Tiitto, R., Aphalo, P.J., 2004c. Boron mobility in deciduous forest trees in relation to their polyols. New Phytol. 163, 333–339. Leite, V.M., Brown, P.H., Rosolem, C.A., 2007. Boron retranslocation in coffee trees. Plant Soil 290, 221–229.

2068

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069

Leite, S.M.M., Marino, C.L., Bonine, C.A.V., 2010. Response of Eucalyptus grandis and E. grandis × E urophylla clones to boron suppression. Scientia Forestalis 38, 19–25. Levine, A., Tenhaken, R., Dixon, R., Lamb, C., 1994. H2 O2 from the oxidative burst orchestrates the plant hypersensitive disease resistance response. Cell 79, 583–593. Lewis, D.H., 1980. Boron, lignification and the origin of vascular plants – a unified hypothesis. New Phytol. 84, 209–229. Liakopoulos, G., Karabourniotis, G., 2005. Boron deficiency and concentrations and composition of phenolic compounds in Olea europaea leaves: a combined growth chamber and field study. Tree Physiol. 25, 307–315. Liakopoulos, G., Stavrianakou, S., Nikolopoulos, D., Karvoni, E., Vekkos, K.-A., Psaroudi, V., Karabourniotis, G., 2009. Quantitative relationships between boron and mannitol concentrations in phloem exudates of Olea europaea leaves under contrasting boron supply conditions. Plant Soil 323, 177–186. Liu, P., Yang, Y.A., 2000. Effects of molybdenum and boron on membrane lipid peroxidation and endogenous protective systems of soybean leaves. Acta Bot. Sinica 42, 461–466. Loomis, W.D., Durst, R.W., 1992. Chemistry and biology of boron. BioFactors 3, 229–239. Lorenz, K., Preston, C.M., Raspe, S., Morrison, I.K., Feger, K.H., 2000. Litter decomposition and humus characteristics in Canadian and German spruce ecosystems: information from tannin analysis and 13 C CPMAS NMR. Soil Biol. Biochem. 32, 779–792. Luiro, J., Kukkola, M., Saarsalmi, A., Tamminen, P., Helmisaari, H.-S., 2010. Logging residue removal after thinning in boreal forests: long-term impact on the nutrient status of Norway spruce and Scots pine needles. Tree Physiol. 30, 78–88. Mälkönen, E., Derome, J., Kukkola, M., 1990. Effects of nitrogen inputs on forest ecosystems. Estimations based on long-term fertilization experiments. In: Kauppi, P., Anttila, P., Kenttämies, K. (Eds.), Acidification in Finland. SpringerVerlag, Berlin, pp. 325–347. ´ Zˇ ., Uehlein, N., Kaldenhoff, R., Zwiazek, J.J., Weiss, M., Hampp, R., et al., Marjanovic, 2005. Aquaporin in poplar: what a difference a symbiont makes! Planta 222, 258–268. Marschner, H., 1995. Mineral Nutrition of Higher Plants. Academic Press, 889 pp. Marschner, H., Dell, B., 1994. Nutrient uptake in mycorrhizal symbiosis. Plant Soil 159, 89–102. Matoh, T., 1997. Boron in plant cell walls. Plant Soil 193, 59–70. Matoh, T., Ochiai, K., 2005. Distribution and partitioning of newly taken-up boron in sunflower. Plant Soil 278, 351–360. Melin, E., 1952. Transport of labelled nitrogen from an ammonium source to pine seedlings through mycorrhizal mycelium. Svensk Bot Tidskr. 46, 281–285. Minato, K., Morita, T., 2005. Blackening of Diospyros genus xylem in connection with boron content. J. Wood Sci. 51, 659–662. Mitchell, R.J., Garrett, H.E., Cox, G.S., Atalay, A., 1986. Boron and ectomycorrhizal influences on indole-3-acetic acid levels and indole-3-acetic acid oxidaseand peroxidase activities of Pinus echinata Mill. roots. Tree Physiol. 1, 1–8. Mitchell, R.J., Garrett, H.E., Cox, G.S., Atalay, A., 1990. Boron and ectomycorrhizal influences on mineral nutrition of container-grown Pinus echinata Mill. J. Plant Nutr. 13, 1555–1574. Mitchell, R.J., Garrett, H.E., Cox, G.S., Atalay, A., Dixon, R.K., 1987. Boron fertilization, ectomycorrhizal colonization, and growth of Pinus echinata seedlings. Can. J. For. Res. 17, 1153–1156. Miwa, K., Fujiwara, T., 2010. Boron transport in plants: co-ordinated regulation of transporters. Ann. Bot. 105, 1103–1108. Möttönen, M., Aphalo, P.J., Lehto, T., 2001a. The role of boron in drought resistance in Norway spruce (Picea abies) seedlings. Tree Physiol. 21, 673–681. Möttönen, M., Lehto, T., Aphalo, P.J., 2001b. Growth dynamics and mycorrhizas of Norway spruce (Picea abies) seedlings in relation to boron availability. Trees 15, 319–326. Möttönen, M., Lehto, T., Aphalo, P.J., Kukkola, M., Mälkönen, E., 2003. Response of mature stands of Norway spruce (Picea abies) to boron fertilization. For. Ecol. Manage. 180, 401–412. Möttönen, M., Lehto, T., Rita, H., Aphalo, P.J., 2005. Recovery of Norway spruce (Picea abies) seedlings from repeated drought as affected by boron nutrition. Trees 19, 213–223. Müntzenberger, B., Kottke, I., Oberwinkler, F., 1995. Reduction of phenolics in mycorrhizas of Larix decidua Mill. Tree Physiol. 15, 191–196. Musetti, R., de Toppi, L.S., Martini, M., Ferrini, F., Loschi, A., Favali, M.A., Osler, R., 2005. Hydrogen peroxide localization and antioxidant status in the recovery of apricot plants from European Stone Fruit Yellows. Eur. J. Plant Pathol. 112, 53–61. ˜ Nable, R.O., Banuelos, G.S., Paull, J.G., 1997. Boron toxicity. Plant Soil 198, 181–198. Nehls, U., 2008. Mastering ectomycorrhizal symbiosis: the impact of carbohydrates. J. Exp. Bot. 59, 1097–1108. Noiraud, N., Maurousset, L., Lemoine, R., 2001. Transport of polyols in higher plants. Plant Physiol. Biochem. 39, 717–728. Noppakoonwong, R.N., Bell, R.W., Dell, B., Loneragan, J.F., 1993. An effect of shade on the boron requirement for leaf blade elongation in black gram (Vigna mungo). Plant Soil 155/156, 317–320. Nylund, J.-E., 1987. The ectomycorrhizal infection zone and its relation to acid polysaccharides of cortical cell walls. New Phytol. 106, 505–516. O’Neill, M.A., Ishii, T., Albersheim, P., Darvill, A.G., 2004. Rhamnogalacturonan II: structure and function of a borate cross-linked cell wall pectic polysaccharide. Annu. Rev. Plant Biol. 55, 109–139.

O’Neill, M., Eberhard, S., Albersheim, P., Darvill, A., 2001. Requirement of borate cross-linking of cell wall rhamnogalacturonan II for Arabidopsis growth. Science 294, 846–849. Orozco-Cárdenas, M.L., Nárvaez-Vásquez, J., Ryan, C.A., 2001. Hydrogen peroxide acts as a second messenger for the induction of defense genes in tomato plants in response to wounding, systemin and methyl jasmonate. Plant Cell 13, 179–191. Parr, A.J., Loughman, B.C., 1983. Boron and membrane function in plants. In: Robb, D.A., Pierpoint, W.S.A. (Eds.), Metals and Micronutrients: Uptake and Utilization by Plants. Academic Press, New York, pp. 87–107. Perica, S., Brown, P.H., Connell, J.H., Nyomora, A.M.S., Dordas, C., Hu, H., Stangoulis, J., 2001. Foliar boron application improves flower fertility and fruit set of olive. HortScience 36, 714–716. Peterson, L.A., Newman, R.C., 1978. Influence of soil pH on availability of added boron. Soil Sci. Soc. Am. J. 40, 280–282. Pfeffer, H., Dannel, F., Römheld, D., 1998. Are there connections between phenol metabolism, ascorbate metabolism and membrane integrity in leaves of borondeficient sunflower plants? Physiol. Plant. 104, 479–485. Pfeffer, H., Dannel, F., Römheld, V., 1999. Are there two mechanisms for B uptake in sunflower? J. Plant Physiol. 155, 34–40. Pietiläinen, P., 1984. Foliar nutrient content and 6-phosphogluconate dehydrogenase activity in vegetative buds of Scots pine on a growth disturbance area. Comm. Inst. For. Fenn. 123, 1–18. Plassard, C., Dell, B., 2010. Phosphorus nutrition of mycorrhizal trees. Tree Physiol. 30, 1129–1139. Poss, J.A., Grattan, S.R., Grieve, C.M., Shannon, M.C., 1999. Characterization of leaf boron injury in salt-stressed Eucalyptus by image analysis. Plant Soil 206, 237–245. Poss, J.A., Grattan, S.R., Suarez, D.L., Grieve, C.M., 2000. Stable carbon isotope discrimination: an indicator of cumulative salinity and boron stress in Eucalyptus camaldulensis. Tree Physiol. 20, 1121–1127. Pourcel, L., Routaboul, J.M., Cheynier, V., Lepiniec, L., Debeaujon, I., 2007. Flavonoid oxidation in plants: from biochemical properties to physiological functions. Trends Plant Sci. 12, 29–36. Power, P.P., Woods, W.G., 1997. The chemistry of boron and its speciation in plants. Plant Soil 193, 1–13. Preston, C.M., Nault, J.R., Trofymow, J.A., Smyth, C., CIDET working group, 2009. Chemical changes during 6 years of decomposition of 11 litters in some Canadian forest sites. Part 1. Elemental composition, tannins, phenolics, and proximate fractions. Ecosystems 12, 1053–1077. Quamme, H.A., 1995. Deep supercooling in buds of woody plants. In: Lee Jr., R.E., Warren, G.J., Gusta, L.V. (Eds.), Biological Ice Nucleation and Its Applications. The American Phytopathological Society, Minnesota. Räisänen, M., Repo, T., Lehto, T., 2004. Boorin puutoksen vaikutus kuusen talvenkestävyyteen. Metsäntutkimuslaitoksen Tiedonantoja 934, 41–46. Räisänen, M., Repo, T., Lehto, T., 2006a. Effect of thawing time, cooling rate, and boron nutrition on freezing point of the primordial shoot in Norway spruce buds. Ann. Bot. (Lond.) 97, 593–599. Räisänen, M., Repo, T., Lehto, T., 2007. Cold acclimation was partially impaired in boron deficient Norway spruce seedlings. Plant Soil 292, 271–282. Räisänen, M., Repo, T., Lehto, T., 2009. Cold acclimation of Norway spruce roots and shoots after boron fertilization. Silva Fenn. 43, 223–233. Räisänen, M., Repo, T., Rikala, R., Lehto, T., 2006b. Does ice crystal formation in buds explain growth disturbances in boron-deficient Norway spruce? Trees 20, 441–448. Raitio, H., Rantala, E.M., 1977. Microscopic and macroscopic symptoms of a growth disturbance in Scots pine. Commun. Inst. For. Fenn. 91, 5–30. Reid, R.J., Hayes, J.E., Post, A., Stangoulis, J.C.R., Graham, R.D., 2004. A critical analysis of the causes of boron toxicity in plants. Plant Cell Environ. 25, 1405– 1414. ˜ Reguera, M., Wimmer, M., Bustos, P., Goldbach, H.E., Bolanos, L., Bonilla, I., 2010. Ligands of boron in Pisum sativum nodules are involved in regulation of oxygen concentration and rhizobial infection. Plant Cell Environ. 33, 1039–1048. Reinikainen, A., Silfverberg, K., 1983. Significance of whole-tree nutrient analysis in the diagnosis of growth disorders. Comm. Inst. For. Fenn. 116, 48–58. Rikala, R., 2003. Kannttaisiko kasvuhäiriöalueille istutettavia taimia booritankata jo taimitarhalla? Taimiuutiset 3/2003, 6–7, Finnish Forest Research Institute. Rikala, R., Vuorinen, M., 2004. Koivikoiden booripitoisuuden ja kasvuhäiriöiden yhteys. Finnish Forest Research Institute Research Papers 934, 64–68. Rikala, R., Vuorinen, M., 2005. Boorin levitysajankohdan vaikutus kivennäismaan kuusikon neulasten booripitoisuuteen. Metsätiet. Aikakausk. 1/2005, 51–56. Robinson, B.H., Green, S.R., Chancerel, B., Mills, T.M., Clothier, B.E., 2007. Poplar for the phytomanagement of boron contaminated sites. Environ. Pollut. 150, 233–255. Ruiz, J.M., Bretones, G., Baghour, M., Ragala, L., Belakbir, A., Romero, L., 1998. Relationship between boron and phenolic metabolism in tobacco leaves. Phytochemistry 48, 269–272. Ruiz, J.M., Garcia, P.C., Romero, L., 1999. Response of phenolic metabolism to the application of carbendazim plus boron in tobacco. Physiol. Plant. 106, 151–157. Rummukainen, A., Julkunen-Tiitto, R., Räisänen, M., Lehto, T., 2007. Phenolic compounds in Norway spruce as affected by boron nutrition at the end of the growing season. Plant Soil 292, 13–23. Ruuhola, T., Yang, S., 2006. Wound-induced oxidative responses in mountain birch leaves. Ann. Bot. 97, 29–37. Ruuhola, T., Yang, S., Ossipov, V., Haukioja, E., 2008. Foliar oxidases as mediators of the rapidly induced resistance of mountain birch against Epirrita autumnata. Oecologia 154, 725–730.

T. Lehto et al. / Forest Ecology and Management 260 (2010) 2053–2069 Ruuhola, T., Rantala, L., Neuvonen, S., Yang, S., Rantala, M.J., 2009. Effects of longterm simulated acid rain on a plant–herbivore interaction. Basic Appl. Ecol. 10, 589–596. Ruuhola, T., Salminen, J.-P., Haviola, S., Yang, S., Rantala, M.J., 2007. Immunological memory of mountain birches: effects of phenolics on performance of the autumnal moth depend on herbivory history of trees. J. Chem. Ecol. 33, 1160–1176. Ryan, P.J., 1989. Boron retention within a catena of rhyolitic soils and its effect on radiata pine growth and nutrition. Aust. J. Soil Res. 27, 135–148. Ryan, J., Singh, M., Yau, S.K., 1998. Spatial variability of soluble boron in Syrian soils. Soil Tillage Res. 45, 407–417. Ryyppö, A., Repo, T., Vapaavuori, E., 1998. Development of frost hardiness in roots and shoots of Scots pine seedlings at non-freezing temperatures. C. J. For. Res. 28, 557–565. Saarsalmi, A., Tamminen, P., 2005. Boron, phosphorus and nitrogen fertilization in Norway spruce stands suffering from growth disturbances. Silva Fenn. 39, 351–364. Saarsalmi, A., Mälkönen, E., Kukkola, M., 2004. Effects of wood ash fertilization on soil chemical properties and stand nutrient status and growth of some coniferous stands in Finland. Scand. J. For. Res. 4, 217–233. Saarsalmi, A., Kukkola, M., Moilanen, M., Arola, M., 2006. Long-term effects of ash and N fertilization on stand growth, tree nutrient status and soil chemistry in a Scots pine stand. For. Ecol. Manage. 235, 116–128. Saarsalmi, A., Tamminen, P., Kukkola, M., Hautajärvi, R., 2010. Whole-tree harvesting at clear-felling: Impact on soil chemistry, needle nutrient concentrations and growth of Scots pine. Scand. J. For. Res. 25, 148–156. Sakya, A.T., 2000. Boron nutrition of eucalypt seedlings, role of ectomycorrhiza fungi. M.Phil. Thesis. Murdoch University, Australia. Sakya, A.T., Dell, B., Huang, L., 2002. Boron requirements for Eucalyptus globulus seedlings. Plant Soil 246, 87–95. Shorrocks, V.M., 1997. The occurrence and correction of boron deficiency. Plant Soil 193, 121–148. Sillanpää, M., 1990. Micronutrient Assessment at Country Level: An International Study. FAO Soils Bulletin No. 63. FAO, Rome. Skinner, M.F., Han, C.S., Singh, A.P., 2003. Boron deficiency and tracheid properties of Pinus radiata. N. Z. J. For. Res. 33, 273–280. Smith, S.E., Read, D.J., 2008. Mycorrhizal Symbiosis. Academic Press, third ed. Sonmez, O., Aydemir, S., Kaya, C., 2009. Mitigation effects of mycorrhiza on boron toxicity in wheat (Triticum durum) plants. N. Z. J. Crop Horti Sci. 37, 99–104. Stangoulis, J.C., Brown, P.H., Bellaloui, N., Reid, R., 2001. The efficiency of boron utilization in canola. Aust. J. Plant Physiol. 28, 1109–1114. Stavrianakou, S., Liakopoulos, G., Karabourniotis, G., 2006a. Boron deficiency effects on growth, photosynthesis and relative concentrations of phenolics of Dittrichia viscosa (Asteraceae). Environ. Exp. Bot. 56, 293–300. Stavrianakou, S., Liakopoulos, G., Karvonis, E., Resta, E., Karabourniotis, G., 2006b. Low-boron acclimation induces uptake of boric acid agaist a concentration gradient in root cells of Olea europaea. Funct. Plant Biol. 33, 189–193. Stone, E.L., 1990. Boron deficiency and excess in forest trees: a review. For. Ecol. Manage. 37, 49–75. Sutinen, S., Vuorinen, M., Rikala, R., 2006. Developmental disorders in buds and needles of mature Norway spruce, Picea abies (L.) Karst, in relation to needle boron concentrations. Trees 20, 559–570. Sutinen, S., Aphalo, P.J., Lehto, T., 2007. Does timing of boron application affect needle and bud structure in Scots pine and Norway spruce seedlings? Trees 21, 661–670. Sword, M.A., Garrett, H.E., 1994. Boric acid–phenolic relationships within the Pinus echinata–Pisolithus tinctorius ectomycorrhizal association. Tree Physiol. 14, 1121–1130. Takahama, U., Oniki, T., 1997. A peroxidase/phenolics/ascorbate system can scavenge hydrogen peroxide in plant cells. Physiol. Plant. 101, 845–852.

2069

Takano, J., Miwa, K., Fujiwara, T., 2008. Boron transport mechanisms: collaboration of channels and transporters. Trends Plant Sci. 13, 451–457. Takano, J., Noguchi, K., Yasumor, M., Kobayashi, M., Gajdos, Z., Miwa, K., Hayashi, H., Yoneyama, T., Fujiwara, T., 2002. Arabidopsis boron transporter for xylem loading. Nature 420, 337–340. Tamminen, P., Saarsalmi, A., 2004. Viljavien maiden nuorten kuusikoiden neulasten booripitoisuus Suomessa. Metsätiet. Aikakausk. 3, 271–283. Tanaka, M., Fujiwara, T., 2008. Physiological roles and transport mechanisms of boron: perspectives from plants. Pflügers Arch.-Eur. J. Physiol. 456, 671–677. Timonen, S., Peterson, R.L., 2002. Cytoskeleton in mycorrhizal symbiosis. Plant Soil 244, 199–210. Trumbore, S.E., Gaudinski, J.B., 2003. The secret lives of roots. Science 302, 1344–1345. Turvey, N.D., Carlyle, C., Downes, G.M., 1992. Effects of micronutrients on the growth form of 2 families of Pinus radiata (D. Don) seedlings. Plant Soil 139, 59–65. Tuskan, et al., 2006. The genome of black cottonwood, Populus trichocarpa (Torr. & Gray). Science 313, 1596–1604. Van Hees, P.A.W., Jones, D.L., Finlay, R.D., Godbold, D.L., Lundström, U.S., 2005. The carbon we do not see – the impact of low molecular weight compounds on carbon dynamics and respiration in forest soils: a review. Soil Biol. Biochem. 37, 1–13. Veijalainen, H., Reinikainen, A., Kolari, K.K., 1984. Metsäpuiden ravinneperäinen kasvuhäiriö Suomessa. Folia Forestalia 601, 1–41. Wallander, H., Nilsson, L.O., Hagerberg, D., Bååth, E., 2001. Estimation of the biomass and seasonal growth of external mycelium of ectomycorrhizal fungi in the field. New Phytol. 151, 753–760. Warrington, K., 1923. The effect of boric acid and borax on the broad bean and certain other plants. Ann. Bot. (Lond.) 37, 628–672. Wells, M.L., Conner, P.J., Funderburk, J.F., Price, J.G., 2008. Effects of foliar-applied boron on fruit retention, fruit quality, and tissue boron concentration of pecan. HortScience 43, 696–699. White, J.B., Krause, H.H., 2001. Short-term boron deficiency in a black spruce (Picea mariana [Mill] B.S.P.) plantation. For Ecol. Manage. 152, 323–330. Wikner, B., 1983. Distribution and mobility of boron in forest ecosystems. Commun. Inst. For. Fenn. 116, 131–141. Wojcik, P., Wojcik, M., Lamkowski, K., 2008. Response of apple trees to boron fertilization under conditions of low soil boron availability. Sci. Hortic. 116, 58–64. Wimmer, M.A., Lochnit, G., Bassil, E., Mühlig, K.H., Goldbach, H.E., 2009. Membrane-associated, boron-interacting proteins isolated by boronate affinity chromatography. Plant Cell Physiol. 50, 1292–1304. Wisniewski, M., 1995. Deep supercooling in woody plants and the role of cell wall structure. In: Lee, R.E., Warren, G.J., Gusta, L.V. (Eds.), Biological Ice Nucleation and its Applications. American Phytopathological Society, St Paul, MN, pp. 163–182. Yermiyahu, U., 1995. Boron sorption by soil in the presence of composted organic matter. Soil Sci. Soc. Am. J. 59, 405–409. Yermiyahu, U., Keren, R., Chen, Y., 1988. Boron sorption on composted organic matter. Soil Sci. Soc. Am. J. 52, 1309–1313. Zhao, D., Oosterhuis, D.M., 2002. Cotton carbon exchange, non-structural carbohydrates, and boron distribution in tissues during development of boron deficiency. Field Crops Res. 78, 75–87. Zhao, D., Oosterhuis, D.M., 2003. Cotton growth and physiological responses to boron deficiency. J. Plant. Nutr. 26, 855–867. Zimmermann, M.H., Ziegler, H., 1975. List of sugars and sugar alcohols in sievetube exudate. In: Zimmermann, M.H., Milburn, J.A. (Eds.), Transport in Plants. I. Phloem Transport. Encyclopedia of Plant Physiology. New Series vol. 1. SpringerVerlag, Berlin, pp. 480–503.