Predisposition of trees by air pollutants to low temperatures and moisture stress

Environmental Pollution 87 (1995) 105-117 © 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0269-7491/94/$07.00

ELSEVIER

PREDISPOSITION OF TREES BY AIR POLLUTANTS TO LOW TEMPERATURES A N D MOISTURE STRESS Arthur H. Chappelka School of Forestry, Alabama Agricultural Experiment Station, Auburn University, Auburn, AL 36849, USA

&

Peter H. Freer-Smith Forestry Authority, Forest Research Station, Alice Holt Lodge, Wrecclesham, Farnham, Surrey, UK, GUIO 4LH (Received 12 July 1993; accepted 30 November 1993)

and biotic in nature (Fig. 1), and it is important to understand that trees are exposed to one or more of these factors in a fairly or very severe form at some point in time. Soil moisture deficits and low temperature stress represent two important limiting factors to tree growth and distribution (Kramer, 1983; Kozlowski et al., 1991). On a global basis, plant growth and productivity can be limited by water deficit more than by any other factor (Kramer, 1983), with almost every plant process vulnerable to this stress, depending upon its severity. Plant distribution is severely limited by low temperature stress, with low winter temperatures being responsible for the northern limits of the temperate forests in North America and Europe (Kozlowski et al. 1991). Within the last 20-30 years air pollutants have become a potential threat to forest production worldwide (McLaughlin, 1985). Air pollutants can affect tree growth directly by disruption of biochemical and physiological processes (Winner et al., 1985; Chappelka & Chevone, 1992). Public concern has increased over this issue, due to reports of decreased growth of southern pines in the United States (Sheffield & Cost, 1987), photochemical oxidant effects to forests in the western US (Miller, 1983), decline in health of red spruce (Picea rubens) in the eastern US (Eagar & Adams, 1992) and damage to several forest types in Europe (Blank, 1985). Since air pollutants are known to predispose plants to abiotic and biotic stresses, it is therefore necessary to gain a better understanding of these interactions and their effects on tree growth and productivity. The purpose of this paper is to discuss predisposition of trees by air pollutants to low temperatures and soil moisture stress. We have deliberately approached air pollutant-moisture-low temperature interactions from the viewpoint that air pollutants may induce biochemical, physiological and morphological changes which will predispose plants to low temperatures or moisture stress. In general, this sequence appears accurate both in explaining the mechanisms of a systems effect, and

Abstract

Air pollution can have direct effects on trees. It can cause visible injury to foliage and a disruption of physiological processes, such as photosynthesis and carbon allocation, leading to losses in growth and productivity. This review suggests that of equal or greater importance is the potential of air pollutants to indirectly affect tree growth and vitality by predisposing them to injury from other abiotic and biotic stresses. Predisposition by air pollutants can be the result of a disruption in biochemical processes, such as enzyme activity or production, or physiological factors (e.g. stomatal closure, carbon allocation). Air pollutants such as S02, Os and acidic mists have been implicated as predisposing agents to two of the most important of these stresses: low temperature and soil moisture. Probable mechanisms, as well as implications of predicted changes in global climate will be discussed. INTRODUCTION It is clear that plants respond to climate by three main mechanisms over a wide range of spatial and temporal scales: (1) migration; (2) evolution and adaptation (dependent on genetic heterogeneity and selection pressure); and (3) physiological plasticity. These mechanisms result in a clear correlation between climate and plant distribution (see Woodward, 1987 for a detailed discussion of this topic). All three mechanisms operate as a consequence of the impact of climate on physiological processes. Of the climatic variables, the impacts of temperature and water availability are the most obvious on a large geographic scale. A key which separates areas on the basis of rainfall and temperature can be used to predict plant community structure (Woodward, 1989), with vegetation increasing from absence in extreme desert, through grass and tree mixtures, to dense forest in tropical areas (Holdridge, 1947). Trees are perennial in nature and therefore subject to a multitude of climatic, edaphic and biological factors during a life cycle. These stresses can be both abiotic 105

106

A. H. Chappelka, P. H. Freer-Smith

Above ground

tion, cold (freeze and chilling) injury and photooxidation of pigments (Levitt, 1980). Increased photoBiotic Factors AbioticFactors oxidation of chlorophyll can result from the combinaInsects ~ Temperature Pathogens Humidity tion of solar radiation and low temperatures, above or Genotype SolarRadiation below freezing. Low air temperatures can cause injury Competition Wind Carbondioxide to vegetation by both chilling and freezing stress (Fitter Airpollutants & Hay, 1989; Kozlowski et al., 1991). Plants can be Antioxidants damaged by chilling injury at temperatures several ~.~i:~:~iiii~i~:?~!:~::~?~ !?:~::~?:~:?~::i~:~.;~:~!~ii~::~:~!~::~!~!:~i~ ~ degrees above freezing. In order for freeze injury to i~::~i~::~e~'i!:-~::~:~i~::!ii:.!::!iii:::.:::::.~::!ii::i~ :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: occur, temperatures must be low enough for ice crystals i!!!i~ctieroflor~~i~!iiii!~::~::ii:/./~l~~. ~i::ii::i::i::i::!::i::i::i::i::i::i!i!~::~6isture!i!iiiiliiii::i::i::i!!! ::::::::::::::::::::::::::::::::::::::::::::::::::::::~I :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: to form. Low soil temperatures can injure plants by iiii:ii!Competitionii~!~ili~i!iii~:~:~i~.~i~ ii~:i:i:ii~i~ili!i!ii~iiiii:Aerationi~i:~ii:ii~i!iiiiiiill causing a decrease in root absorption, thus resulting in iiiiiiili::i::iiiiiiiiiiiiiii::iC°:m:P:: i:°:n:ii::iiiiiiiiii shoot dehydration (Fitter & Hay, 1989). Susceptibility to injury in conifers is greatest in the :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ========================================================================================================================= autumn, when the hardening process is incomplete, and in the spring when reserves are depleted and cells and Fig. 1. Factors influencing tree growth and productivity tissues have de-hardened or when newly emerged (after Chappelka & Chevone, 1992). foliage is exposed to unusually late spring frosts (Weiser, 1970). Air pollutants can exacerbate this chronologically in describing decline sequences. Our injury by interfering with or altering one or more of the review addresses this overall hypothesis. The visible metabolic functions involved in the hardening process, symptoms generally observed in regional declines in or by altering the timing of bud-break and growth forest condition include those of low temperature and cessation (Davison et aL, 1988). In addition, air polludrought, rather than direct pollution damage. Clear tants are known to affect stomatal functioning (Darrall, examples of direct injury to forests by air pollutants are 1989) and to accelerate the degradation of epicuticular much more localized when they occur (Smith, 1990). waxes (Barnes et al., 1988) which may reduce the Further discussion of systems effects and of the relative ability of trees to withstand winter desiccation. importance of predisposing, weakening and damaging factors can be found in Schulze (1989). Photo-oxidation o f pigments Bleaching of conifer needles during the winter has been LOW TEMPERATURE-AIR POLLUTION INTERreported by Linder (1972) and Oquist (1983). This ACTIONS injury is caused by photo-oxidation of chlorophylls due Plant productivity is limited more by water availability to the formation of free radicals from the combination than by any other factor, on a global basis (Kramer, of solar radiation and low temperatures, either above 1983), but low temperature is the main factor that limor below freezing (Oquist, 1983, 1986). its plant distribution (Parker, 1963). Between 15 and Air pollutants may exacerbate this process by con20% of the world's arable land is limited by low temtributing to pigment destruction (Davison et al., 1988). peratures. Annual crop damage in the US due to frost Ozone impairs plant metabolism through the formation or freezing temperatures has been estimated at over one of free radicals which give rise to lipid peroxidation billion dollars (White & Hass, 1975). (Mehlhorn & Wellburn, 1987; Heath, 1988). Ozone, The winter hardening process in trees involves a therefore, may accentuate the photo-oxidation of series of molecular, metabolic and physiological pigments due to the solar radiation-low temperature interaction described previously (Alscher et aL, 1989b; processes that condition cells for exposure to cold Chappelka et al., 1990). Trees exposed to these conditemperatures (Aronsson et al., 1976; Levitt, 1980; Oquist, 1986). These factors include a reduction in tions may be damaged due to a depletion in antiphotosynthesis, increased hydrolysis of starch to oxidants or substrates resulting from the combination of year-long 03 exposures and low winter temperatures soluble sugars, changes in cell ultrastructure, and an increase in production of antioxidant substrates and (Alscher et al., 1989a; Hausladen et al., 1990; Doulis et al., 1993) and/or an increase in free radical production proteins. caused by a light-low temperature-O3 interaction Low temperature stress may be exhibited in several ways, including photo-oxidation of pigments, chilling (Chappelka et al., 1990). Research in this area is limited (Table 1). and freezing injury, and winter desiccation injury Chappelka et al. (1990) observed needle bleaching on (Fitter & Hay, 1987; Kozlowski et al., 1991). These newly emerging foliage of loblolly pine (Pinus taeda) factors can affect plants singly, simultaneously or exposed to above-ambient 03 concentrations after sequentially. passage of a late season (April) cold front. Ozone Biochemicaland physiologicalresponses alone, or in combination with freezing temperatures, Trees are exposed to a wide variety of stresses in the appeared to have little effect on membrane permewinter, including soil moisture deficits, low temperaability, as demonstrated by the lack of increased tures and intense solar radiation resulting in desiccaelectrolyte leakage. They determined that the observed

Predisposition o f trees by air pollutants to low temperatures and moisture stress

107

Table 1. Effects of pollutant exposure on photo-oxidation of pigments in trees

Species

Pollutant a

Effect

Loblolly pine

CF, NF, NF × 1.7, NF × 2.5 03

Visibleinjury (bleached needles occurred on trees exposed to 03 at above-ambient concentrations and was significantly greater in the O3-sensitive family)

Reference Chappelka et al. (1990)

"CF = charcoal-filtered air: NF = non-filtered air. injury symptoms were from photo-destruction of chlorophylls, rather than direct freezing or desiccation injury. Since injury occurred during exposure to elevated 03 concentrations, it cannot be concluded from their study that previous 03 exposure caused an exhaustion of antioxidant reserves which predisposed plants to cold injury. Further research is needed to determine if these mechanisms are involved in the induction of the type of injury observed with loblolly pine (Chappelka et al., 1990). Chilling and freezing stress

Actively growing plants can be injured or killed at temperatures just above or below 0°C. However, many tree species are adapted to survive temperatures of -25°C or lower during dormancy (Salisbury & Ross, 1978). Some temperate and most tropical species of trees are sensitive to cold damage (chilling injury) at temperatures above freezing (Kozlowski et al., 1991). Chilling injury can result from one or a combination of three factors: (1) indirect injury from metabolic function; (2) direct injury from increased cellular membrane permeability; and (3) desiccation injury resulting from Chilling Stress

I

, Diff. in Ea (slopes of Arrhenius plots) between enzymes

I Protein .2 denaturation or dissociation

Phase transition .1 of membrane lipids

I

I

Sudden chilling

I Gradual chilling

t

t

Membrane fracture

Solidification of membrane

I

,

*aPermeability increase of cellular membranes

Permeability .4 decrease to water (roots)

I

I

leakage of

Excess water loss over absorption

cell c o n t e n t s

I increase in E8 es of membrane enzymes in chloroplasts and mitochondria

I Inhibited*e photosynthesis and respiration

I metabolic* 7 disturbance Direct

injury

4

J

Secondary Water stress

indirect

injury

Injury

Fig. 2. Probable alterations leading to three different kinds of chilling injury: * areas where air pollutants may exacerbate chilling injury (after Levitt, 1980); "Wolfenden and Wellburn (1991); 2 Heath 0988); 3 Evans and Ting (1973); 4 Kaufmann 0975); 5 Aono et al. (1991); 6 Darrall 0989); 7 Chen and Wellburn 0989); Darrall (1989).

a reduction in water mobility, due to higher viscosity and lower cytoplasmic permeability. Water then freezes in the roots or other conducting tissues, thus inducing a deficit in water (Levitt, 1980). The pathways that these three factors can follow to induce chilling injury are shown in Fig. 2. Chilling injury and the mechanisms involved in this type of cold stress are reviewed in detail by Lyons (1973), Levitt (1980) and Koziowski et el. (1991). Light and chilling stresses often interact in order to injure plants (Kozlowski et el., 1991). For example, light exacerbates chilling injury to photosynthesis in chilling-sensitive plants (Powles et al., 1983). This type of injury can thus occur in conjunction with photooxidation, and can be exacerbated by a free radical producing air pollutant, such as 03 (Chappelka et al., 1990). Although the potential for air pollutants to predispose trees to chilling injury was demonstrated by Chappelka et el. (1990), very little research exists on the predisposition of plants by air pollutants to chilling injury. Areas where air pollutants may exacerbate chilling injury are shown in Fig. 2. Air pollutants have been shown to affect membrane permeability and alter protein structure (Heath, 1988), change photosynthesis and respiration rates (Darrall, 1989) and to disturb metabolic functions such as carbohydrate synthesis (Barnes et al., 1990; Spence et el., 1990; Amundsen et al., 1991) and ethylene production (Chen& Wellburn, 1989). Research in this area is warranted, since plants are very susceptible to cold injury in the autumn and spring, before and after winter hardening. Air pollutants may cause temporal changes in physiology during the autumn and spring seasons resulting in alterations in the extent of damage at these times (Fig. 2), even at temperatures above 0°C, (Chappelka et al., 1990). Freezing injury occurs when plants are exposed to temperatures below the freezing point of water. Injury occurs with the formation of extracellular ice, altered lipid-protein interaction and/or protein denaturation, membrane leakage and alterations in mitochondria membrane processes and chloroplasts (see Fig. 3; Palta & Li, 1978; Levitt, 1980). Most studies examining the effects of air pollutants on plant response to low temperature have involved freeze injury. Similarly to plant predisposition to chilling injury, air pollutants can alter biochemical (Heath, 1988) and physiological functions (Darrall, 1989) of plants, thereby exacerbating freezing injury (Fig. 3).

108

A. H. Chappelka, P. H. Freer-Smith Extracellular freezing

I

I Dehydration

I

i

Mechanical

Osmotic

I

I

° 1 D e n a t u r a t i o n of membrane proteins

I

I

I

Altered lipid-protein *2 interactions

I

I Inactivation of potassium and sugar pumps

I Leakage of solutes *~

I I

t

Infiltration

Loss of turgot

of tissue

Lack of oxygen

Exchange of K for

ca

I

I

[

Swelling of A l t e r a t i o n s *~ Cellular protoplasm in mitochondria membrane breakdown .4 and chloroplasts

'

Death

L

Fig. 3. Hypothetical series of events leading to injury and death during freeze-thaw cycles: * areas where air pollutants may exacerbate freezing injury (after Palta & Li, 1978; Levitt, 1980); i Heath (1988); 2 Mehlhorn et al. (1990); 3 Heath (1988); 4 Evans and Ting (1973); 5 Miyake et al. (1984). Several pollutants have been demonstrated to alter tree response to freezing stress (Keller, 1978; Lucas et al., 1988; Fowler et al., 1989). Lucas et al. (1988) exposed 2-year-old seedlings of Sitka spruce (P. sitchensis) to increasing 03 concentrations during the growing season. The seedlings were harvested in November and December and evaluated for frost hardiness by placing detached shoots in freezers and subjecting them to a series of regulated freezing temperatures. The plant samples exposed to these treatments and harvested in November exhibited a significant 03 effect,, with sensitivity to freeze injury increasing with 03 concentration. There were no differences among treatments in tolerance to freeze injury in the samples collected in December. The authors concluded that by December all the seedlings had completed winter hardening, but that this process was disrupted in the earlier collection, indicating that autumn frosts may damage Sitka spruce exposed to high summer 03 concentrations. Edwards et al. (1990) fumigated 1-year-old loblolly pine seedlings with sub-ambient, ambient, and twice ambient 03 concentrations for one growing season. Detached needles were exposed to different freezing temperatures during October, January and March, and injury was ascertained by determining the relative diffusate electrical conductivity. Similarly to Lucas et al. (1988), the authors found that seedlings exposed to twice-ambient 03 concentrations were less hardened in the autumn and spring, compared with the other treatments. No visible symptoms of freeze injury were induced by 03. Other studies (Table 2) using Norway spruce (P. abies), red spruce, avocado (Persea americana) and grapefruit (Citrus paradisi) tend to support the hypothesis that 03 can affect the winter hardening process (Brown et al., 1987; Barnes & Davison, 1988; Eissenstat et al., 1991; Fincher & Alscher, 1992).

Red spruce exposed to ambient 03 on Whitetop Mountain, Virginia, US did not exhibit a decrease in cold tolerance compared with those exposed to subambient 03 levels (De Hayes et al., 1991). Senser (1990) observed no increase in visible symptoms of freeze injury in Norway spruce after 03 exposure (Table 2), but these freeze exposures did not begin until the plants were 'hardened' during the winter months. Other air pollutants have also been shown to affect winter hardening and subsequent freeze injury in several tree species (Table 2). SO2 fumigations in outside chambers reduced frost tolerance of Norway spruce seedlings during the following spring (Keller, 1978). Freer-Smith and Mansfield (1987) exposed Sitka spruce seedlings to SO2, NO2 and combinations of the two during the dormant period. Seedlings were then subjected to a range of decreasing night-time temperatures (4°C to -15°C). Seedlings exposed to SO2 exhibited a decrease in bud survival with decreasing temperatures, and seedlings exposed to both pollutants showed some needle injury at low night-time temperatures. Acidic mists have caused a reduction in the freeze tolerance (Table 2) of red spruce (Fowler et al., 1989; Cape et al., 1991). Fowler et al. (1989) exposed red spruce seedlings to increasingly acidic mists from July to December in chambers containing air that was charcoal-filtered to remove ambient 03. Excised shoots were exposed to different freezing temperatures and then examined for differences in visible injury and electrical conductivity. Seedlings treated with the most acidic mists exhibited the greatest injury and at the highest temperatures. Cape et al. (1991) reported that a delay, or reduction in timing or extent of autumnal frost hardening was associated with increased concentrations of S O 4 and to a lesser extent, (NH4)2 ions in acidic mists. Eamus and Murray (1993) demonstrated that mists containing SO4 caused irreversible stomatal opening and thus increased mid-winter freeze sensitivity of Norway spruce. These results are important, since SO4 is the predominant anion in acidic deposition in the regions where red and Norway spruce generally grow (Georgii et al., 1984; Zemba et al., 1988). Two additional studies (DeHayes et al., 1991; Vann et al., 1992) demonstrated that red spruce exposed to ambient cloud water are less frost tolerant during the winter than those from which the water was excluded (Table 2). As shown in Fig. 3, probable mechanisms responsible for air pollution predisposition of trees to freeze injury include altered lipid composition, membrane leakage of solutes, changes in chlorophyll pigments, carbohydrates and antioxidants. Wolfenden and Wellburn (1991) found a difference in fatty acid composition of O3-treated plants compared with non-treated seedlings during the autumn, and related this to a change in winter hardening induced by 03. Barnes and Davison (1988) observed a decrease in chlorophyll fluorescence in O3-treated seedlings after

Predisposition o f trees b y air p o l l u t a n t s to low temperatures a n d m o i s t u r e stress

109

Table 2. Effects of pollutant exposures on freeze tolerance in trees

Species

Pollutant

Effect

Reference

Norway spruce

SO2

Trees exposed to SO2 exhibited increased frost sensitivity in spring

Keller (1978)

Sitka spruce

SO2, NO2, SO, + NO2

Buds from SO2-treated trees were more sensitive to freeze injury than those from controls. SO2 + N O 2 induced some needle injury

Freer-Smith & Mansfield (1987)

Norway spruce

03

No effect of 03 on frost tolerance, based on visible injury

Senser(1990)

Norway spruce

03

03 induced significant injury to seedlings post-freezing; chlorophyll fluorescence reduced by 03 in two clones post-freeze

Barnes & Davison (1988)

Norway spruce

03

Visible injury observed on O3-treated trees after November frost

Brown et al. (1987)

Red spruce

03

Mesophyll cells disrupted in O3-treated plants after frosts during 1st year, but not 2nd. No differences in visible injury observed

Fincher & Alscher (1992)

Sitka spruce

03

More visible injury due to freezing temperatures for O3-treated seedlings harvested in November. No differences observed in December harvests

Lucas et al. (1988)

Avocado, grapefruit

03

Threefold ambient 03 caused an increase in electrolyte leakage and visible injury in freeze-treated trees. Effects near ambient were minimal

Eissenstat et al. (1991)

Loblolly pine

03

Seedlings exposed to two-fold ambient 03 were less hardened in autumn and spring than other treatments. Electrolyte leakage was greater for these seedlings

Edwards et al. (1990)

Red spruce

AM ~

Seedlings exhibited increased freeze injury as mist acidity increased. More common in early autumn

Fowler et al. (1989)

Red spruce

AM"

Frost hardening delayed by AM. ~ Related to concentrations of sulphate and ammonium in the mists

Cape et al. (1991)

Red spruce

AM"

Sulphate caused stomata to irreversibly open. Frost hardening delayed by AM a containing sulphate

Eamus & Murray (1993)

Red spruce

Cloud water (ambient AM ~)

Red spruce seedlings exposed to ambient cloud water were less cold tolerant than those to which ambient cloud water was excluded

De Hayes et al. (1991)

Red spruce

Cloud water (ambient AM")

Red spruce branches exposed to ambient cloud water were less cold tolerant than those to which ambient cloud water was excluded

Vann et al. (1992)

"AM = acidic mist. exposure to freezing temperatures, indicating injury to cell membranes (e.g. plasma membrane, thylakoid membranes). Since fluorescence was detected, but no visible injury observed, the data indicate that ozoneinduced injury to cell membranes occurs much earlier than visible injury. Several studies have shown alterations in carbohydrate contents of seedlings exposed to 03 (decreases in raffinose and other ethanol-soluble carbohydrates) and have related these changes to a decrease in freeze

tolerance, since these carbohydrates can act as cryoprotectants (Alscher et al., 1989a; Barnes et al., 1990; A m u n d s o n et al., 1991). Hausladen et al. (1990) and Doulis et al. (1993) observed an alteration in antioxidants such as ascorbic acid, glutathione and superoxide dismutase with 03 fumigation and related these changes to a decrease in frost tolerance of seedlings. Feiler (1985) with SO2, Edwards et al. (1990) and Barnes and Davison (1988) with 03 and Fowler et al. (1989) with acidic mist observed an increase in

110

A. H. Chappelka, P. H. Freer-Smith

Table 3. Effects of pollutant exposures on winter desiccation in trees

Species Pollutant

Effect

Norway 03, AM~ Slight decrease in surface waxes increased leaf spruce area/weight ratio Waxes change, occluded Norway O3 stomata common in spruce ozone-treated seedlings

Reference Barnes & Brown (1990) Barnes et al. (1988)

dition of these species, especially at the northern limits of their range. These results may have further implications, due to potential global climate changes. Elevated CO2 and concomitant warmer temperatures may extend the growing season of several tree species (Miller et aL, 1987). Possible increases in autumn and spring frosts, combined with elevated 03 and increased radiation, could magnify this type of injury (Krupa & Manning, 1988).

~AM = acidic mist. MOISTURE

electrolyte leakage after freeze treatments, indicating damage to one or more cellular membranes, and an increase in membrane permeability. Winter desiccation injury In addition to direct injury induced by chilling and freezing temperatures, trees can be damaged by water deficits induced by cold temperatures (Fig. 3). As temperatures decrease, water mobility is reduced due to higher viscosity and lower cytoplasmic permeability. Water then may freeze in the roots or other conducting tissue, thus inducing water deficits (Fig. 2). Environmental conditions such as bright sunlight and high vapor pressure may exacerbate this process. Air pollutants may accelerate winter desiccation by disrupting stomatal function (Darrall, 1989) and accelerating cuticular erosion of needles (Barnes et aL, 1988; Barnes & Brown, 1990) (Table 3). Careful examination of winter injury to determine the importance of pollutant-desiccation interactions in frost tolerance is needed. Summary

Low temperatures, even above freezing, can cause damage to trees by injuring foliage, stems, and in some cases, roots. This condition can be exacerbated by air pollutants, especially in the autumn and spring months, before and after 'winter hardening.' Ozone and acidic mists have been reported to increase winter injury in several tree species. This is important, since both pollutants can be regional in nature and may occur in the same area. Very little research exists on the combined effects of these pollutants on cold tolerance in trees. Air pollutants affect a tree's ability to tolerate cold temperatures in several ways: photo-oxidation of pigments; membrane leakage; reductions in antioxidants and intermediate carbohydrates; alterations of pigments; and winter desiccation. More than one of the above processes may be altered by exposure to air pollutants during the cold hardiness period. The vast majority of research in this area has occurred with high elevation, cold tolerant species (e.g. red spruce). The cold tolerance of loblolly pine and other low elevation species tested have also been altered by air pollutants. The implications of these results are important and may influence the forest con-

STRESS-AIR POLLUTANT INTER-

ACTIONS Having first highlighted the relationship between intercepted radiation and plant growth some 20 years ago (Monteith, 1977), Monteith has recently predicted plant productivity based on radiation and water availability experienced at a site. In an important review of climatic constraints on crop production, Monteith and Elston (1994) have recently shown that production can be considered to be limited by four constraints. Two of these, radiation and rainfall, are resources, and two, temperature and saturation vapor deficit, are modifiers. This synthesis allows the period of time during the growing season and the extent to which precipitation limits production to be identified for a particular site (Fig. 4). The potential production for each month, as limited by precipitation, is calculated from rainfall, mean saturation deficit and biomass:water ratio (BWR). BWR is commonly referred to as water-use efficiency. Saturation deficit is important because it has tight control on stomatal conductance. Monteith and Elston's analysis (1994) is a way of synthesizing those controls which we know, from empirical observation, are of importance in limiting potential and actual production. Water availability and physiological adaptations to water deficit are clearly important. Actual productivity falls significantly SUTTON BONINGTON C3

i Apr.

July

Oct.

Fig. 4. Potential monthly biomass production for eastern England as a function of precipitation or radiation (after Monteith & Elston, 1994).

Predisposition of trees by air pollutants to low temperatures and moisture stress

111

below potential production because the model does not allow for other variables that may constrain BWR or limit stomatal conductance. Examples of these variables include the effect of water supply on nutrient availability in soil and potential impacts of air pollution. For both factors, the interaction of effects with those of water availability are likely to be important. The condition of some European forests and of forests in the US has declined over the last 20 years (Blank, 1985; McLaughlin, 1985). For a number of these areas there is now evidence that the decline was the result of combined effects of deposition of various pollutants, depending on the region, and of adverse climatic conditions. In north Germany, the Black Forest, the Bavarian Forests and Vosges Mountains, there is general agreement that pollutant effects are mediated via soil processes (Schulze, 1989; Ulrich, 1989; Zoettl & Huettl, 1991). The direct effects of gaseous air pollutants clearly have a role in the more polluted areas and direct effects of 03 episodes are important on a regional basis. Elucidation of the role of O3 remains impossible. However, the monitoring of forest condition has shown that deterioration of forests occurs in drought years and that decline can be checked or reversed in wet years (Prinz et al., 1987). It was suggested that 03 could cause increased transpiration during dry periods, thereby exacerbating the effects of water deficits (Prinz et al., 1987). Although Britain is normally considered to have plentiful and well distributed rainfall, the condition of European beech (Fagus sylvatica) in southern Britain is a good example of the influence of drought. Annual surveys have shown that crown condition deteriorated in dry years, as £or example in 1989 and 1990 (Innes & Boswell, 1991). These observations were supported by detailed measurements of growth (Lonsdale, 1986) which demonstrated that mild drought can lead to reduced root, shoot and leaf growth in southern England alad that a single dry season can depress the vigor of beech for several successive years. Since 03 episodes are more common in dry years (Krupa & Manning, 1988), data collected from. the forest cannot separate the effects of these two important factors or determine the nature of their interactions. For this we must turn to the experimental data.

Fig. 5. Generalized time course of gross and adaptive changes in plants in response to a gradual development of water stress in the field. Band width at a given time represents the relative magnitude of the particular response. The starting position of each band on the time scale is indicative of the water stress threshold for eliciting the response. Band shape reflects variations in response with increasing stress duration and intensity (after Bradford & Hsiao, 1982).

Biochemical and physiological responses Plant responses to water availability and water deficit have been extensively studied. Water shortage has dramatic effects on the physiology of the plant. The processes which may be altered were detailed by Bradford and Hsiao (1982). Studies of the course of events occurring as the intensity of drought increases show, surprisingly, that restriction of leaf and shoot growth may be the first effect (Fig. 5). This response is then followed by altered root to shoot ratio, osmotic adjustments and only then by stomatal closure. At extreme water deficits, wilting, leaf senescence and finally, as might be expected, abscission occur. The

Leaf growth and water relations Wright et al. (1987) reviewed the physiological responses of plants to SO2, NO~ and 03 in order to identify responses that have implications for drought resistance. These authors presented important new information demonstrating that small concentrations of SO2 and NO2 (down to 20 ppb) could severely affect the ability of leaves to conserve water. Water loss was observed using drying curves with excised leaves from plants exposed to between 20 ppb and 90 ppb of both pollutants. In Betula pubeseens and B. pendula, water loss was substantially quicker from leaves exposed to air pollutants as compared to leaves grown in clean air,

i

Water stress intensity Restriction of canopy development

i I

~

I

]

Increase in growth of roots relative to shoots Osmotic

adjustment

Stomatol closure

w..n

or

"~

S

I i

Leaf senescence

~tB

Leaf death by dessication

1 i

Stress starts

TIME

removal of the transpiring leaf area reduces the water consumption of the plant. Figure 5 illustrates the range of processes which are effected as drought intensifies and therefore identifies where effects of air pollution have the potential to interact with those of drought (Table 4). The traditional view is that decreased water availability results in decreased water potential and turgor in plants, and that this hydraulic signal leads to observed plant responses. However, there is increasing evidence for an additional, fine-tuning mechanism by which stomata may partially or completely close and leaf growth may slow so that shoot water potentials and turgors are maintained even when soil moisture deficits occur (see review by Davies et al., 1990). This fine-tuning appears to rely on a chemical or plant growth regulator signal from roots, rather than a hydraulic signal.

A. H. Chappelka, P. H. Freer-Smith

112

Table 4. Reports of where pollutant exposures have been shown to modify drought response mechanisms

Drought response a Restriction of canopy development Altered root : shoot ratio

Effect of pollution

Reference

Current ambient concentrations have transient effects on leaf growth of poplar Pollution increases specific root length (length per unit mass) of beech and spruce 0 3 alters allocation in P. taeda, favoring more photoassimilate retention in the stems

Taylor & Frost (1992)

Solute potentials higher in polluted air. Starch accumulation common in conifers Drought stress and 03 caused a decrease in water and osmotic potential. No osmotic adjustment occurred.

Taylor & Dobson (1989)

Stomatal closure

Opening and closure common depending on concentration and pollutant, see Table 5

Maier-Maercker & Koch (1992)

Leaf wilting and rolling

Pollutant exposure accelerates leaf drying

Wright et al. (1987)

Leaf senescence

Common response to pollutant exposure

Freer-Smith (1983); Chappelka & Chevone (1992) (review)

Leaf death (abscission)

Common response to pollutant exposure

Mooi (1981); Cheppelka & Chevone (1992) (review)

Osmotic adjustments

Taylor et al. (1989); Taylor & Davies (1990) Spence et al. (1990); Friend & Tomlinson (1992)

Roberts & Cannon (1992)

~Afler Bradford and Hsiao (1982). and the effects were related to exposure concentration. Transpiration rates were increased in those leaves exposed to SO2 and NO2. The time course of the drying curves indicated ~that such effects were due to accelerated cuticular loss or, alternatively, disruption of the leaf epidermis to such an extent that stomatal closure was prevented (i.e. loss of epidermal cell turgor). This second mechanism was supported by scanning electron micrographs of the leaf surfaces and by measurements of transpiration on attached leaves. According to the Lockhart model of leaf growth (Lockhart, 1965), cell (and thus leaf) growth is dependent on turgor-induced irreversible extension. Once a critical turgor (the yield threshold) has been exceeded, the cell or leaf will expand at a rate which is dependent on the extensibility of the cell wall and is linearly related to turgot. Loss of cell turgor and increased water loss from leaves might be expected to decrease leaf growth. Taylor and Frost (1992) have looked closely at the effects of ambient air pollution in southern England on leaf growth in hybrid poplar (Populus nigra x deltoides), using the Lockhart model as a basis for their investigation. These authors concluded that, as for stomatal conductance (see below), current ambient pollutant concentrations are within the range where no effects or small transient effects on leaf growth occur. Sensitivity of poplar leaf growth to air pollution varies over the growing season and with the prevailing weather conditions. Final leaf size increased regardless

of air quality as daily maximum temperatures declined in association with periods of rainfall. This suggests that cooler, wet weather results in accelerated leaf growth, possibly due to altered stomatal conductance and increased leaf cell turgor pressure. These and other similar studies suggest that while exposure to polluted air can influence leaf water relations (Wright et al., 1987), at current ambient concentrations the effects of gaseous air pollution on leaf growth are relatively small and would tend to be masked quickly by direct effects of even mild water deficit (Taylor & Frost, 1992). Root growth relative to shoot growth Work on herbaceous plants and trees has clearly demonstrated that exposure to gaseous air pollutants can decrease root to shoot ratios (Chevone et al., 1990; Chappelka & Chevone, 1992). SO2 and O3 alter translocation such that a greater proportion of photoassimilate is retained in the shoots for defense and repair (Spence et al., 1990; Friend & Tomlinson, 1992). Root growth and biomass production has consistently been shown to be reduced in response to gaseous air pollutants (Chappelka & Chevone, 1992). As for a number of the physiological effects on trees which are described here, once senescence and abscission occur, such altered allocation of dry matter is marked due to a significant loss of leaves. A decreased root to shoot ratio may be expected to exacerbate susceptibility to water deficits because a relatively larger shoot system

Predisposition o f trees by air pollutants to low temperatures and moisture stress

113

Table 5. Effects of pollutant exposure on stomatal conductance (gs) of trees

Species

Pollutant

Effecff

Reference

Sitka spruce, Norway spruce

03

03 uptake increased with increased gs. 03 exposure increased gs and decreased WUE

Freer-Smith & Dobson (1989)

European beech, hybrid poplar, Norway spruce, Sitka spruce

Unfiltered ambient air

At current ambient concentrations pollutants can inhibit and stimulate gs in trees

Freer-Smith & Taylor (1992) (review)

Norway spruce, Sitka spruce

03 and drought

Drought decreased gs and hence 03 uptake. No effects of WUE

Dobson et al. (1990)

European beech

03 episodes and drought

03 and drought decreased gs. Drought + 03, stomata failed to close fully, giving interactions in effect

Pearson & Mansfield (1993)

Fraser fir (Abiesfraseri)

03, drought

Drought decreased gs. WUE improved with water stress

Tseng et al. (1988)

"WUE = water use efficiency.

would be dependent for water supply upon a relatively smaller root system (Table 4). A system for growing trees and other crop plants in long tubes placed into the ground to allow normal soil temperature and exploration of depth, along with accessibility to roots for analysis has been developed. Initially, this approach was used to show that root growth in Zea mays was stimulated during soil moisture deficit (Sharp & Davies, 1989). Recently, the same technique has been used to grow F. sylvatiea in opentop chambers fumigated with charcoal-filtered and non-filtered air so that the effects of ambient pollution concentrations on root morphology could be detected (Taylor & Dobson, 1989). Ambient pollutant concentrations experienced for a whole growing season at a site in southern England had no effect on total root mass. However, when careful measurements were made down the root profile, it was found that at a depth of 20 cm and below the roots of plants in polluted air were longer. Since root dry mass was not affected, specific root length (root length per unit dry weight) was increased by pollutant exposure. Roots were longer and thinner for trees grown in polluted air. The effects occurred for well-watered and unwatered plants and without an interaction between air quality and water availability. Similar morphological effects were also observed in roots of Sitka spruce (Taylor et al., 1989). Stomatal responses As would be expected for diffusive flow driven by concentration gradients, there are direct linear relationships between stomatal conductance and the stomatal uptake of SO2, NOx and 03 (Freer-Smith, 1985; FreerSmith & Dobson, 1989). The role of stomatal conductance in pollution resistance mechanisms has been of interest for a number of years (Mansfield & FreerSmith, 1984), and it is clear that factors such as drought and large atmospheric saturation vapor

deficits, which result in stomatal closure, can decrease pollutant uptake. It is now well established that gaseous air pollutants influence stomatal conductance and a recent review of effects on the gas exchange of trees (Freer-Smith & Taylor, 1992) concluded that at current ambient concentrations of gaseous air pollutants both inhibitions and stimulations of stomatal conductance can be observed. Acidic mists have also been shown to influence stomatal function; Fluckiger et al. (1988) showed, for example, that acidic mist of pH 3 will considerably lessen night-time stomatal closure of F. sylvatica. Similarly, 03 is known to disrupt stomatal control. For example, Reich & Lassoie (1985) have shown that 2 months at 125 ppb results in lower wateruse efficiency in hybrid poplar (P. deltoides × trichocarpa). As in the experiments of Wright et al. (1987) with SO2 and N O 2 (previously described), stomatal closure in poplar was poor and sluggish after exposure to 03 and excised leaves wilted more quickly than those of control plants. Clearly, the concern is that long-term exposure to low levels of gaseous air pollution may disrupt stomatal regulation and thus enhance sensitivity to water deficit (Table 5). This type of effect has been demonstrated recently by Pearson and Mansfield (1993). European beech were grown in charcoal-filtered air or in base-line 03 concentrations (30 ppb) with episodes of 60, 80, 100 and 120 ppb superimposed on this baseline. Ozone exposure resulted in stomatal closure and the imposition of drought also resulted in stomatal closure in both the polluted and control plants. However, of particular interest, stomatal closure caused by drought was less marked for plants exposed to 03 than in those grown in clean air. There were highly significant interactions between the effects of 03 and water deficit. The stomata of trees exposed to 03 responded less effectively tO drought, with pollution lessening their ability to close.

A. H. Chappelka, P. H. Freer-Smith

114

Table 6. Investigations of interactions in effects of pollution and drought effects on growth of trees

Species

Pollutant

Effecta

Reference

European beech

Ambient pollution and drought

Ambient pollution decreased root weight and increased specific root length. Effects of drought similar in both air quality treatments

Taylor & Davies (1990)

European beech

0 3 and

03 decreased the negative impact of drought on root weight (03 increased A and gs)

Davidson et al. (1992)

Ponderosa pine

03

Water-stressed ponderosa pine, was protected from 03 by decreases of gs

Beyers et al. (1992)

Ponderosa pine

03

Less 03 induced leaf injury in ponderosa pine exposed to water deficit

Temple et al. (1992)

Shortleaf pine

03

03 uptake continued to be detrimental to physiology and growth even in severe water deficit

Flagler et al. (1993)

Fraser fir

03

03 exposure results in no biomass change. Photosynthesis decreased in all treatments. No water stress-O3 interaction

Tseng et al. (1988)

Red spruce

03 and drought

03 effects were more deleterious with drought than controls

Roberts & Cannon (1992)

drought

( P. echinata)

aA = photosynthesis; gs -- stomatal conductance. L e a f senescence and abscission

Accelerated leaf senescence and early abscission have long been known to be key effects of long-term exposure of trees to small concentrations of gaseous air pollutants (Table 4). Such effects consistently have been observed in broadleaves and conifers exposed to SO2 (Garsed et al., 1979; Mooi, 1981; Garsed & Rutter, 1982; Freer-Smith, 1983). In recent years, these effects have also been observed with 03 (Chappelka & Chevone, 1992). Models have suggested that such effects may have a greater impact on tree growth than short-term effects of 03 episodes (Mohren et al., 1992). Surveys of tree condition in woodlands and forests are normally based on crown density as the main parameter. Crown density is a non-specific symptom and the result of combined effects of canopy development, branch form, senescence and abscission of leaves and branch dieback. The abiotic factors responsible for premature yellowing and necrosis of tree foliage were reviewed by Freer-Smith and Taylor (1992). Clearly, early leaf loss, although it may have other seriofis effects on trees, will leave a smaller leaf area from which water is lost by transpiration. The experiments of Wright et al. (1987) described earlier are good examples of such an effect. Summary

Forest observations commonly show declines in tree conditions in hot, dry summers. Carefully controlled experimental work is needed to establish the role of air pollutants and their interacting effects with drought in such declines. To date, only a few studies have examined the interactions of pollution and drought on trees. Drought appears to produce more dramatic impacts than do realistic concentrations of air pollutants. How-

ever, in contrast to work with herbaceous/crop plants, 03 effects still occur in trees experiencing water deficits (Pearson & Mansfield, 1993). Studies of mechanisms demonstrate a wide range of interactions, depending on pollutant dose and on other environmental factors. The concept of drought closing stomata and protecting plants from pollutant exposure (Tingey & Hogsett, 1985) is an oversimplification for woody plants. Similarly, the antagonistic interaction between drought and pollution, in which drought decreases productivity so that an air pollutant such as 03 no longer exerts a measurable effect on yield (Chevone et aL, 1990) is not appropriate as a model for interactions in woody plants (Flagler et aL, 1993). Trees with long life cycles will commonly experience many periods of drought and episodes of pollutant exposure of varying duration, timing and severity. Interactions are inevitably more complex and the evidence reviewed suggests that they are also of greater significance. Together, the papers reviewed have made a strong case for the importance of pollutant and drought interactions. Disruptions of metabolic responses to drought caused by (pre)exposure to air pollutants have been established in a number of controlled experiments (Table 6). These results are consistent with the observations made in the forest; pollutants, especially 03, can disrupt normal responses to water deficits so that the two factors occurring together can result in serious effects on tree condition. CONCLUSIONS Air pollutants have been shown to alter cold tolerance in several tree species. No studies to date have ascertained whether this subsequently affects growth. Several

Predisposition o f trees by air pollutants to low temperatures and moisture stress mechanisms are implicated in the decrease in cold tolerance in trees. It appears that other environmental conditions, such as light and v a p o r pressure deficit, m a y influence which mechanism occurs. Observational and controlled studies have demonstrated that symptoms of injury may occur at ambient or near-ambient pollutant concentrations. An air pollutant-cold tolerance interaction may exist and may be important in understanding changes in forest condition. Only a few studies have examined drought-pollution effects on tree growth. Studies of mechanisms (e.g. leaf growth, senescence) show a wide range of interactions depending on pollutant exposure and other environmental factors. Forests commonly show declines in tree condition during hot-dry summers. Overall, these conclusions strongly support a possible role for air pollutant-drought interactions.

ACKNOWLEDGEMENTS The authors would like to thank R. Alscher, S. Krupa, T. Mansfield, H. Neufeld and L. Samuelson for reviewing an earlier draft of this manuscript and J. L. Monteith and C h a p m a n & Hall for permission to reproduce Fig. 4. A l a b a m a Agricultural Experiment Station Journal No. 9-933623.

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