Branch growth and leaf gas exchange of Populus tremula exposed to low ozone concentrations throughout two growing seasons

Environmental

Pollution 79 (1993) l-7

BRANCH GROWTH AND LEAF GAS EXCHANGE OF Populus tremula EXPOSED TO LOW OZONE CONCENTRATIONS THROUGHOUT TWO GROWING SEASONS Rainer Matyssek’, Theodor Keller” & Takayoshi Koikeb a Swiss

Federal Institute of Forest, Snow and Landscape Research, Ztircherstr. Ill. CH-8903 Birmensdorf ZH, Switzerland b Forestry and Forest Products Research Institute, Hokkaido Research Center, Sapporo 062, Japan

(Received 17 June 1991; accepted 2 September 1991)

Abstract During

two consecutive

growing

seasons, the same potted

individuals of European aspen (Populus tremula), grown from root cuttings of one clone, were fumigated with either ambient air or ozone concentrations of 0 (control), 0.05 or O,I $itre litrel. Structure and biomass of the annually formed branches were analysed after excision at the end of each season. Only at 0.1 plitre lit& was branch weight reduced, and crooked axes occurred in each season. During the second season, branch length and leaf sizes were strongly reduced, while many leaves displayed yellowish deficiency symptoms and lowered cation concentrations. Such leaves contrasted to those showing characteristic O,-bronzing. Although foliage density was enhanced due to reduced branch length, the area of attached foliage was limited by the small leaf sizes, necrotic leaves and premature leaf loss. During mid-summer of the second fumigation period, photosynthetic capacity, carboxylation ejiciency and water-use eficiency (WUE) declined in (attached) yellowish and bronze leaves at 0.1 ditre litrel, whereas green leaves at 0.05 plitre litre’ displayed accelerated senescence in late summer while maintaining WUE. It is concluded that the d@erences in branch growth between the two growing seasons were caused in part by internal changes in those plant organs (root and basal stem), which had experienced both fumigation periods.

INTRODUCTION The root : shoot biomass ratio is often found to be reduced in plants exposed to air pollutants (Mooney & Winner, 1988). Under such stress, carbon allocation may favour the green to the below-ground biomass in order to counterbalance, in part, the consequences of declining photosynthesis and premature leaf loss on whole-plant production (Mooney et al., 1988). Also, a reduced assimilate transport in the phloem may limit root growth (Spence et al., 1990). In particular, trees Environ. Pollut. 0269-7491/92/$05.00 0 1992 Elsevier Science Publishers Ltd, England. Printed in Great Britain

with mesophytic leaves sensitive to air pollutants (Reich, 1987) may display such changes in allocation. In birch under low O3 stress, we have found both reduced whole-plant production and root : shoot ratios accompanied by declining gas exchange and structural breakdown of leaves, reduced foliage area and deformed phloem tissue in leaf petioles and the stem (Matyssek et al., 1991, 1992). What are the consequences for tree growth, if such root limitation occurs year after year? Even though resolution of this question is essential for the long-term survival of plants, fumigations have rarely been conducted for more than one growing season (Pye, 1988), mostly because of logistical problems. The latter are often associated with clipping above-ground plant parts after each growing season (Wang et al., 1986), e.g. to cope with the space restrictions of fumigation cabinets. Such biomass removal does, however, bias the root : shoot ratio of subsequent years. Despite the shortcomings of branch clipping, an attempt was made in this study to fumigate the same aspen plants during two subsequent growing seasons with low O3 concentrations, as found in rural regions of Central Europe. The need for branch excision after each season obviously prohibits quantitative interpretation of root : shoot biomass ratios as bioindications of O3 stress. Although the roots were not directly analysed, comparison of the growths of new branches in the two seasons were considered valid, as plants were subjected to the same O3 regime each year, but were possibly affected by changes in root function. During the branch growth of the second season analytical experiments on leaf gas exchange were conducted (cf. Lange et al., 1987; Matyssek et al., 1991). MATERIALS AND METHODS Plants and treatments In March 1986, root cuttings of one clone of Populus tremula L. (European aspen; clone ‘Birmensdorf’ 21-5; cuttings from the offspring of a mother tree at 900 m above sea level) were potted into a mixture of peat, bark compost, loamy soil and styrene flakes in the ratio

2

R. Matyssek, T. Keller, T. Koike

of 6: 3 : 1 : 1 (v/v; lo-litre pots). All plants were well watered throughout the experiment and equally fertilized at the beginning of each growing season. Throughout 1986, all plants were kept in ambient air, where they developed straight, branchless stems. The latter were cut back to a height of 30 cm at the end of this growing period. In April 1987, the plants were transferred into field fumigation chambers (see Keller, 1976). Five chambers (1 plant per chamber) were each continuously supplied with either: (a) charcoal-filtered air (control); (b) ambient air (of the rural region of Birmensdorf near Zurich, 550 m above sea level; mean monthly ozone concentration during summer, 0.03 plitre litrel); (c) 0.05 plitre 0, litre 1 or (d) 0.10 plitre O3 litrei. For the latter two regimes, ozone was generated from pure oxygen (Fischer ozone generator, model 502) and continuously added to charcoal-filtered air, while O3 concentrations were monitored at stem height in all four treatments (Monitor Labs, model 8810). Each of the four fumigations was run with the same set of plants during the growing seasons of 1987 (fumigation period 1 = ‘FPl’) and 1988 (fumigation period 2 = ‘FP2’). About 10 branches developed from the basal stem of each tree by the autumn of 1987 (FPl), when they were excised at their bases to allow a similar number of branches to grow during 1988 (FP2) from the previous year’s buds. The plants were kept in ambient air between the two fumigation periods from October 1987 through April 1988. Biomass assessment In the autumn of 1987 (FPl) and 1988 (FP2), all branches in a tree were excised and analysed in terms of basal diameter, length and dry weight. In addition, leaf number and (one-sided) foliage area (Delta-T area meter MK2) were determined in September 1988 (FP2), together with the annual area increments of the basal stem. Throughout FP2, the annual course of length growth in all branches and the number of attached necrotic and shed leaves per tree were recorded at 3-5-week intervals. Each time, all branches were classified into four categories, which were defined in each 0, regime by the relative proportion of three representative leaf sizes (large, medium, small) in a branch’s foliage: categories (l), 50% of branch leaves large + 50% medium; (2) 100% medium; (3) 50% medium + 50% small; (4) 100% small. This allowed calculation of the area of attached foliage per tree throughout the growing season as based on the number of attached branch leaves. Gas exchange experiments These were conducted in 1988 (FP2) on attached fully grown, either 2-week-old or lCweek-old branch leaves, with a thermoelectrically climate-controlled cuvette system (Walz), which was installed in the field close to the fumigation chambers. We investigated those leaves which showed either no visible injury (as control leaves in Fig. 1A); or displayed the O,-characteristic ‘bronze’ discoloration (Fig. 1B; without large necroses as in Fig. 1C); or a yellowish colour (Fig. 1D; leaves

developing bronze necroses after the appearance of the yellowish discoloration were not included in analyses). Both discolorations occurred on the same individual plant. The evening before the measurement, plants were randomly chosen from the treatments and were brought to the measuring site. During the gas exchange experiments, the plants were shielded from direct sunlight and rain (experimental procedures and equipment as described by Matyssek et al., 1991). Steady-state responses of leaves to non-limiting cuvette conditions (light intensity, I > 1200 pmol photons m-2 s-1; leaf temperature, T, = 19°C; leaf/air difference of water vapour mole fraction Aw = 9 mmol molll) provided a measure of the water-use efficiency (WUE) at ambient CO, concentration, c, = 340 plitre litrel, the carboxylation efficiency (CE) and the maximum CO1 assimilation rate (A,,, at c, = 1500 plitre litre-I). C, was adjusted by a mass flow-controlled CO2 -dispensing system while CE was calculated from the linear (Walz), correlation between the CO, assimilation rate and the CO, concentrations of the mesophyll intercellular spaces, c;, in the range ci < 100 klitre litre--1 (cf. Farquhar & Sharkey, 1982). CE and c, are apparent values with respect to potential non-homogeneity in leaf function (cf. Terashima et al., 1988). The intensity, Z, of the artificial light source used (8 ‘Multi-Mirror’ halogen bulbs, General Electric) was measured with a GaAsP photodiode (Hamamatsu G 1118) after calibration with a quantum sensor (LI-190, LICOR), and leaf temperature, T,, was registered with a 0.1 mm chromeValume1 thermocouple. Gas exchange rates were based on the one-sided leaf area (including discoloured leaf parts; stomata only on abaxial leaf sides, Gtinthardt-Goerg, pers. comm.). Chemical analysis The leaf concentrations of nitrogen, N, were assessed with a Carlo Erba NA1500 analyser, those of cations, S and P by ICP-AES (inductive coupled plasmaatomic emission spectroscopy), and that of chlorophyll according to Lichtenthaler & Wellburn (1983; with acetone 80%, v/v, as solvent). RESULTS Stem and branch growth In 1986, when branchless stems had developed from the root cuttings under ambient air, the annual increment in the basal stem area was similar in all plants (Fig. 2). When the plants were exposed to different ozone fumigations during the 2 successive years (after excision of the upper stem at the end of 1986), the area increments increased under all but the 0.1 plitre litrei regime (Fig. 2). In that case, the increment declined toward zero in 1988 (FP2). In contrast to the stem growth, the total length of all developing branches per tree was rather similar in all plants at the end of 1987 (FPl; Fig. 3A). However, the weight increment of the branches at 0.10 plitre litrei, and thus the biomass investment per unit of branch length were only about one-third of those in

3

Branch growth and gas exchange in aspen under 2-year ozone stress Populus tremula 300

In 1987 (FPI) and 1988 (FP2), these lowered ratios at 0.10 plitre lit& were accompanied by a reduced leaf number, which, beginning in June, was due to the O3 -induced leaf loss (Fig. 4A; 1987 not shown). This loss was confined to about 30% of all leaves formed during the rest of the summer (Fig. 4A, C; 1988). Despite leaf loss, the foliage density was enhanced at 0.1 plitre litre-i (Fig. 4B), due to the limited growth in the length of the branches (cf. Fig. 3D). Remarkably, trees of all treatments had formed a similar total number of leaves throughout the season regardless of the Oj concentrations (Fig, 4C). The resulting foliage area of a tree exposed to 0.10 plitre O3 litre-i after leaf loss was reduced to a higher extent than the number of attached leaves (Fig. 4A, D). This effect was caused by a mean (one-sided) leaf area of 35 cm2 as compared with 75 cm2 in the other treatments, whereas the leaf sizes had been similar in all plants in 1987 (FPl). Additionally, the attached foliage area at 0.10 plitre litre 1 was impaired by

1

fumigation treatments during summer

all plants in ambient air

F7

E 250

.E

I .

; ; 200 Sv) .:

150

s E !?

u 100

.E = 2 2

0 ambient air 0.05

l

50

1

0'

1988

1987

1986

End of annual assimilation period Fig. 2. Increment of the basal stem area (10 cm above ground) at the end of the years specified (1987 = FPl; 1988 = FP2), given as means + SD of five trees each.

the other treatments (Fig, 3B, C). By the end of 1988 (FP2), the current-year branches (after excision of those formed in 1987) differed between the 0.1 plitre litrel and the other O3 treatments in both length and weight (Fig. 3D, E). Again, the ratio of the total branch weight to branch length was drastically lowered at 0.10 plitre litre 1 (Fig. 3F) and, as in 1987 (FPl), resulted in very crooked branch axes.

1988

Populus tremula

gibe

Ozone zinc. (ctl 1-l) oambient air A 0.10

Populus tremula I

1

15001

1

I

1

900. 011

600 -

I

300 -

J

J

A

S

I

3.0>

D

A

I

I

5

M

Oozone cont. @I I-‘)

200.

0

l

control 0 ambient

0.05 A 0.10

loo-

P U

+a+

4 B

I 0--

{

2015-

P

p M

10

105-

E

4

+

C

F

I

O1987

1988

End of annual~assimilation period Fig. 3. Total length (A, D) and total dry weight (B, E) of all annually formed branches in a tree, and the ratio of total weight to total length (C, F) at the end of 1987 (FPl) and 1988 (FP2); means * SD of five trees each (arrangement of treatment symbols as in Fig. 2).

J

J

A

S

Seasonal courses of A, number of attached leaves; B, attached leaves per unit of branch length; C, sum of all leaves formed (attached + shed); D, total attached foliage area; E, relative proportion of necrotic leaves in a tree (occurring under the 0.1 plitre 1itre-i 0s regime only); means f SD of five trees each as determined on May 19, June 8, July 5 and 25, August 18 and September 22, 1988 (FP2; means without bars have SD smaller than symbol width). Treatment symbols are grouped around each date for graphical reasons. In D, foliage areas were calculated (see ‘Methods’), except for those measured on September 22.

4

R. Matyssek, T. Keller, T. Koike Populus tremula

Table 1. Nutrient and chlorophyll concentrations of leaves in mid-summer of 1988 (second fumigation period; means + standard deviation) Concentrations 0 Control 0.05

P (mg g-l) S (mg &I) N (%) (Mid-summer) (Late summer) Chlorophyll (I-Lgcm-2)

1

c 5

(plitre lit&) 0.1 0.1

5

‘L

Yellowish

2.2kO.6 1.3kO.3

2.0f0.7 1.3f0.4

2.3fO.l 15+0,7

3.0f0.6 1.6kO.2

2.6 f 0.1 1.2 f 0.1

2.6 * 0.2 1,2+0.1

1.9fO.l

3.0+0.1 -

5,5* 1.2

7.9k5.2

I

0

0.05

0

0.10

A l

/

0.10 (yellow) = Y

-

A

.

40-

6 .k

//>:

f

258k5.7

21.3k2.4

I

0 control

a I!E

3.7f0.7 3.7f0.5 3.7kO.4 2.4+0.1 20.2k9.3 20.5f4.2 21.3k4.6 10.9k2.2 32.3 f 5.0 36.3 f 9.7 26.9 f 3.8 20.2 + 5.8

1988 I

ozone cont. (PI I-‘)

80

Visual symptoms* None Bronzing

None** Mg (mg gl) Ca (mg &I) Fe (/*g 0

0,

2-week-old leaves

.

‘3 5co F B

:i z/

.

A/ / /AA

tir 0 0

* Leaves of the ‘ambient air’ fumigation did not visually differ from the control and the 0.05 plitre litrem’ O3 treatment, and thus, were not included in the analysis. ** Nutrient concentrations of green control leaves are consistent with non-limiting nutritional status in Populus (Bergmann, 1986).

0

I 0

1 8

Maximum CO, assimilation rate,

1 16

-1

A_, (pmol mz s”)

Fig. 5. Carboxylation efficiency (CE) as related to the maximum CO2 assimilation rate (A,,,) of leaves without visible injury and of yellowish 2-week-old leaves during midsummer 1988 (FP2); CE, which is the amount of CO2 fixed (nmol) per CO2 experimentally provided (pmol), represents the linear slope between the CO2 assimilation rate and the CO, concentration in the mesophyll intercellular spaces, ci, at ci < 100 plitre lit+ (further cuvette conditions as described in ‘Methods’); each point represents one leaf.

a progressively increasing proportion of discoloured and necrotic leaves, found only under that 0, regime (Fig. 4E; 1988). Leaf analysis

During both fumigation periods, only leaves under the 0.1 plitre litrel O3 treatment displayed the O,characteristic symptom of bronze discoloration (Fig. 1B). However, during FP2, yellowish leaves with symptoms of nutrient deficiency also developed under that treatment (Fig. lD), even though all treatments had been fertilized in the same way. In fact, cation concentrations were reduced in yellowish leaves (Table l), while these concentrations were not clearly affected

in bronzed

and

in the green

leaves

of the other

0,

regimes. Among the non-metals, only N was enhanced independently of the leaf symptoms under the 0.1 plitre lit& regime, while chlorophyll declined in both bronzed and yellowish leaves (Table 1). Bronzed leaves were shed without yellowing. However, yellowing was sometimes followed by the formation of bronze necroses, but such leaves were not subjected to further analysis.

Populus tremula 2-weekaId

ozone cont. (ctl I-‘) 0 0 control 0.05 _ _/

? 3 d E

12-

ozone cont. (pl I-*)

0

0 control ’ 0.05 A 0.10 (bronze)

l

:‘E ‘; mu, =_N E’ .a E

8_

g-5

4-

4 0.10 A 0.10 (yellow)

.

474 * B l -_ -

.

-.

/-

. .’ ,.’

E 00.3

1988

14-week-old leaves

leaves

A

, .’

,“A’ 0-l

0

A I

120

240

8

! 0

Stomata1 conductance, gHzO, at Aamb

A

A

120

240

(mm01 rn-‘~-~)

Fig. 6. CO2 assimilation rate (Aa& at CO2 concentration c, = 340 plitre lit& as related to the stomata1 conductance, gHIOat Aamb(non-limiting cuvette conditions given in ‘Methods’). A, 2-week-old leaves in mid-summer 1988 (FP2); B, 14-week-old leaves in September 1988 (FP2); each point represents one leaf, which was either without visible injury or had yellowish (as in A; see Fig. 1D) or bronze symptoms (as in B; see Fig. 1B). No uninjured 14-week-old leaves were left in September under the 0.10 plitre lit+ 0, regime.

Branch growth and gas exchange in aspen under 2-year ozone stress

A_ was similar in green 2-week-old leaves of all 0) treatments, although the apparent CE tended to be lowered at 0.1 plitre litre-1 (Fig. 5). However, as soon as leaves became yellowish, CE and A, declined in parallel to each other. Due to the lowered CE of the green leaves at 0.1 plitre litrel their CO* assimilation rate, Aamb,at c, = 340 plitre litrel tended to decrease at a stomata1 conductance (g,.& similar to that of the other O3 treatments (Fig. 6A). This behaviour of gas exchange at 0.1 plitre litre-1 reflects a declining wateruse efficiency (WUE; cf. Schulze & Hall, 1982), which became lowest in the yellowish leaves. At O-1 plitre litrel such decline in WUE also occurred in ICweekold leaves with bronze discoloration (Fig. 6B). In contrast to the young leaves at O-05 plitre litrel (Fig. 6A), Aam,,and gHZowere reduced in green lCweek-old leaves of that O3 regime while maintaining a similar WUE to that of the control (Fig. 6B). DISCUSSION

did branch growth differ between the two O3 fumigation periods (FPl and FP2)? To affect branch growth of the aspen clone at all, the O3 concentration not only needed to be higher than in the ambient air (0.03 plitre litrel) but, perhaps, substantially to exceed 0.05 plitre litre1 in both FPl and FP2. Only at 0.1 plitre litrel were the annual weight and length increments and leaf sizes of branches found to be limited. However, reductions in lengths and leaf size occurred only during FP2. With regard to growth and foliar loss, this aspen clone proved to be less O,-sensitive than sensitive clones of American aspen (Populus tremuloides; Keller, 1988). Length increment is determined, in part at least, by the assimilate reserves accumulated during the preceding growing season (Dickson, 1989; Odin, 1972). In fact, length growth was similar in all treatments during FPl, the year after all plants had grown under the same conditions (i.e. in ambient air). However, the weight (radial) growth of branch and stem axes is supplied from the current-year production, and thus is limited by processes reducing assimilate availability, e.g. fruit formation (Eis et al., 1965), defoliation (Ericsson et al., 1980; Matyssek et al., 1992) or O3 stress (Reich, 1987). Therefore at the 0.1 plitre litre-1 0, regime, a limited assimilate pool during FPl apparently reduced the weight : length ratio of branches, while the declining storage of reserves (in root and lower stem) inhibited the length increment of the branches formed in subsequent FP2. Such branches differed from those of O3 regimes ~0.1 plitre litre1 by having a crooked shape. In straight branches, weight : length ratios above 90 mg cm-l were associated with high ratios of total diameter : pith diameter at the stem basis (about >4; Fig. 7). However, in crooked branches, the weight : length ratios below 60 mg cm-l were translated into small diameter ratios (~4, as the total diameter was small relative to the soft, similarly sized pith of straight branches). Thus, the reduced How

8.

Populus tremula

(end of 1987)

1 0

8-

0-

4.. 20-r 0

.‘t bent axes

Ai \ 1

Ozone akentration (pl P) l 0.05 O control 1. 0.10 0 ambient air

300 50 loo 160 zoo 250 Branch dry weight / branch length (mg cm-‘)

Fig. 7. Ratio of total diameter : pith diameter of a branch, as related to the ratio of branch dry weight : branch length; the diameter ratio was determined as 10 cm above. the branch basis. Each point represents one individual branch at the end of 1987 (FPl); only the 2 strongest branches each (i.e. those with widest diameter) are shown of a tree.

diameter ratio may reflect a weakened mechanical branch stability, which was mediated by the suppressed latewood formation at 0.10 plitre lit& (Fig. 8) and may have resulted in the crooked growth pattern. Similarly, as in birch (Matyssek et al., 1991, 1992), the annual production at 0.1 plitre lit& was limited by the reduced foliage area after Oj caused impairment and premature loss of leaves, as generally the total foliage area available determines plant growth (Khmer, 1991; Matyssek & Schulze, 1987). In addition, ozone reduces phloem transport (Spence et al., 1990), which especially limits the root (and basal stem) and thus contributes to lowered root : shoot biomass ratios (Mooney & Winner, 1988; Matyssek et al., 1992). Therefore, a decreasing reverse storage in the root and basal stem of the aspen plants can be assumed, although stimulated length growth of roots may not be excluded (Taylor et al., 1989). In fact, root length may better represent root function (soil exploitation) than does root weight; root function nevertheless requires carbon investment. Possibly ozone reduced the weight : length ratio of roots in a similar way as in branches, because a disturbed root function was indeed indicated in the aspen plants. The latter appeared during FP2 under the 0.1 plitre litrel O3 regime in visually nutrient-deficient yellowish leaves with lowered cation concentrations. The gas exchange of such leaves displayed decreasing WUE, as has been found in trees on nutrient-deficient soils (Kiippers et al., 1985; Beyschlag et al., 1987). Leaves, which had changed to bronzing by direct O3 impact without yellowing, also declined in WUE, but their cation concentrations did not differ from green leaves. Cation concentrations were even found to be raised in green spruce needles after seasonal ozonation at 0.1 plitre litre1 (Keller dz Matyssek, 1990). In these needles, as well as in both yellowish and bronze aspen leaves, N was also raised, although pure oxygen was used to generate ozone (i.e. no air contamination with N,Os; cf. Brown & Roberts, 1988). It seems that, unlike leaf fall in autumn, O,-caused leaf loss is coupled with loss of nutrients (Matyssek et al., 1991).

R. Matyssek,

T. Keller,

T. Koike

Fig ,. 8. Cross-sections through representative branches at 10 cm above branch basis; branch from control tree (A, B) and fr‘om 0.1 plitre lit& Or regime (C, D). Xylem structure of central cross-section (A, C) is compared with that of distal position (B, D):

no late wood formation in D comparable with that in B. Nutrient deficiency and O3 stress cause a decline in photosynthetic capacity and in CE (Ktippers et al., 1985; Sasek & Richardson, 1989). However, the apparent CE obtained under Or stress may be biased by the structural breakdown of leaves (Matyssek et al., 1991). In late summer, green poplar leaves under the 0.05 plitre litrei O3 regime displayed declining Aamb and gHzO, while maintaining WUE as in the control. The stable WUE of these ozonated leaves may still reflect structural integrity (Matyssek et al., 1991). The latter seems to be required for interpreting their declining gas exchange as accelerated senescence (Schulze & Hall, 1982; Thomas & Stoddart, 1980). Overall, the development of the aspen clone during FP2 was determined not only by the current O3 fumigation, but apparently also by those plant organs (root and basal stem) which had experienced FPl. Without assuming functional changes in those organs, the cation deficiency, reduced leaf size and length growth of branches during FP2 cannot be explained. Although 2-year fumigations may involve experimental limitations (Pye, 1988), they can nevertheless elucidate potential principles of cumulative year-by-year stress caused by air pollutants. ACKNOWLEDGEMENTS

We gratefully acknowledge the technical assistance of Mr U. Btihlmann, Mr P. Bleuler, Mr A. Burkart and

the chemical analyses by Mrs M. Guecheva and Professor Dr H. Sticher, Swiss Federal Institute of Technology, Zurich. The support in microscopical work by Dr F. H. Schweingruber and Mr W. Schoch is highly appreciated, also the use of experimental equipment provided by Dr R. Hasler. We thank Dr W. Landolt and Dr P. Schmutz for helpful suggestions concerning the manuscript, and Mrs M. J. Sieber for editing the English text. REFERENCES

Bergmann, W. (1986). Erniihrungsst&ungen bei Kulturpjlanzen, visuelle und analytische Diagnose. VEB Gustav Fischer Verlag, Jena, pp. 306. Beyschlag, W., Wedler, M., Lange, 0. L. & Heber, U. (1987). Einfluss einer Magnesiumdtingung auf Photosynthese und Transpiration von Fichten an einem MagnesiumMangelstandort im Fichtelgebirge. Allg. Forst. Z., 42, 738-41.

Brown, K. A. $t Roberts, T. M. (1988). Effects of ozone on foliar leaching in Norway spruce (Picea abies L. Karst): confounding factors due to NO, production during ozone generation. Environ. Pollut., 55, 55-73. Dickson, R. E. (1989). Carbon and nitrogen allocation in trees. Ann. Sci. For., 46, 63147. Eis, S., Garman, E. H. & Ebell, L. F. (1965). Relation between cone production and diameter increment of Douglas fir (Pseudotsuga menziesii (Mirb.) France), grand fir (Abies grandis (Do@.) Lindl.), and western white pine (Pinus monticola Dougl.). Can. .I. Bat., 43, 1553-9. Ericsson, A., Larsson, S. & Tneow, 0. (1980). Effects of early

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Caption for Colour Tip-in between pages 2 and 3

Fig. 1. Leaf discoloration of Populus tremula (from Matyssek et al., 1990). a, control leaf (also representative for green leaves without visible injury of other fumigation treatments); bd, leaves of 0.1 plitre lit& O3 regime; b, O,-characteristic ‘bronze’ discoloration; c, like b but advanced stage of decline (not included in gas exchange experiments); d, nutrient deficiency symptom of leaves with lowered cation concentrations (see ‘Results’).