yield responses of wheat to ozone exposure as modified by drought-induced differences in ozone...

YIELD RESPONSES OF WHEAT TO OZONE EXPOSURE ASMODIFIED BY DROUGHT-INDUCED DIFFERENCES IN

OZONE UPTAKE

S. KHAN and G. SOJA∗Department Environmental Research, Austrian Research Centers Seibersdorf, Seibersdorf, Austria

(∗ author for correspondence, e-mail: [email protected], Fax: +43 50 550 3520)

(Received 16 January 2002; accepted 25 March 2003)

Abstract. Over a period of two years greenhouse experiments were carried out to quantify theinteraction ozone exposure × water stress in winter wheat (Triticum aestivum L. cv. Perlo). Assess-ment of effects carried out on various yield parameters showed that abundant water supply made theplants most sensitive to ozone exposure. In well-watered plants (75% of soil water capacity, s.w.c.),the AOT40 ozone exposure doses of 26.8 and 24.9 µmol mol−1 hr−1 (ppm.h) caused grain yieldreductions by 35 and 39%. No reductions of yields were observed at severe water stress (35% ofs.w.c.) condition. The decrease in ozone responsiveness under drought can be explained by a distinctreduction in ozone uptake (18 vs. 2 mmol m−2 in well-watered vs. severely stressed plants at thesame ozone exposure). The calculations of ozone uptake were based on repeated measurements ofleaf conductance. Generally curvi-linear regression functions explained the dependence of relativeyield on ozone and on water stress better than multiple simple linear regression functions. Theconsideration of ozone uptake instead of ozone exposure improved the performances of the modelsfurther. For explaining grain yield, 96.8% of the variances could be explained by a model resultingfrom curvi-linear regression fitting. A suggestion for calculating correction factors to modify criticallevels in the case of limited water supply is presented.

Keywords: air pollutant, critical level, growth, ozone, pollutant uptake, stomatal conductance, waterstress, winter wheat, yield

1. Introduction

Although ozone has an important role in protecting the biosphere by absorbingharmful ultraviolet radiation in the upper stratosphere, in the troposphere it is aphytotoxic air pollutant. The detrimental effects of photochemical oxidant mixtureson plants were first recognised in the late 1940s (Middleton et al., 1950). Thistropospheric ozone becomes progressively important in many parts of the world.The annual mean concentration of ozone in the northern hemisphere has increasedfrom about 0.01 µmol mol−1 100 yr ago to about 0.02 µmol mol−1 today. Duringthe last few decades an increase of about 1% per year has been observed even inremote sites of the northern hemisphere (Anfossi and Sandroni, 1994).

Wheat is regarded as a crop species especially sensitive to ozone (Selldén andPleijel, 1995). The European open-top chambers network led to the definition ofa critical level for ozone injury in wheat, based on the accumulated ozone dose

Water, Air, and Soil Pollution 147: 299–315, 2003.© 2003 Kluwer Academic Publishers. Printed in the Netherlands.

300 S. KHAN AND G. SOJA

AOT40 (Sanders et al., 1995). Relating yield losses and AOT40, a maximum yieldloss of 5% was calculated to occur at 3.0 µmol mol−1 hr−1 (Fuhrer, 1996).

The effects of ozone on plants do not only depend on the concentration patternsand exposure doses but also on the interaction with environmental factors. Factorsthat influence the stomatal conductance can alter the flux of ozone into intercellularspaces of the leaf. Water stress is believed to be one of the most important modi-fying factors of ozone sensitivity. As water stress is accompanied by decreasedstomatal conductance, less ozone reaches the inner leaf tissues, thereby decreas-ing ozone injury to the leaves. In wheat as well as in other crops reduced wateravailability frequently caused reductions of ozone injury because lower stomatalconductances decrease ozone uptake rates (Tingey and Hogsett, 1985; Heagle etal., 1987). A comparison of data from drought-stressed and well-watered ozone-exposed plants demonstrated drought protection via reduced stomatal conductancein crops like wheat (Mortensen, 1990; Soja et al., 1996), soybean (King and Nel-son, 1987), cotton (Heagle et al., 1988), tomato (Hassan et al., 1999), grasslandspecies (Bungener et al., 1999), and in forest species like Fraxinus (Reiner etal., 1996), Picea (Karlsson et al., 1997), Betula (Pääkkönen et al., 1998), Abies(Retzlaff et al., 2000), and Pinus (Panek and Goldstein, 2001). In some experi-ments (Fangmeier et al., 1994b; Grüters et al., 1995) no clear interactions wereobserved between ozone effects on wheat and water stress when only moderatedrought treatments were applied.

The next step in describing ozone effects on plants quantitatively has to be thecomprehensive consideration of environmental parameters which modify ozonesensitivity. Soil moisture deficit is one of these parameters (King, 1987), withphenology, photon flux density, air humidity, and temperature as additional co-factors determining ozone uptake into the leaf. The inclusion of these modifyingparameters in existing dose-effect models may lead to either an improved conceptof critical levels (Level II) or to an assessment of pollutant uptake doses.

The objective of this study was a quantitative assessment of the modificationof ozone effects in winter wheat by water stress, leading to a correction factorfor ozone critical levels. Accompanying measurements of stomatal conductanceshould allow for the calculation of ozone uptake. Consequently the yield effects ofozone which were modified by water stress could be related to the ozone exposuredose in comparison to the cumulative ozone uptake.

2. Materials and Methods

A closed-chamber experiment was set up with two ozone levels in environmentallycontrolled greenhouse compartments adapted as fumigation chambers. The studywas designed as a two-factorial pot experiment (n = 8) with one cultivar of winterwheat (Triticum aestivum cv. Perlo) known for its sensitivity to ozone. Seeds weregerminated at +20 ◦C for 24 hr and immediately afterwards vernalized for 60 days

MODIFICATION OF OZONE EFFECTS BY WATER STRESS IN WHEAT 301

at +1 ◦C. These vernalized seedlings were planted in 15 L-pots with a standard soilmix (Einheitserde ED 73) and thinned to 15 plants per pot.

Four levels of water stress treatments were imposed by different amounts ofirrigation water for each pot. The soil humidity levels were kept constant at 75%of soil water capacity (w.c.) in the control treatment, and at 60% w.c., 45% w.c.,and 35% w.c. in the moderate to severe stress treatments. Soil water capacity wasdetermined as the ratio of actual water content to maximum capillary capacity. Soilhumidity levels were adjusted three times a week gravimetrically. Corrections fordry matter increases of the plants were made by parallel harvests of separate potsin each fumigation treatment.

Fumigation started one month after emergence and lasted for three months.Ozone was generated from pure oxygen by electrical discharge (Fisher 502, Meck-enheim, Germany). Ozone concentrations in the fumigation compartments wereheld constant at 80 nL L−1 (± 6 nL L−1) from 9 a.m. to 5 p.m. with a continuousozone analyzer (Horiba APO 350E, Horiba Europe, Langenfeld, Germany) con-trolling the output of the ozone generator. The ozone concentration in the controltreatment (without ozone addition) fell in the range of O3 concentrations lowerthan average ambient air in summer, similar to charcoal filtered open-top chambers.Ozone concentrations in both treatments were continuously monitored. The air incontrol and ozone fumigation compartments was stirred with fans during the 7 h-period of fumigation in order to create an atmosphere with decreased leaf boundarylayer resistance similar to the conditions in open top-chambers and similar to theleaf cuvette of the plant gas exchange measurement system.

Leaf conductance (gl) was measured with a gas exchange analyser (LI-COR6000, Lincoln, Nebraska, U.S.A.) at the wheat flag leaf. The calculation of theuptake doses was based on leaf conductance measurements of the different wa-ter supply treatments and on established functions of light intensity effects ongl (Emberson et al., 2000). The ozone flux (mol O3 m−2 s−1) was calculated bymultiplying the stomatal conductance for water (mol H2O m−2 s−1) with the ozoneconcentration (mol mol−1) and 0.613 to account for the different diffusities ofozone and water.

For statistical analysis (two-way ANOVA, Duncan-test, regression fitting) thesoftware tools WinSTAT (G. Greulich Software, Staufen, Germany) and Table-Curve 3D (SPSS Inc.) were used.

3. Results

3.1. HEIGHT OF TILLERS

Both water stress and ozone significantly reduced final height of the wheat plants(height of the tillers including ear; Tables I and II). Only in severely water-stressedplants (35% w.c.) no ozone effects on the final height were detected. Significant


interactions between water stress and ozone were observed for the height of theplants in the first year, with marginal significance also in the second year of theexperiment (Table III). The height reductions caused by severe soil moisture deficit(33 to 40%) were much more pronounced than those induced by ozone (7 to 16%).

3.2. ABOVE GROUND DRY WEIGHT

The total dry weight was most affected by decreasing water capacity (Tables I andII). Statistical analysis (Table II) showed significant interactions between waterstress and ozone in both experimental years. At the highest soil moisture level (75%of w.c.) total above ground dry matter of ozone-fumigated plants was reduced byabout 35% whereas at 60% of soil w.c. it decreased by 13% in the first year and by18% in the second year. At 45 and 35% of water capacity the differences were stillsmaller.

3.3. NUMBER OF EARS/PLANT

The ozone and drought treatments had also significant effects on the number ofears in both years. Ozone-induced reductions in the number of ears under highwater supply (75% of soil w.c.) were 17% in the first and 22% the second year.At moderate water supply (45% of soil w.c.) these reductions were 16 and 3%.However, no ozone-induced reductions were observed at severe water deficit.

3.4. GRAIN WEIGHT/PLANT

Grain yield was reduced by the ozone treatment as well as by insufficient wa-ter supply (Tables I and II). Reduction in grain weight was primarily associatedwith the reduction in grain number (p < 0.001). Regarding the water stress andO3 treatments, statistical analysis showed significant interactions in both years(Table III). Ozone-induced reduction in grain weight was more pronounced underabundant water supply than under water stress. In the first year, a 35% ozone-induced reduction in seed weight was observed at 75% of water capacity; in thesecond year it was 39%. These reductions in grain weight were approaching tozero with increasing severity of water stress. At moderate water supply (60% ofsoil w.c.) the ozone-induced reduction of grain yield was still similar (20%) to thedrought-induced reduction (19%). With increasing water stress, however, droughteffects rose dramatically whereas ozone effects diminished.

3.5. NUMBERS OF GRAINS/PLANT

In both years significant differences (p < 0.001) in the number of grains wereobserved for both water stress and ozone treatments. The increase in grain numberwas associated with an increase in the number of ears. Treatments also showedsignificant interactions of water stress and O3 on the number of grains in both


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(cm

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10.7

±0.

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81.5

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2.40

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53a

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0.23

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TABLE III

Statistical significance (P-values) of two-way analyses of variance for yield and yield components ofwinter wheat as affected by the factors water stress treatment and fumigation level

Parameter First year Second year

Water Fumigation Inter- Water Fumigation Inter-

stress action stress action

Height of the crops <0.001 <0.001 0.001 <0.001 <0.001 0.066

Above ground dry <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

weight/plant

Number of ears/plant 0.018 <0.001 <0.001 <0.001 <0.001 <0.001

Number of grain/plant 0.006 <0.001 <0.001 <0.001 <0.001 <0.001

Weight of grains/plant <0.001 <0.001 <0.001 <0.001 <0.001 <0.001

1000 Grains weight <0.001 0.036 <0.001 <0.001 <0.001 0.320

Harvest index <0.001 0.283 0.010 <0.001 0.152 0.682

years. Ozone reduced the number of grains in well-watered plants by 28% in thefirst and by 29% in the second year. At severe water stress conditions the grainnumber remained unaffected (Tables I and II).

3.6. THOUSAND-GRAIN WEIGHT

Ozone exposure as well as severe drought condition decreased thousand-grainweight significantly. At 75% of soil water capacity ozone caused reductions in1000-grain weight by 11 and 14% in both experimental years. At severe waterstress condition (35% soil w.c.) these values were 8 and 10% (Tables I and II).

3.7. HARVEST INDEX

The harvest index refers to the economic yield of a crop to the total dry matterof harvest. It is defined as the proportion of the biological yield represented bythe economic yield. Because the effects of ozone on grain yield and total drymatter were similar, no significant effect of ozone on harvest index could be de-tected whereas water stress caused significant (p < 0.001) decreases in both years(Tables I and II).

3.8. OZONE EXPOSURE AND OZONE UPTAKE DOSES

The ozone doses during the fumigation periods were expressed as both AOT40exposure doses and as uptake doses. For the individual water stress treatments theAOT40 values naturally were identical though by about 7% lower in the second


TABLE IV

Ozone exposure doses (AOT40 in µmol mol−1 hr−1) and ozone absorbed doses (in mmol m−2;without treshold) in both experimental years

Year Drought treatment AOT40 (µmol mol−1 hr−1) Ozone uptake (mmol m−2)

(% of soil Control Ozone Control Ozone

water capacity) treatment treatment treatment treatment

1 75 1.5 26.8 4.9 18.3

60 1.5 26.8 4.0 14.7

45 1.5 26.8 2.4 8.7

35 1.5 26.8 0.5 1.9

2 75 1.3 24.9 4.4 17.7

60 1.3 24.9 3.6 14.3

45 1.3 24.9 2.1 8.5

35 1.3 24.9 0.5 1.9

experimental period (Table IV). This homogeneity sharply contrasted with thelarge differences in ozone absorption between the drought treatments. Severe soilmoisture deficit reduced the ozone uptake doses by up to 90%. These reductionsbecame apparent at both control and ozone treatments. The dependence of stomatalconductance on soil water capacity could be expressed by the function

y = 179.6 – 5618/x (r2 = 0.96) ,

with y = relative conductance and x = soil water capacity (in %; for 33 ≤ x <70). This equation is derived from several conductance measurement campaignswhich are summarized in Figure 2. At soil water capacities below 60%, leaf sto-mates were increasingly closed. The comparison of fumigated and control plantsshows that under well-watered conditions 80 nL L−1 ozone decreased stomatalconductance by 10 to 20% compared to non-fumigated plants.

3.9. YIELD BEHAVIOUR IN RESPONSE TO OZONE AND SOIL WATER SUPPLY

The results were used to derive functions with yield responses as dependent vari-ables and ozone and water stress as independent variables. Several families ofregression functions were established, which differed either in the use of ozoneexposure (Tables V and VII) and ozone uptake dose (Tables VI and VIII) or inthe application of multiple linear regression functions (Tables V and VI) and cur-vilinear regressions (Tables VII and VIII). The regression fitting for grain yieldand above ground dry matter gave more satisfying results than for the other yield


Figure 1. Relationship (z = a + b/x1.5 + cy3; a = 1.408, b = 265.0, c = 0.00006154) between relativewheat grain yield (z), average soil water capacity (x) and ozone uptake in three months (y; r2 = 0.97).The database was derived from the yield and ozone flux data as given in Tables I, II, and IV.

parameters, usually with coefficients of determination of higher than 80%. Thethousand grain weight and the number of ears per plant were not well explained byany descriptive combination of ozone and drought. The use of the ozone uptakedose instead of the ozone exposure dose increased the regression performanceslightly for grain yield and above ground dry matter but not for the other dependentvariables. The performance gains were higher when curvilinear regression modelswere applied than when simple multiple regressions were fitted. Generally the per-formance of curvilinear models was superior to that of simple linear regressions,topping at an r2 of 0.968 for explaining grain yield in dependence on soil watercapacity and ozone uptake (Table VIII).

4. Discussion

Both ozone exposure and water stress reduced the growth and yield of winter wheatsignificantly. Water-stressed plants were smaller than well-watered ones, had lessleaf dry matter and less leaf area, had substantially less weight, and yielded signi-ficantly less grain. Water stress did not influence leaf senescence in the same wayas ozone. In the present study, the reduction in the number of grains was associatedwith the reduction in the number of ears. Fangmeier et al. (1994b) found in their


Figure 2. Conductance of wheat flag leaves as influenced by soil water capacity and ozone fumigation(see Table IV). Measurements were taken at 4 occasions (each n = 8) between 117 and 124 days afteremergence.

TABLE V

Yield parameters of wheat as influenced by parallel exposures to drought and ozone.Multiple linear regression models for individual yield parameters of wheat as de-pendent variables and soil water capacity and ozone exposure dose as independentvariables

Yield parameter (relative Function (x = soil water % of variance

to non-stressed capacity in %, y = ozone dose explained by

control plants) AOT40 in µmol mol−1 hr−1) the model

Grain yield = –0.257 + 0.0165 x –0.00580 y 80.8

Above ground dry matter = –0.044 + 0.0132 x – 0.00530 y 78.7

Thousand grain weight = 0.667 + 0.00479 x – 0.00336 y 46.6

Number of grains per plant = –0.0149 + 0.0138 x – 0.00264 y 69.8

Number of ears per plant = 0.308 + 0.00918 x – 0.00300 y 48.2

Harvest index = 0.239 + 0.0114 x – 0.000223 y 62.8


TABLE VI

Yield parameters of wheat as influenced by parallel exposures to drought and ozone.Multiple linear regression models for individual yield parameters of wheat as de-pendent variables and soil water capacity and ozone uptake dose as independentvariables


to non-stressed capacity in %, y = uptake explained by

control plants) dose in mmol mol−2) the model

Grain yield = –0.446 + 0.0206 x – 0.0166 y 81.9

Above ground dry matter = –0.219 + 0.0170 x – 0.0155 y 80.5

Thousand grain weight = 0.578 + 0.00639 x – 0.00643 y 41.3

Number of grains per plant = –0.0889 + 0.0152 x – 0.00577 y 69.3

Number of ears per plant = 0.227 + 0.0107 x – 0.00613 y 46.9

Harvest index = 0.252 + 0.0108 x + 0.00234 y 63.1

TABLE VII

Yield parameters of wheat as influenced by parallel exposures to drought and ozone. Curvilinearregression models for individual yield parameters of wheat as dependent variables and soil watercapacity and ozone exposure dose as independent variables


to non-stressed capacity in %, y = ozone dose explained by

control plants) AOT40 in µmol mol−1 hr−1) the model

Grain yield = 1.147 – 1095/x2 – 0.03656 y0.5 90.7

ln (above ground dry matter) = 0.2288 – 1475/x2 – 0.01014 y 88.8

Thousand grain weight = 1.003 – 3.728E+14 e−x – 0.02115 y0.5 74.9

Number of grains per plant = 1.132 – 913.4/x2 – 4.491E-06 y3 78.4

Number of ears per plant = 0.9153 – 5.993E+14 e−x – 0.01901 y0.5 61.7

Harvest index = 3.310 – 0.2313 x0.5 – 1772/y2 85.1

experiment with ozone and water stress in wheat that the yield loss mainly resultedfrom reductions in the number of ears per plant. Fuhrer et al. (1989) deduced fromtheir ozone exposure experiment that the size of grains rather than the number ofgrains per ear was primarily the cause for the yield reduction. They also observeda reduction of ears per area at high ozone concentrations. The reductions in grainyield were primarily attributed to reductions in grain filling, as measured by 1000grain weight under ozone fumigation and also at severe water stress conditions.Basically three fundamental processes can limit grain yield: (a) source of activity(photosynthesis in the green organs linked to grain filling) (b) translocation of


TABLE VIII

Yield parameters of wheat as influenced by parallel exposures to drought and ozone. Curvi-linear regression models for individual yield parameters of wheat as dependent variables andsoil water capacity and ozone uptake dose as independent variables


to non-stressed capacity in %, y = uptake explained by

control plants) dose in mmol mol−2) the model

Grain yield = 1.408 – 265.0/x1.5 – 6.154E-05 y3 96.8

Above ground dry matter = 1.626 – 47.65/x – 5.895E-05 y3 96.6

Thousand grain weight = 1.061 – 4.955E+14 e−x – 0.04475 y0.5 73.9

Number of grains per plant = 1.187 – 1037/x2 – 0.0004833 y2 79.2

Number of ears per plant = 1.148 – 786.8/x2 – 3.232E-05 y3 62.1

Harvest index = –1.838 + 33.90/x0.5 – 703.2/x1.5 85.0

photosynthates from source to sink, and (c) sink activity of the growing grain(quantitatively dominated by the capacity of starch synthesis and storage in theendosperm). Fangmeier et al. (1994b) found that thousand-grain weight was notaffected by water stress. Finnan et al. (1996) observed a significant effect of ozoneon 1000-grain weight in their study with spring wheat. Also in other experimentsreductions in yield, which were entirely attributable to reductions in seed weighthave been reported (Amundson et al., 1987; Pleijel et al., 1991). Soja and Soja(1995) studied ozone sensitivity differences on winter wheat and found that eargrowth was most affected, followed by stem growth and total leaf production.

The results described here show that water stress is a very potent modifier ofozone injury in wheat. Ozone caused the most severe yield reductions only if theplants had optimal water supply (60 to 75% of w.c.). Ozone-induced grain yielddecreases were more pronounced under high water supply (35% in the first and39% in the second year) but at 35% of w.c. the ozone-induced reduction of grainyield was approximately zero. These differences in injury severity can be explainedby the inherent differences in ozone uptake caused by the induction of stomatalclosure under drought conditions. Uptake of ozone by plants is largely controlledby stomata (Kerstiens and Lendzian, 1989). Consequently, advanced ozone fluxmodels increasingly consider the effects of conductance-modifying parameters likesoil water supply in the calculations of uptake rates (Emberson et al., 2000).

Contrary to the results of the present study, Fangmeier et al. (1994a) did not ob-serve significant interactions between water supply and ozone stress in experimentscomparing four ozone levels and two water supply levels. The reason might be thechoice of the water stress treatments. In our experiments, covering a very widerange of water supply levels, we found a relatively broad range of intermediate soilwater capacities (45–60%) where there was little or no variability in modifying


the extent of ozone-induced yield reductions. Only distinctly higher or lower watersupply resulted in either increased (at 75% of s.w.c.) or lowered (at 35% of s.w.c.)yield reductions due to ozone. These varying yield modifications constituted thesignificant interactions we observed in our experiments. If both water supply treat-ments of Fangmeier et al. (1994a) fell within the medium range with low variationsin the modification potential, they might not have detected significant interactions.

The question by how much soil water stress conditions modify the crop re-sponse to ozone was further analysed by establishing regression models (Tables V–VIII, Figure 1). The slightly better performance of models using an ozone uptakedose instead of the exposure dose might be caused by the consideration of ad-ditional environmental parameters like light intensity when stomatal conductancewas measured. Flux-based models may include parameters like temperature, light,vapour pressure deficit and phenology which are mechanistically relevant to theozone uptake into the leaves (Grünhage and Haenel, 1997; Grünhage et al., 2000;Pleijel et al., 2000). However, correction factors for such uptake-modifying factorsmay also be applied to exposure-effect models. In the case of soil moisture deficit,such correction factors have been derived from precipitation and evapotranspirationmodels either to modify the critical level of ozone exposure (Fuhrer, 1995) or to cal-culate directly a corrected ozone flux into the leaves (Emberson et al., 2000). Ourresults and conclusions, however, are based on own experiments, performed withwinter wheat, which hitherto has not been as extensively studied in the literature asspring wheat.

The quantification of the influence of soil water stress on ozone injury is essen-tial for a realistic assessment of the risk to crop yields under future ozone scenarios.A more frequent occurrence of drought conditions in Central Europe is one of thepossible consequences of global climate change which has direct effects on veget-ation and agricultural production. Prolonged drought periods are accompanied bymeteorological conditions favouring ozone production in the troposphere. But suchclimatic extremes would also lower soil water content, plant stomatal conductancesand ozone uptake into the leaves. Soja et al. (2002) observed that 80 nL L−1 ozonedecreased wheat stomatal conductance by about 15% in open top-chamber condi-tions, which is in accordance with the results presented here. However, Grantz etal. (1999) traced back the ozone effects on vapour phase conductivity to reducedroot hydraulic conditions and to impaired phloem loading. To which extent thismodel, developed for cotton, also refers to wheat remains to be determined.

The modifications of the effects of elevated ozone concentrations in the fieldhave to be included into models of ozone-induced yield or growth reductions.Otherwise the predicted ozone injury will be biased towards exaggerated ozoneeffects. The efforts to reach the so-called ‘Level II’ of the concepts of CriticalLevels rely on a quantitative consideration of such factors modifying the ozoneuptake into the leaves. The knowledge of quantitative data for relating soil watersupply and ozone sensitivity is one of the keys for a realistic assessment of theozone risk for crops. The consideration of such modifying factors may be based


Figure 3. Dependence of ozone-induced grain yield reduction on soil water capacity. The expectedrelative yields (dash-dot-lines) were calculated on the basis of the current critical level for ozone I(AOT40 of 3 µmol mol−1 hr−1 corresponding to a risk of 5% yield reduction; Fuhrer, 1996). Y1 =experimental year 1, Y2 = experimental year 2. Regression: y = 120.8–0.497x – 3.55 x2 (r2 = 0.91)with y = relative wheat grain yield and x = soil water capacity (in %; with 35 ≤ x ≤ 75).

on the introduction of correction factors which are either applied to the calculationalgorithm of ozone doses or to the critical levels themselves. For the second of thesetwo possibilities a correction factor has been developed which was derived fromthe water stress impact on ozone-induced yield reductions as shown in Figure 3.The data presented there were transformed to critical level (CL) exceedances inrelation to yield reduction and used for calculating the factor df considering themodification potential of drought stress. It is suggested to re-calculate the CL usedto assess the extent of CL exceedances according to

CLcorrected = CL/df

df = –0.0287 – 0.00244 x + 0.0001887 x2 (r2 = 0.99) ,

with x = soil water capacity (in %) and 0.1 ≤ df ≤ 1.0 (corresponding to 35 ≤ x ≤80).


5. Conclusions

From the above experiment the following can be concluded:

– Plants under water stress react by closing stomata. This reduced leaf conductiv-ity is responsible for reduced uptake of ozone. Thus it decreases the sensitivityto ozone.

– Well-watered wheat plants are more ozone-sensitive than water-stressed plants.Consequently, this modifying potential of water stress for biological effects ofozone should be included in the calculation of critical levels.

– Regression models that use ozone uptake instead of ozone exposure in com-bination with soil water conditions describe the yield responses of wheat satis-fyingly.

– For mapping purposes, we suggest a function for correcting the critical levelfor ozone to be used in risk assessments.

Acknowledgements

The authors gratefully acknowledge financial support by the Austrian BMWV(FTSP-Programm) and by the Austrian academic exchange service (ÖAD). Specialthanks to H. R. Bolhàr-Nordenkampf for helpful discussions and valuable advice.

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