Growth responses to ozone in plant species from wetlands

Environmental and Experimental Botany 44 (2000) 39–48

Growth responses to ozone in plant species from wetlands

J. Franzaring *, A.E.G. Tonneijck, A.W.N. Kooijman, Th.A. DueckPlant Research International, P.O. Box 16, 6700 AA Wageningen, The Netherlands

Received 20 October 1999; received in revised form 9 February 2000; accepted 12 February 2000

Abstract

Ten wet grassland species were fumigated with four concentrations of ozone (charcoal-filtered air, non-filtered airand non-filtered air plus 25 or 50 nl l−1 ozone) in open-top chambers during one growing season to investigate thelong-term effect of this air pollutant on various growth variables. Only Eupatorium cannabinum showed ozone-relatedfoliar injury, while five species reacted with significantly ozone-enhanced senescence. Premature senescence wasparalleled by a significant ozone-induced reduction of green leaf area in Achillea ptarmica, E. cannabinum andPlantago lanceolata. At the intermediate harvest performed after 28 days shoot weights were significantly decreasedby ozone in A. ptarmica and increased in Molinia caerulea. At the final harvest performed at the end of the growingseason two other species, Cirsium dissectum and E. cannabinum had a significantly reduced shoot weight due to ozone.Root biomass was determined only at the intermediate harvest. The root:shoot ratio (RSR) was significantly reducedin C. dissectum, while it increased in M. caerulea. Seven of the species developed flowers during the experiment. Whileno significant ozone effects on flowering date and flower numbers were detected, flower weights were significantlyreduced in E. cannabinum and P. lanceolata. © 2000 Elsevier Science B.V. All rights reserved.

Keywords: Cirsio-Molinietum; Foliar injury; Natural vegetation; Ozone sensitivity; Premature senescence

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1. Introduction

Concentrations of tropospheric ozone in ruralareas are higher on average compared to urbanareas, posing a phytotoxic risk to crops and natu-ral vegetations. In Europe, critical levels for ozoneare currently being proposed within the frame-work of the UN/ECE to protect crops, forestsand natural vegetation against adverse effectsof rising concentrations of ozone (Fuhrer and

Achermann, 1999). This has resulted in broadresearch on the response of a significant numberof plant species from the European flora to ozone.Some short-term experiments were performed infumigation cabinets in the 1980s in which morethan 200 European herbaceous species were ex-posed to elevated ozone concentrations (Corneliuset al., 1985; Ashmore et al., 1987). In such screen-ing experiments, visible injury has been frequentlyused as a reliable response parameter. However,chronic responses due to longer lasting episodesof elevated ozone may have greater ecologicalsignificance to a vegetation than responses toacute exposures to ozone.

* Corresponding author.E-mail address: [email protected] (J. Fran-

zaring).

S0098-8472/00/$ - see front matter © 2000 Elsevier Science B.V. All rights reserved.

PII: S0 098 -8472 (00 )00052 -6

J. Franzaring et al. / En6ironmental and Experimental Botany 44 (2000) 39–4840

A number of species from various Europeanvegetation types have also been included in long-term experiments to date. Bergmann et al. (1996,1998) and Pleijel and Danielsson (1997) studiedthe ozone sensitivity of a number of annual andbiennial ruderals. Ashmore et al. (1996) concen-trated on a range of dry calcareous grasslandspecies while Grub et al. (1997), Bungener et al.(1999) fumigated perennial species from managedpastures. No published information so far existson the ozone sensitivity of species from wetlands(including fen meadows and mires) although rem-nants of such ecosystems are under special protec-tion throughout Europe. Changes in hydrologyand eutrophication have already had negative ef-fects in the past decades (Joyce and Wade, 1998),but are there any effects known of the long-termimpact of ozone on wetlands? The uptake ofozone and other air pollutants is inherently cou-pled to the gas exchange of a plant (Reich, 1987)and in studies on crops it has been shown thatreadily transpiring plants grown under moist soilconditions are more susceptible to ozone thanplants grown under a slight drought stress. It canbe hypothesised that vegetations from perma-nently wet environments are at a greater risk toadverse ozone concentrations than plants andvegetations growing in a dry habitat. Further-more, the importance of relative growth rates andleaf morphology in relation to the ozone sensitiv-ity of plant species requires testing.

To study the ozone sensitivity of wetland spe-cies, an experiment was performed using commontaxa from the Dutch flora. Ten perennial herbsand grasses from extensively managed wet grass-lands were used in a fumigation experiment withopen-top chambers to investigate the ozone sensi-tivity in terms of growth responses. The results ofthe experiment will be presented and discussed inthis paper.

2. Material and methods

2.1. Culti6ation and fumigation

Seeds of ten plant species (Table 1) werecollected in the institute garden at Wageningen

in 1996. Plants in this garden originated fromremnants of fen-meadows (Cirsio-Molinietumplant community, Schaminee et al., 1996) in theEastern Netherlands. Seeds were germinated inwashed sand in a greenhouse in spring 1997.After 3 weeks one seedling per pot wastransplanted into 3 l pots (for the intermediateharvest) and 5 l pots (for the final harvest).These pots were filled with a sand:pottingmixture (1:2). The commercially availablepotting mixture (Lentse 3) had a pH of 5.5 andconsisted of 70% peat and 30% river clay, towhich 1.5 kg m−3 slow release NPK (12:14:24)was added at the start of the experiment. Noadditional nutrients were supplied during theexperiments.

The open-top chambers (OTCs) and thefumigation system were previously described byDueck (1990). Four ozone concentrations wereused in duplicate: charcoal-filtered air (CF),non-filtered air (NF), non-filtered air plus 25 nll−1 O3 (NF+ ) and non-filtered air plus 50 nll−1 O3 (NF+ + ). Chambers were arranged intwo blocks and the treatments were randomlyplaced within each block. Ozone was generatedfrom pure oxygen via electric discharge (Sorbiosgenerator) and added to the NF air from 10:00to 19:00 CET using massflow controllers.Ozone-levels were measured sequentially in theOTCs with an ozone analyser (Monitor Labsmodel 8810). Ozone exposure levels arepresented as seasonal daytime mean values andas accumulated exposures over a threshold of 40nl l−1 (AOT40). The AOT40 is expressed as nll−1 h and is calculated as the sum of differencesbetween the hourly ozone concentrations andthe cut-off threshold of 40 nl l−1 when theglobal radiation exceeds 50 Wm−2 (Karenlampiand Skarby, 1996). The experiment commencedon 20 May 1997 with ten pots (five 5 l and five3 l pots) per species randomly arranged in eachof the eight chambers. The OTCs were 6 m2

large and mutual shading effects of the growingplants were avoided by re-mixing the remainingpots after the intermediate harvest, which wasperformed after 4 weeks. An automatic wateringsystem was used to supply water to the pots.

J. Franzaring et al. / En6ironmental and Experimental Botany 44 (2000) 39–48 41

2.2. Visual assessment and har6ests of plants

During the course of the experiment visualassessments of the plants were made once a week.Numbers of flowers and senescent leaves werecounted and the plants were observed for visibleinjuries. After 28 days (intermediate harvest, 16June) five plants per species and treatment wereharvested to determine leaf numbers, leaf areaand dry weights of leaves, stems, roots and flow-ers. Dry weights were determined by drying theplant material at 80°C for 48 h. Relative growthrates (RGR) and specific leaf areas (SLA) weredetermined according to Hendry and Grime(1993).

A final harvest was performed with the otherfive plants per species at a time depending on thespecies’ phenology (Table 1). For the seven spe-cies developing flowers, harvests were performedwhen seed ripening had begun. Succisa pratensis,Carex nigra and Danthonia decumbens did notproduce flowers and were harvested between 10

September and 8 October. Areas of green leaveswere measured in all species except C. nigra andD. decumbens. Dry weights and numbers of green,senescent and dead leaves were determined in allspecies, except the numbers of leaves in Achilleaptarmica, C. nigra and D. decumbens. The propor-tions of dead and senescent leaves in relation tototal leaves (number and biomass) were consid-ered as senescence parameters. Dry weights ofstems and flowers of all plants were determined.

2.3. Data processing and statistics

Data were processed separately for the twoharvests. To test for significant ozone treatmenteffects, one-way analyses of variance (ANOVA)were performed on the untransformed data foreach species. Data on percentage senescence androot:shoot ratios (RSR) was arc-sin transformedprior to the analysis according to Sokal and Rohlf(1981). Analyses followed a randomised block-de-sign with the four ozone treatments placed atrandom within each of the two blocks.

Table 1Accumulated exposures over a threshold of 40 nl l−1 (AOT40, in ml l−1 h)a and seasonal daytime mean ozone concentrations(10:00–19:00 CET, in nl l−1) for exposures of 4 weeks (intermediate harvest) and a growing season (final harvest) of ten wetgrassland species

Exposure untilb Treatmentc

CF NF NF+ NF++

Intermediate harvest0 7.516 JuneAll species 3.30.8

Final harvest2.7A. ptarmica L. 11.019 August 25.102.9C. nigra (L.) Reichard 12.530 September 29.30

26.311.52.9C. dissecturm (L.) Hill 009 SeptemberD. decumbens (L.) DC. 0 2.9 12.5 29.608 OctoberE. connabinum L. 26.111.42.9026 August

7.21.3 17.7029 JulyL. flos-cuculi L.2.3 9.7 22.9L. salicaria L. 13 August 0

15 September 0M. caerulea (L.) Moench 2.9 11.8 27.2P. lanceolata L. 30 July 0 1.3 7.3 18.1

26.410 September 11.5S. pratensis Moench 2.90

Seasonal 9 h daily means (nl l−1)Intermediate harvest 16 June 4.5 35.5 58 77.5

8 OctoberWhole season 1997 3 33.5 53.5 77

a Identical to AOT40 in ppm h−1.b The experiment commenced on 20 May 1997.c CF, charcoal-filtered air; NF+, non-filtered air plus 25 nl l−1 O3; NF++ non-filtered air plus 50 nl l−1 O3.


Fig. 1. Contribution (%) of senescent leaves to total leafbiomass in five plant species exposed to four levels of ozonefor a growing season. Species are from left to right A. ptarmica(black bars), C. nigra (striped bars), E. cannabinum (grey bars),M. caerulea (dotted bars) and P. lanceolata (white bars). Forsignificance levels see Table 2.

nl l−1 O3 in the second week of August. Duringthe whole season, AOT40 in the NF treatmentremained below the 3 month critical level of 3 mll−1 h, which was proposed by the UN-ECE(Karenlampi and Skarby, 1996). The summer of1997 thus represents a summer with low ozoneexposure.

In the NF+ + treatments ozone concentra-tions occasionally reached 120 nl l−1 in early Juneand 150 nl l−1 in August. There were no signifi-cant differences between the treatment replicates.The AOT40 and exposure duration at the finalharvest differed between species (Table 1) and theAOT40 varied between 17.7 ml l−1 h for Lychnisflos-cuculi and 29.6 ml l−1 h for D. decumbens inthe NF+ + treatments.

3.2. Visible injury and senescence

Foliar injury was first observed in Eupatoriumcannabinum in the NF+ + chambers 4 weeksafter the onset of the fumigation. The ozone-re-lated spots appeared on the upper surface of thefirst order leaves. Leaves that were produced laterin the season did not show foliar injury. In themiddle of the season, small whitish spots wereobserved in the centre of leaves of L. flos-cuculi.These rather un-specific symptoms occurred onlyin some plants from the NF+ + treatment andwere accompanied by a structural change of themesophyll, which appeared to be water-soaked.

Until the intermediate harvest no signs ofozone-enhanced senescence were observed at theweekly visual assessments. At the final harvestsenescence appeared to be significantly affected byozone in five species (Table 2, Fig. 1). Ozone-en-hanced senescence was paralleled by a significantreduction of green leaf area in A. ptarmica, E.cannabinum and Plantago lanceolata (Fig. 2).

3.3. Growth responses

ANOVAs were calculated separately for thetwo harvests for each of the ten species (Table 2).Results of significant ozone treatment effects arepresented for the response parameters shootbiomass, leaf area and number, and root andflower biomass.

Fig. 2. Green leaf area in three plant species exposed to fourlevels of ozone for a whole season. Species are from left toright A. ptarmica (black bars), E. cannabinum (grey bars) andP. lanceolata (white bars). For significance levels see Table 2.

3. Results

3.1. Ozone concentrations

In the beginning of June, mean hourly concen-trations of ambient ozone exceeded 70 nl l−1 foronly a few days, but reached a maximum of 100

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Table 2Effects of ozone treatment on various growth parameters of wet grassland species determined at two harvests (a) and plant traits of the ten wet grassland species determined in ozone-free air (b)a

Species ba

Final harvest (for harvest dates see Table 1)a Traits in O3 free airIntermediate harvest (after 28 days)a

RootShoot Root: RGR SLA SenescencebLeaf Shoot FlowerLeaf Leaf Stem SLAFlower RGR(g g−1 day−1)numberd (cm−2 g−1)weightareac weightdweightweight shootweightnumberarea

Leaf Leaf Percentweightnumber weight

A. ptarmica 0.0030.037 0.048 n.d. n.s. B0.001 n.s. 0.002 n.s. n.s. n.s. 0.197 2360.046 0.002 0.005 0.023n.s. 0.001 n.d. 0.001 B0.001 n.s. n.d. n.p.n.s. n.p.C. nigra n.p. 0.123 150n.s.n.s.0.044n.s.n.s. n.s. n.s. n.s. n.s. 0.046C. disseclum n.s.n.s. n.s. n.s. n.s. 0.091 159n.s. n.s. 0.027 0.001n.s. n.s. n.d. n.s. n.s. n.s. n.d. n.p.n.s. n.p.D. decumbens n.p. 0.070 147n.s.n.s.n.s.n.s.n.s. n.s. B0.001 B0.001 0.002 0.006 B0.001 0.020E. cannabinum n.s.n.s. 0.047 0.176 364n.s. n.s. n.s. n.s.n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s.n.d. n.s.L. flos-cuculi n.s. 0.104 192n.d.n.s.n.s.n.s.n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. n.s. 0.158L. salicaria 244n.s. n.s. n.s. 0.040 n.s.0.001 n.s. 0.030 0.032 0.013 n.s. n.s. n.s.0.044 n.s.0.004 n.s. 0.087 202M. caerulea 0.005 0.003 0.027n.s. n.s. 0.002 B0.001 B0.001 n.s. 0.008 n.s. n.s. 0.007 0.110P. lanceolata 167n.s. n.s. n.s. n.s. n.s.n.s. n.s. 0.003 0.007 n.s. n.s. n.s. n.p. n.p. n.p. 0.090n.s. 185S. pratensis n.s. n.s. n.s. n.s.

a Values represent the P-values from ANOVAs indicating significant treatment effects. N.d., not determined; n.s., not significant; n.p., not present.b Refers to senescent and dead leaves and their proportion of leaf total.c Green leaf area at final harvest.d Refers to flowers or inflorescences.


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3.4. Shoot biomass

Four species showed significant shoot biomassresponses to ozone. Growth reductions were ob-served in A. ptarmica at the intermediate harvestand in C. dissectum and E. cannabinum at the finalharvest (Fig. 3). A significant growth stimulationoccurred in M. caerulea at the intermediate har-vest. Moreover, P. lanceolata showed a trendtowards growth reductions at both harvests andC. nigra a trend towards growth stimulations atthe final harvest, but in both species these werenot statistically significant.

3.5. Leaf area and number

Total leaf area could only be determined at theintermediate harvest before any senescence wasobserved. Leaf area was significantly affected byozone in three species. In A. ptarmica it wasreduced and in C. nigra and M. caerulea it wassignificantly increased by ozone (P-levels in Table2). Leaf number was determined at the intermedi-ate harvest and was reduced by ozone in A.ptarmica and increased in M. caerulea (P-levels inTable 2).

3.6. Roots and flowers

Belowground biomass was determined only atthe intermediate harvest. Root weights were sig-nificantly affected by ozone in A. ptarmica, C.dissectum, L. salicaria and M. caerulea. However,RSR was affected in only three species. In A.ptarmica a strong reduction occurred in the NF+treatment, while a significant decrease in RSR dueto elevated ozone was observed in C. dissectum.An increase of RSR in the elevated ozone treat-ments was obtained in M. caerulea (Figs. 3 and4).

Visual assessments of the plants did not revealany effects of ozone on the timing, duration andextent of flowering. At the final harvest sevenspecies had produced flowers. Flower numberswere not affected by ozone, but significant ozonetreatment effects on flower weight were observedin E. cannabinum and P. lanceolata. In the formerspecies mean flower biomass was highest in theNF+ treatments, while in the latter plants fromthe NF treatments produced highest flowerbiomass.

4. Discussion

During the course of a whole growing season,premature senescence was the most common re-sponse to ozone in the present experiments, butthere were significant growth responses after ashorter exposure to ozone. Although, none of thespecies showed signs of premature senescence inthe first 4 weeks of the experiment, A. ptarmicahad already a significantly reduced shoot weightdue to ozone at the intermediate harvest, while M.caerulea was significantly stimulated in growthafter the first 4 weeks. At this point, significantozone effects on root biomass were observed infour species and RSR was affected in three spe-cies, indicating that the carbon allocation patternof wet grassland plants might be altered by ozone.Such effects have before been noted in a numberof tree and crop species, while for herbaceousplants from the native European flora, Reilingand Davison (1992a,b) were the first to showchanges in RSR. A large proportion of photosyn-

Fig. 4. RSR of C. dissectum (black bars), M. caerulea (greybars) and A. ptarmica (white bars) exposed to four levels ofozone for 28 days. For significance levels see Table 2.


thates of grassland species is allocated to thebelowground plant parts, which serves these spe-cies to cope with poor nutrient and wateravailability. Allocation patterns will also greatlyinfluence the competition between species in wetgrassland communities (Aerts et al., 1991). Onecould thus hypothesise that a decrease in root orshoot biomass (or the ratio between these) due toozone, might cause a changed competitive abilityof a species in a plant community. While a reduc-tion of root and shoot weights by ozone wasobserved in C. dissectum, M. caerulea showed theopposite reaction, possibly increasing its competi-tive ability in plant communities under rising lev-els of ozone.

While the foliar injury observed in floweringplants of L. flos-cuculi could not be clearly at-tributed to ozone, the strong injury in E. can-nabinum caused a substantial reduction of theassimilating area. The loss of green leaf area mayhave resulted in the significantly reduced shootbiomass towards the end of the growing period.While the thin young leaves of Eupatorium ap-peared to be very sensitive to ozone in the earlysummer, later in the season plants produced ro-bust leaves, which did not show ozone-relatedinjury. These findings demonstrate the need toaccount for seasonal and life stage related differ-ences in ozone sensitivity. Opposed to the findingsof the present study, Carlsson et al. (1996) foundyoung pea leaves to be less responsive than olderleaves whereas Reiling and Davison (1992b)found that Plantago major reacted similarly toshort ozone exposures at different developmentalstages.

With regard to growth responses, the presentexperiments indicated differential effects in thesame species depending on the exposure duration.The significant responses in shoot biomass of A.ptarmica and M. caerulea observed at the interme-diate harvest were not significant at the finalharvest, although the same trends still persisted.In E. cannabinum and C. dissectum shoot weightsappeared to be significantly reduced only after awhole season of ozone exposure, indicating that inthese species ozone effects were cumulative.

While vegetative plant parts were significantlyaffected in four wet grassland species, ozone ef-

fects on reproduction organs were less pro-nounced. Timing and duration of flowering andflower numbers remained unaffected in our exper-iments, indicating that ozone does not influencethe phenology of wet grassland species. In con-trast to our results, direct effects of ozone on theflowering in Brassica species were shown by Stew-art et al. (1996). However, flower weights weresignificantly affected by ozone in E. cannabinumand P. lanceolata in the present study. The poten-tial impact of ozone on the biomass of reproduc-tive organs was previously shown in a number ofshort-lived European ruderals (Bergmann et al.,1998). While annuals depend on seed output, thesurvival of perennials like the wet grassland spe-cies in the present study depends on vegetativereproduction in the first place. However, the eco-logical significance of ozone effects on generativeand vegetative reproduction has not been muchaccounted for because most fumigation studieswere finished before flowering and seed ripeningof the plants.

4.1. Possible causes and effects of different ozonesensiti6ity

Half of the species showed premature senes-cence due to ozone. The occurrence of ozone-en-hanced senescence has been reported severaltimes, but the physiological mechanisms for itremain unclear. Senescence seems to be a com-plex, highly regulated process in the life of a leafand results in the re-mobilisation of compoundsto other parts of the plant (Buchanan-Wollaston,1997). In the individual plant premature senes-cence due to ozone might have strong implica-tions, eventually shortening the vegetative phaseand reducing the general vitality. In a plant com-munity a differential response in terms of en-hanced senescence might lead to disadvantages forsome and advantages for other species.

In order to understand, the species-specific sen-sitivity to ozone and addressing general ecologicalimplications of ozone impacts on natural vegeta-tion some authors have used the concept of eco-logical strategies (Harkov and Brennan, 1982;Sellden and Pleijel, 1995; Pleijel and Danielsson,1997; Bungener et al., 1999). Fast growing, com-


petitive species have larger gas exchange rates,which might result in larger ozone responses. Theconcept of strategies is based on Grime et al.(1988), who classified plant species as competitors(C), stress tolerators (S) and ruderals (R). In ourexperiments, the slow-growing and most sclero-phyllous (lowest RGR and SLA, see Table 2,right-hand side) D. decumbens was the most ozonetolerant species as it did not show any ozone-re-lated symptoms in terms of premature senescenceor growth reductions. These observations are wellin line with Grime et al. (1988) who classifiedDanthonia as a stress tolerator, but contrast re-sults of Ashmore et al. (1996), who found agrowth stimulation due to ozone. Another species,S. pratensis was also classified as a stress toleratorby Grime et al. (1988) and our results support thiswith regard to the ozone effects as this species didnot show any significant growth responses. Again,the low RGR and SLA of this species suggest thatplant traits, as expressed for example by growthstrategy sensu Grime, to a certain extent deter-mine the relative ozone sensitivity of plant species.

This relationship could also be observed in thecase of the two most responsive species, E. can-nabinum and A. ptarmica. The thin leaves (highestSLA) of the fast growing (high RGR) Eupatoriummight be the morphological and physiologicaltrigger of strong leaf injury and consequently forthe ozone-induced reduction in shoot biomass.Likewise, the high RGR in A. ptarmica accompa-nied by a high SLA could have determined theadverse ozone effects present in this species. Bothozone sensitive species are classified as competi-tive ruderals (CR/CSR) by Grime et al. (1988)indicating that species with a strong ruderal com-ponent might be strongly affected by ozone. Gen-erally speaking, our experiments indicate that fastgrowing thin leaved taxa might take up largerdoses of ozone than slow growing sclerophyllousspecies.

Two of the species in our experiments, M.caerulea and C. nigra are classified as competitivestress tolerators (Grime et al., 1988). The first hada significantly increased shoot biomass in re-sponse to ozone in the present experiment whilethe second did not significantly respond to ozone.It is interesting to note that although Molinia had

a significantly increased shoot weight due toozone at the intermediate harvest a higher rate ofsenescence was observed at the end of the season.

The growth stimulation due to ozone in thisspecies raises the question if Molinia will have acompetitive advantage over other species underrising levels of ozone. Although, the species growsrelatively slowly, its strong competitive ability insemi-natural vegetation may be brought about byits growth form (monoclonal spread via rhi-zomes), longevity and stress tolerance. While N-and P-eutrophication were often found to be themain cause for an increased competitiveness of M.caerulea throughout Europe (Egloff, 1987; Heiland Bruggink, 1987; Aerts, 1989; Aerts and deCaluwe, 1989; Berendse, 1994), the results of ourstudy indicate that increasing ozone concentra-tions might be an additional factor to consider.However, the possible influences of gaseous airpollutants on community structures and plantbiodiversity need to be tested in field-trials inorder to establish such a relationship.

Overall, the results of this fumigation experi-ment indicate a diverse reaction pattern of wetgrassland species to ozone. While prematuresenescence is a common response to ozone,growth reductions seem to be less widespread andmay in part be related to ecological plant strategyand growth rates. However, growth stimulationsdue to ozone were only observed in M. caerulea.Comparable responses were observed earlier inother grass species (Ashmore et al., 1996; Grub etal., 1997), but the physiological mechanisms forthese and ecological consequences are not yetunderstood.

Acknowledgements

J. Franzaring was funded by a grant from theEU-TMR Programme (ENV4-CT97-5056). Keesvan den Dries (Meteorology and Air Quality,Wageningen UR) is thanked for supplying dataon climate. Rob Geerts and Jacques Withagen(Plant Research International) are acknowledgedfor supplying seed material and assisting withstatistical analyses, respectively.


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Growth responses to ozone in plant species from wetlands