V'CTO Phytol. (1997), 135, 361-367

Growth of 27 herbs and grasses in relationto ozone exposure and plant strategy

BY H. P L E I J E L * AND H. D A N I E L S S O N

Swedish Environmental Research Institute (IVL), P.O. Box 47086, S-402 58 Goteborg,Sweden

{Received 5 February 1996; accepted 12 September 1996)

SUMMARY

Potted specimens of 27 Swedish native herbs and grasses were exposed to three different ozone concentrations, CF(charcoal filtered air), NF (non-filtered air) and NF+ (1-5 x non-filtered air) in open-top chambers from 2 Julyto 5 August 1994. The species represented a wide range of different plant strategies according to the C-S-R model.The results show that the stress-tolerators, S, had a smaller mean relative growth rate, R, during the exposureperiod and also a smaller accumulation of biomass compared with other strategies. The species with theintermediate strategy, CSR, had a R similar to that of the species with a large component of C, the competitorstrategy, and/or R, the ruderal strategy, whereas the net accumulation of biomass was smaller in the CSR group.This difference between R and net growth for the CSR species compared to the species of other strategies, can beexplained by a slower growth for the CSR group during the establishment phase. In the present investigation theresponse to ozone was very small regardless of plant strategy, although there was a weak trend towards slowergrowth with higher ozone concentration, 18 out of 27 species having a greater growth in CF than in NF-I-. Thegrowth of one typical stress-tolerant species, Festuea ovina L., was stimulated significantly by ozone. Visibleinjury, most likely caused by ozone, was noticed in three species, Daetylis glomerata L., Dactylis aschersonianaGraebn. and Phleum alpinum L. Although there were no major restrictions to plant growth in terms of water,nutrients or space in the present open-top chamber experiment, the variability in all climatic factors was as largeas in the ambient air. This might harden the plants and make them less sensitive to ozone compared to plantsgrown in the laboratory under controlled and optimal conditions with low variability.

Key words: Ozone, herbs, grasses, plant strategy, biomass, open-top chambers.

A problem with the use of visible leaf injury as theINTRODUCTION ^^ .^ ^^^^^ parameter of ozone impact, is that there

Present ozone concentrations have been shown to is only a weak correlation between visible injury andcause visible injury and yield loss in different crop other aspects of plant life such as plant growth whichspecies (Heck, Taylor & Tingey, 1988; Jager et a/., are of more fundamental importance for plant1993) as well as various negative physiological effects survival and reproduction (Reiling & Davison, 1992).in trees (Lucas et al, 1988; Wallin, Skarby & There seems to exist a reserve capacity in manySellden, 1990). The number of studies on the impact plants that allows a limited amount of leaf injury toof ozone on herbs and grasses of natural and semi- occur without any great influence on the growth rate,natural plant communities is relatively limited. The C-S-R model of plant strategies (Grime,Threshow & Stewart (1973) showed that, by fu- 1979; Grime, Hodgson & Hunt, 1988) offers themigating plants using small closed chambers in the opportunity to classify plant species with respect tofield, additions of ozone resulted in visible injury in resource capture, allocation and responses to dis-a number of species and that a substantial variation turbance and stress. For example the stress-toleratorin sensitivity existed between species with respect to strategy (S) is characterized by a slower maximumthis type of effect. In the United States, ozone potential relative growth rate than that of theepisodes in certain areas are high enough to cause competitor strategy (C) and the ruderal strategy (R)visible injury in some plant species, although the (Grime, 1979). In a British investigation, the ozoneextent of injury varies between years in relation to effects on growth and other parameters of approx. 30ozone concentrations and water availability (Show- native species were investigated (Reiling & Davison,man, 1991). 1992). Significant negative effects of ozone on plant

. „ , , , , J, ,, J growth were found in a number of species and a* To whom correspondence should be addressed. , , . , . . .E-mail: hakan.pleijel@ivl.se tendency towards higher sensitivity to ozone in

362 H. Pleijel and H. Danielsson

Table 1. The species included in the present study and their strategies according to the C-S-R model

Grass species Strategy Herb species Strategy

Alopecurus pratensis L.Anthoxanthum odoratum L.Briza media L.Bromus arvensis L.Dactylis aschersoniana Graebn.Dactylis glomerata L.Festuca ovina L.Festuca pratensis HudsonPhalaris arundinacea L.Phleum alpinum L.Poa annua L.Poa palustris L.; Roth

CSR-CSR-CSRS

c*CSR*CSR-C

sCSRCs*RCSR-R*

Agrostemma githago L. R*Anthyllis vulneraria L. CSRCentaur ea cyanus L. R*Chenopodium album L. R-CRChrysanthemum segetum L. RDianthus deltoides L. S*Hieracium pilosella L. CSR-SHypochoeris radicata L. CSRLeontodon hispidus L. SPapaver rhoeas L. RPlantago lanceolata L. CSRPlantago media L. CSR*Rumex acetosa L. CSRSenecio vulgaris L. RSilene vulgaris (Moench) Garcke CSR

The strategies were taken from Grime (1979) and Grime, Hodgson & Hunt (1988) where possible. For the rest of thespecies, marked with *, the strategy classification was suggested by the authors of the present paper.

plants with a large component of the C- or R-strategy was observed, whereas the few S-strategistsdid not respond to the ozone exposures used. It wasshown by Hunt et al. (1991), in an investigation of 27plant species native to England, that the influence ofenhanced CO2 concentrations was much more pro-nounced in C-strategists than in the other strategies.Thus, there may exist important links between plantstrategy and plant response to atmospheric change.The generality of the relationship between plantstrategy and response to ozone in a plant has,however, recently been questioned, based on thefinding of a rather high ozone sensitivity in a numberof herb species from chalk grassland of the stress-tolerating group (Warwick & Taylor, 1995).

The aim of the present investigation was to test theeffects of ozone on plant growth in 27 native Swedishplant species, representing a wide range of plantstrategies according to the C-S-R model (Grime,1979; Grime etal, 1988).

MATERIALS AND METHODS

Experimental site

The experiment was made at Ostad, 40 km north-east of Gothenburg, Sweden (57°54'N, 12°24'E).The experimental site is c. 60 m above sea level. Nomajor air pollutant sources that might have affectedthe results are located in the vicinity of theexperimental site.

Plant material and strategies

Seeds were obtained from the Nordic Gene Bank (10species), from Vastakra Angsfro AB (14 species),from Weibulls AB (one species) or were collected innatural populations in south-west Sweden (Senecio

vulgaris L., Silene vulgaris (Moench) Garcke). The27 plant species to be studied were selected torepresent a wide range of plant strategies (Grime,1979; Grime et al., 1988). The plants tested andtheir strategies according to the C-S-R model arelisted in Table 1.

The seeds were sown on 11 June 1994 in a soilwith fine structure (80 % Sphagnum peat and 20 %clay, nutrients: N 120, P 60, K 140 g m"^ and micro-nutrients, pH 5-5-6-5) and placed in two chamberswith charcoal-filtered air. After 3 wk the plants werepotted, individually into 3-5 1 containers with soilhaving a coarser structure (80 % Sphagnum peat and20% clay, nutrients: N 180, P 100, K 210 g m"^ andmicronutrients, pH 5-5-6-5). After replanting, thepots were placed in the open-top chambers. Three,or in some cases two, pots of each species wereplaced in each chamber.

Open-top chambers

The open-top chambers in the experiment were3-1 m in diameter and 2-6 m tall, including thefrustum. Air from a continuously operating fan wassupplied through a metal duct and a manifold whichdistributed the incoming air in the chamber. Thefiow rate corresponded to approx. two air changes ofthe chamber volume per minute.

Treatments

Three concentrations of ozone were used, with threereplicate chambers per treatment: charcoal filteredair (CF), non-filtered air (NF) and non-filtered airplus extra ozone (NF + ). For the control of thechamber effect, one ambient air plot was also used,denoted AA. The extra ozone in the NF + treatment,approx. 1-5 times the NF concentration, was added

Growth of 27 plant spp. In relation to ozone 363

Table 2. AOT40 (ppbh, for daylight hours), 7-hnean (ppb, 1100-1800, local time) and 24-h meanppb) of ozone concentrations in the different

treatments for the exposure period (2 JulyS August1994)

AA lm CF NF NF-f-

A O T 4 0 ( p p b h ) 4809 0 3913 1145024-h mean (ppb) 32 11 33 437-h mean (ppb) 53 17 49 73

each day between 0800 and 2000 hours, local time.Ozone was generated by electrical discharge withpure oxygen. The ozone was distributed into thechambers via Teflon® tubes from the generator tothe fans. The exposure lasted from 2 July to 5August 1994.

Monitoring of air pollutants

Ozone concentrations were monitored using aThermo Environmental Model 49 ozone monitor,based on the u.v. absorption measurement principle.The ozone gas analyser was calibrated once a monthusing a portable ozone generator (Monitor Labs8500). Air was sampled at 1-5 m above the ground inthe centre of the chamber and drawn through 50 mlong Teflon tubes to a time-share system which was

connected to the ozone analyser. The ambientconcentration of ozone was sampled at one point 1 mabove the ground.

Climate monitoring

The air temperature and the r.h. were measured witha Rotronic YA-100 hygrometer/thermometer at12m above the ground in the ambient air. Thephotosynthetically active radiation (PAR) wasmeasured in the ambient air at 1 m above the groundusing a Li-Cor LI-190SA Quantum sensor. Ozoneand climate data were recorded with a CampbellScientific CR 10 data logger.

Harvest and analysis of plant material

The above-ground biomass was harvested by cuttingall the plants at the surface of the soil at the finalharvest, and for seedlings of each species atreplanting. The plants were examined for visibleozone injury (at the final harvest only), the f. wt wasmeasured and then the plants were dried at 70 °C toconstant weight. The total above-ground biomasswas measured and the mean relative growth rate (R)was calculated using the formula:

R =

160 -

14-0 -

U)0!

O

XI

Z]

o

oSi<

16-0 -

Bromus Phalaris Alopecurus Dactylis Poaarvensis arundinacea pratensis glomerata palustris

^ 30-0-

Poa Centaurea Papaver Chenopodiumannua cyanus rhoeas album

Agrostemma Chrysanthemum Seneciogithago segetum vulgaris

totntD

O

laT3c

o<

Festucapratensis

Anthyllisvulneraria

Hypochoerisradicata

Piantago Dactylislanceolata aschersoniana

Piantago Rumex Silenemedia acetosa vulgaris

1-0-

0-5-

Anthoxanthumodoratum

Brizamedia

Festuca Leontodon Hieraciumovina hispidus pilosella

Dianthus Phleumdeltoides alpinum

Figure 1. Above-ground biomass (g d. wt) for plants with (a) C, CSR-C, CSR-R, R and R-CR strategiesand (b) CSR, CSR-SR, CSR-S and S growth strategies according to the CSR system in the different ozonetreatments: H C F (charcoalfiltered air): S N F (non-filtered air) and • NF-I- (non-filtered air with additionof extra ozone), means+ SE.

364 H. Pleijel and H. Danielsson

IOC

Bromus Phalaris Alopecurus Dactylis Poaarvensis arundinacea pratensis glomerata palustris

ss

Festuca Hypochoeris Plantago Dactylispratensis radicata lanceolata aschersoniana

Anthyllis Plantago Rumex Silenevulneraria media acetosa vulgaris

35-0 -

Poa Centaurea Papaver Chenopodiumannua cyanus rhoeas album

Agrostemma Chrysanthemum Seneciogithago segetum vulgaris

Anthoxanthum Festuca Leontodon Hieraciumodoratum ovina hispidus pilosella

Briza Dianthus Phleummedia deltoides alpinum

Figure 2. Mean relative growth rate, R (% wk'^) for plants with (a) C, CSR-C, CSR-R, R and R-CR and(b) CSR, CSR-SR, CSR-S and S growth strategies according to the CSR system in the different ozonetreatments: (key as in Fig. 1) means ±SE.

where W^ and W^ are the dry weights at the finalharvest and at replanting, respectively, and t^ and t^are the number of days from sowing to the finalharvest and to replanting, respectively (Hunt, 1990).Values of R are presented as % wk"\ The flowersfrom five species, which started to flower before theend of the experiment, were cut at the final harvestand counted, and the d. wt was determined as above.The harvest index (proportion of above-groundbiomass found in reproductive structures) and thetotal flower biomass were calculated.

A one-way analysis of variance (ANOVA) wasperformed for all harvest parameters according toParker (1979). The significance of the differencebetween treatments was calculted using the LSD-test if the ANOVA was significant (only one case).The chamber was regarded as the true replicate.Thus, there were three replicates of each treatmentand the value of each replicate for a certain speciesconsisted of an average of three, in some cases two,pots per replicate.

RESULTS

Climate

The climatic conditions in south-west Swedenduring the experimental period in the summer of1994 were characterized by considerable sunshine

(average PAR during daylight hours =839 fimol m^^ s" )̂ with unusually high temperatures(mean of daily maximum 28-2 °C) and low r.h.(average daytime r.h. 64%). The plants were keptwell watered throughout the experiment by dailyirrigation in the open-top chambers and irrigationthree to five times per week for the plants in theambient air plot.

Ozone concentrations

The ozone concentrations in the different treatmentsduring the exposure period are presented in Table 2.In the NF treatment, the 7 h mean (1100-1800hours, local time) was 49 ppb, i.e. 4 ppb lower thanin the ambient air (AA). The corresponding ozoneconcentrations in the CF and N F + treatments were17 ppb and 73 ppb, respectively.

Visible ozone injury

The examination of the plants during the finalharvest revealed that three of the plant species hadvisible injuries on their leaves possibly caused by theexposure to ozone: Phleum alpinum (necrotic flecks),Dactylis glomerata and Dactylis aschersoniana(reddish flecks). These symptoms were most pro-nounced in the NF-I- treatment and least in the CFtreatment.

Growth of 27 plant spp. In relation to ozone 365

Biomass, relative growth rate and harvest index

Figure 1 a, b shows the above-ground biomass andFigure 2 a, b the mean relative growth rates, R, forall 27 species. The results show that the response toozone was small. Festuea ovina, however, had apositive response to extra ozone, which was signifi-cant at the 5 % level but not at the 1 % level.

In Figures 3 and 4, the species are grouped bytheir strategies according to the C-S-R model. Thegroup consisting of plants with a high component ofthe S strategy had the lowest above-ground biomassas well as the lowest R. The above-ground biomassof the CSR strategists was lower than that of thegroup with R, C and related strategies, while the Rdid not differ between these two groups.

Ii: 1-2

< 1-0(0CO

0-8

0-6

0-2

R, C, R-CR,CSR-R, CSR-C

CSR SR-CSR, S-CSR, S

Figure 5. The ratio between the total above-groundbiomass of plants grown in the ambient air (AA) and in theNF (non-filtered air) open —top chambers. Error barsshow SE.

18-0« 16-0E 14-01 - 1 2 - 0•? s 10-0

<uoa 1i111il

6-04-02-0

R, C, R-CR, CSR-C, CSR-R CSR SR-CSR, S-CSR, S

Figure 3. Average above-ground biomass (g d. wt) for allspecies included in the study grouped by growth strategyaccording to the CSR system in the different ozonetreatments: (key as in Fig. 1) means+ SE.

30-0

25-0

20-0

15-0

10-0

5-0

IOC11

IRx

111XX

Rx?<XXX/Qs1i

11R, C, R-CR, CSR-R, CSR-C CSR SR-CSR, S-CSR, S

Figure 4. Average relative growth rate, R (% wk )̂ for allspecies included in the study grouped by growth strategyaccording to the CSR system in the different ozonetreatments: (key as in Fig. 1) means+ SE.

The harvest index (percentage above-groundbiomass found in the reproductive structures at finalharvest) of five species with easily separable flowerswhich started to flower during the experiment isshown in Table 3. The harvest index did not differsignificantly between ozone treatments for any of theplants studied.

Chamber effects

The water content of plants grown in NF-chamberswas lower than in the plants placed in the ambient airplot in 21 species of 25. The average water content ofNF plants at harvest was 78-5 % whereas it was82-5 % for the plants in the ambient air (AA). Earlierstudies (Pleijel, 1993) show that the temperature ishigher and the r.h. is lower in chambers than inambient air. In Figure 5, it can be seen that theabove-ground biomass was more negatively affectedby the chamber efl'ect in the R and C strategists,possibly by the drier air in the chamber, while thegrowth of both the CSR and S strategists wasstimulated by the chamber effect, possibly because ofthe higher temperatures in the chamber.

DISCUSSION

The plant strategy concept of Grime (1979) predictsthat the growth potential is smaller in stress-tolerators than in competitors and ruderals. This

Table 3. Harvest index for five species with easily separable flowers

CF NF NF-I-

Species

Agrostemma githagoPlantago mediaPlantago lanceolataChrysanthemum segetumCentaurea cyanus

Average

13-27-45-9

14-538-4

±SE

1-401-000-312-742-66

Average

10-89-26-0

14-832-5

+ SE

0-661-301-591-052-39

Average

12-87-67-2

13-632-5

±SE

0-461-061-691-171-36

366 H. Pleijel and H. Danielsson

suggestion was supported by the results of thepresent investigation (Figs 3, 4). The representativesof the S strategy had a lower mean relative growthrate, R, and a lower accumulation of biomasscompared to the other strategies. The representativesof the intermediate CSR strategy had a R which wassimilar to the group with strong components of the Rand C strategies, but the net accumulation of biomassbetween sowing and harvest was smaller in the CSRgroup. This discrepancy between R and net growthfor the CSR strategy, compared with R and C, can beexplained by the slower growth during the es-tablishment phase. The average d. wt of the CSRstrategists at the start of exposure was 6 mg, whereasthe corresponding figure for the group with R, C andrelated strategies was 22 mg. For the plants be-longing to the S group the corresponding weight was7 mg. Thus, the CSR strategists tended to be slowerstarters, partly because of lower seed weight, thanthe species in the R and C strategy group.

In the present investigation the response to ozonewas small, regardless of plant strategy. No stat-istically significant negative effects of ozone wereobserved. Thus, the hypothesis that the response toozone is related to plant strategy was not supportedby the present investigation. None of the plantsresponded strongly to the ozone treatments. Therewas visible injury, possibly caused by ozone, in threespecies. This effect on Phleum alpinum agrees wellwith what has been found in Norway with the samespecies where Mortensen (1993) found it to be themost ozone sensitive among a selection ofScandinavian subalpine plants. There were, how-ever, no significant treatment effects on growth inthe three species with visible injury. Although therewas a weak trend towards slower growth with higherozone concentration, with 18 out of 27 speciesproducing more biomass in CF than in NF-I-, itcannot be stated from the present experiment thatambient ozone exerts an important stress on nativeSwedish herbs and grasses. A different selection ofspecies however, might have produced a differentresult.

A significant stimulation of the growth rate wasobtained in one typical stress-tolerator, Festucaovina. Recently, Warwick & Taylor (1995) found anegative ozone effect on the allometric root:shootcoeflicient on this plant species when exposed tosimilar ozone concentrations in closed chambers.Root growth was not studied in the present ex-periment. There is evidence in the literature thatroot growth is more affected by ozone than is shootgrowth (Cooley & Manning, 1987). It cannot bediscounted that effects on root biomass were presentin the experiment and that the stimulation of shootgrowth in Festuca ovina was at the expense of anegative effect on root growth. Warwick & Taylor(1987) found significant negative ozone effects alsoon other plant species of the stress tolerant type.

which casts further doubt on the possibility topredict ozone sensitivity of plant species from growthstrategy.

One tnay ask why no large negative ozone effectswere observed in the present experiment, when sucheffects were found in a number of species in theexperiment of Reiling & Davison (1992). The ozonetreatments in the two experiments were comparablein the sense that in the Newcastle experiment theozone concentrations were < 5 nl 1~̂ and 70 nl 1~̂ ,7 h d"\ respectively, the latter not very differentfrom our 7-h mean in the NF-j- treatment of73 nl 1"̂ . On the other hand, the work by Reiling &Davison (1992) differed from the present experimentin that they used a square-wave exposure andconstant light. In the present study, light and ozoneconcentration varied with the ambient dynamics ofthese factors. Another important difference was thatin the Newcastle experiment plants were only grownfor 2 wk after germination, whereas in the presentstudy seedlings were kept in charcoal-filtered air forthe first 3 wk before exposure started. Out of the sixspecies in common between the two experiments,two were significantly negatively affected by ozone inthe Newcastle experiment. In these two cases, as inthe remaining four, the effects were smaller than10%. In three cases the effect was close to zero inboth experiments. Overall, however, the differencebetween the two experiments was not very large forthe species in common. The general pattern ofinsensitivity in the Swedish experiment and sen-sitivity of a number of species in the Englishexperiment, might be explained by the Swedishexperiment containing fewer sensitive species.Alternatively, a systematic difference in the ex-perimental set-up might account for the difference inresponse pattern.

The Newcastle experiment was performed inclosed chambers in the laboratory, whereas theSwedish experiment used open-top chambers in thefield with larger variations in climatic conditions andozone concentrations. The more unpredictable en-vironment in the field could possibly have hardenedthe plants against various types of stress. Exposure ata very early growth stage, as in the experiment byReiling & Davison (1992), might also lead to largereffects. A further possibility is that the dry air in theSwedish experiment led to a smaller uptake of ozoneowing to the effect of the vapour pressure deficit onstomatal aperture.

Quite consistently, screenings of herbs and grassesfor ozone sensitivity performed in closed chambershave resulted in substantial and significant negativeozone effects on at least some of the screened species(Mortensen, 1992,1993; Reiling & Davison, 1992;Warwick & Taylor, 1995). The possibility that theenvironment of closed growth chambers over-estimates air pollution effects should be testedexperimentally.

Growth of 27 plant spp. In relation to ozone 367

ACKNOWLEDGEMENTS

Thanks are due to Patrik Alstromer at Ostad sateri for theavailability and preparation of the investigation site. Wealso would like to express our gratitude to Fbe and JuriSild for valuable assistance at the field site. Financialsupport from the Swedish National Environment Pro-tection Board is gratefully acknowledged.

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