A range of plant species typical of semi-natural grasslands were tested for their sensitivity to short-term ozone injury under normal and reduced irrigation, and in relationship to air vapour pressure deficit. Potted specimens of 24 herbs, legumes and grasses were exposed during two seasons to four O$ treatments in open-top chambers. The ozone treatments were : (a) charcoal-filtered air ; (b) charcoal-filtered air plus ozone to match ambient levels ; (c) charcoal-filtered air plus O $ to ambient levels 1n5 and (d ) charcoal-filtered air with ozone added to twice ambient levels during selected episodes of 7-13 d. During these ozone episodes, half of the plants in each ozone treatment received reduced irrigation (dry treatment) while the rest was kept under full irrigation (wet treatment). Type and date of first occurrence of leaf injury were noted during individual growth periods. Plants were harvested three times per year, and the percentage of injured leaves was recorded. Depending on species, injury symptoms were expressed as flecking (O $ -specific injury), leaf yellowing or anthocyanin formation. Carum carvi and most species of the Fabaceae family (Onobrychis sativa, Trifolium repens, Trifolium pratense) were found to be most responsive to O $ , injury occurring after only a few days of exposure in treatment (b). An episodic reduction in irrigation tended to reduce the expression of O $ -specific symptoms, but only in species for which a reduction in soil moisture potential and an associated reduction in stomatal conductance during the dry episodes were observed. In other species, the protection from O $ injury seemed to be of little importance. Using artificial neural networks the injury response of nine species was analysed in relation to Species, stomatal conductance, ozone as AOT40 (accumulated exposure above a threshold of 0n04 ppm for periods with global radiation 50 W m −# (Fuhrer et al., 1997)), mean relative growth rate, air vapour pressure deficit and global radiation. In the model with all factors, Species was most important, and when Species was omitted, stomatal conductance was the most important determinant for leaf injury to occur, whereas mean relative growth rate was less important. With no plant-related factors included, air vapour pressure deficit and AOT40 were of highest importance. Only in eight species was a positive relationship found between these two factors during the five days before the onset of injury, indicating increasing protection from ozone with increasing air vapour pressure deficit in some but not all species. These data show that across a range of grassland species, leaf injury caused by elevated levels of ozone is most likely to occur in species with high stomatal conductance and that protection from ozone during dry periods is species-specific and depends on a reduction in stomatal conductance due to a decrease in soil moisture potential. Protection under increased vapour pressure deficit can occur in some but not all species, depending on the relationship between stom...

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Plant species typical of semi-natural grasslands were tested for their sensitivity to long-term ozone effects on growth under two irrigation regimes, and the relationship between ozone sensitivity, relative growth rate, and plant strategy was analysed. Individuals of 24 grasses, herbs and legumes were grown in pots and exposed for two seasons to four levels of ozone in open-top chambers : (a) CF, charcoal-filtered air, (b) CFAA, charcoal-filtered air plus ozone to match ambient levels, (c) CFj, charcoal-filtered air plus ozone added to match 1.5 times ambient levels and (d) CFjj, charcoal-filtered air with ozone added to match twice ambient levels during selected episodes of 7-13 d. During the ozone episodes, one half of the plants received reduced irrigation (dry treatment) while the rest was kept well watered (wet treatment). The effect of ozone on the relative growth rate was determined during the initial growth period during the first season. Above-ground plant dry weight was determined three times during each season, and stubble dry weight at the end of the experiment. In most species, the effect of ozone on relative growth rate was not significant, except in Centaurea jacea and Lychnis flos-cuculi for which a significant stimulation of relative growth rate was found. There was a significant negative relationship between percentage relative growth rate and relative growth rate measured in the CF treatment, indicating that in species growing faster in clean air relative growth rate tended to be more sensitive to ozone than in slowergrowing species. After the first re-growth period during the first season, a significant effect of ozone on aboveground biomass was found in five species, i.e. a decrease in Chrysanthemum leucanthemum, Trifolium pratense and T. repens, and an increase in Silene dioıW ca. During the second season no significant ozone effect was detected, except for an increase in stubble mass in C. leucanthemum. In 10 species, the dry treatment caused a significant decrease in biomass, but in Plantago lanceolata during the first year, and in Knautia arvensis and T. repens during the second year, an ozoneiirrigation interaction was observed. Exposure-response patterns in the wet treatment varied between species. In some species growth tended to be increased in charcoal-filtered air plus ozone to ambient levels, in others either a continuous increase or decrease in biomass was observed with increasing ozone. Trifolium repens showed the lowest AOT40 (ozone accumulated above a threshold of 40 ppb) corresponding to a 10% change in biomass. The comparison between exposure-response pattern between the wet and dry treatments revealed reduced, increased, or unchanged ozone sensitivity. The shift was related to plant strategy and not to the specific sensitivity to the dry treatment. The specific response to both ozone and dry treatment tended to be related to Grime's triangular classification of plant strategies, i.e. the C-S-R model. In species with a large component of C (competitor strategy), and\o...

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