Leaf injury characteristics of grassland species exposed to ozone in relation to soil moisture condition and vapour pressure deficit

A range of plant species typical of semi-natural grasslands were tested for their sensitivity to short-term ozone injury under normal and reduced irri gation, and in relationship to air vapour pressure deficit. Potted specimen s of 24 herbs, legumes and grasses were exposed during two seasons to four O-3 treatments in open-top chambers. The ozone treatments were: (a) charcoa l-filtered air; (b) charcoal-filtered air plus ozone to match ambient level s; (c) charcoal-filtered air plus O-3 to ambient levels 1.5 and (d) charcoa l-filtered air with ozone added to twice ambient levels during selected epi sodes of 7-13 d. During these ozone episodes, half of the plants in each oz one treatment received reduced irrigation (dry treatment) while the rest wa s kept under full irrigation (wet treatment). Type and date of first occurr ence of leaf injury were noted during individual growth periods. Plants wer e harvested three times per year, and the percentage of injured leaves was recorded. Depending on species, injury symptoms were expressed as flecking (O-3-specific injury), leaf yellowing or anthocyanin formation. Carum carvi and most species of the Fabaceae family (Onobrychis sativa, Trifolium repe ns, Trifolium pratense) were found to be most responsive to O-3, injury occ urring after only a few days of exposure in treatment (b). An episodic redu ction in irrigation tended to reduce the expression of O-3-specific symptom s, but only in species for which a reduction in soil moisture potential and an associated reduction in stomatal conductance during the dry episodes we re observed. In other species, the protection from O-3 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 0.04 ppm for peri ods with global radiation greater than or equal to 50 W m(-2) (Fuhrer et al ., 1997)), mean relative growth rate, air vapour pressure deficit and globa l radiation. In the model with all factors, Species was most important, and when Species was omitted, stomatal conductance was the most important dete rminant for leaf injury to occur, whereas mean relative growth rate was les s important. With no plant-related factors included, air vapour pressure de ficit and AOT40 were of highest importance. Only in eight species was a pos itive relationship found between these two factors during the five days bef ore the onset of injury, indicating increasing protection from ozone with i ncreasing air vapour pressure deficit in some but not all species. These da ta show that across a range of grassland species, leaf injury caused by ele vated levels of ozone is most likely to occur in species with high stomatal conductance and that protection from ozone during dry periods is species-s pecific and depends on a reduction in stomatal conductance due to a decreas e in soil moisture potential. Protection under increased vapour pressure de ficit can occur in some but not all species, depending on the relationship between stomatal conductance and air vapour pressure deficit.