INTERSPECIFIC AND INTRASPECIFIC DIFFERENCES IN OZONE SENSITIVITY IN SEMINATURAL PLANT-COMMUNITIES

The sensitivity of native species from different sites in Switzerland was tested by exposing soil blocks with intact semi-natural grassland vegetation to elevated ozone under climate-controlled conditions. The aim was to study inter- and intraspecific differences in sensitivity, and to relate the differences between and within species to difference s in leaf conductance and site conditions. The selected sites reflecte d different situations in terms of grassland type, management, climate and soil, and ozone exposure. The plant response was analyzed on the basis of visible leaf injury. The species were classified according to the appearance of visible injury after six days of exposure to ozone with a maximum daily 1-hr mean concentration of 100 ppb (sensitive spe cies), or after an additional three-day exposure to 150 ppb (intermedi ate species). Species without visible injury were classified as resist ant. The majority (48 %) of the 31 species tested was classified 'inte rmediate'. Sensitive species included mainly legumes (Trifolium repens , T. pratense, Lathyrus pratensis, Ranunculus friesianus, Lotus cornic ulatus), In the case of T. repens and T. pratense, large differences e xisted between ecotypes, but no clear relationship was found between s ensitivity and site conditions. From the ecotype of T. repens with the highest sensitivity, a sensitive and a resistant clone could be ident ified. In general, differences in ozone sensitivity between species an d within species could be related to differences in leaf conductance, i.e. highest sensitivity was found in species, ecotypes, or clones wit h the highest leaf conductance. It is concluded that in semi-natural g rassland communities the large differences in ozone sensitivity presen ts a potential for natural selection for tolerance. Long-term exposure may thus affect the clonal composition of species, or the species com position of the community. This possibility needs to be tested in futu re experiments with intact plant communities.