DIFFERENT RESPONSES TO OZONE OF TOBACCO, POPLAR, BIRCH, AND ALDER

Plants of an ozone sensitive tobacco clone (Nicotiana tabacum L. var. BelW3), cuttings of poplar (Populus x euramericana var. Dorskamp) and birch (Betula pendula Roth), and seedlings of alder (Alnus glutinosa ( L.) Gaertn.) were grown in the field or in the field fumigation chambe rs during one growing season. Twenty chambers (each with one plant per species) were used for four treatments with either filtered air, or f iltered air with added ozone (75 ppb) from 07:00 to 19:00, or from 19: 00 to 07:00, or continuously. Tobacco did not respond to ozone applied during the nighttime, but leaf injury symptoms of plants grown in the field under shade appeared at an ozone dose similar to that applied d uring the daytime. The leaves of the deciduous trees showed injury sym ptoms in all ozone treatments. In the tree species the stomatal pores were open at 06:00 (closed in tobacco), but they were narrowed at 10:0 0 under ozone as compared to filtered air. The time span until ozone-i nduced leaf injury symptoms or premature leafless occurred, was influe nced by the species, the season, and the ozone regime. Similarities an d differences, in particular in the starch metabolism or the formation of cell wall exudates, could be traced from early symtoms to cell col lapse in all species. In contrast to agricultural plants, ozone impact on the leaves of deciduous trees is as important during dusk, night a nd dawn as during daytime.