PHYSIOLOGICAL, STOMATAL AND ULTRASTRUCTURAL OZONE RESPONSES IN BIRCH (BETULA-PENDULA ROTH.) ARE MODIFIED BY WATER-STRESS

The physiological, stomatal and ultrastructural responses to ozone and drought of ozone-sensitive and more ozone-tolerant birch (Betula pend ula Roth,) clones were studied singly and in combination, in a high-st ress chamber experiment and in a low-stress open-field experiment. In the chamber experiment, well watered (WW), moderately watered (MW) or drought-stressed (DS) saplings were exposed for 36 d to 0 or 130 mmol mol(angle 1) ozone. In the open-field experiment, well watered or drou ght-stressed saplings were grown for one growing season in ambient air or exposed to 1.8 x ambient ozone, Drought stress reduced growth rate , stomatal conductance, stomatal density and the proportion of starch and thylakoids in chloroplasts, but stimulated net photosynthesis, Rub isco and chlorophyll quantity at the end of the growing season, and in creased the size and density of plastoglobuli. Ozone fumigations cause d more variable, clone- and exposure-dependent responses in growth, de creased stomatal conductance and net photosynthesis, an increased numb er of stomata, visible and ultrastructural chloroplast injuries, and e nhanced autumn yellowing of the leaves, Ozone-induced changes in plast oglobuli, starch and thylakoids resembled drought responses. The two e xperiments revealed that, depending on the experimental conditions and the variable, the response to drought and ozone stress can be! indepe ndent, additive or interactive. Drought protected the plants from ozon e injuries under high-stress conditions in the chamber experiment. In the low-stress, open-field experiment, however, enhanced ozone damage was observed in birch saplings grown under restricted water supply.