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 pendula Roth.) clones were studied singly and in combination, in a high‐stress 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 nmol mol ∠1 ozone. In the open‐field experiment, well watered or drought‐stressed saplings were grown for one growing season in ambient air or exposed to 1·8 × ambient ozone. Drought stress reduced growth rate, stomatal conductance, stomatal density and the proportion of starch and thylakoids in chloroplasts, but stimulated net photosynthesis, Rubisco and chlorophyll quantity at the end of the growing season, and increased the size and density of plastoglobuli. Ozone fumigations caused more variable, clone‐ and exposure‐dependent responses in growth, decreased stomatal conductance and net photosynthesis, an increased number of stomata, visible and ultrastructural chloroplast injuries, and enhanced autumn yellowing of the leaves. Ozone‐induced changes in plastoglobuli, starch and thylakoids resembled drought responses. The two experiments revealed that, depending on the experimental conditions and the variable, the response to drought and ozone stress can be independent, additive or interactive. Drought protected the plants from ozone 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.