Physiological and ultrastructural responses of birch clones exposed to ozone and drought stress

Saplings of birch (Betula pendula) clones were exposed to drought and ozone stress in controlled chambers and open-field conditions. In the chamber experiment, the plants exposed to high drought stress were more resistant to 130 ppb ozone treatment than the well- and moderately watered plants. On the contrary, in the more realistic field conditions, exposure to low drought stress strengthened the negative effects of slightly elevated ozone. Photosynthetic senescence was delayed despite ozone in drought-stressed plants in the late summer. Specific differences between ozone and drought stress were found particularly in ultrastructural changes of chloroplasts and mitochondria.