EFFECTS OF OZONE ON BIRCH (BETULA-PENDULA ROTH) CLONES

Saplings of five birch clones (Betula pendula Roth.) were exposed to o zone doses 1.2 - 1.7 x higher than tile ambient under field conditions during growing seasons 1991-1993. The clones were also fumigated with 40 ppb - 150 ppb ozone in three chamber experiments.?he effects of ni trogen supply on ozone responses were studied using field fumigation s ystem and three levels of nitrogen supply (low, medium, high). The ozo ne-sensitivity was clone-spesific. Even low ozone exposure decreased t he biomass production of the most sensitive done, whereas the most tol erant clone showed unaffected growth rate. hi all clones, ozone fumiga tion accelerated autumn senescence of leaves, increased ultrastructura l injuries, especially in chloroplasts, and increased diffusion resist ance and stomatal density of leaves. Ozone-induced changes in tile tis sue and fine structure of leaves were interpreted as enhanced senescen ce. ?he plants receiving high nitrogen supply were more resistant to o zone than the plants under lower nitrogen availability. Significant in teractions between ozone and nitrogen supply were found in growth and leaf anatomical parameters. The ozone sensitivity and the growth rate of birch clones seem to be related to biochemical and ultrastructural changes of chloroplasts, as reduced growth rate of the most sensitive clone was accompanied by lower contents of Rubisco small subunit and c hlorophyll, and lower net photosynthesis. Fast enlarging and young mat ure leaves without acclimation were more sensitive to ozone fumigation than the leaves emerging under ozone stress.