The ozone sensitivity of birch (Betula pendula) in relation to the developmental stage of leaves

Clonal birch (Betula pendula Roth.) seedlings at three different developmental stages were fumigated for 12 h daily with 50, 90 and 130 ppb ozone for 25 d. When transferred to fumigation chambers, the leaves of plant group 1 were almost expanded, in group 2 they were fast enlarging, and in group 3 all the leaves emerged under the exposure. The plants in which the leaves emerged under the ozone stress were the most resistant to ozone. In groups 1 and 2 ozone treatments reduced the growth of leaf biomass, and induced more visible injuries on leaves and ultrastructural symptoms in chloroplasts than they did in group 3. The stomatal density of leaves increased in response to ozone in all the groups. The effects of ozone on stomatal conductance were complex. In groups 1 and 2 the 50 ppb ozone exposure increased stomatal conductance whereas lowered conductances were observed in group 3 after fumigation with 90 and 130 ppb ozone. The results suggest that the leaves developing under ozone fumigation were better able to tolerate the ozone stress, as indicated by unaffected or stimulated growth and small amounts of visible and ultrastructural injuries and stomatal changes. It is possible to regard the changes as acclimation reactions.