VARIATION IN OZONE SENSITIVITY AMONG CLONES OF BETULA-PENDULA AND BETULA-PUBESCENS

Forty clones of Betula pendula and 6 clones of Betula pubescens, origi nating from southern and central Finland, were ranked in order of ozon e sensitivity according to visible injuries, growth and leaf senescens e under low ozone exposure. The plants were fumigated in natural clima tic conditions using an open-air exposure system during two growing se asons. Control plants were grown under ambient air, and the elevated-o zone exposures were 1.6x the ambient in 1994 and 1.7x the ambient in 1 995. The differences in ozone sensitivity among clones were large. Ozo ne tolerance was related to thicker leaves and higher stomatal density as compared to sensitive clones. Ultrastructural ozone-induced sympto ms were found in chloroplasts of sensitive clones. Increased number of visibly injured leaves on fumigated plants was correlated with reduce d leaf formation, foliage area, shoot dry wt and number of stomata, an d increased yellowing of leaves. The results suggest that a considerab le proportion of birch trees, showing high sensitivity to ozone, are a t risk if ambient ozone exposures increase. (C) 1997 Elsevier Science Ltd.