INDUCTION OF GENES FOR THE STRESS PROTEINS PR-10 AND PAL IN RELATION TO GROWTH, VISIBLE INJURIES AND STOMATAL CONDUCTANCE IN BIRCH (BETULA-PENDULA) CLONES EXPOSED TO OZONE AND OR DROUGHT/

Well watered (WW) or drought-stressed (DS) saplings of ozone-sensitive and ozone-tolerant (less sensitive) birch (Betula pendula Roth) clone s were exposed for 43 d to 0 nl l(-1) or 100 nl l(-1) ozone. Relative growth rates of leaves, stem, and roots, leaf discolouration, stomatal conductance and induction of genes encoding stress-related proteins P R-10, PAL and a LEA-group protein BP8 were determined. In general, bot h ozone and drought stress, singly and in combination, increased trans cript levels of PR-10 in both clones. This was related to lower induct ion of PAL (except in older leaves of the tolerant clone), and increas ed proportions of visibly injured and yellowed leaves in ozone-exposed plants. The clones differed in their stomatal conductance and growth responses. In the less sensitive clone 2, ozone did not affect growth rates, but high stomatal conductance was observed in WW ozone-exposed plants. The more sensitive clone 5 showed, on the contrary, reduced gr owth rates and low stomatal conductance in WW ozone plants. Interestin gly, clone 2 was sensitive to drought stress alone, whereas clone 5 wa s highly sensitive to ozone and drought stress experienced together. T he results show that appearance of visible injuries (necrotic flecks) and enhanced yellowing of leaves coincided with the induction of genes for stress proteins PR-10 and PAL. The short-term growth responses, h owever, seemed to be separate processes. Additionally, stomatal conduc tance was related to leaf injuries and growth rates in a complicated m anner, emphasizing the complex nature of ozone sensitivity/tolerance m echanisms in birch.