Well watered (WW) or drought-stressed (DS) saplings of ozone-sensitive and ozone-tolerant (less sensitive) birch (Betula pendula Roth) clones were exposed for 43 d to 0 nl l −" or 100 nl l −" ozone. Relative growth rates of leaves, stem, and roots, leaf discolouration, stomatal conductance and induction of genes encoding stress-related proteins PR-10, PAL and a LEA-group protein BP8 were determined. In general, both ozone and drought stress, singly and in combination, increased transcript levels of PR-10 in both clones. This was related to lower induction of PAL (except in older leaves of the tolerant clone), and increased 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 ozoneexposed plants. The more sensitive clone 5 showed, on the contrary, reduced growth rates and low stomatal conductance in WW ozone plants. Interestingly, clone 2 was sensitive to drought stress alone, whereas clone 5 was highly sensitive to ozone and drought stress experienced together. The 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, however, seemed to be separate processes. Additionally, stomatal conductance was related to leaf injuries and growth rates in a complicated manner, emphasizing the complex nature of ozone sensitivity\tolerance mechanisms in birch.Key words : Betula pendula Roth (birch), clones, ozone, drought, stress proteins.
Forest trees grow in a dynamic environment and are continuously exposed to several abiotic and biotic stress factors (Lefohn, 1992). Atmospheric ozone and drought are potential stress factors for forest trees in the USA (e.g. McLaughlin & Downing, 1996) and Europe including Scandinavia (e.g. Pearson & Mansfield, 1993 ;Karlsson et al., 1995 ; * To whom correspondence should be addressed. E-mail : Elina.Paakkonen!messi.uku.fi Pa$ a$ kko$ nen, 1996 ; Pa$ a$ kko$ nen, Holopainen & Ka$ renlampi, 1997 a). Physiological, structural, and biochemical responses of birch to ozone have recently been studied in several open-field and chamber experiments. Typical ozone responses in ozonesensitive birch clones were impaired shoot growth, accelerated leaf senescence, visible leaf injuries, ultrastructural chloroplast injuries, increased stomatal density and lower stomatal conductance (Pa$ a$ kko$ nen et al., 1993(Pa$ a$ kko$ nen et al., a,b, 1996 a ; Pa$ a$ kko$ nen, 1996). Lower net photosynthesis and decreased Rubisco and chlorophyll contents have 296 E. PaW aW kkoW nen and others also been observed during open-field fumigations . In the chamber exposures, on the other hand, increased activities of superoxide dismutase, peroxidase and glutathione reductase, induction of PAL (phenylalanine ammonia-lyase), accumulation of free pu...