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Rapid response of antioxidant enzymes to O3-induced oxidative stress in Populus tremuloides clones varying in O3 tolerance

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Forest Genetics





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The concentration of atmospheric ozone (O3), formed at the interaction of n itrogen oxid es, volatile hydrocarbons and ultraviolet rad iation, is increasing globally (FOWLER et al. 1999) due fossil fuel consumption. While O3 levels are increasing in the atmosphere, in fact none of it has been detected inside the leaf (LAISK et al. 1989). Ozone readily reacts with water giving rise to reactive oxygen species (ROS) hydrogen peroxide and super- oxide anion radical (GRIMES et al. 1983; KANOFSKY & SIMA 1995). ROS and especially radicals can non- specifically react with lipids, pro teins and nuc leic acids (DAVIES 1995) thus disrupting the structural and func- tional integrity of the cell. W hile the increased rise in tropospheric O3 and the resulting increase in ROS is the result of human activities, the active oxygen species are also produced naturally during the normal metabolism of photosynthesizing cells. All green plants as well as other aerobic organisms have evolved antioxidant defense systems to combat the inevitable ROS (ALSCH- ER et al. 1997). The antioxidant systems can be enzy- matic or non-enzymaticROS scavengers that require the activation of specific metabolic pathways and invest- ment of energy. It has been shown that plants growing under the conditions of oxidative stress have elevated levels of antioxidants (Rao et al. 1996; POLLE 1997; NOCTOR & FOYER 1998). The principle of the action of the antioxidants is simple, but the mechanism of regula- tion of their synthesis and activity is not well estab- lished. An aspect of doubt is whether antioxidants can provideprotection against briefhigh-O3 episodes orare they useful for protecting plants from only long-term chronic exposure. In current experiment we subjected two aspen (Populus tremuloides Michx.) clones, differ- ing in O3 tolerance, to acute O3 episodes of up to six hours to test the inducibility of the enzymes of the Halliwell-Asada antioxidant pathway (Fig. 1) and catalase, the major scavenger of high levels of hydrogen peroxid e.