Glutathione (GSH), a major antioxidant in most aerobic organisms, is perceived to be particularly important in plant chloroplasts because it helps to protect the photosynthetic apparatus from oxidative damage. In transgenic tobacco plants overexpressing a chloroplast-targeted ␥ -glutamylcysteine synthetase ( ␥ -ECS), foliar levels of GSH were raised threefold. Paradoxically, increased GSH biosynthetic capacity in the chloroplast resulted in greatly enhanced oxidative stress, which was manifested as light intensity-dependent chlorosis or necrosis. This phenotype was associated with foliar pools of both GSH and ␥ -glutamylcysteine (the immediate precursor to GSH) being in a more oxidized state. Further manipulations of both the content and redox state of the foliar thiol pools were achieved using hybrid transgenic plants with enhanced glutathione synthetase or glutathione reductase activity in addition to elevated levels of ␥ -ECS. Given the results of these experiments, we suggest that ␥ -ECS-transformed plants suffered continuous oxidative damage caused by a failure of the redox-sensing process in the chloroplast. INTRODUCTIONPlants, like all aerobic organisms, possess an array of hydrophilic and lipophilic antioxidants, such as glutathione, ascorbic acid (vitamin C), phenolic isoflavanoid compounds, ␣ -tocopherol (vitamin E), and the carotenoids, including the xanthophylls (Fryer, 1993; Mullineaux and Creissen, 1996). The reduced forms of these compounds, together with antioxidant enzymes, scavenge reactive oxygen species (ROS) and other products of oxidative reactions. These enzymes include subcellular compartment-specific isoforms of superoxide dismutase (SOD), catalase, ascorbate peroxidase (APX), glutathione S -transferase/glutathione peroxidase (GST/GPX), dehydroascorbate reductase, monodehydroascorbate free radical reductase, and glutathione reductase (GR). Several reduction-oxidation (redox) cycles that scavenge ROS in different subcellular compartments and that involve these enzymes and antioxidants have been proposed (e.g., the ascorbate-GSH cycle). The reducing equivalents for these reactions are derived ultimately from photosynthetic electron transport (Foyer and Halliwell, 1976; Mullineaux and Creissen, 1997). Thus, the degree of reduction of major antioxidant pools is generally considered to reflect the redox status of the tissue in question and is consequently an indicator of oxidative stress.Glutathione, either as GSH or as GSSG (glutathione disulfide; oxidized glutathione), is regarded as a key component of antioxidant defenses in most aerobic organisms, including plants (Foyer et al., 1997). However, the high (i.e., millimolar) concentration of GSH in the chloroplast (Foyer and Halliwell, 1976; Law et al., 1983; Bielawski and Joy, 1986) is in apparent conflict with its proposed roles: the regeneration of ascorbate (Foyer and Halliwell, 1976), reduction of lipid hydroperoxides (Mullineaux et al., 1998), and regulation of chloroplast gene expression by thiol-mediated modulation of RNA polymeras...

In comparison with the effects of extended drought periods or severe nutrient stress, those of ozone are generally much milder, at least with respect to growth. However, there is substantial evidence from experiments, in the main using young saplings, that O $ does impose a stress on forest trees under European conditions. Decreased chlorophyll contents and photosynthetic rates, changes in carbon allocation, increased antioxidant activity, and reductions in biomass due to O $ have often been recorded, particularly in fast-growing species. Furthermore, O $ appears to weaken the trees' resilience to a range of biotic and abiotic stresses. Interactions between O $ and climatic stress, in particular drought and frost hardiness, are likely to result in potentially detrimental effects.A link between the occurrence of O $ and forest damage is not unequivocally established in Europe, and the problem remains of extrapolating and\or scaling up from studies on seedlings to predict responses to O $ of mature trees and forest stands, because we know so little about acclimation to O$ . An accurate assessment is also lacking of the magnitude of the O $ effect on European trees both in terms of the forest areas affected and its extent. In this review we suggest that C allocation is the key factor underlying the responses of trees to O $ . Stomata also play a key role, since the acquisition of C must be achieved while an effective control over water consumption is retained.Key words : Seedlings, mature trees, growth, drought, frost, ozone. Statistical evaluation of annual surveys of forest condition in Europe (UNECE, ICP Forests) over the last decade has shown that tree crown condition in the main damage areas cannot be wholly explained by the location, nature and climate of the site (Mu$ ller-Edzardz et al., 1997). This might imply an important role for external factors such as air pollutants in the decline of tree health. One quarter of the coniferous trees assessed were damaged ( 20 % defoliation) and damage was worst in central

Glutathione (GSH), a major antioxidant in most aerobic organisms, is perceived to be particularly important in plant chloroplasts because it helps to protect the photosynthetic apparatus from oxidative damage. In transgenic tobacco plants overexpressing a chloroplast-targeted gamma-glutamylcysteine synthetase (gamma-ECS), foliar levels of GSH were raised threefold. Paradoxically, increased GSH biosynthetic capacity in the chloroplast resulted in greatly enhanced oxidative stress, which was manifested as light intensity-dependent chlorosis or necrosis. This phenotype was associated with foliar pools of both GSH and gamma-glutamylcysteine (the immediate precursor to GSH) being in a more oxidized state. Further manipulations of both the content and redox state of the foliar thiol pools were achieved using hybrid transgenic plants with enhanced glutathione synthetase or glutathione reductase activity in addition to elevated levels of gamma-ECS. Given the results of these experiments, we suggest that gamma-ECS-transformed plants suffered continuous oxidative damage caused by a failure of the redox-sensing process in the chloroplast.

Six pairs of O3‐sensitive and O3‐tolerant cultivars, clones or populations of different plants (tobacco, plantain, clover, radish, poplar and loblolly pine) were taken through identical but short episodic exposures to O3 in a controlled‐environment fumigation system. Emissions of ethene and concentrations of polyamines, total phenols and ascorbate as well as levels of reduced glutathione and ascorbate in fumigated and clean air controls were determined in these various cultivars, clones and populations. A large number of significant differences were detected between the various sensitive and tolerant pairs, but it is clear that different sequences of response involving these parameters occur in these various plant pairs to account for their individual O3 sensitivity or tolerance. Some O3‐tolerant plants have increased polyamines, others total phenols and some appear to be able to form reduced ascorbate and glutathione more rapidly. In the case of ethene emissions, however, all O3‐sensitive selections produce more ethene when fumigated while O3‐tolerant ones either reduce their rates of emissions below those of clean air‐grown controls or at least keep them at the same level.

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