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Changes in CuZn-SOD activity and content in isolated wheat chloroplasts under the light, and the involvement of protease(s) and/or active oxygen species in this process were studied. Both SOD activity and content decayed with exposure time to photooxidative stress. Ascorbate, a H2O2 scavenger, prevented photooxidation-associated inactivation of SOD, while benzoate, a .OH scavenger, prevented SOD degradation. Wheat chloroplasts incubated in the dark did not hydrolyze exogenous or endogenous SOD, either H2O2-pretreated or not. Protease inhibitors did not prevent SOD degradation under photooxidative treatment, suggesting that plastid protease(s) did not participate in this process. Purified chloroplast CuZn-SOD was exposed to H2O2 and O2- or .OH-generating systems. O2- had no effect on either SOD activity or stability (estimated by native PAGE). H2O2 up to 700 microM inhibited SOD in a dose-dependent manner and induced charge/mass changes as seen by native PAGE. .OH also reduced SOD activity by inducing its fragmentation. High levels of active oxygen, as can be generated under strong stress conditions, could directly inactivate and degrade chloroplastic SOD.

The participation of the antioxidant system in the drought tolerance of wheat cultivars (Triticum aestivum L.) was studied under field and in vitro conditions. Under field conditions, drought tolerance was evaluated by the capacity to maintain the grain yield under drought, which was higher in cvv. Elite and La Paz than in the sensitive cvv. Oasis and Cruz Alta. Tolerant cultivars showed lower relative water content (RWC) and lower above-ground vegetative biomass than sensitive cultivars. Field assays did not show a clear correlation between water-stress tolerance and antioxidant system behaviour. However, when leaves of cvv. with contrasting drought tolerance were subjected to osmotic stress in vitro, clear differences in the antioxidant system activity and oxidative damage between cvv. were observed. In the tolerant cultivar Elite, it was possible to observe an increase in ascorbate peroxidase (APX), superoxide dismutase (SOD) and glutathione reductase (GR) activities, a higher glutathione (GSH) and ascorbate content and less oxidative damage than in the sensitive cultivar Oasis, which showed no changes or only slight decreases in the enzyme activities. These results indicate that water stress tolerance is in part associated with the antioxidant system activity, and suggest that the behaviour of the antioxidant systemin vitro assays can be used as an early selection tool.

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