Responses of Antioxidative Systems to Drought Stress in Pendunculate Oak and Maritime Pine as Modulated by Elevated CO2

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Abstract
The aim of the present study was to investigate the effects of an enhanced CO2 concentration alone or in combination with drought stress on antioxidative systems of a deciduous (oak; Quercus robur) and an evergreen (pine; Pinus pinaster) tree species. The seedlings were grown for one season in a greenhouse in tunnels supplied with 350 or 700 [mu]L L-1 CO2. The experiment was repeated in a second year. Antioxidants, protective enzymes, soluble protein, and pigments showed considerable fluctuations in different years. Elevated CO2 caused significant reductions in the activities of superoxide dismutases in both oak and pine. The activities of ascorbate peroxidase and catalase were also reduced in most cases. The activities of dehydroascorbate reductase, monodehydroascorbate radical reductase, glutathione reductase, and guaiacol peroxidase were affected little or not at all by elevated CO2. When the trees were subjected to drought stress by withholding water, the activities of antioxidative enzymes decreased in leaves of pine and oak grown at ambient CO2 and increased in plants grown at elevated CO2 concentrations. The present results suggest that growth in elevated CO2 might reduce oxidative stress to which leaf tissues are normally exposed and enhance metabolic flexibility to encounter increased stress by increases in antioxidative capacity.