Effect of Acute O3 Exposure on Leaf Photosynthesis of Paddy Rice Grown at Elevated CO2 Concentration

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Leaf gas exchange, chlorophyll fluorescence, and ascorbic acid contents of rice grown at ambient and elevated CO2 concentrations (400 and 800 cm3 m−3, expressed as C400 and C800) were examined under three O3 concentrations (0, 0.1 and 0.3 cm3 m−3, expressed as O0, O0.1 and O0.3) to elucidate the effects of acute O3 exposure. The net photosynthetic rate, maximum and operating quantum efficiencies of photosystem II decreased immediately after O3 exposure, but these were ameliorated by C800. The acclimation of stomatal conductance to elevated CO2 occurred in rice leaves of C800 plants. Therefore, it was anticipated that O3-induced injury of photosynthesis in the C800+O0.3 plants was more suppressed than in C400→C800+O0.3 plants (grown at C400+O0 and received acute C800+O0.3 exposure). However, no significant difference was found between these plants. The contents of total ascorbic acid, reduced form of ascorbic acid (AA) and dehydroascorbic acid, and redox state of ascorbic acid were unaffected by C800 when they were expressed per leaf area. However, the total ascorbic acid and AA contents per fresh and dry mass bases were decreased by C800 because the specific leaf weight was increased significantly by C800. Therefore, long-term exposure of rice plants to elevated CO2 might have little effect on antioxidant systems.