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The chronic effects of ambient levels of O3 on photosynthetic components and radical scavenging system of leaves at different positions on the main stem of Japanese rice (Oryza sativa L.) were investigated. Two cultivars of Japanese rice (Koshihikari and Kinuhikari) were planted in plastic boxes filled with flooded Andisol in 9 chambers on 15 May, and exposed daily to charcoal-filtered air or O3 at 60 or 100 nl l -1 (ppb) (10:00-17:00) from 30 May to 20 September 2007. The activity and concentration of ribulose 1, 5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of both cultivars were not significantly affected by exposure to O3. However, the concentrations of chlorophyll in the lower leaves on the main stem of both cultivars were significantly reduced by the exposure to 100 ppb O3, while the exposure to O3 significantly increased the activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDAR) and glutathione reductase (GR); regardless of the leaf position in the two cultivars. Although no significant combined effect of O3 and cultivar on the concentration of chlorophyll, the activity and concentration of Rubisco, or the concentration of antioxidants and activity of radical scavenging enzymes was observed, there was a highly positive correlation between the AOT40 (accumulated exposure over a threshold of 40 ppb) of O3 and the relative activity of APX, MDAR or GR in leaves exposed to O3 to that in leaves exposed to charcoal-filtered air of both cultivars. These results suggest that APX, MDAR and GR played important roles in the antioxidative response of Japanese rice to O3. However, the detoxification capacity of reactive oxygen species by APX, MDAR and GR was insufficient to prevent the adverse effects of ambient levels of O3 on the concentration of chlorophyll in the two cultivars of Japanese rice.

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