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To clarify the relationship between cultivar difference in the sensitivity of net photosynthesis to ozone (O3) and the reactive oxygen species (ROS) scavenging system in wheat ( Triticum aestivum), we investigated the effects of chronic exposure to ambient levels of O3 on gas exchange rates, activity and concentration of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco), activity of ROS scavenging enzymes and concentration of antioxidants of the flag leaf in two Japanese winter wheat cultivars (Norin 61 and Shirogane-komugi). Although the net photosynthetic rate of the flag leaf in Norin 61 was not significantly reduced by exposure to O3, that in Shirogane-komugi was significantly reduced by the exposure to O3 during the anthesis and early grain-filling stages. In the two cultivars, stomatal diffusive conductance to H2O of the flag leaf was not significantly affected by the exposure to O3. The exposure to O3 induced significant reductions in the activity and concentration of Rubisco, activities of catalase (CAT) and monodehydroascorbate reductase (MDAR) and concentrations of reduced form of ascorbate and total glutathione of the flag leaf in Shirogane-komugi. It was concluded that the sensitivity of net photosynthesis of flag leaf to O3 is higher in Shirogane-komugi than in Norin 61, and the difference in the sensitivity to O3 between the two cultivars is mainly due to that in the effects of O3 on the detoxification ability of ROS, mainly determined by the activity of ROS scavenging enzymes, such as CAT and MDAR. [ABSTRACT FROM AUTHOR]