Evaluation of O3 effect on net photosynthetic rate in flag leaves of rice (Oryza sativa L.) by stomatal O3 flux and radical scavenging enzyme activities

To evaluate the effects of O3 on the net photosynthetic rate in flag leaves of rice (Oryza sativa L.) based on stomatal O3 flux and reactive oxygen species (ROS) scavenging enzyme activities, the Japanese rice cultivar Koshihikari was exposed to O3 for 1 month after heading in O3-exposure chambers. During the exposure period, rice plants were exposed to charcoal-filtered air or O3 at 60 or 100 nl l(ppb) daily (10:00–17:00). Light-saturated net photosynthetic rate (Asat) and ROS scavenging enzyme activities in the flag leaves were periodically measured during the exposure period. The O3 exposure significantly reduced Asat on the 20th and 31st days after the beginning of O3 exposure (DAE). The degree of O3-induced reduction in Asat was explained by the cumulative flux of O3 compared with concentration-based O3 indices. The activities of ROS scavenging enzymes such as ascorbate peroxidase and catalase at 32 DAE were lower than at 1 DAE. During the latter half of the O3 exposure period (21–31 DAE), the daily average stomatal O3 flux was also lower than that during the first half of the period (1–20 DAE), which was mainly caused by lower photosynthetic photon flux density, lower air temperature and leaf senescence. These results suggest that the balance between stomatal O3 flux and leaf cellular detoxification capacity in the flag leaves of rice, which determines the degree of O3 damage, might have been similar between the first half and latter half of the O3 exposure period.