Impact of growth light environment on oxygen sensitivity in rice: Pseudo-first-order response of photosystem I photoinhibition to O 2 partial pressure.

Photosynthetic organisms generate reactive oxygen species (ROS) during photosynthetic electron transport reactions on the thylakoid membranes within both photosystems (PSI and PSII), leading to the impairment of photosynthetic activity, known as photoinhibition. In PSI, ROS production has been suggested to follow Michaelis-Menten- or second-order reaction-dependent kinetics in response to changes in the partial pressure of O 2 . However, it remains unclear whether ROS-mediated PSI photoinhibition follows the kinetics mentioned above. In this study, we aimed to elucidate the ROS production kinetics from the aspect of PSI photoinhibition in vivo. For this research objective, we investigated the O 2 dependence of PSI photoinhibition by examining intact rice leaves grown under varying photon flux densities. Subsequently, we found that the degree of O 2 -dependent PSI photoinhibition linearly increased in response to the increase in O 2 partial pressure. Furthermore, we observed that the higher photon flux density on plant growth reduced the O 2 sensitivity of PSI photoinhibition. Based on the obtained data, we investigated the O 2 -dependent kinetics of PSI photoinhibition by model fitting analysis to elucidate the mechanism of PSI photoinhibition in leaves grown under various photon flux densities. Remarkably, we found that the pseudo-first-order reaction formula successfully replicated the O 2 -dependent PSI photoinhibition kinetics in intact leaves. These results suggest that ROS production, which triggers PSI photoinhibition, occurs by an electron-leakage reaction from electron carriers within PSI, consistent with previous in vitro studies.