In vivo deleterious effects specific to reactive oxygen species on photosystem I and II after photo-oxidative treatments of rice (Oryza sativa L.) leaves

Under various photo-oxidative conditions in the presence of methyl viologen (MV), diethyldithiocarbamic acid (DDC, a superoxide dismutase (SOD) inhibitor), and hydroxyurea (HU, an ascorbate peroxidase (APX) inhibitor), the causal relationships of the reactive oxygen species (ROS) with the photo-oxidative damages in photosystems in vivo were investigated in rice leaves. In the MV-treated leaves, damages were mainly observed in photosystem (PS) II with a decrease in the maximum photochemical capacity of PSII, Fv/Fm, and the degradation of D1 protein, a core component of PSII. For PSI, the extent of P700 photo-oxidation, P700+, decreased slightly but PsaA/B proteins, core components of PSI, were barely affected. In contrast, DDC, which is expected to produce O2−, caused a greater decrease in P700+, while the decrease in Fv/Fm was relatively small in the DDC-treated leaves. Interestingly, the deleterious effect of MV on PSII was largely masked in the presence of DDC, but instead the decrease in P700+ was more pronounced in the MV/DDC-treated leaves. The damages in PSII by the treatment of MV were mainly caused by the accumulated H2O2 rather than OH, because dimethylsulfoxide, a hydroxyl radical scavenger, did not protect against the decrease of Fv/Fm in the MV-treated leaves. Unexpectedly, the ROS responsible for the HU-induced photo-oxidative damages in PSII was OH rather than H2O2. However, the production of OH did not imply a significant increase in the level of cellular catalytic free metals. Moreover, it was possible to differentiate various photo-oxidative stresses in vivo based on the degradation pattern of D1 protein. In the present study, it was further discussed, how different ROS were responsible for the differential damages in photosystems in vivo under various photo-oxidative conditions, and how the differential inhibition of protective machineries contributed to the production and accumulation of different ROS.