The effect of various abiotic stresses on photosynthetic apparatus is inevitably associated with formation of harmful reactive oxygen species (ROS). In this review, recent progress on ROS production by photosystem II (PSII) as a response to high light and high temperature is overviewed. Under high light, ROS production is unavoidably associated with excitation energy transfer and electron transport in PSII. Singlet oxygen is produced by the energy transfer form triplet chlorophyll to molecular oxygen formed by the intersystem crossing from singlet chlorophyll in the PSII antennae complex or the recombination of the charge separated radical pair in the PSII reaction center. Apart to triplet chlorophyll, triplet carbonyl formed by lipid peroxidation transfers energy to molecular oxygen forming singlet oxygen. On the PSII electron acceptor side, electron leakage to molecular oxygen forms superoxide anion radical which dismutes to hydrogen peroxide which is reduced by the non-heme iron to hydroxyl radical. On the PSII electron donor side, incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. Under high temperature, dark production of singlet oxygen results from lipid peroxidation initiated by lipoxygenase, whereas incomplete water oxidation forms hydrogen peroxide which is reduced by manganese to hydroxyl radical. The understanding of molecular basis for ROS production by PSII provides new insight into how plants survive under adverse environmental conditions.

Significance: Reactive oxygen species (ROS) damage the D1 and D2 reaction center proteins of Photosystem II in a process known as photoinhibition. Photoinhibition is an unavoidable consequence of excitation energy transfer and electron transport. The ROS responsible for oxidative damage, the sites of ROS production, and the residues oxidatively modified have not been determined. In this work, we identify HO ^• as being produced on both the oxidizing and reducing sides of the photosystem. O ₂ ^•− also appears to be produced at either Pheo _D1 or Q _A . Additionally, residues on the D1 and D2 proteins were identified that are oxidatively modified during a photoinhibitory timecourse. Finally, we propose plausible pathways for the propagation of protein oxidation events in the D1 and D2 proteins.