Photoinhibition of PSI and PSII in Nature and in the Laboratory: Ecological Approaches

Light is indispensable for plants to photosynthesize organic matters. Almost all the organisms including animals on our planet eventually rely on this plant function for their energy as well as the materials forming their bodies. Paradoxically, light often damages the photosynthetic apparatus. This phenomenon is called photoinhibition and has been attracting attention of many photosynthesis researchers. Although the term photoinhibition had been used almost synonymously to refer to photoinhibition of photosystem II (PSII), it was recently shown that PSI is susceptible to fluctuating light. First, we compare two PSII photoinhibition hypotheses: the Mn/(Two-step) hypothesis and Excess-Y(NO) hypothesis. The former claims that the oxygen-evolving complex (OEC) is primarily damaged, while the latter claims excess excitation energy in PSII directly damages D1 function. Both can be induced in the laboratory and may parallelly occur in the same leaf. Because OEC is damaged by ultraviolet (UV) or blue light, UV screening substances in the leaf epidermis plays a crucial role. It is also indicated that the rate of PSII repair in PSII damaged by the Mn/(Two-step) mechanism is much slower than that by the Excess-Y(NO) mechanism. It appears then plants should avoid PSII photoinhibition by the Mn/(Two-step) hypothesis. Photoinhibition of photosystem I (PSI) in cucumber, a model chilling sensitive plant, and that in the mutant of PROTON GRADIENT 5 are compared. The effects of fluctuating light, which naturally occurs in the field, on PSI photoinhibition are also discussed. The PSI photoinhibition in these three cases can be explained by similar scenarios. When reduced P700 donates electrons to O2, oxidative damage is induced. The mechanisms that protect PSI from photoinhibition all contribute to oxidation of P700 to P700+, a safe quencher. When a leaf is suddenly exposed to strong light, spillover of excitation energy from PSII to PSI protects both PSII and PSI from photoinhibition. In all these situations, far-red (FR) light plays essential roles in PSI protection. As FR light not only protects PSI but also enhances photosynthesis, especially in low light phases in the fluctuating light, the roles of FR light in photosynthesis should be fully examined. Other ecologically important problems that should be solved in the future are also pointed out.