Photoinhibition of Photosynthetic Electron Transport
Light, the primary driving force of photosynthesis, is a highly energetic and potentially dangerous substrate, which can affect all components of the photosynthetic apparatus and induce secondary destructive processes. Light-induced decline of photosynthetic activity, generally known as photoinhibition, is a general phenomenon in all photosynthetic organisms under conditions when the metabolic processes can not keep up with the electron flow produced by the primary photoreactions. The main factors responsible for the light sensitivity of the photosynthetic apparatus are the presence of excited pigment molecules, oxygen, and the high oxidizing potential of electron donors. Although light-induced damage occurs in all pigmented photosynthetic complexes the main site of photodamage is the Photosystem II complex, with smaller effect on Photosystem I and on the light harvesting antenna. Photodamage of the photosynthetic apparatus can be prevented efficiently by excitation quenching in the antenna, which dissipates excess excitation energy into heat. Photoprotection can also be provided by partly photoinhibited Photosystem II complexes, which work as dissipation centers, as well as by non-radiative charge recombination in the reaction center, which bypasses the triplet chlorophyll formation. Photodamage of Photosystem II can be repaired efficiently via a complex process that includes degradation of light damaged D1 protein subunit of the reaction center by thylakoid bound proteases, followed by resynthesis and membrane insertion of the newly made subunits, and, finally, the ligation of redox cofactors. The damage–repair cycle of the Photosystem II complex provides a highly dynamic system for the regulation of plant acclimation to rapidly changing light conditions.