Photoinhibition

Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII. In living organisms, photoinhibited PSII centres are continuously repaired via degradation and synthesis of the D1 protein of the photosynthetic reaction center of PSII. Photoinhibition is also used in a wider sense, as dynamic photoinhibition, to describe all reactions that decrease the efficiency of photosynthesis when plants are exposed to light.

The first measurements of photoinhibition were published in 1956 by Bessel Kok. Even in the very first studies, it was obvious that plants have a repair mechanism that continuously repairs photoinhibitory damage. In 1966, Jones and Kok measured the action spectrum of photoinhibition and found that ultraviolet light is highly photoinhibitory. The visible-light part of the action spectrum was found to have a peak in the red-light region, suggesting that chlorophylls act as photoreceptors of photoinhibition. In the 1980s, photoinhibition became a popular topic in photosynthesis research, and the concept of a damaging reaction counteracted by a repair process was re-invented. Research was stimulated by a paper by Kyle, Ohad and Arntzen in 1984, showing that photoinhibition is accompanied by selective loss of a 32-kDa protein, later identified as the PSII reaction center protein D1. The photosensitivity of PSII from which the oxygen evolving complex had been inactivated with chemical treatment was studied in the 1980s and early 1990s. A paper by Imre Vass and coworkers in 1992 described the acceptor-side mechanism of photoinhibition. Measurements of production of singlet oxygen by photoinhibited PSII provided further evidence for an acceptor-side-type mechanism. The concept of a repair cycle that continuously repairs photoinhibitory damage, evolved and was reviewed by Aro et al. in 1993. Many details of the repair cycle, including the finding that the FtsH protease plays an important role in the degradation of the D1 protein, have been discovered since. In 1996, a paper by Tyystjärvi and Aro showed that the rate constant of photoinhibition is directly proportional to light intensity, a result that opposed the former assumption that photoinhibition is caused by the fraction of light energy that exceeds the maximum capability of photosynthesis. The following year, laser pulse photoinhibition experiments done by Itzhak Ohad's group led to the suggestion that charge recombination reactions may be damaging because they can lead to production of singlet oxygen. The molecular mechanism(s) of photoinhibition are constantly under discussion. The newest candidate is the manganese mechanism suggested 2005 by the group of Esa Tyystjärvi. A similar mechanism was suggested by the group of Norio Murata, also in 2005.

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