Photoinhibition of photosynthesis

Photoinhibition of photosynthesis is a widespread phenomenon in oxygenic photosynthetic organisms that can result in large decreases in photosynthetic productivity. In the widest context photoinhibition of photosynthesis can be defined as the light-induced decrease in CO2 assimilation, which would include the effects of photo-oxidative radical damage to many components of the photosynthetic apparatus that can occur in environmentally stressed organisms at high irradiances. However, quite frequently photoinhibition of photosynthesis is used to refer specifically to light-induced damage to the PSII reaction centre, which precedes the onset of the more severe radical-induced damage to other components of the photosynthetic apparatus. In the past decade it has become established that an essential, intrinsic feature of the photosynthetic apparatus is the light-induced decrease in the quantum efficiency of photosynthesis as irradiance increases; when leaves are exposed to increasing photon flux densities an increasing proportion of the absorbed energy is lost as heat, thereby reducing the quantum efficiency of the photosynthetic processes. Although this light-induced decrease of photosynthetic efficiency is not associated with damage to components of the photosynthetic apparatus and is, in fact, a mechanism to protect from the damaging effects of high light, it does constitute a light-dependent depression of photosynthetic potential and warrants consideration in any treatise on photoinhibition of photosynthesis. In 1956 Kok defined photoinhibition as the light-dependent reduction in photosynthetic efficiency and this still perhaps provides the most useful working definition of photoinhibition of photosynthesis for leaves and whole organisms.