Photoinhibition

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Photoinhibition is the process by which the capacity for photosynthesis in plants is reduced in strong light (above the light saturation point). Photoinhibition is not caused by high light per se, but rather too much light energy absorbed compared with the photosynthetic capacity, i.e. any excess energy that the photosystem can not handle is damaging. Too much light energy affects (photosystem II (P680)) more than photosystem I (PSI), and it has been hypothesized that the excess energy damage either the reducing or oxidizing side of PSII, blocking the flow of electrons or the splitting of water respectively (Hall & Rao 1999).

Photoinhibition is often reversible, i.e. dynamic photoinhibition, and does in that case not inflict permanent damage to the photosystem. However, severe photoinhibition over a long time may cause highly reactive free oxygen radicals to form, which degrade photosynthetic components, i.e. chronic photoinhibition or photodamage. Particularly vulnerable is one of the main core proteins of photosystem II, protein D1, encoded by the gene psbA.

Plants and algae have several mechanisms to protect against photoinhibition, e.g. through the xanthophyll cycle.