Photoinhibition

Photoinhibition is a reduction in a plant's (or other photosynthetic organism's) capacity for photosynthesis caused by exposure to strong light (above the saturation point). Photoinhibition is not caused by high light per se, but rather absorption of too much light energy compared with the photosynthetic capacity, i.e. any excess energy that the photosystem cannot 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 damages either the oxidizing (donor) or reducing (acceptor) side of PSII, damaging the water oxidizing complex on the donor side or blocking the flow of electrons to plastoquinone on the acceptor side (Hall & Rao 1999).

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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.

References

• Hall, D. O. & K. K. Rao, 1999, Photosynthesis. – Cambridge Univ. Press, Cambridge.

See also

• Photosynthetic reaction centre
• Photosynthesis
• Chlorophyll
• Light reaction
• Kautsky effect
• Anthocyanin