Abstract
Photosystem II (PSII) utilizes light energy but is particularly sensitive to light. This paradoxical phenomenon is referred to as photodamage or photoinhibition of PSII and several hypotheses have been proposed, such as the "acceptor-side" and "donor-side" mechanisms. However, none of them can fully explain the in vivo observations that photodamage to PSII is proportional to light intensity, independent of electron transport or reactive oxygen species, and dominant to UV/blue light. Recently, increasing evidence has revealed that the oxygen-evolving complex is the primary site of damage by light. This model suggests that light is an inevitable damaging factor for PSII. Given that the activity of PSII is determined by the balance between damage and repair in vivo, it appears that repair is a main mechanism of the protection of PSII from light. The role of repair in the protection of PSII from other environmental factors will also be discussed.
