Figure 4

Proposed mechanism of PSII photodamage in high light. (a) Active PSII absorbs photons. (b) The pool of acceptors is fully reduced. (c) Repetitive turnover P680⁺Q_A ⁻ induces formation of P680 triplet or side reactions as described by Vass⁸. (d) Accumulation of ROS occurs at several parts of PSII and they diffuse to several sites. (e) Inactivation of the Mn₄CaO₅ cluster by ROS. (f) Once the Mn₄CaO₅ cluster is inactive, the Mn²⁺ ion is released. (g) In parallel to inactivation to the Mn₄CaO₅ cluster, ROS may deactivate the acceptor side activity on a gradual way but lower rate ROS. (h) As a consequence of oxidative stress the three remaining Mn²⁺ ions are released; however, at this step, PSII is completely inactive.