Abstract
Excess photon energy against photosynthesis produces alternative electron flow (AEF). AEF consists of both O2-dependent electron flow driven by the water-water cycle (WWC) and O2-independnt one by unknown mechanism. We found cyclic electron flow within PSII of thylakoid membranes (CEF-PSII) as a candidate for O2-independent AEF. CEF-PSII activity was detected by the simultaneous measurements of both a quantum yield of PSII and an activity of linear electron flow. The expression of CEF-PSII activity required both ΔpH across thylakoid membranes and the reduced plastoquinone. CEF-PSII also operated in intact chloroplasts, and its activity increased with the increase in light intensity and corresponded to WWC activity. These results suggest that CEF-PSII functions in the dissipation of excess photon energy as an electron sink as WWC does. In fact, we found that CEF-PSII alleviated PSII photoinhibition by stimulating electron flow in PSII.
