Cyclic electron flow within PSII protects PSII from its photoinhibition in thylakoid membranes from spinach chloroplasts

Cyclic electron flow within PSII (CEF-PSII) was proven to alleviate the photoinhibition of PSII. We set the conditions where CEF-PSII functioned or did not, by adding nigericin to the reaction mixture for the dissipation of DeltapH across thylakoid membranes, and then the thylakoids were illuminated. When CEF-PSII did not function and the activity of linear electron flow (LEF) was low, light-treated thylakoid membranes largely lost the activity of LEF. The inactivation of LEF was due to the loss of the activity of PSII, but not that of PSI. The inactivation of PSII was suppressed, when CEF-PSII functioned or LEF was enhanced. These results imply that CEF-PSII contributes to the protection of PSII from its photoinhibition with LEF, as an electron sink.