Enhancement of cyclic electron flow around PSI at high light and its contribution to the induction of non-photochemical quenching of chl fluorescence in intact leaves of tobacco plants

Non-photochemical quenching (NPQ) of Chi fluorescence is a mechanism for dissipating excess photon energy and is dependent on the formation of a DeltapH across the thylakoid membranes. The role of cyclic electron flow around photosystem I (PSI) (CEF-PSI) in the formation of this DeltapH was elucidated by studying the relationships between O-0-evolution rate [V(O-2)], quantum yield of both PSII and PSI [(D(PSII) and (D(PSI)], and Chi fluorescence parameters measured simultaneously in intact leaves of tobacco plants in CO2-saturated air. Although increases in light intensity raised V(O-2) and the relative electron fluxes through both PSII and PSI [Phi(PSII)xPFD and Phi(PSI)xPFD] only Phi(PSI)xPFD continued to increase after V(O-2) and Phi(PSII)xPFD became light saturated. These results revealed the activity of an electron transport reaction in PSI not related to photosynthetic linear electron flow (LEF), namely CEF-PSI. NPQ of Chi fluorescence drastically increased after Phi(PSII)xPFD became light saturated and the values of NPQ correlated positively with the relative activity of CEF-PSI. At low temperatures, the light-saturation point of (D(PSII)xPFD was lower than that of Phi(PSI)xPFD and NPQ was high. On the other hand, at high temperatures, the light-dependence curves of Phi(PSII)xPFD and Phi(PSI)xPFD corresponded completely and NPQ was not induced. These results indicate that limitation of LEF induced CEF-PSI, which, in turn, helped to dissipate excess photon energy by driving NPQ of Chi fluorescence.