Increasing cyclic electron flow mediated by NDH is related to heat tolerance under low light in grape leaves

Examination of the effects of high temperature (42 °C) on the photoinhibition of photosystem II (PSII) in grape leaves revealed that the extent of photoinhibition of PSII was lower in the light (200 μmol m^-2 s^-1) than in the dark. Heat stress in the dark induced severe injury in the grapevines, as determined by the critical temperature (Tc). The maximal efficiency of PSII photochemistry (Fv/Fm) decreased significantly in the dark, but it decreased much less in the light. In addition, there was a lower level of degradation of the D1 protein in the light than in the dark. Furthermore, the NAD(P)H dehydrogenase (NDH)-dependent cyclic electron flow (CEF) was remarkably enhanced in the light, but it was suppressed in the dark. The half-time of P700⁺ re-reduction (t1/2) was reduced moer in in the light than in the dark during heat stress. Compared to the control leaves, the antimycin A (AA)-treated leaves showed much less of a decrease in Fv/Fm in the light than in the dark during heat stress; however, this increase seemed to disappear in methyl viologen (MV)-treated leaves. Based on these results, we propose a significant physiological function of the NDH-dependent CEF pathway under low light is the protection of PSII against heat-induced photoinhibition.