Photosystem I photoinhibition induced by fluctuating light depends on background low light irradiance

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Fluctuating light (FL) can induce the preferential photoinhibition of photosystem I (PSI) in angiosperms. However, the underlying mechanisms have not been fully clarified. In this study, we examined the effects of low-light phases on FL-induced photoinhibition of PSI in Bletilla striata. We found that PSI was highly over-reduced within the first 20 s after transition from 59 to 1455 μmol photons m−2 s−1. However, such severe over-reduction of PSI was not observed in FL transition from 132 or 272–1455 μmol photons m−2 s−1. As a result, FL (59–1455) induced stronger PSI photoinhibition than FL (132–1455) and FL (272–1455). During low-light phase, a higher light intensity induced a higher proton gradient (ΔpH) across the thylakoid membranes, favoring the rapid formation of ΔpH after light increased and preventing an over-reduction of PSI. These results indicate that the low-light phase significantly affects the extent of PSI photoinhibition in FL. After transition from 59 or 132–1455 μmol photons m−2 s−1, CEF activity first increased and then decreased to the steady-state. However, such transient CEF stimulation was not observed during the transition from 272–1455 μmol photons m−2 s−1. Furthermore, the CEF activation in the high-light phase was highly dependent on the PSI reduction state. We conclude that CEF activation is finely regulated to optimize the trade-off between photoprotection and light use efficiency in FL.