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

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 mu mol photons m(-2) s(-1). However, such severe over-reduction of PSI was not observed in FL transition from 132 or 272-1455 mu mol 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 (Delta pH) across the thylakoid membranes, favoring the rapid formation of Delta pH 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 mu mol 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 mu mol 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.

Automated summary

This study revealed that low-light phases significantly affect the extent of PSI photoinhibition in fluctuating light conditions, with a higher light intensity inducing a higher proton gradient across thylakoid membranes. The activation of cyclic electron flow (CEF) is finely regulated to optimize the balance between photoprotection and light use efficiency in fluctuating light environments. Additionally, CEF activation in high-light phases is highly dependent on the PSI reduction state.