PSI photoinhibition and protection at low temperatures and in fluctuating light: An 30 ecophysiological vista
In green leaves in vivo, photosystem I (PSI) was thought to be resistant to photoinhibition. In 1994, it was found that, in chilling-sensitive plants such as cucumber (Cucumis sativus L.), exposing leaves to moderate constant light at temperatures below 10 degrees C induces PSI photoinhibition with little effect on PSII. Since the beginning of this century, PSI photoinhibition induced by fluctuating light (alternating high and low light, FL) or repetitive saturating pulses (rSPs) has been reported in many species. In this review, we compare the causes and consequences of PSI photoinhibition in cucumber at chilling temperatures with those of PSI photoinhibition induced by FL or rSPs. Changes in chloroplasts during these photoinhibitory treatments are followed, focusing on where and how reactive oxygen species are produced and which parts of PSI are damaged. We also discuss the roles of two pathways of cyclic electron flow around PSI, the NDH and PGR5/PGRL1 pathways, particularly in suppression of PSI photoinhibition. Lastly, we discuss problems with methods for measuring PSI parameters and future research directions from an ecophysiological viewpoint.