Different Strategies for Photosynthetic Regulation under Fluctuating Light in Two Sympatric Species.

Fluctuating light can cause selective photoinhibition of photosystem I (PSI) in angiosperms. Cyclic electron flow (CEF) around PSI and electron flux from water via the electron transport chain to oxygen (the water-water cycle) play important roles in coping with fluctuating light in angiosperms. However, it is unclear whether plant species in the same genus employ the same strategy to cope with fluctuating light. To answer this question, we measured P700 redox kinetics and chlorophyll fluorescence under fluctuating light in two () Pftzer (Orchidaceae) species, and . After transition from dark to high light, displayed a rapid re-oxidation of P700, while displayed an over-reduction of P700. Furthermore, the rapid re-oxidation of P700 in was not observed when measured under anaerobic conditions. These results indicated that photo-reduction of O mediated by the water-water cycle was functional in but not in . Within the first few seconds after an abrupt transition from low to high light, PSI was highly oxidized in but was highly reduced in , indicating that the different responses of PSI to fluctuating light between and was attributed to the water-water cycle. In , the lack of the water-water cycle was partially compensated for by an enhancement of CEF. Taken together, and employed different strategies to cope with the abrupt change of light intensity, indicating the diversity of strategies for photosynthetic acclimation to fluctuating light in these two closely related orchid species.