The water-water cycle facilitates photosynthetic regulation under fluctuating light in the epiphytic orchid Dendrobium officinale

Fluctuating light can cause selective photoinhibition of photosystem I (PSI) in angiosperms. However, the strategies for photosynthetic regulation under fluctuating light in wild epiphytic plants are little known. We here compared PSI and PSII performances under fluctuating light in the epiphytic orchid Dendrobium officinale and the terrestrial orchid Bletilla striata. Upon dark-to-light transition, D. officinale showed a rapid re-oxidation of P700, but this rapid re-oxidation of P700 was not observed in B. striata. Within the first 40 s after transition from dark to actinic light, B. striata showed much higher activation of cyclic electron transport (CET) than D. officinale. However, this strong activation of CET could not prevent the over-reduction of PSI. Therefore, the rapid re-oxidation of P700 in D. officinale was attributed to pseudo-cyclic electron transport (water-water cycle). Within the first 60 s after transition from low to high light, PSI was highly oxidized in D. officinale but was highly reduced in B. striata. Concomitantly, B. striata showed much higher stimulation of CET. These results indicate that water-water cycle significantly regulates PSI redox state under fluctuating light in the epiphytic orchid D. officinale. By comparison, the terrestrial orchid B. striata employs CET to cope with fluctuating light, similar to the phenotype of Arabidopsis thaliana. Taking together, these two orchids used different strategies for photosynthetic regulation to cope with fluctuating light.