Photosynthesis under fluctuating light in the CAM plant Vanilla planifolia

Photosynthetic induction after a sudden increase in illumination affects carbon gain. Photosynthetic dynamics under fluctuating light (FL) have been widely investigated in C3 and C4 plants but are little known in CAM plants. In our present study, the chlorophyll fluorescence, P700 redox state and electrochromic shift signals were measured to examine photosynthetic characteristics under FL in the CAM orchid Vanilla planifolia. The light use efficiency was maximized in the morning but was restricted in the afternoon, indicating that the pool of malic acid dried down in the afternoon. During photosynthetic induction in the morning, electron flow through photosystem I rapidly reached the 95% of the maximum value in 4-6 min, indicating that V. planifolia showed a fast photosynthetic induction when compared with C3 and C4 plants reported previously. Upon a sudden transition from dark to actinic light, a rapid re-oxidation of P700 was observed in V. planifolia, indicating the fast outflow of electrons from PSI to alternative electron acceptors, which was attributed to the O₂ photo-reduction mediated by water-water cycle. The functioning of water-water cycle prevented photosystem I over-reduction after transitioning from low to high light and thus protected PSI under FL. In the afternoon, cyclic electron flow was stimulated under FL to fine-tune photosynthetic apparatus when photosynthetic CO₂ was restricted. Therefore, water-water cycle cooperates with cyclic electron flow to regulate the photosynthesis under FL in the CAM orchid V. planifolia.