The decline in photosynthetic rate upon transfer from high to low light is linked to the slow kinetics of chloroplast ATP synthase in Bletilla striata.

Upon a sudden transition from high to low light, the rate of CO assimilation (A) in some plants first decreases to a low level before gradually becoming stable. However, the underlying mechanisms remain controversial. The activity of chloroplast ATP synthase (g) is usually depressed under high light when compared with low light. Therefore, we hypothesize that upon a sudden transfer from high to low light, the relatively low g restricts ATP synthesis and thus causes a reduction in A. To test this hypothesis, we measured gas exchange, chlorophyll fluorescence, P700 redox state, and electrochromic shift signals in Bletilla striata (Orchidaceae). After the transition from saturating to lower irradiance, A and ETRII decreased first to a low level and then gradually increased to a stable value. Within the first seconds after transfer from high to low light, g was maintained at low levels. During further exposure to low light, g gradually increased to a stable value. Interestingly, a tight positive relationship was found between g and ETRII. These results suggested that upon a sudden transition from high to low light, A was restricted by g at the step of ATP synthesis. Taken together, we propose that the decline in A upon sudden transfer from high to low light is linked to the slow kinetics of chloroplast ATP synthase.