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 2 assimilation ( A N ) 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 H + ) 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 H + restricts ATP synthesis and thus causes a reduction in A N . 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 N 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 H + was maintained at low levels. During further exposure to low light, g H + gradually increased to a stable value. Interestingly, a tight positive relationship was found between g H + and ETRII. These results suggested that upon a sudden transition from high to low light, A N was restricted by g H + at the step of ATP synthesis. Taken together, we propose that the decline in A N upon sudden transfer from high to low light is linked to the slow kinetics of chloroplast ATP synthase.