In vivo regulation of proton motive force during photosynthetic induction

Thylakoid proton motive force (pmf) plays a central role in photosynthesis. However, the regulation of pmf during photosynthetic induction is not well known. In the present study, in order to investigate the importance of cyclic electron flow (CEF) and chloroplastic ATP synthase in pmf formation during photosynthetic induction, we examined chlorophyll fluorescence, P700 signal, electrochromic shift signal for leaves of a low-light species Panax notoginseng and a high-light species Bletilla striata. During the initial induction phase at both low and high light, the activation of CEF associating with low proton conductivity of the chloroplastic ATP synthase (gH+) resulted in the rapid buildup of proton motive force (pmf) in both species. Furthermore, during photosynthetic induction at high light, the increase in gH+ in B. striata was accompanied with increases in linear electron flow (LEF) and CEF, leading to a stable pmf. Under high light, the high-light species B. striata, with much higher LEF and CEF than P. notoginseng, showed significantly lower pmf owing to the higher gH+. This result suggested that, under high light, CEF mainly contributed to ATP synthase in B. striata but favored lumenal acidification in P. notoginseng. Taking together, during photosynthetic induction, the coordination between CEF and the activity of chloroplastic ATP synthase finely regulated the pmf levels, optimizing photosynthesis and photoprotection.