Regulation of Chloroplast ATP Synthase Modulates Photoprotection in the CAM Plant Vanilla planifolia .

Generally, regulation of cyclic electron flow (CEF) and chloroplast ATP synthase play key roles in photoprotection for photosystems I and II (PSI and PSII) in C3 and C4 plants, especially when CO 2 assimilation is restricted. However, how CAM plants protect PSI and PSII when CO 2 assimilation is restricted is largely known. In the present study, we measured PSI, PSII, and electrochromic shift signals in the CAM plant Vanilla planifolia . The quantum yields of PSI and PSII photochemistry largely decreased in the afternoon compared to in the morning, indicating that CO 2 assimilation was strongly restricted in the afternoon. Meanwhile, non-photochemical quenching (NPQ) in PSII and the donor side limitation of PSI (Y(ND)) significantly increased to protect PSI and PSII. Under such conditions, proton gradient (∆pH) across the thylakoid membranes largely increased and CEF was slightly stimulated, indicating that the increased ∆pH was not caused by the regulation of CEF. In contrast, the activity of chloroplast ATP synthase ( g H + ) largely decreased in the afternoon. At a given proton flux, the decreasing g H + increased ∆pH and thus contributed to the enhancement of NPQ and Y(ND). Therefore, in the CAM plant V. planifolia , the ∆pH-dependent photoprotective mechanism is mainly regulated by the regulation of g H + rather than CEF when CO 2 assimilation is restricted.