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

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 assimilation is restricted. However, how CAM plants protect PSI and PSII when CO assimilation is restricted is largely known. In the present study, we measured PSI, PSII, and electrochromic shift signals in the CAM plant . The quantum yields of PSI and PSII photochemistry largely decreased in the afternoon compared to in the morning, indicating that CO 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 () largely decreased in the afternoon. At a given proton flux, the decreasing increased ∆pH and thus contributed to the enhancement of NPQ and Y(ND). Therefore, in the CAM plant , the ∆pH-dependent photoprotective mechanism is mainly regulated by the regulation of rather than CEF when CO assimilation is restricted.