Photoinhibition of photosystem I under fluctuating light is linked to the insufficient ΔpH upon a sudden transition from low to high light

Despite photosystem I (PSI) is susceptible to photoinhibition under fluctuating light in wild-type angiosperms, the underlying mechanism is not well known. Because proton gradient (Delta pH) across the thylakoid membranes plays a crucial role in protecting PSI, we hypothesized that PSI photoinhibition under fluctuating light may be linked to the formation of Delta pH. To test this hypothesis, we measured chlorophyll fluorescence, PSI redox state, and the electrochromic shift signal during transition from low to high light in two angiosperms Bletilla striate (Orchidaceae) and Arabidopsis thaliana. The measurement of fast P700 redox kinetics indicated that the fast re oxidation of P700 mediated by photo-reduction of O-2 was absent in B. striate. Furthermore, the redox state of PSI is highly determined by Delta pH in both species. For the first 20 s after transition from low to high light, both species could not build up a sufficient Delta pH, which was accompanied by the over-reduction of P700. During prolonged illumination at high light, the sufficient Delta pH made PSI to be highly oxidized. These results demonstrated that PSI photoinhibition under fluctuating light in wild-type angiosperms occurred mainly at the first 20s after an increase in light intensity, which was caused by the insufficient Delta pH.