PSI photoinhibition is more related to electron transfer from PSII to PSI rather than PSI redox state in Psychotria rubra

Although it has been believed that wild-type plants are capable of protecting photosystem I (PSI) under high light, our previous study indicates that PSI is sensitive to high light in the shade-established tree species Psychotria rubra. However, the underlying physiological mechanisms are unclear. In this study, we examined the roles of electron transfer from PSII to PSI and PSI redox state in PSI photoinhibition in P. rubra by treatments with lincomycin (Lin), diuron (DCMU), and methyl viologen (MV). After exposure to 2000 mu mol photons m(-2) s(-1) for 2 h, PSI activity decreased by 35, 29, 3, and 49 % in samples treated with H2O, Lin, DCMU, and MV, respectively. Meanwhile, the MV-treated samples showed higher P700 oxidation ratio than the H2O-treated samples, suggesting the PSI photoinhibition under high light was accompanied by high levels of P700 oxidation ratio. PSI photoinhibition was alleviated in the DCMU-treated samples but was accelerated in the MV-treated samples, suggesting that PSI photoinhibition in P. rubra was mainly controlled by electron transfer from PSII to PSI. Taking together, PSI photoinhibition is more related to electron transfer from PSII to PSI rather than PSI redox state in P. rubra, which is different from the mechanisms of PSI photoinhibition in Arabidopsis thaliana and cucumber.