Photosystem I (I) photoinhibition suppresses plant photosynthesis and growth. However, the mechanism underlying I photoinhibition has not been fully clarified. In this study, in order to investigate the mechanism of I photoinhibition in higher plants, we applied repetitive short-pulse () illumination, which causes I-specific photoinhibition in chloroplasts isolated from spinach leaves. We found that treatment caused I photoinhibition, but not II photoinhibition in isolated chloroplasts in the presence of O2. However, chloroplastic superoxide dismutase and ascorbate peroxidase activities failed to protect I from its photoinhibition. Importantly, I photoinhibition was largely alleviated in the presence of methyl viologen, which stimulates the production of reactive oxygen species () at the stromal region by accepting electrons from I, even under the conditions where CuZn-superoxide dismutase and ascorbate peroxidase activities were inactivated by KCN. These results suggest that the production site, but not the production rate, is critical for I photoinhibition. Furthermore, we found that not only superoxide (O2−) but also singlet oxygen (1O2) is involved in I photoinhibition induced by href="#def7" rid="def7" class=" def">rSP treatment. From these results, we suggest that href="#def5" rid="def5" class=" def">PSI photoinhibition is caused by both O2− and 1O2 produced within the thylakoid membranes when electron carriers in href="#def5" rid="def5" class=" def">PSI become highly reduced. Here, we show, to our knowledge, new insight into the href="#def5" rid="def5" class=" def">PSI photoinhibition in higher plants.
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