| Summary: | In view of the current and expected future rise in atmospheric CO<sub>2</sub> concentrations, we examined the effect of elevated CO<sub>2</sub> on photoinhibition of photosystem I (PSI) under fluctuating light in <i>Arabidopsis thaliana</i>. At 400 ppm CO<sub>2</sub>, PSI showed a transient over-reduction within the first 30 s after transition from dark to actinic light. Under the same CO<sub>2</sub> conditions, PSI was highly reduced after a transition from low to high light for 20 s. However, such PSI over-reduction greatly decreased when measured in 800 ppm CO<sub>2</sub>, indicating that elevated atmospheric CO<sub>2</sub> facilitates the rapid oxidation of PSI under fluctuating light. Furthermore, after fluctuating light treatment, residual PSI activity was significantly higher in 800 ppm CO<sub>2</sub> than in 400 ppm CO<sub>2</sub>, suggesting that elevated atmospheric CO<sub>2</sub> mitigates PSI photoinhibition under fluctuating light. We further demonstrate that elevated CO<sub>2</sub> does not affect PSI activity under fluctuating light via changes in non-photochemical quenching or cyclic electron transport, but rather from a rapid electron sink driven by CO<sub>2</sub> fixation. Therefore, elevated CO<sub>2</sub> mitigates PSI photoinhibition under fluctuating light at the acceptor rather than the donor side. Taken together, these observations indicate that elevated atmospheric CO<sub>2</sub> can have large effects on thylakoid reactions under fluctuating light.
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