Abstract
When plants are exposed to light intensities in excess of those utilized in photosynthesis, NPQ is induced as a mechanism for protection of PSII. In contrast, increasing evidence has been reported on the enhancement of cyclic electron flow around PSI (CEF1) under excess light conditions, raising a new question as to how the over-acidification of thylakoid lumen avoided under prolonged stress conditions. We found that the ε subunit of the chloroplast ATP synthase complex was specifically decomposed in the leaves of wild watermelon subjected to drought/excess light stresses, suggesting the enhanced leakage of protons from thylakoid lumen to stroma. The proton leakage triggered by the decomposition of the ε subunit may therefore serve as a mechanism for PSI over-excitation quenching, i.e., the avoidance of hyper-accumulation of protons translocated by CEF1, enabling the continuous CEF1 and energy dissipation under excess light conditions.
