Chloroplastic ATP synthase optimizes the trade-off between photosynthetic CO2 assimilation and photoprotection during leaf maturation

In the present study, we studied the role of chloroplastic ATP synthase in photosynthetic regulation during leaf maturation. We measured gas exchange, chlorophyll fluorescence, P700 redox state, and the electrochromic shift signal in mature and immature leaves. Under high light, the immature leaves displayed high levels of non-photochemical quenching (NPQ) and P700 oxidation ratio, and higher values for proton motive force (pmf) and proton gradient (Delta pH) across the thylakoid membranes but lower values for the activity of chloroplastic ATP synthase (g(H)(+)) than the mature leaves. Furthermore, g(H)(+) was significantly and positively correlated with CO2 assimilation rate and linear electron flow (LEF), but negatively correlated with pmf and Delta pH. Delta pH was significantly correlated with LEF and the P700 oxidation ratio. These results indicated that g(H)(+) was regulated to match photosynthetic capacity during leaf maturation, and the formation of pmf and Delta pH was predominantly regulated by the alterations in g(H)(+). In the immature leaves, the high steady-state Delta pH increased lumen acidification, which, in turn, stimulated photoprotection for the photosynthetic apparatus via NPQ induction and photosynthetic control. Our results highlighted the importance of chloroplastic ATP synthase in optimizing the trade-off between CO2 assimilation and photoprotection during leaf maturation.