Balancing light use efficiency and photoprotection in tobacco leaves grown at different light regimes

In this study, we have compared photosynthetic regulation in tobacco leaves (Nicotiana tabacum L.) grown under high light (HL) and low light (LL) by measuring chlorophyll fluorescence, P700 redox state and electrochromic shift signals. Under high light, HL-plants had much higher linear electron flow and cyclic electron flow (CEF) around photosystem I (PSI) than LL-plants. Meanwhile, HL-plants showed significantly lower proton gradient (Delta pH) across the thylakoid membranes, owing to the increased activity of chloroplast ATP synthase. The relationships between Delta pH, non-photochemical quenching (NPQ) and PSI donor side limitation [Y(ND)] differed between HL- and LL-plants. At a given Delta pH, HL-plants displayed higher NPQ and Y(ND). Furthermore, at a given CEF, LL-plants showed higher Delta pH and Y(ND). These results indicate that HL-plants down-regulate Delta pH to increase light use efficiency by enhancing the activity of chloroplast ATP synthase. In contrast, LL-plants decrease the activity of chloroplast ATP synthase to up-regulate Delta pH and to favor photoprotection. Our findings suggest that the coordination of CEF and chloroplast ATP synthase balances light use efficiency and photoprotection at different growth light conditions.