Plasticity in roles of cyclic electron flow around photosystem I at contrasting temperatures in the chilling-sensitive plant Calotropis gigantea

Cyclic electron flow (CEF) around photosystem I is thought to balance the ATP/NADPH energy budget and protect photosynthetic apparatus. However, the plasticity in roles of CEF at contrasting temperatures is not well understood. We examined photosynthetic electron flow, non-photochemical quenching (NPQ), PSI redox state, and electrochromic shift (ECS) signal at 25 °C (normal temperature) and 4 °C (low temperature) for leaves of a chilling-sensitive tree species Calotropis gigantea. We found that electron flow through PSII (ETRII) was largely depressed at 4 °C irrespective of light condition, whereas CEF was enhanced under light intensities below 500 μmol photons m−2 s−1 but suppressed under light intensities above 760 μmol photons m−2 s−1. Meanwhile, both NPQ and PSI donor side limitation [Y(ND)] were enhanced at 4 °C. Moreover, the relationships between the rate of CEF and values for NPQ and Y(ND) changed between 4 °C and 25 °C. At low temperature, the proton gradient across thylakoid membranes (ΔpH) was enhanced and the thylakoid proton conductivity was depressed. Based on these results, we propose that, at normal growth temperature such as 25 °C, CEF contributes mainly to the balancing of ATP/NADPH energy budget required by primary metabolism. In contrast, at low temperature such as 4 °C, the main function of the CEF-dependent proton motive force is lumenal acidification, favoring photoprotection for photosynthetic apparatus. The plasticity in roles of CEF at contrasting temperatures may be regulated by the activity of thylakoid ATP synthase.