Abstract
Higher plant chloroplasts induce large energy dependent non-photochemical quenching (qE) after rapid light-to-dark transition for avoiding photodamage. In Chlamydomonas reinhardtii, however, state transitions are the dominant photoprotection mechanism instead of qE. The qE activity is far less than its maximum capacity observed under stress conditions such as high light and low CO2 concentrations. Chlamydomonas raudensis, which inherently lacks state transition ability, shows high qE as in higher plants, suggesting that the functions of state transitions could suppress qE activity. Dark adapted C. reinhardtii in State 2 shows large cyclic electron flow, though accumulation of proton gradient in the thylakoid lumen is slow. The small proton gradient can be explained by the large proton conductivity at the ATP synthase. We propose a new model of photoprotection, where the phosphorylation/dephosphorylation of the proteins in state transitions affect stromal inorganic phosphate concentration for regulating the ATP synthase conductivity.
