Contribution of photosynthetic electron transport, heat dissipation, and recovery of photoinactivated photosystem II to photoprotection at different temperatures in Chenopodium album leaves

Temperature dependence of photoinhibition and photoprotective mechanisms (10-35°C) was investigated for Chenopodium album leaves grown at 25°C under 500 μmol quanta m^{-2 s-}1. The fraction of active photosystem II (PSII) was determined after photoinhibitory treatment at different temperatures in the presence and absence of lincomycin, an inhibitor of chloroplast-encoded protein synthesis. In the absence of lincomycin, leaves were more tolerant to photoinhibition at high (25-35°C) than at low (11-15°C) temperatures. In the presence of lincomycin, the variation in the tolerance to photoinactivation became relatively small. The rate constant of photoinactivation (k_pi) was stable at 25-35°C and increased by 50% with temperature decrease from 25 to 11°C. The rate constant of recovery of inactivated PSII (k_rec) was more sensitive to temperature; it was very low at 11°C and increased by an order of magnitude at 35°C. We conclude that the recovery of photoinactivated PSII plays an essential role in photoprotection at 11-35°C. Partitioning of light energy to various photoprotective mechanisms was further analyzed to reveal the factor responsible for k_pi. The fraction of energy utilized in photochemistry was lower at lower temperatures. Although the fraction of heat dissipation increased with decreasing temperatures, the excess energy that is neither utilized by photochemistry nor dissipated by heat dissipation was found to be greater at lower temperatures. The k_pi value was strongly correlated with the excess energy, suggesting that the excess energy determines the rate of photoinactivation.