Cyclic electron flow around photosystem I is required for adaptation to high temperature in a subtropical forest tree, Ficus concinna *

Dissipation mechanisms of excess photon energy under high-temperature stress were studied in a subtropical forest tree seedling, Ficus concinna. Net CO2 assimilation rate decreased to 16% of the control after 20 d high-temperature stress, and thus the absorption of photon energy exceeded the energy required for CO2 assimilation. The efficiency of excitation energy capture by open photosystem II (PSII) reaction centres (F v′/F m′) at moderate irradiance, photochemical quenching (q P), and the quantum yield of PSII electron transport (Φ PSII) were significantly lower after high-temperature stress. Nevertheless, non-photochemical quenching (q NP) and energy-dependent quenching (q E) were significantly higher under such conditions. The post-irradiation transient of chlorophyll (Chl) fluorescence significantly increased after the turnoff of the actinic light (AL), and this increase was considerably higher in the 39 °C-grown seedlings than in the 30 °C-grown ones. The increased post-irradiation fluorescence points to enhanced cyclic electron transport around PSI under high growth temperature conditions, thus helping to dissipate excess photon energy non-radiatively