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 CO(2) 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 CO(2) 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 (Phi(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 degrees C-grown seedlings than in the 30 degrees 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.