Abstract: 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.

Key words: Ficus concinna, High-temperature stress, Chlorophyll fluorescence, Photosynthesis, Cyclic electron transport around photosystem I, Dissipation of excitation energy

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