Abstract: O₂ plays an important role in photosynthesis and organic matter accumulation as a byproduct of oxygen-releasing photosynthesis and as a competitor to the carboxylation reaction of ribulose 1,5-diphosphate (RuBP). We used a LI-6400-40 photosynthetic analyzer to measure the fluorescence and gas exchange of soybean (Glycine max) leaves at 2%, 11%, 21% and 31% O₂ concentrations. A mechanistic model for the photosynthesis-light response was used to fit the light-response curve of the electron transport rate. The results showed that there were significant differences in the maximum electron transfer rate (J_max) of soybean leaves under different O₂ concentrations, with greater J_max values under higher O₂ concentrations. The J_max of soybean leaves at 31% O₂ concentration was 2.15 times higher than that at 2% O₂ concentration. The mechanism model was used to elucidate the underlying reason for the substantial impact of O₂ concentration on J_max, which can be attributed to the modulation of O₂ concentration on effective light energy cross-section of chlorophyll. Furthermore, the minimum average life-time of harvesting pigment molecules (τ_min) in soybean leaves at 31% O₂ concentration was far shorter than that at 2% O₂ concentration, which may lead to a higher exciton utilization efficiency (Φ) in the former. This study provides a theoretical basis and research tools for investigating the influences of O₂ concentrations on the electron transfer rate of soybean leaves and the physical characteristics of chlorophyll molecules.

Key words: Glycine max, eigen-absorption cross-section, effective light absorption cross-section, photosynthetic mechanistic model, electron transport rate