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Changes in chlorophyll fluorescence, P700⁺-absorbance and gas exchange during the induction phase and steady state of photosynthesis were simultaneously examined in rice (Oryza sativa L.), including the rbcS antisense plants. The quantum yield of photosystem II (ΦPSII) increased more rapidly than CO₂ assimilation in 20% O₂. This rapid increase in ΦPSII resulted from the electron flux through the water–water cycle (WWC) because of its dependency on O₂. The electron flux of WWC reached a maximum just after illumination, and rapidly generated non-photochemical quenching (NPQ). With increasing CO₂ assimilation, the electron flux of WWC and NPQ decreased. In 2% O₂, WWC scarcely operated and ΦPSI was always higher than ΦPSII. This suggested that cyclic electron flow around PSI resulted in the formation of NPQ, which remained at higher levels in 2% O₂. The electron flux of WWC in the …