Photosynthetic characteristics in rice (Oryza sativa L.) leaves were examined after treatment with low temperature (15 deg C) and high irradiance (1,500 mu mol quanta m(-2) s(-1)). Decreases in quantum efficiencies in PSII (phi PSII) and PSI (phi PSI) and in the rate of CO2 assimilation were observed with a decrease in the maximal quantum efficiency of PSII (Fv/Fm) by simultaneous measurements of Chl fluorescence, P700(+) absorbance and gas exchange. The decreases in phi PSII were most highly correlated with those in CO2 assimilation. Although the initial (the activity immediately measured upon extraction) and total (the activity following pre-incubation with CO2 and Mg(2+)) activities of ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) decreased slightly, the maximal activity (the activity following treatment with SO4(2-)) of Rubisco remained almost constant. These results indicate that the decrease in CO2 assimilation rate with the decreasing Fv/Fm was not caused by a decrease in Rubisco activity but rather by a decrease in RuBP regeneration capacity which resulted from the decrease in the rate of the linear electron transport. On the other hand, the decrease in (phi PSI was very small and the ratio of phi PSI to phi PSII increased. The de-epoxidation state of xanthophyll cycle pigments also increased. Thus, the cyclic electron transport around PSI occurred in photoinhibited leaves.