| In order to reveal the physiological mechanism of the reduction of photosynthesis under water stress, the change of photosynthetic and chlorophyll fluorescence characteristics and key enzymes activities of photosynthetic apparatus after 150 min 30% PEG-6000 stress were investigated using rice seedling under hydroponic culture. The result indicated that stomatal conductance decreased drastically, while intercellular CO2 concentration, net photosynthetic rate, transpiration rate, and chlorophyll fluorescence parameters decreased gradually by 0～60 min of stress. During 60～120 min of stress, stomatal conductance decreased slowly, RuBP carboxylase activity began to rapidly decreased, intercellular CO2 concentration began to rise, non-photochemical quenching continued rise, other photosynthetic fluorescence parameters continued to drop. After 120 min of stress, non-photochemical quenching began to decline, RuBP carboxylase activity and intercellular CO2 concentration showed a trend of declining and increasing respectively, while other photosynthetic fluorescence parameters declined. These results suggest that photosynthesis during 0～60 min of stress and 60～120 min of stress, and after 120 min of stress were regulated by stomatal limitation, combination of stomatal limitation and non-stomatal limitation, and non-stomatal limitation, respectively. |