Abstract: Water stress, induced by adding polyethyleneglycol (PEG) 6000, reduced the rate of CO2-dependent O2 evolution.In the Presence of uncoupling reagents water stress had little effect on the photosynthetic electron transport in mesophyll cells of Iwpoea batatas, but in the absence of uncoupling reagents it promoted the electron transport and displayed pronounced uncoupling effect. Water stress iujured the structure of chloroplast coupling factor, inhibited ATP synthesis, deplayed the photosynthetic lag phase of mesophyll cells of Impoea batatas.