Abstract:Water stress inhibited the photosynthetic O2 evolution rate of wheat leaves. It was shown that water stress decreased the electron transport rate, the activities of photophosphorylation and, coupling factor, and, the synthesis of ATP in chloroplasts. PS Ⅱ electron transport was more senstitive to water stress than PS Ⅰ. The reduction in photophosphorylation activity might be the results of reduction in electron transport rate and coupling factor activity, as well as the uncoupling effect of water stress on chloroplasts. The uncoupling effect could be due to the inhibition of light induced proton translocation in chloroplasts.