The effctts of osmotic dehydration in wheat leaves (Triticum aestivum L.cv. Long-chun No.10) on the photochemical function and protein metabolism of PSII were studied with isolated thylakoid and PSIImembranes. The results indicated tha PSIIwas rather resistant to water stress as mild water deficit in leaves did not sigificantly affect its activity. However, extreme stress conditions such as 40% decrease in relative water content (RWC) or 1.8Mpa in water potential (Ψw) caused ca 50% reduction in O2 evolution and ca 25% inhibtion of DCIP (2.6-dichloophenol indophenol) photore-duction of PSII.In addition. it was found that the inhibited DCIP photreduction of PSII, suggesting that water stress did not affect electron donation to PSII. Urea-SDS-PAGE and westen blot analysis showed that the steady state levls of major PSII proteins, including the DI and D2 proteins in the PSII reaction center, declined on a chlorophyll basis with increasing water stress, possibly as a result of increased degra-chlorophyll bisis with increasing water stress, possibyl as a result of increased degra-duction. In vitro translation experiments and quantitative analysis of chlorplast RNAs indicated that the potntial synthesis of chloroplast proteins from their mRNAs was impaired by water stress. From the results it is concluded that the effects of water stress on PSII protein metabolism, especially on the reation center protenins, may account for the damage to PSII photochemistry.