The effects of exogenous spermidine(Spd) on lipid peroxidation,relative plasma membrane permeability, photosystemⅡ(PSⅡ) gene expression and PSIl photochemical activity in water-stressed wheat seedlings were investigated.The decrease in relative water content(RWC),ChI content,and 2,6-dichlorophenol indophenol (DCIP) photoreduction of PSIl,and increases in electrolyte leakage of plasma membranes and malonyldialdehyde(MDA) in water-stressed leaves was alleviated by Spd pretreatment.Furthermore,Western and Northern blot analysis showed that decreases in the PSIl major proteins D1,D2 and LHCⅡand the transcripts of corresponding genes psbA,psbD and cab were also alleviated by Spd pretreatment under water stress.These results suggest that the application of exogenous Spd protects PSⅡagainst water stress at both the transcriptional level and the translational level,and allows PSⅡto retain a higher activity level during water stress.The protective role of Spd in the photosynthetic apparatus also is discussed. The effects of exogenous spermidine (Spd) on lipid peroxidation, relative plasma membrane permeability, photosystem II (PSII) gene expression and PSIl photochemical activity in water-stressed wheat seedlings were investigated. The decrease in relative water content (RWC), ChI content, and 2,6-dichlorophenol indophenol (DCIP) photoreduction of PSI1, and increases in electrolyte leakage of plasma membranes and malonyldialdehyde (MDA) in water-stressed leaves was alleviated by Spd pretreatment. Further, Western and Northern blot analysis showed that decreases in the PSI1 major proteins D1, D2 and LHC Ⅱ and the transcripts of corresponding genes psbA, psbD and cab were also alleviated by Spd pretreatment under water stress. These results suggest that the application of exogenous Spd protects PSⅡagainst water stress at both the transcriptional level and translational level, and allows PSIIto retain a higher activity level during water stress. protective role of Spd in the photosynthetic app aratus also is discussed.