Abstract:The effects of exogenous spermidine (Spd) on lipid peroxidation, relative plasma membrane permeability, photosystem II (PSII) gene expression and PSII photochemical activity in water-stressed wheat seedlings were investigated. The decrease in relative water content (RWC), Chl content, and 2,6-dichlorophenol indophenol (DCIP) photoreduction of PSII, 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 PSII major proteins D1, D2 and LHCII 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 PSII against water stress at both the transcriptional level and the translational level, and allows PSII to retain a higher activity level during water stress. The protective role of Spd in the photosynthetic apparatus also is discussed.(Author for correspondence.Tel：028 8541 1175; E-mail:honghuilin@hotmail.com)