Abstract: The objective of the study is to investigate the effect of exogenous salicylic acid（SA）on superoxide dismutase（SOD）and polyphenol oxidases（PPO）isozymes changes and endogenous SA content in pear（Pyrus bretschneideri‘Yali’）leaves and to examine the response of NPR1 gene expression to SA treatment. The results showed that SA at 0.200 mmol · L^-1 increased content of total SA was increased in pear leaves. SA treatment increased the content of free states SA with the increase of SA concentration gradually，and the contents were significantly lower than that of control. The content of binding states SA was reduced with the increase of SA concentration gradually，but all treatments were significantly higher than that of control. The isozymes analysis results showed that expression of SOD and PPO isozymes were enhanced，but not new enzymes bands were detected in pear leaves. The experiment of pathogen inoculation suggested that SA at 0.200 mmol · L^-1 could induce the resistance of pear leaves to Physalospora piricola Nose significantly. SA treatment enhanced the activities of SOD and PPO，and the real-time fluorescent quantitative PCR analysis indicated that expression of NPR1 gene were enhanced in pear leaves while was inhibited in stem.

Key words: pear, salicylic acid, isozyme, NPR1 gene, Physalospora piricola Nose