Abstract:This study aims to elucidate the physiological response of apple (Malus domestica) cultivars Fuji and Gala to exogenous salicylic acid (SA). The effect of exogenous SA application on malondiadehyde (MDA) contents, defensive enzymes activities and pathogenesis-related (PR) gene expression were determined in Fuji and Gala leaves by spraying method. The results showed that 0.2 mmol·L-1 SA treatment did not significantly affect the MDA contents in Fuji and Gala leaves when compared with the control. However, the activities of catalase (CAT), polyphenol oxidase (PPO), peroxidase (POD) and phenylalanine ammonialyase (PAL) were all significantly promoted in apple leaves after SA treatment. In Fuji and Gala leaves, the maximum activities of four defensive enzymes were 1.31 to 2.06 and 1.16 to 3.13 times more than controls, respectively. In addition, the expression of PR5 and PR8 was significantly up-regulated in SA treated apple leaves. Gene expression was highest at three days post SA treatment in Fuji leaves, while reached the peaks at one day after treatment in Gala leaves. The results in this study indicated that exogenous SA could increase defensive enzymes activities and induce PR genes expression, whereas, the levels and maintaining times of defensive enzymes activities as well as responses of PR genes were different in Fuji and Gala leaves, which provides a theoretical basis for using plant induced resistance to control apple leaf diseases.