Abstract: This paper studied the effects of exogenous salicylic acid (0, 0.1, 0.5, 1.0, and 2.0 mmol·L^-1) on the physiological characteristics of longan (Dimocarpus longana ‘Fuyan’) seedlings under acid rain (pH 3.0) stress. Under the stress of the acid rain, the seedling’s leaf superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities as well as the leaf total chlorophyll, soluble protein, and soluble sugar contents decreased obviously, and the leaf malondialdedyde content had a remarkable increase, suggesting the toxic effect of the acid rain on the longan seedlings. Applying 0.1-1.0 mmol·L^-1 of exogenous salicylic acid improved the SOD, POD, and CAT activities and the total chlorophyll, soluble protein, and soluble sugar contents significantly, and decreased the malondialdedyde content. Low concentrations of exogenous salicylic acid reduced the oxidative damage caused by the acid rain stress, and 0.5 mmol·L^-1 of salicylic acid had the best effect. As compared with the acid rain stress alone, applying 0.5 mmol·L^-1 of salicylic acid improved the SOD, POD, and CAT activities by 144%, 440%, and 132%, and increased the total chlorophyll, soluble protein, and soluble sugar contents by 62%, 105%, and 177%, respectively, and decreased the malondialdedyde content by 35%. It was suggested that low concentrations of exogenous salicylic acid (0.1-1.0 mmol·L^-1) could alleviate the toxic effect of acid rain stress on longan seedlings via activating the seedlings leaf antioxidant enzyme activities and reducing oxidative stress, while high concentration of exogenous salicylic acid (2.0 mmol·L^-1) lowered the mitigation effect.

Key words: survival rate, growth, Sepia pharaonis, digestive enzyme., re-feeding, starvation