Abstract: With root injection and foliar spray, this paper studied the effects of different concentrations salicylic acid, brassinolide, chitosan and spermidine on the growth, morphogenesis, and physiological and biochemical characters of cucumber (Cucumis sativus L.) seedlings under 200 mmol·L^- NaCl stress. The results showed that at proper concentrations, these four exogenous chemical activators could markedly decrease the salt stress index and mortality of cucumber seedings, and the decrement induced by 0.01 mg·L^-1 brassinolide was the largest, being 63.0% and 75.0%, respectively. The activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) increased significantly, resulting in a marked decrease of malondialdehyde (MDA) content and electrolyte leakage.The dry weight water content and morphogenesis of cucumber seedlings improved, and the stem diameter, leaf number, and healthy index increased significantly. All of these suggested that exogenous chemical activators at proper concentrations could induce the salt resistance of cucumber, and mitigate the damage degree of salt stress. The salt resistance effect of test exogenous chemical activators decreased in the sequence of 0.005～0.05 mg·L^-1 brassinolide, 150～250 mg·L^-1 spermidine, 100～200 mg·L^-1 chitosan, and 50～150 mg·L^-1 salicylic acid.

Key words: Karst forest, Human disturbance, Degradation evaluation, Guizhou