Abstract: To investigate the effects of exogenous cinnamic acid on reactive oxyen metabolism in cucumber leaves under low light stess ， cucumber seedlings（ Cucumis sativus cv. Jinchun No. 4）were pretreated with 50 μ mol cinnamic acid for 2 d and then were cultivated at normal light intensity of 500 μmol/（m·s）and low light intensity of 80 μmol/（m·s）for 6 d. We investigated if cinnamic acid could protect cucumber plantlets under low light stress，and examined whether the protective effect related to antioxidant enzymes and lipid-peroxidation regulation or not. The results showed that at 2 d，exogenous cinnamic acid decreased levels of malonaldehyde（MDA），superoxide radical（O₂^.-）and hydrogen peroxide（H₂O₂），elevated the activities of antioxidant enzymes superoxide dismutase（SOD），glutathione peroxidase（GSH- Px）and ascorbate peroxidase（APX）in cucumber leaves. At 8 d，the combination of cinnamic acid pretreatment and low light decreased the contents of MDA，H₂O₂ and O₂^.-，enhanced the activities of SOD，catalase（CAT），APX and GSH-Px compared with non-pretreated plants. The exogenous cinnamic acid is able to moderate antioxidant enzyme activities and decrease lipid peroxidation in certain extent，it reduces stress intensity at low light．

Key words: cucumber, tolerance to low light, cinnamic acid, physiological indexes