Coronatine Enhances Chilling Tolerance in Cucumber (Cucumis sativus L.) Seedlings by Improving the Antioxidative Defence System

Coronatine (COR) is a new plant growth regulator that mimics the biological activities of methyl jasmonate. We determined whether COR enhanced chilling tolerance of cucumber (Cucumis sativus L. cv. Jinchun 4) seedlings and if such tolerance was correlated with changes in the activity of antioxidant enzymes. COR was applied to seedlings at two-leaf stage at 0 (Control), 0.1, 1, 10, and 100 nm. Seedlings were then subjected to chilling stress at 5 +/- 1 degrees C for 4 days. Seedlings treated with COR showed significant higher tolerance to chilling stress and the optimal concentration was 1-10 nm. Compared with control, the chilling injury index (CII) of the seedlings treated with COR at 1 and 10 nm was decreased by 44.9 % and 24.5 %, respectively, while the membrane chilling stability (MCS) expressed as the change of relative conductance was increased by 37.2 % and 17.0 %, respectively. The malondialdehyde (MDA) content in leaves treated with COR at 1 nm was decreased by 39.7 %, and the O-2(-) production rate and H2O2 content reduced by 28.6 % and 8.5 %, respectively. Treatment with COR at 1 nm increased the activities of superoxide dismutase (SOD) in leaves by 34.4 %, catalase (CAT) by 58.7 % and ascorbate peroxidase (APX) by 23.0 % under low temperature. The 1,1-diphenyl-2-picrylhydrazyl (DPPH) radical scavenging activities were also significantly improved by 28.9 % and 21.3 % following treatment with COR at 1 and 10 nm, respectively. The overall results suggest that COR enhanced chilling tolerance in cucumber seedlings by improving the antioxidative defence system.