Caffeic acid protects cucumber against chilling stress by regulating antioxidant enzyme activity and proline and soluble sugar contents

To investigate the physiological mechanism underlying chilling stress mitigation by exogenous caffeic acid (CA), we pretreated Cucumis sativus cv. Jinchun no. 4 seedlings with CA for 2 days, followed by exposure to normal (25/18 °C) or cold (15/8 °C) temperatures for 1 day. We chose 25 μM as the optimum CA concentration, since it produced lower levels of superoxide anion radical, hydrogen peroxide and malondialdehyde in chilling-stressed leaves than other concentrations of CA. Chilling treatment caused 50 % of the second leaves be withered, reduced the relative water content in the leaves and inhibited plant growth. Pretreatment with 25 μM CA alleviated the damaging effects of chilling. When the CA-pretreated seedlings were exposed to chilling, the superoxide dismutase, guaiacol peroxidase, catalase glutathione peroxidase, monodehydroascorbate reductase, ascorbate peroxidase, glutathione reductase and dehydroascorbate reductase activities were higher than those produced by chilling treatment alone, which coincided with increased transcript levels of copper/zinc superoxide dismutase, glutathione peroxidase and manganese superoxide dismutase genes; these results are consistent with the increased contents of ascorbate and glutathione. The application of 25 μM CA also increased the contents of endogenous CA and ferulic acid, as well as proline and soluble sugars, in chilling-stressed leaves. Therefore, exogenous CA treatment increases endogenous CA levels, induces antioxidant enzyme activity and reduces the levels of reactive oxygen species under chilling stress, thus protecting cucumber from chilling stress. Soluble sugars and proline are involved in the CA-mitigated chilling stress response.