Exogenous cinnamic acid regulates antioxidant enzyme activity and reduces lipid peroxidation in drought-stressed cucumber leaves

To investigate the physiological mechanisms of drought stress mitigated by exogenous cinnamic acid (CA), cucumber seedlings were pretreated with 50 μM CA for 2 days and then were subjected to 10% polyethylene glycol (PEG) 6000. We examined if CA protects plants from PEG-induced drought stress, and whether the protective effect is related to antioxidant and lipid-peroxidation regulation. 2 days of CA application enhanced the activities of guaiacol peroxidase (GPX), glutathione peroxidase (GSH-Px), dehydroascorbate reductase (DHAR) and glutathione reductase (GR) and increased the levels of ascorbate, proline, soluble sugar, vanillic acid (VA) and CA in leaves. After CA-pretreated leaves were exposed to drought, the activities of superoxide dismutase (SOD), catalase, ascorbate peroxidase, monodehydroascorbate reductase, GPX, GSH-Px, DHAR and GR were higher than under drought stress alone, while the levels of reduced glutathione, ascorbate, proline, soluble sugar, VA and CA in leaves were more. The combination of exogenous CA and drought led to higher transcript levels of Cu/Zn-SOD and Mn-SOD genes and decreased contents of malonaldehyde and hydrogen peroxide, but drought had adverse effects on them. Furthermore, the combined effects of exogenous CA and drought made 61.67% leaf edges dried, while drought resulted in 95.83% withered leaves. We conclude CA pretreatment leads to higher contents of CA and VA in drought-stressed leaves and thereby results in higher antioxidant activities directly or indirectly via proline and soluble sugar, thus increasing drought tolerance of cucumber. We also propose soluble sugar can reduce reactive oxygen species and decrease lipid peroxidation when exogenous CA mitigates drought stress.