Cinnamic acid pretreatment enhances heat tolerance of cucumber leaves through modulating antioxidant enzyme activity

To investigate the physiological mechanism of heat stress mitigated by exogenous cinnamic acid (CA), seedlings of a cucumber cultivar ( Cucumis sativus cv. Jinchun no. 4) were pretreated with CA for 2   d and then were exposed to normal (25/18   °C) and elevated (42/38   °C) temperatures for 3   d. 50   μM CA was chosen since it improved growth more than other CA concentrations under heat stress. At 2   d, supply of 50   μM CA elevated the activities of antioxidant enzymes superoxide dismutase (SOD, EC 1.15.1.1), catalase (CAT, EC 1.11.1.6), guaiacol peroxidase (GPX, EC 1.11.1.7), glutathione peroxidase (GSH-Px, EC 1.11.1.9), ascorbate peroxidase (APX, EC 1.11.1.11), monodehydroascorbate reductase (MDHAR, EC 1.6.5.4), dehydroascorbate reductase (DHAR, EC 1.8.5.1) and glutathione reductase (GR, EC 1.6.4.2) in cucumber leaves, and it also increased the contents of reduced glutathione (GSH) and ascorbate (AsA). When the CA-pretreated cucumber seedlings have been exposed to heat stress, their antioxidant activities were changed further and were higher than heat treatment alone. Compared to heat treatment, the combination of CA pretreatment and heat also increased relative water content (RWC) of leaves, enhanced levels of proline and soluble sugars, elevated contents of CA and vanillic acid (VA) in leaves, and decreased levels of malonaldehyde (MDA), superoxide radical (O 2 − ) and hydrogen peroxide (H 2 O 2 ), while it resulted in 8.33% of leaf edges being dried up. Meantime, heat had adverse effects on plant growth, RWC, CA and VA contents in leaves, and it as well increased the levels of MDA, O 2 − and H 2 O 2 and made the dried leaf edges to be 79.17%. We conclude that pretreatment with 50   μM CA elevates CA and VA contents in leaves, enhances antioxidant activities under heat stress and decreases lipid peroxidation to some extent, accompanying with increasing proline and soluble sugar levels in heat-stressed leaves, thus the pretreatment enhances heat tolerance of cucumber seedlings.