Exogenous p-hydroxybenzoic acid regulates antioxidant enzyme activity and mitigates heat stress of cucumber leaves

To elucidate a physiological mechanism of heat stress mitigated by p-hydroxybenzoic acid (PHBA), seedlings of Cucumis sativus cv. Jinchun no. 4 were watered for 2 d with the Hoagland nutrient solution containing 0.5 mM PHBA, and then they were watered with the Hoagland nutrient solution only and were subjected to normal (25/18 degrees C) and elevated (42/38 degrees C) temperatures for 3 d. We investigated whether PHBA could protect cucumbers from heat stress and whether the protective effect was associated with regulation of antioxidant enzymes and lipid peroxidation. 0.5 mM PHBA was chosen since it improved growth inhibition and decreased levels of superoxide radical (O-2(center dot-)), hydrogen peroxide (H2O2) and malonaldehyde (MDA) under heat stress more than other concentrations of PHBA. At 2 d, supply of 0.5 mM PHBA elevated the activities of superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (EC 1.11.1.11), catalase (EC 1.11.1.6), monodehydroascorbate reductase (EC 1.6.5.4), dehydroascorbate reductase (EC 1.8.5.1) and glutathione reductase (EC 1.6.4.2) in cucumber leaves. When the PHBA-pretreated seedlings were exposed to heat, the antioxidant enzyme activities were changed further and were higher than those in heat treatment alone, and this was consistent with the increased transcript levels of Cu/Zn-SOD, Mn-SOD and guaiacol peroxidase (GPX) genes and coincided with the enhanced contents of reduced glutathione. Heat increased the levels of O-2(center dot-), H2O2 and MDA and made 62.5% of leaf edges being dried up. Compared to heat treatment, the combination of 0.5 mM PHBA and heat decreased the levels of O-2(center dot-), H2O2 and MDA, while it resulted in the dried leaf edges to be 29.17%. We conclude that pretreatment with 0.5 mM PHBA enhances antioxidant enzyme activities under heat stress: thus, it decreases lipid peroxidation to some extent and enhances heat tolerance of cucumber seedlings. (c) 2012 Elsevier B.V. All rights reserved.