Hydrogen peroxide pretreatment alters the activity of antioxidant enzymes and protects chloroplast ultrastructure in heat-stressed cucumber leaves

Abstract To elucidate the physiological mechanism of heat stress mitigated by exogenous hydrogen peroxide (H 2 O 2 ) further, seedlings of Cucumis sativus cv Lvfeng no. 6 were subjected to two temperatures (42/38 and 25/18 °C) after pretreatment with 1.5 mM H 2 O 2 . We investigated whether exogenous H 2 O 2 could protect chloroplast ultrastructure under heat stress (42/38 °C) and whether the protective effect was associated with the regulation of antioxidant enzymes. The heat condition disintegrated the membranes of 71.4% chloroplasts in the leaf cells and resulted in the elevated levels of malondialdehyde (MDA) and endogenous H 2 O 2 . When H 2 O 2 pretreatment was combined with the heat stress, the abnormal chloroplasts occurred at a rate of 33.3%, and the contents of MDA and endogenous H 2 O 2 were decreased. Heat stress and exogenous H 2 O 2 both increased the activities of antioxidant enzymes such as glutathione peroxidase (GSH-Px, EC 1.11.1.9), dehydroascorbate reductase (DHAR, EC 1.8.5.1), monodehydroascorbate reducatase (MDHAR, EC 1.6.5.4), and glutathione reductase (GR, EC 1.6.4.2). The combined effect of heat stress and H 2 O 2 pretreatment led to higher activity of antioxidant enzymes including superoxide dismutase (SOD, EC 1.15.1.1), ascorbate peroxidase (APX, EC 1.11.1.11), GSH-Px, DHAR, MDHAR and GR in comparison to the heat treatment alone. We propose that exogenous H 2 O 2 increases antioxidant enzyme activities in cucumber leaves, decreases lipid peroxidation, and thus protects the ultrastructure of chloroplasts under heat stress.