Effects of high temperature on activities and gene expression of enzymes involved in ascorbate–glutathione cycle in apple leaves

Higher plants growing in natural environments experience various stresses. In order to characterize the response of antioxidative system to high temperature, 2-year-old potted apple (Malus domestica Borkh.) plants were exposed to 28 °C as control or high temperature at 40 °C, respectively. The thermal dependent malondialdehyde (MDA) content, hydrogen peroxide (H2O2) content, the activity of protective enzymes, as well as the gene expression of dehydroascorbate reductase (DHAR, EC 1.8.5.1), ascorbate peroxidase (APX, EC 1.11.1.11) and glutathione reductase (GR, EC 1.6.4.2) at control or high temperature were determined after 0, 2, 4, 6 and 8 h treatment. The MDA and H2O2 concentrations in apple leaves increased at the high temperature. The content of total ascorbate, reduced ascorbic acid (AsA), total glutathione (GSH) was the highest at 2 h in the high temperature treatment, followed by a continuous decline with further increases in treatment duration. Activity of protective enzymes reached the highest point at 4 h of high temperature treatment duration, and then decreased. Gene expression of DHAR, APX and GR showed the same changes as the enzyme activities. The results suggest that the ascorbate–glutathione cycle is up-regulated in response to high temperature, but its regulation ability declines obviously after reaching the maximum with further increases in treatment duration.