Abstract: Objective: To ascertain the role of the ascorbic acid-reduced glutathione (AsA-GSH) cycle in scavenging excessive H2O2 in melons induced by salicylic acid (SA) during postharvest storage. Methods: Melon (cv. Yujinxiang) fruit were soaked in 4 mmol/L SA for 10 min, and stored at ambient temperature. Malondialdehyde (MDA) content and the accumulation level of reactive oxygen species, and the activity of superoxide dismutase (SOD) and catalase (CAT) were determined as well as enzyme activities involved in the AsA-GSH cycle and the contents of substrate and product. Results: SA treatment reduced MDA content, resulting in a 14.6% reduction in MDA content compared with the control group on day 10. SA significantly increased superoxide anion radical production rate and H2O2 content in the fruit (P < 0.05). On day 2 after the treatment, the production rate of superoxide anion radical in the treated group was 1.9 times higher in than that in the control at the same time point, and H2O2 content was 29.7% higher than that in the control on day 6. The chemical increased SOD activity and inhibited CAT activity, suggesting that scavenging of H2O2 is not dependent on the enzymatic system but other systems. Moreover, SA increased the activity of ascorbate peroxidase, glutathione reductase, monodehydroascorbate peroxidase, and dehydroascorbate reductase as well as the contents of ascorbic acid and oxidized glutathione, and decreased the contents of dehydroascorbic acid and reduced glutathione. Conclusion: SA treatment promoted oxygen burst in melons and reduced MDA content. Excessive H2O2 induced by SA was scavenged mainly through the AsA-GSH cycle.

Key words: melon; salicylic acid; reactive oxygen species; ascorbic acid-reduced glutathione cycle