Pre-harvest sodium nitroprusside sprays enhance resistance against diseases in harvested muskmelons

Abstract: 【Objective】Muskmelon is an important cash crop, due to its unique appearance and intrinsic quality. However, postharvest rots of the muskmelon fruit are serious. Several fungi are involved in the decay of muskmelon fruit. Among them, pink rot caused by Trichothecium roseum is a major fungal disease in China. Although pink rot can be effectively controlled by application of chemical fungicides, the excessive use of chemical fungicides will lead to pesticide residues in fruit, pathogen resistance and environmental pollution. Therefore, a new strategy needs to be developed for controlling the rots of postharvest muskmelon fruit. In this study, we evaluated the effects of sodium nitroprusside(SNP) sprays during fruit development on disease resistance against pink rot in harvested muskmelon fruit, and explored their effect on fruit ripening and energy charge level.【Methods】The cultivar‘Manao'was used as material. The plants were sprayed with SNP at 0.5 mmol · L^-1 for four times at the stage of young fruit(14 days after flowering), the early stage of enlargement(21 days after flowering), the late stage of enlargement(28 days after flowering) and the mature stage(2 days before harvest). Fruit with the same maturity, uniform size, with no pests and diseases were selected as experimental materials. The fruit surface was cleaned with running water, then dipped in 2% sodium hypochlorite solution for 2 mins for surface disinfection, and finally rinsed and stored at ambient temperature(25±2 ℃; RH 55%-60%). T. roseum for inoculation was cultivated in potato dextrose agar(PDA) and preserved in 4 ℃. The effects of SNP sprays on disease incidence and lesion diameter on the inoculated muskmelon fruit during ambient storage were evaluated. Meanwhile, the respiratory intensity, ethylene production, the activities of malate dehydrogenase(MDH) and succinate dehydrogenase(SDH), and energy change were determined at harvest and during storage.【Results】SNP sprays reduced the decay incidence of harvested muskmelon fruit inoculated with T. roseum, which were 47.4% and 29.1% lower than the control at 6 and 8 days after inoculation, respectively. Similarly, the lesion diameter of the treated fruit was also significantly lower than the control, which was 30.3% and 37.4% lower than the control at 2 and 6 days after inoculation, respectively. SNP sprays reduced ethylene release at harvest. However, no significant difference was found in respiration rate. SNP delayed the peaks of respiration and ethylene release by two days during storage. The respiration rate and ethylene release of treated fruit were significantly lower than the control at the early and the middle stages during storage, being 18.3% and 26.4% lower at the 6 th day of storage, respectively. SNP sprays had no significant effects on MDH and SDH activities at harvest. However, the activities of MDH and SDH increased significantly during storage, being 54% and43.5% higher than the control at the 4 th and 6 th days of storage, respectively. In addition, SNP delayed the decline of ATP content, increased the AMP content, and thus increased energy charge level. The ATP content at day 4 and energy charge at day 6 in the treated fruit were 73.8% and 48.5% higher than the control, respectively.【Conclusion】The disease resistance of harvested muskmelon fruit could be effectively enhanced by spraying with SNP during fruit development. This effect is closely related to NO delaying fruit ripening, increasing the activities of the key mitochondrial enzymes and maintaining high energy charge levels.