The induction of pre-harvest acetylsalicylic acid treatment on postharvest resistance of Hami melon fruit

Astract:【Objective】Hami melon (Cucumis melo L.) is a typical local fruit in the Northwest of China, however, postharvest rot of this fruit is a serious concern. Many fungi are involved in the decay of Hamimelons, among them, pink rot caused by Trichothecium roseum is an important postharvest disease in theNorthwest of China. The disease not only breaks down the quality of the fruit and causes serious posthar-vest losses, but it also accumulates mycotoxins in the fruit. Although pink rot can be effectively controlledby the application of chemical fungicides, the use of chemical fungicides will lead to pesticide residues inthe fruits and also cause pathogen produce resistance and environmental pollution. Therefore, a safer andmore effective strategy needs to be developed. In this study, an experiment is conducted to assay the ef-fects of acetylsalicylic acid (ASA) sprayed during fruit development as an induced resistance in the har-vested fruit and to also study if phenylpropane metabolism and antioxidant enzymes contribute to inducedresistance in the fruit at harvest and during storage. 【Methods】The muskmelon, cultivar‘XizhoumiNo.17', was used as the material. The plants were sprayed with ASA at 1.0 mmol·L^-1 four times duringthe young fruit period (14 d after flowering) , enlarging period (21 d after flowering) , netting period (28 d after flowering) and mature period (preharvesting 48 h) . We selected commercially mature and uniformsized fruit, without diseases and pests, for our experimental materials. The first step was to remove dustand foreign bodies from the fruit surface using a tap water rinse and then to soak in 2% sodium hypochlo-rite solution for 2 mins for surface disinfection, then rinse and dry pack at ambient temperature (25±2 ℃;RH 55%-60%) for storage. T. roseum was isolated from the onset of the typical fruit, using a separationand purification of Potato Dextrose Agra (PDA) , in 4 ℃ as part of the conditions for preservation. The ef-fects of treatment on the lesion diameter of the inoculated melon fruit and natural incidence of melon fruitduring ambient storage were evaluated. Meanwhile, the activities of the key enzymes and metabolite con-tents of the phenylpropane metabolism were determined at harvest and during storage, as well as the activi-ties of peroxidase and polyphenol oxidase.【Results】Preharvest treatment effectively inhibited the develop-ment of the lesion diameters of the inoculated fruit with Trichothecium roseum and significantly reducedthe natural incidence of the fruit during ambient storage. After 7 days of inoculation, the lesion diametersof the treatment fruit were 40.7% lower than the control. The natural incidence of the treated melons wasonly half of the control after 28 days of storage. Moreover, the activity of cinnamic acid-4-hydroxylaseand the content of flavonoid and lignin were significantly increased by treatment at harvest. Comparedwith the control, the ASA treatments enhanced 34.4% of the cinnamic acid-4-hydroxylase activities andincreased by 30% and 7.05% the contents of flavonoid and lignin at harvest. The activities of phenylala-nine ammonia lyase, cinnamic acid-4-hydroxylase and 4-coumaryl coenzyme A ligase and the contentsof total phenolics, flavonoids and lignin were significantly higher than the control during storage. The ac-tivities of the phenylalanine ammonia lyase, cinnamic acid-4-hydroxylase and 4-coumaryl coenzyme A li-gase were 34.4%, 84.5% and 56.3% higher than the control after 21, 14 and 7 days of storage, respective-ly. After 21 days of storage, the contents of the total phenolics, flavonoids and lignin were 10.3%, 17.6%and 10.1% higher than the control. Moreover the treatment also notably induced the peroxidase activity atharvest and enhanced the activity of peroxidase and polyphenol oxidase during storage. Compared withthe control, the treatment activity of peroxidase was greater than 64%. During storage, the activities of per-oxidase and polyphenol oxidase were 0.35 and 2.3 folds higher than the control after 7 and 28 days of stor-age.【Conclusion】The postharvest disease resistance of Hami melon could be effectively induced by ASAsprayed during fruit development, the partial mechanism is closely related to the activation of phenylpro-panoid metabolism and the increase of the activities of peroxidase and polyphenol oxidase. Therefore, pre-harvest ASA sprayed four times during fruit development is an effective measure to enhance the diseaseresistance of Hami melon during ambient temperature storage.