Studies On Regulation And Physiological Mechanism Of Melatonin Treatment On Ripening And Senescence Of Kiwifruit

In this study,regulation and physiological mechanism of 0.05 mmol L^-1 melatonin(MT)treatment on ripening and senescence of‘Xuxiang'kiwifruit during ambient storage were investigated.The studies will provide theoretical basis and technical support for postharvest kiwifruit preservation.The main results are as follows:1.Effect of MT treatment on ripening and senescence of kiwifruit was studied.Results showed that MT treatment inhibited respiration rate,maintained lower weight loss and soluble solid content,and slowed firmness and color losses in kiwifruit.MT treatment weakened the activity of pyruvate decarboxylase and alcohol dehydrogenase,and reduced the accumulation of pyruvate,acetaldehyde and ethanol,and thus maintaining the flavor and quality of kiwifruit.This indicates that MT is an effective ripening and senescence delaying factor for kiwifruit.2.Effect of MT treatment on respiratory metabolism and membrane lipid peroxidation of kiwifruit was studied.Results showed that MT treatment inhibited the activity of cytochrome c oxidase and succinate dehydrogenase,and activated NADK activity which promoted the transformation of NAD to NADP and the accumulation of NADPH,thereby partly changing the respiration pathway from glycolysis-tricarboxylic acid cycle to pentose phosphate pathway,ultimately decreased respiration consumption in kiwifruit.Also,MT treatment suppressed the production of superoxide anions and the accumulation of hydrogen peroxide and malondialdehyde,and inhibited the activity of phosphatase D and lipoxygenase,in turn alleviated the peroxidation of membrane lipid in kiwifruit.This indicates that MT treatment is beneficial to maintain lower basal metabolic activity and oxidative damage,which makes a necessary foundation for kiwifruit to retard ripening and senescence.3.Effect of MT treatment on redox homeostasis of kiwifruit was studied.Results showed that MT treatment activated superoxide dismutase,catalase and peroxidase activity,maintained higher activity of key enzymes in ascorbate-glutathione(As A-GSH)cycle which mainly included ascorbate peroxidase,dehydroascorbate reductase,glutathione reductase and monodehydroascorbate reductase,as well as higher As A and GSH levels in kiwifruit.The content of endogenous MT was also higher in MT-treated fruit.This indicates that MT plays a role in delaying ripening and senescence in kiwifruit by mediating redox homeostasis.4.Effect of MT treatment on phenolic and proline metabolism of kiwifruit was studied.Results showed that MT treatment induced the activation of enzymes related to phenolic and proline anabolism which includes shikimate dehydrogenase,phenylalanine ammonia lyase and?-1-pyrroline-5-carboxylic acid synthase,and the inactivation of polyphenol oxidase and proline dehydrogenase related to its catabolism,contributing to the higher contents of total phenolic compounds and proline in kiwifruit.Phenolic compounds and proline can act as an effective line of defense taking part in reactive oxygen scavenging by providing hydrogen and/or decomposing peroxides,which plays an important role in postponing ripening and senescence in kiwifruit by MT treatment.