Optimal Preharvest Melatonin Applications to Enhance Endogenous Melatonin Content, Harvest and Postharvest Quality of Japanese Plum

Plum is one of the most produced stone fruits worldwide. Melatonin is an environmentally eco-friendly substance that, in low concentrations, activates defence systems against biotic and abiotic stresses. This substance is considered a tool that could increase fruit quality. This study aimed to evaluate the impact of different preharvest foliar applications with different melatonin concentrations (0.1, 0.3, 0.5 mmol L-1) to enhance melatonin content and shelf life of 'Primetime' plum. To this purpose, two and three applications were carried out at different critical stages of fruit growth. Different quality characteristics such as size, colour, titratable acidity, total soluble solids, ripening index, respiration rate, ethylene production rate, anthocyanins and total antioxidant activity, as well as endogenous melatonin content, were tested at harvest and after 40 days of cold storage. Results showed that 'Primetime' plums that received 3 applications of 0.5 mmol L-1 enhanced endogenous melatonin content at harvest and showed less softening, delayed darkening, higher anthocyanin concentration and total antioxidant activity after 40 days of storage. Therefore, the concentration of 0.5 mmol L-1 melatonin in 3 applications was effective in improving the quality of 'Primetime' plums.