Combined Pre- and Postharvest Melatonin Treatments Improve the Functional Quality of the Sweet Cherry cv. ‘Sunburst’

Chronic metabolic disorders have increased recently due to changes in dietary habits and lifestyle. Red-coloured fruits, such as sweet cherries, are rich in anthocyanins and other (poly)phenolic compounds with health-promoting properties, which has garnered growing scientific interest. Melatonin elicitation has emerged as a promising strategy to improve the functional quality of these fruits. This research investigates, for the first time, the combined effect of pre- and postharvest melatonin treatments, followed by a cold storage (2 °C) of 21 days, on the endogenous melatonin and phenolic compound levels of 90 sweet cherries (<i>n</i> = 3) from the 'Sunburst' cultivar and harvested from 9 trees per treatment. Single preharvest or postharvest melatonin treatments increased the endogenous melatonin content via direct absorption and activation of key biosynthetic genes, while they reduced anthocyanin, hydroxycinnamic acid, and flavonol levels, likely due to a ripening-delaying effect at harvest. Nevertheless, the combined treatment increased endogenous melatonin levels 5-fold compared to harvest and increased all measured polyphenolic compound levels, including a 29% rise in total anthocyanins reverting the delay in the ripening process. These effects suggest upregulation of genes in the phenylpropanoid pathway and could improve fruit's functional quality. The response to melatonin is cultivar- and dose-dependent. Future research should investigate genetic and transcriptomic validation to confirm these potential effects and assess whether increased bioactive compound content would translate into measurable human health benefits.