Melatonin Uncouples Ripening Promotion from Softening and Enhances the Polyphenol Profile in Postharvest ‘Reineta’ Apples (Malus domestica)

Melatonin regulates fruit ripening and postharvest quality, but its effect on the phenolic profile of apples and its interaction with ethylene remain unclear. We evaluated exogenous melatonin (10, 50, 500 µM), 1-aminocyclopropane-1-carboxylic acid (ACC) and their combination on quality parameters and the polyphenolic profile of ‘Reineta’ apples (Malus domestica) during 35-day postharvest ripening at 22 ± 2 °C. Sugars, firmness, color and ethylene production were determined, together with total phenols, flavonoids and eleven polyphenols quantified by HPLC-DAD in skin and flesh. Melatonin accelerated ripening—advancing ethylene production, increasing sugars and promoting color development—while significantly delaying softening. ACC reproduced the ethylene and color responses but did not modify firmness or phenolic content, whereas the melatonin–ACC combination synergistically enhanced ethylene production without compromising texture or coloration. Melatonin attenuated the postharvest decline in phenols, flavonoids and individual polyphenols, mainly in skin and on chlorogenic acid, procyanidin B2 and quercetin glycosides; 50 and 500 µM were most effective. Melatonin thus uncoupled rising ethylene from firmness loss within the climacteric cascade, improving fruit quality through a protective mechanism additional to the ethylene pathway, plausibly linked to its antioxidant capacity, and making this indoleamine a promising tool for modulating apple ripening after harvest.