Melatonin Boosts the Phytochemical Profile of Blood Oranges, Enhancing (Poly)phenol and Endogenous Melatonin Content, Through Pre- and Postharvest Treatments

Nowadays, increasing consumer awareness of the link between diet and health has underscored the value of bioactive compounds in preventing metabolic disorders. In this frame, blood oranges are highly appreciated for their unique phytochemical profile, including anthocyanins along with flavanones with recognised health-promoting benefits. This study explores, for the first time, the combined effect of preharvest and postharvest melatonin treatments, along with cold storage, on the accumulation of endogenous melatonin, phenolic compounds and vitamin C in blood orange (Citrus sinensis L. Osbeck) cv. ‘Sanguinelli’. HPLC-QqQ-MS/MS and HPLC-DAD methodologies were used to analyse the bioactive compounds in leaves, flavedo, albedo and juice. The findings demonstrate that exogenous melatonin is an effective strategy to enhance the functional quality of blood oranges. Preharvest treatments increased endogenous melatonin levels in the leaves, flavedo and albedo, either by positive absorption or enhancement of biosynthetic pathways. However, postharvest melatonin treatments combined with cold storage led to the highest endogenous melatonin accumulation, suggesting a synergistic effect between exogenous supply and stress-induced biosynthesis. Furthermore, melatonin treatments promoted the accumulation of flavanones, anthocyanins and vitamin C, although cold storage remained the key driver of anthocyanin biosynthesis. These findings highlight the potential of melatonin as a natural elicitor to improve the functional quality of blood oranges, identifying preharvest melatonin treatment at 1 mM combined with cold storage as the most effective strategy. Nonetheless, further studies on bioavailability and bioactivity are required to determine whether these enhancements translate into greater health benefits for consumers.