Pre- and Postharvest Melatonin Treatment: A Comparative Study of Lemon Cultivars on Melatonin and Flavanone Content During Cold Storage

Adverse environmental and postharvest conditions challenge the functional quality of lemons, an economically vital citrus crop. Melatonin (MEL) has emerged as an effective regulator of plant stress responses and secondary metabolism. This study evaluated the effects of pre- and postharvest MEL treatments, combined with cold storage, on the fruit quality of two lemon cultivars (‘Fino’ and ‘Verna’). The research focused specifically on endogenous MEL and flavanone dynamics. Three experimental conditions were assessed: (a) preharvest MEL application at 0.1 and 1 mM; (b) preharvest treatment followed by cold storage; and (c) combined pre- and postharvest MEL treatment followed by cold storage. Preharvest treatments increased endogenous MEL at harvest in a dose- and cultivar-dependent manner. Specifically, 1 mM being optimal for ‘Fino’, while 0.1 mM was more effective for ‘Verna’. During cold storage, ‘Fino’ fruit, characterized by low basal endogenous MEL levels, showed a marked increase in MEL accumulation, suggesting the stimulation of biosynthesis. In contrast, ‘Verna’ fruit, which had initially high endogenous MEL content, exhibited a pronounced decline, indicating MEL consumption to counteract oxidative stress. Flavanone content increased dose-dependently after preharvest treatment and was preserved during storage in ‘Fino’ but declined in ‘Verna’. These findings demonstrate that the fruit cultivar must be considered a critical factor in MEL-based strategies, as identical treatments may yield markedly different outcomes even within the same species.