Melatonin as an inhibitor of sweet cherries ripening in orchard trees.

Although melatonin effects on postharvest fruit ripening have been studied in some detail, information is still scarce during pre-harvest. Here, we examined whether or not melatonin may exert a regulatory role during sweet cherries ripening in orchard trees. We evaluated (i) the endogenous variations in melatonin contents, in comparison to those of well-known phytohormones such as ABA, salicylic acid and jsamonic acid, by ultrahigh performance liquid chromatography coupled to tandem mass spectrometry (UHPLC-MS/MS) during fruit ripening over two consecutive years, and (ii) to what extent melatonin treatments at low and high concentrations (at 10-4 M and 10-5 M, respectively) influence fruit ripening on the tree. Endogenous melatonin contents decreased in parallel to those of salicylic acid and jasmonic acid, while ABA contents increased as fruit ripening progressed, thus suggesting an inhibitory role for melatonin in fruit ripening. Furthermore, melatonin treatment at 10-5 M, which transiently increased endogenous melatonin contents at physiological concentrations, delayed anthocyanin accumulation, thus confirming an inhibitory regulatory role for melatonin in fruit ripening. We also found that the endogenous contents of cytokinins, but not those of ABA were transiently affected by melatonin treatment at 10-5 M. It is concluded that melatonin may delay sweet cherries ripening in orchard trees, probably exerting a modulatory role through a hormonal cross-talk. These results have important implications for the use of melatonin in the control of the timing of sweet cherries ripening in orchard trees.