Exogenous L-tryptophan treatment increases the concentrations of melatonin and aroma compounds in grape berries and wine

L-Tryptophan (Trp) is a substrate for the biosynthesis of melatonin, and melatonin is a signal molecule that alters the secondary metabolite profile; whether Trp treatments promote the biosynthesis of melatonin to regulate aroma compounds in grape berries and wine remains unclear. Here, the content of melatonin was higher in Marselan grapes and wine than that in other grape cultivars. Marselan grapes were subjected to 30 different Trp treatments. The content of melatonin and aroma compounds in Marselan grapes was increased by several treatments. Increases in the content of volatiles were mainly driven by increases in aldehydes. Hexanal, the most abundant aldehyde compound, was the main contributor to increases in aldehydes following Trp treatment. The results of our study indicate that the root application of 250 mg/L L-tryptophan during the fruiting expansion stage and the spraying application of 50 mg/L L-tryptophan spray during the veraison stage were the optimal treatments because the content of melatonin and aroma compounds, as well as other basic quality parameters, were highest in Trp-treated grape berries and wine in these treatments. Overall, these two effective Trp treatments could be used to enhance the content of melatonin and aroma compounds in Marselan grapes, and this could increase the economic value of this cultivar.