Abstract

The worldwide consumed leafy vegetable lettuce is mostly cultivated in both controlled environments and open fields, which contains various health-promoting compounds. Glycine, one of the most abundant free amino acids in soil, can be absorbed and utilized by plants. However, the sole application of glycine significantly improves lettuce quality while adversely affecting yield. Therefore, we explored a nitrogen fertilization strategy using glycine combined with nitrate, maintaining a constant total nitrogen input, to establish an approach that enhances crop quality. In this study, lettuce cultivars ‘Green Lollo Rossa’ and ‘Red Lollo Rossa’ were hydroponically cultured in nutrient solutions containing varying concentrations of glycine and nitrate. These solutions included 0 mM glycine and 9 mM NaNO3 (G0), 2.25 mM glycine and 6.75 mM NaNO3 (G2.25), 4.5 mM glycine and 4.5 mM NaNO3 (G4.5), and 6.75 mM glycine and 2.25 mM NaNO3 (G6.75). The results indicated that the G4.5 treatment induced the greatest levels of amino acids, glycosylated quercetin derivatives, luteolin derivatives, and phenolic acids, while decreasing some organic acids compared to the G0 counterparts, without biomass loss in ‘Red Lollo Rossa’. While 6.75 mM glycine can improve the nutritional quality of lettuce, but it resulted in a loss in yield in ‘Green Lollo Rossa’. These findings suggest that partial replacement of nitrate with an appropriate concentration of glycine can enhance the nutritional quality, and could be applied as an important strategy in lettuce cultivation by balancing the quality and yield. © 2025