Abstract: To enhance the quality of hydroponic lettuce, this study examined the effect of adding different concentrations of glycine and humic acid five days before harvest on the physical properties, nutritional components (phenols, soluble monosaccharides, and sesquiterpene lactones), and flavor of three lettuce varieties, Huqian (green), Rebaweiya (red), and SAGC88 (red). The results showed that treatment with low concentrations of glycine (3 mmol/L) and humic acid (50 mg/L) significantly increased the firmness of lettuce by 25.1% to 33.8% and promoted the accumulation of soluble monosaccharides and phenols relative to the control group. The overall quality of Huqian treated with high concentrations of glycine and humic acid, Rebaweiya treated with low concentrations of glycine and humic acid, and SAGC88 treated with high concentrations of glycine and humic acid exceeded that of the control group. Sensory evaluation showed that the sweetness and consumer preference of the treated groups were significantly improved. The sucrose content was significantly positively correlated with the sweetness (r = 0.68) and consumer preference (r = 0.85), while the contents of phenolic acids were significantly negatively correlated with the consumer preference (r < −0.70). The ratio of total sugar to total phenols was significantly negatively correlated with the bitterness (r = −0.74) and significantly positively correlated with the sweetness (r = 0.73) and consumer preference (r = 0.74), indicating that increasing the sugar-to-phenol ratio can result in a balanced flavor. The study demonstrated that regulating the concentrations of glycine and humic acid can improve the taste of lettuce by increasing the sugar-to-phenol ratio, providing a basis for enhancing the quality of hydroponic lettuce.

Key words: lettuce; nutrient solution; nutritional components; flavor; antioxidant; sensory evaluation