Combined application of different nitrogen forms enhances nutritional quality and secondary metabolite accumulation in tomato (Solanum lycopersicum L.) fruits

Nitrogen deficiency and improper nitrogen form ratios often limit tomato fruit quality and yield. Optimizing nitrogen form and ratio may provide an effective solution for improving tomato nutritional value and production quality. In this study, to identify the optimal nitrogen form and ratio that can improve tomato fruit quality, 13 treatments were established under the same nitrogen level (15 mM) with varying ratios of nitrogen forms (nitrate nitrogen: ammonium nitrogen: urea nitrogen). The treatments included CK (0:0:0), T1 (100%:0:0), T2 (0:100%:0), T3 (0:0:100%), T4 (75%:25%:0), T5 (50%:50%:0), T6 (25%:75%:0), T7 (0:25%:75%), T8 (0:50%:50%), T9 (0:75%:25%), T10 (75%:0:25%), T11 (50%:0:50%), and T12 (25%:0:75%). The results showed that, compared with CK, the contents of total amino acids, glutamic acid, soluble sugars, N, Cu, Mn, and Zn were significantly increased under T4 treatment. Principal component analysis based on 20 quality indicators ranked T4 treatment as the highest. In conclusion, the T4 treatment (75% nitrate-N: 25% ammonium-N: 0% urea-N) was identified as the most favorable for improving overall tomato fruit quality, providing a theoretical basis for the scientific application of nitrogen in high-quality tomato cultivation. • The application of nitrogen in different forms can enhance tomato fruit quality. • The optimal nitrogen ratio was determined through multivariate statistical analysis. • A ratio of 75% NO 3 - -N to 25% NH 4 + -N is optimal for enhancing tomato fruit quality.