Dynamic Regulation of Nitrogen and Organic Acid Metabolism of Cherry Tomato Fruit as Affected by Different Nitrogen Forms

Cherry tomatoes (Lycopersicon esculentum Mill., cv. hongyangli) were hydroponically cultivated in a greenhouse to determine the effect of different nitrogen (N) forms on organic acid concentration and the activities of related enzymes involved in nitrogen and organic acid metabolism during cherry tomato fruit development. The results showed that fruit nitrate reductase (NR) activity was much higher following treatment with 100% NO3- and 75% NO3- +25% NH4+ than with 100% NH4+ except at maturity. Glutamine synthetase (GS) activity trended downward during fruit development under all three treatments. Plants fed 100% NH4+ had the lowest fruit citrate and malate levels at maturity, with the highest malate concentration at an early stage. The activity of phosphoenolpyruvate carboxylase (PEPC) was found to be in accord with the malate concentration with every N source. Under all three N forms, the citrate synthase (CS) activity peaked one week before the citrate concentration.