Abstract:Cherry tomatoes were grown in a matrixnutrient solution coculture systems with 100% nitrate (NO-3), 100% ammonium (NH+4) and 75% NO-3∶25% NH+4 as the N source. The effects of the nitrogen (N) forms on the contents of organic acids, the activities of phosphoenolpyruvate carboxylase (PEPC), citrate synthase (CS) and malate dehydrogenase (MDH), and their transcriptional expression patterns were investigated during the development of the fruit. The results are as follows: 1) During the development of the fruit, citrate contents under the treatments of 100% NO3-and 75%NO-3∶25% NH+4 are always significantly higher than that under the treatment of 100% NH+4, while the contents of citrate and malate in mature fruits under the 75% NO-3∶25% NH+4 treatment are lower than those of the 100% NO-3 treatment. 2) During the development of the fruit, the citrate content is related to the activity of CS under all treatments and shows the tendency with a single peak, although the peak values are different at the different stages. The content of malate is positively correlated with PEPC activity under the 100% NH+4, in contrast, the contents of malate and citrate are positively correlated with MDH activity under the 100% NO-3 and 75%NO-3∶25% NH+4. 3) At the enlargement stage and mature stage, the expression of PEPC genes (PPC1 and PPC2) supplied by 100% NO-3 and combined 75%NO-3 with 25% NH+4 are both significantly higher than that of the 100% NH+4. The activity of MDH is related to the mMDH expression level and the malate accumulation during the development of the fruit. Our results suggest that an appropriate ratio of NO-3/NH+4 could reduce organic acid contents in ripe fruits of tomato, and different exogenous N forms have different effects on the activities of PEPC and MDH and the expression pattern of corresponding genes.