Differences in fruit quality between Jinqiu Shatangju tangerine (Citrus reticulata 'Jinqiu Shatangju') grafted on two types of rootstocks and the relationship with absorption, distribution, and utilization of nitrogen

Excessive amounts of nitrogen are frequently applied in citrus production in China, and different rootstocks affect the absorption of nutrients and fruit quality of citrus. This study used 15N tracing technology and 3-year-old Jinqiu Shatangju tangerine (Citrus reticulata 'Jinqiu Shatangju', abbr. JST) grafted on two rootstocks of trifoliate orange (Poncirus trifoliata [L.] Raf., abbr. TO) and Ziyang Xiangcheng (Citrus junos Sieb. ex Tanaka, abbr. ZYXC) as the plant materials. Conventional urea and urea labeled with 15N were applied. The results showed that the total content of nitrogen in all the organs of JST grafted on ZYXC was higher than that of the TO. The two types of rootstocks had a significant impact on fruit quality, and the total soluble solids and titratable acid of the fruit significantly negatively correlated with the total nitrogen content in multiple organs. During fruit maturation, there was a relatively consistent distribution of Ndff (nitrogen derived from fertilizer) in various organs of the JST grafted on both types of rootstocks. There were significantly higher amounts of Ndff in various organs of the JST grafted on ZYXC than on TO. There was a significantly higher ratio of nitrogen supplied to the fruit of JST grafted on ZYXC than that of TO, while the opposite was found in the leaves. This indicated that ZYXC absorbed and utilized more nitrogen fertilizer than TO, and this could be the reason for the difference in fruit quality between the two rootstocks of JST.