Effects of nitrogen fertilization on growth characteristics and absorption,distribution and utilization of NH₄¹⁵NO₃ of five apple rootstocks

Five kinds of oneyearold apple rootstocks, M. hupehensis Rehd., M. micromalus Makino, M. micromalus (Willd) Borkh., M. micromalus (Ledeb.) Roemer and M. baccata Borkh. were used to study the growth and the characteristics of absorption, distribution and utilization of labeled NH415NO3 of the rootstocks. The results show that with the increase of nitrogen application rate, the biomass, root surface area, total root length and root tips of the five rootstocks are increased significantly and the degrees of the increases are different in the five rootstocks, and the growth of aboveground is more sensitive to nitrogen than that of underground. The biomass of M. hupehensis Rehd. is the largest among the five rootstocks and the root surface area, total root length and root tips of M. micromalus (Willd) Borkh are the largest, while those of M. baccata Borkh. are the lowest. With the increase of nitrogen application rate, the changes of 15N utilization ratios of the five rootstocks are quite different, and the15N utilization ratio of is significantly correlated with root surface area. The 15N utilization ratio of M. micromalus (Willd) Borkh. is the largest, while that of M. baccata Borkh. is the lowest. The Ndff values of different organs in the low N（N1） treatment are in order: rootsleavesstems, while in the high N （N2） treatment are: leaves rootsstems. In the five rootstocks, the15N distribution ratio of leaves is the highest, and that of the shoots is the least.