Effects of different N rates on growth and absorption, allocation
and utilization of urea-¹⁵N of young apple trees with different interstocks

【Objectives】 Efficient and intensive dwarf and dense planting are the popular trend of apple industry, however, the matching fertilization technology specific for different stocks and scions with different growth characteristics is absent in China. In efficient or over fertilization of nitrogen often leads to premature senescence and excessive vegetative growth, stunting the extension of the dwarf and dense planting tachnology. So there is urgent need to identify the relationship between the apple tree growth and the absorption, allocation and utilization of nitrogen. 【Methods】 Pot experiment was carried out and one-year old interstock apple trees, M. hupehensis Rehd.(Gongteng Fuji/SH28, Gongteng Fuji/SH38 and Gongteng Fuji/CG24) were used as materials. Three levels of15N stable isotopes: no nitrogen(N0), proper nitrogen(N100)and over-dose nitrogen(N200), were designed to test the growth and absorption, allocation and utilization of the stocks. 【Results】 There were significant differences in biomass and15N utilization rates of young apple trees among the N treatments. The highest biomass and15N utilization were obtained in the over-N input level(N200) for SH28 , which are less dwarfed, those were in proper N treatment(N100) for SH38 and CG24, which are fully dwarfed, and the negative effect in over-dose nitrogen treatment. Significant differences of15N distribution could be seen among different types of interstocks of apple trees under different N treatments. For less dwarfed SH28 , the15N was partitioned more to the aboveground part in over-N input treatment, for fully dwarfed SH38 , more15N was partitioned to aboveground part in no N and proper N input levels, and more to the roots in over N input treatment. For CG24, more15N was partitioned to roots in the proper N treatment, which is significantly higher than in no N and over N treatments. 【Conclusions】 All the interstock morphology, N input levels and their interaction significantly affects the growth and15N utilization of young apple trees. The effect degrees are found in order of interstock morphology＞N application level＞interaction of the two factors. The effects of interaction of interstock varieties and N levels on the root-shoot ratio and15N distribution are significantly higher than those of interstock varieties and N application levels. With the increase of the dwarfing potential, the promoting effect to the growth, the response rate and time of nitrogen are all decreased.