Effect of different N application rates on plant growth,¹⁵N -urea utilization and hormone content of dwarf apple trees

【Objectives】 The dwarfed interstocks are been used to realise the dwarfed apple cultivation, which facilitate early fruiting whereas might lead to presenility of apple trees. Few studies have been focused on the demand of nitrogen as well as the relationship between growth and endogenous hormone of dwarfed interstocks. Thefore, effects of different nitrogen application levels on growth, nitrogen uptake and utilization and endogenous hormone of dwarfed interstocks were studied. 【Methods】Using the 15N isotope tracer technique, three-year-old Fuji/SH6/Malus hupehensis Rehd was selected as test materials. Threre were three N application rates, 50 kg/hm2, 100 kg/hm2 and 200 kg/hm2 (N50, N100 and N200). At the shoot vigorous growing period and slow growth period, the hormones in stem tips and fine roots were measured using enzyme-linked immunoassay(ELISA). In the shoot stop growing, the length of the spring shoots and autumn shoots was measured, then whole plants were parsed, fresh weight and dry weight of each part were weighed, and the plant total nitrogen and 15N abundance were determined. 【Results】The results show that the plant growth and nitrogen uptake are significantly related to the N application levels. The total fresh weight, fine root biomass and total N content of the plants in the N50 treatment are the lowest, followed by N100 treatment, while those in the N200 treatment are the highest. The utilization rates of 15N are just the opposite, N50N100N200. The N application has great influence on dynamic changes and values of GA, ZR, ABA and IAA. During the vigorous growing period and slow growth period of the shoots, the contents of IAA and GA in stem tips and fine roots are as follows: N50 N100 N200, but the contents of ZR and ABA are declined along with the increase of nitrogen: N50＞N100＞N200. Compared with the vigorous growing period, except that the contents of GA of fine roots(N50 is 5.13, N100 is 5.68 and N200 is 6.17 ng/g, FW)are increased, the contents of IAA、GA and ZR in each organ are significantly reduced during the slow growth period of shoots. In the two periods, the ratios of ZR/GA and ABA/GA in stem tips and fine roots are the highest under N50 treatment, followed by the N100 treatment, and the ratios of the N200 treatment are the lowest. The values of (GA+ZR+IAA)/ABA of different N application rates are different in the same organs, meanwhile the ratio of the N200 treatment is significantly higher than those of other treatments. 【Conclusions】With the increase of nitrogen fertilizer in the range of N 50 kg/hm2 to 200 kg/hm2, hormones, total nitrogen and biomass in dwarfed interstock apple saplings are increased significantly, while the 15N utilization rate, ZR/GA and ABA/GA ratios are gradually reduced. In this study, the appropriate amount of nitrogen application for apple trees is recommended as 100 kg/hm2, it could promote the growth of fine roots and the utilization of nitrogen, delay senescence and promote flowering, ensure sufficient and balance supply of nitrogen in intensive cultivation in dwarfed apple under this condition.