Nitrogen and phosphorus absorption and utilization characteristics
of apple rootstocks under different phosphorus levels

【Objectives】Phosphorus is an indispensable element in promoting NO-3 reduction and assimilation. Excessive phosphorus tends to increase free amino acid and amides, and then depress protein synthesis. Therefore, the absorption and utilization characteristics of nitrogen and phosphorus were studied using three apple rootstocks, aiming to pick out efficient rootstocks on nitrogen absorption and utilization under different P levels.【Methods】 Three kinds of one-year-old apple rootstocks, Malus hupehensis Rehd, M. micromalus Makino and M. prunifolia(Willd)Borkh., were labeled with 15N-urea in this study. P2O5 application levels were 0(P0), 50(P1), 100(P2), 150(P3) and 200(P4) kg/hm2 respectively with triplicates. The plants were sampled and divided into roots, stems and leaves, then dried, grinded and weighed. P contents of different organs were determined using vanadium molybdate yellow colorimetric method, the total N was measured by Kjeldahl method, and the abundance of15N was tested by MAT-251 mass spectrometer.【Results】 The P absorption efficiency of the rootstocks are increased with P2O5 levels in the P0-P1 treatments, then decreased under the high P2O5 rate. The Ndff values of M. prunifolia(Willd)Borkh. are high in the P0-P2 treatments, and the N utilization efficiency is the highest in P1 treatment(6.39%), while the percentages of N derived from fertilizer in the total N of the organs (Ndff) and N utilization efficiency in the P3 treatment are the lowest. The Ndff of M. micromalus Makino varies slightly with P levels, and the N utilization efficiencies in the P3-P4 treatments present high levels. The Ndffs of Malus hupehensis Rehd. are decreased after an increase with the phosphorus application, with minimum in P1 and maximum in P2, and the Ndff values of roots, stems and leaves are 5.21%, 4.55% and 5.79% respectively, which exceed those of M. prunifolia(Willd)Borkh. and M. micromalus Makino significantly. The N utilization efficiency of Malus hupehensis Rehd. is the highest in the P2 treatment(4.86%), and then follows a downtrend.【Conclusions】The N utilization rate of M. prunifolia(Willd)Borkh. which is native to northern Shaanxi plateau, is high under low phosphorus condition, thus can be a reference to the apple rootstocks applied in this region from the perspective of P utilization. M. micromalus Makino presents a high N utilization rate under high phosphorus level, hence can be used in high phosphorus apple orchard in Bohai Bay. N utilization of Malus hupehensis Rehd. is high in medium phosphorus level, along with the strong resistance and graft compatibility, so it’s widely used in newly built orchard in the hills.