Annual utilization and allocation of urea-¹³C by M. hupehensis Rehd. under different N rates

【Objective】 Unreasonable use of nitrogen often causes imbalance of C/N, excessive vegetative growth, premature senescence of apple trees and decreases the yield and fruit quality as a result. Utilization and allocation characteristics of photosynthates under different nitrogen application levels should be studied to ensure steady growth and fruiting of apple trees. 【Methods】 A pot experiment was conducted with two-year-old M. hupehensis Rehd.(Yan Fu3/M26) seedlings as test materials. Urea was labeled with 13C andapplied in three levels of N 0, 100 and 200 kg/hm2. The plants were sampled after 72 h of 13C labeling and then divided into roots, branches, interstocks, new shoots and leaves, then dried, grinded and weighed. The abundance of 13C, the ZR(trans-Zeatin-riboside) and ABA(abscisic acid) contents, the soluble starch, leaf area and chlorophyll contents were determined. 【Results】 Compared in the N0 treatment, both the rational application rate of nitrogen fertilizer(N100) and overuse rate(N200) promote the tree biomass, the area and chlorophyll contents of leaves, the promoting effect of N100 is gradually appeared with the growth stages. At the spring shoot growing stage(SSG) and spring shoot stop growing stage(SSS), the number of fine root in N100 treatment is the highest, followed by N200 and N0 treatments successively; at the autumn shoot growing stage(ASG), the fine roots of the N0 treatment are increased significantly to the highest. The leaf starch content of the N0 treatment is higher than those of the N100 and N200 treatments at all the growing stages. At the early stage of nitrogen application, the ZR contents of leaves are in order of N200＞N100＞N0, gradually, the ZR content of the N200 treatment is decreased significantly. In comparison, no significant differences are found on the ABA contents among the treatments at the early fertilization stage, and the ABA content of the N0 treatment is increased and maintains at high level until the late growing stage. Significant differences can be seen in the root to shoot ratio and photosynthate partition among different nitrogen treatments at different growing stages. The root 13C partitions in the N100 and N200 treatments are significantly increased to 285.35% and 217.98%(compared with the N0 treatment) at the spring shoot growing stage, more is distributed to the aboveground in the N0 treatment. At the stage of autumn shoot growing, the distribution of photosynthate to roots in the N0 treatment is improved, while those in the N100 and N200 treatments are decreased to 71.98% and 41.26% of the N0 treatment. 【Conclusions】 The impact on the growth and the photosynthate partition under different nitrogen levels is closely related to the synthesis of ZR and ABA. The nitrogen application significantly improves the partition of photosynthate in roots by large amount syntheses of ZR, and the root to shoot ratio and root biomass are significantly increased at the long time scales. More photosynthates convert into starch under the no nitrogen treatment during the growing stage is caused by ABA synthetic differences.