Dynamics of growth and nitrogen accumulation and utilization of young apple trees

Taking 6-year-old Yanfu3/SH6/Malus hupehensis Rehd. as the test material, the dynamics of plant growth and nitrogen (N) accumulation under seven periods from germination stage to fruit maturity stage were examined by destructive analysis. The absorption, utilization, and distribution of fertilizer N were studied by ¹⁵N isotope tracer technique to clarify the N accumulation dynamics of apple trees and the maximum efficiency period of fertilizer N, and to provide theoretical basis for scientific application of N fertilizer. The results showed that the net accumulation of dry matter was 4.51 kg in germination stage (March 25) to fruit maturity stage (210 d after budbreak), with fruit accounting for 66.5%, the leaves and new shoots accounting for 20.2%, and the perennial organs accounting for 13.3%. The dry matter accumulation in 30-60 d after budbreak was the fastest, accounting for 42.9% of the whole treatment period. The fruit dry matter accumulation in 120-180 d after budbreak was the fastest, which accounted for 70% of the whole treatment period. The total N accumulation of the plant was 29.1 g, which increased rapidly in the 30-60 d and 120-180 d after budbreak by 7.2 g and 12.8 g, respectively accounting for 24.7% and 44% of the whole treatment period. The N accumulation of leaves and new shoots was the fastest in 0-60 d after budbreak, which accounted for 69.1% of the whole period. The N accumulation of fruit was the fastest in 120-180 d after budbreak, accounting for 60.8% of the whole period. The N accumulation of the perennial organ decreased first and then increased, and reached the lowest level at 60 d after budbreak. The ¹⁵N utilization rate of plant differed significantly in different periods which was at a high level in 30-60 d,120-150 d and 150-180 d after budbreak with 2.3%, 4.1% and 4.0% respectively. The ¹⁵N distribution rate in perennial organs in each period showed a high level, that of the new born organ increased first and then decreased which reached the highest level of 38.4% in 30-60 d after budbreak. The fruit reached the highest in 120-150 d and 150-180 d after budbreak by 15.0% and 16.6% respectively. Therefore, the key period of N accumulation in leaves and shoots was 30-60 d after budbreak, and that in fruit was 120-180 d after budbreak. The period with maximum efficiency for fertilizer N was at 30-60 d and 120-180 d after budbreak.