Applying nitrogen fertilizer at 15 days before harvest is suitable for the absorption and distribution of ¹⁵N and accumulation of storage reserves in Fuji apple tree

Objectives We studied the nitrogen uptake, distribution and storage of photosynthate in Fuji apples as affected by autumn fertilization time for efficient and nutrient management in apple production.

Methods Potted 4-year-old Fuji 3/M26/Malus baccata apples were used as materials to conduct a ¹⁵N isotope tracing pot experiment. The five fertilization date treatments were at 45 days before harvest (45BH), 30 days before harvest (30BH), 15 days before harvest (15BH), the harvest day (HD), and 15 days after harvest (15AH). The fertilizer rate for all the five treatments was 5 g of isotopes urea (¹⁵N abundance 10.1%), 16.55 g of common urea and 20 g of monopotassium phosphate. The trees were dissected when the fruit trees had completely entered dormancy, and the ¹⁵N uptake, soluble sugar and soluble protein contents of each tissue were determined.

Results The Ndff values of fruit, annual branches, perennial branches, main stem, and intermediate rootstock of the plant were significantly affected by fertilization date. Ndff values of all organs in 45BH, 30BH and 15BH treatment were higher than HD and 15AH treatment. The plant ¹⁵N uptake and fertilizer utilization were significantly different among the five treatments, with 15BH treatment eliciting the highest overall plant ¹⁵N uptake and fertilizer utilization. The uptake of ¹⁵N in the aboveground tissues were higher in 45BH, 30BH treatment than in HD and 15AH treatment. The ¹⁵N uptake by lateral roots in 15 BH treatment was significantly higher than in 15AH treatment. The soluble sugar content (4.62 g) and distribution ratio (7.46%) in fallen leaves were the lowest in 15BH treatment. The distribution ratio of soluble protein in xylem of trunk in 15BH and HD treatment, which were 6.22% and 6.09%, respectively, were significantly higher than in 45BH, 15AH treatments. The distribution ratio of soluble protein in lateral roots in 15BH treatment was significantly higher than that in 45BH.

Conclusions From a comprehensive view of nutrient uptake and photosynthate storage in apple trees, the appropriate date for autumn fertilization of Fuji apple in this region is around 15 days before harvest.