Studies on leaf senescence and 15N-urea absorption and utilization of ‘Fuji ’apple in response to topdressing nitrogen application

The effects of leaf senescence and the characteristics of absorption and utilization of fifteen-year-old Fuji apple (Malus robusta Rehd) in response to topdressing nitrogen were explored in the study. The results show that the SPAD and activities of SOD, POD and CAT are obviously the highest in one-time topdressing application at full-bloom stage, and then in two-times topdressing application, and the last is the three-time topdressing application. At the flower bud differentiation stage, the two-time topdressing application is the highest, and then the there-times topdressing application, and the lowest is one-time topdressing application. At the fruit maturity stage, the three-time topdressing application is obviously the highest, while the one-time topdressing application is the lowest. At the full-bloom stage and flower bud differentiation stage, the total N contents of leaves of the topdressing nitrogen application are not obviously different, while at the fruit maturity stage, the three-times topdressing application are obviously the highest, and then the two-time topdressing application, and the lowest is the one-time topdressing application. The proportions of N derived from the fertilizer (Ndff) in different organs are significantly different, and the Ndff values of the three-time topdressing application are obviously higher than those of the one or two-time topdressing. The leaf Ndff value is the highest in the one-time topdressing application at the full-bloom stage, the highest value is the two-time topdressing application at the flower bud differentiation stage, and at fruit maturity stage, the highest value is in the three-time topdressing application; Plant total nitrogen, 15N uptake and 15N-urea use efficiency are the highest in the three-times topdressing application at the fruit maturity stage.