Effects of different fruit load on distribution and utilization characteristics of ¹³C and ¹⁵N of apple

【Objective】 Stable isotope 13C and 15N tracer technology was used to study effects of different fruit load on characteristics of distribution and utilization of 13C and 15N of apple. Based on the nutrientabsorption, theory of the effects of fruit load on growth and development of apple was illustrated to provide evidence for fruit thinning and increasing N fertilizer utilization. 【Methods】 The materials were 5-year-old apple tree(Wanglin/SH38/Malus robusta Rehd.), and the fertilizer was applied on March 27, 2013. The method was annular groove fertilization：15N urea 10 g, N 110.33 g, P2O5 143.15 g and K2O 151.26 g each plant. After the fruiting, the fruits were thinned：control, 2/3 fruit load and 1/3 fruit load. The trees were labeled with 13C at the fruit maturity(September 6). 72 hours after labeled, the trees were sampled to determine 13C and 15N abundance. 13C abundance was determined by DELTA V Advantage isotope ratio mass spectrometer, 15N abundance was determined by ZHT-03 mass spectrograph. 【Results】 Compared with the control, the average single fruit weights of the 2/3 fruit load and 1/3 fruit load are increased by 17.68% and 48.57% respectively, root-shoot ratios are increased by 7.69% and 15.38% respectively, however, the average yields per plant are significantly decreased to 50.18% and 78.60% of control, respectively. The unit area average dry yields of the three treatments are significantly different, 0.83 kg/cm2, 0.54 kg/cm2 and 0.33 kg/cm2, respectively. When the fruit load increases, more 13C-assimilate transports to fruit and results in the decrease of the 13C-assimilation in roots. The 13C partitioning rates in fruits of the three treatments(control, 2/3 fruit load and 1/3 fruit load)are 39.81%, 29.25% and 16.46%, and the rates of roots are 16.79%, 19.98% and 24.64%. The 15N use efficiencies of the three treatments are 8.51%, 10.11% and 13.23%. The Ndff values of the three treatments are consistent, and in order： fruitbiennialrootleavesbranch of perennialtrunk. With fruit load increasing, the Ndff values of the fruits are increased, and the values of the control is 2.76%, which is 1.17 and 1.31 times of that of the 2/3 fruit load and 1/3 fruit load, respectively. The 15N partitioning rate is consistent with that of 13C, and when the 15N partitioning rates are high, the 13C partitioning rates are also high in organ.【Conclusions】 With fruit load increasing, more 13C-assimilate is transported to fruits and results in the decrease of the 13C-assimilate in roots, and the 15N utilization is decreased. It is good for apple production that the unit area average dry yield is 0.54 kg/cm2.