Effects of fruit load on growth, development and fruit quality of Fuji apple on dwarfing rootstock

Abstract:【Objective】The effects of different load levels on the branch composition, relative content of chlorophyll (SPAD), leaf area, net photosynthetic rate, fruit quality and yield of Fuji trees on M9T337 dwarfing rootstock with tall-spindle system were studied to determine the appropriate load level, so as to provide reference for reasonable load in production.【Methods】The materials were 5-yearold Fuji trees on M9T337 dwarfing rootstock. Six load-level treatments (Ⅰ: 2 fruits retained per square centimeter of trunk cross-sectional area (TCSA);Ⅱ: 3 fruits retained per square centimeter of TCSA; Ⅲ: 4 fruits retained per square centimeter of TCSA; Ⅳ:5 fruits retained per square centimeter of TCSA; Ⅴ: 6 fruits retained per square centimeter of TCSA;Ⅵ: 7 fruits retained per square centimeter of TCSA) were designed in this experiment. The tree canopy was divided into three heights: lower (less than 1.0 m from the ground), middle (1.0-2.0 m from the ground) and upper (more than 2.0 m from the ground). The experiment was carried out after physiological fruit drop, and the load was determined by the trunk cross- sectional area. The trees with different loads were managed as usual. The development of the trees, the number of different types of branches [Long shoots (＞ 60 cm, ＞ 30-60 cm), middle shoots(＞5-15 cm, ＞15-30 cm), spur shoots (≤5 cm)], SPAD, leaf area, net photosynthetic rate, fruit quality and yield were investigated. Excel was used to sort out the obtained data, and SPSS 22.0 was used for analysis of variance.【Results】With the increase of fruit load, the change trend of the number of long shoots (＞60 cm, ＞30-60 cm), middle shoots (＞5-15cm, ＞15-30 cm) and spur shoots (≤5 cm) and total branches of 5- year- old Fuji trees on M9T337 dwarfing rootstock were not obvious. In this study, it was found that the 5-year-old Fuji on M9T337 dwarfing rootstock with tall-spindle system had 128 branches per tree, the total branches were 3.03×10⁵ per hm² , and the yield was 6.27×10⁴ kg · hm^{- 2} . When the load was 2 fruits retained per square centimeter of TCSA, SPAD was 56.28 and the net photosynthetic rate was 15.50 μmol·m^-2 ·s^-1; when the load was 5 fruits retained per square centimeter of TCSA, SPAD was 59.35, and the net photosynthetic rate of leaves was 17.28 μmol·m^-2 ·s^-1 ; when the load was 7 fruits retained per square centimeter of TCSA, SPAD was 56.77, and the net photosynthetic rate of leaves was 16.45 μmol·m^-2 ·s^-1 . The photosynthetic rate and SPAD of leaves first increased and then decreased with the increase of fruit load. There was a negative correlation between leaf area and fruit load. When the load was 2 fruits retained per square centimeter of TCSA, the leaf area was 34.92 cm² ; When the load was 7 fruits retained per square centimeter of TCSA, the leaf area was 15.12 cm² . The photosynthetic rate, SPAD and leaf area of the same load in different heights of the canopy had no obvious change, indicating that the tall-spindle system had no effect on the photosynthetic rate, SPAD and leaf area. There was a negative correlation between fruit load and single fruit weight. With the increase of load, fruit shape index, soluble solids and L basically showed a downward trend, a and b first increased and then decreased, and fruit quality decreased, but had no effect on vitamin C content, titratable acid and fruit hardness. At the same load, the fruit shape index, hardness, soluble solids, titratable acid and vitamic C content in the upper height of the canopy were higher than those in the middle and lower heights, but there was no obvious difference between the middle and lower heights. In addition to the relationship with canopy, bagging also affected fruit quality and color setting. Bagging promoted coloring but significantly reduced fruit quality. The membership function method was used for comprehensive analysis. The membership function values of each index of different loads were Ⅳ>Ⅲ>Ⅱ>Ⅰ> Ⅴ>Ⅵ in a descending order. When the load of 5-year-old Fuji on M9T337 dwarfing rootstock with tallspindle system was 5 fruits retained per square centimeter of TCSA, the fruit yield, quality, color difference value, leaf net photosynthesis, SPAD, leaf area and other comprehensive indexes ranked the best. 【Conclusion】The suitable loading level of Fuji on M9T337 dwarfing self rootstock with tall spindle system was 5 fruits retained per square centimeter of trunk cross-sectional area. The yield was 6.27×10⁴ kg· hm^-2 , and the total branches amount was 3.03×10⁵ per hm² . The ranking of leaf area index, fruit shape index and yield was the highest by using the membership function method.