Impacts of fruit load on tree growth, leaf gas exchange and fruit development and quality in apple trees grafted on a dwarfing rootstock in southern Xinjiang

Abstract: 【Objective】Impacts of dwarfing rootstock on the performance of apple trees are affected by environmental conditions. There is no precedent for application of dwarfing rootstocks in apple trees in Xinjiang. In addition, fruit load is one of the most important factors that affect the sustainable growth and development of apple trees, and the effects are stage specific. Thus, this research aims to clarify the impacts of fruit load on tree growth, leaf photosynthetic capacity and fruit yield and quality in appletrees grafted on a dwarfing rootstock over growing season in southern Xinjiang. Moreover, impacts of fruit load on total incomes at harvest were estimated.【Methods】Six-year-old Royal Gala/M9-T337 apple trees in southern Xinjiang were used as materials. Fruit loads were controlled according to the number of fruits per cm² of trunk cross-sectional area (No.· cm^-2 ). In total, 5 fruit loads were set, including 0.3 (T1), 2.3 (T2), 4.4 (T3), 6.1 (T4) and 8.2 (T5) No.· cm^-2 . The seasonal dynamics of trunk diameter, branch diameter and leaf gas exchange and diurnal dynamics of leaf gas exchange were measured. The leaf chlorophyll content in various canopy parts were determined. Number of short (＜5 cm), medium (＞5 cm and ＜15 cm), and long (＞15 cm) shoots were counted. Fruit qualities and yield were determined at harvest and the economic incomes were estimated based on fruit diameter distribution.【Results】The cross-sectional areas of trunk and branch started to increase at 40 days after full bloom, irrespective of fruit loads. At 120 days after full bloom, T1 had significantly larger increase in cross-sectional areas of trunk and branch than the other treatments, and corresponding values in T2 and T3 were significantly higher than in T4 and T5. The canopy width started to increase from full bloom. At 120 days after full bloom, canopy width was larger with lower fruit load, and differed significantly among treatments. In addition, the number and proportion of long and medium shoots were higher, and those of the short shoots were lower with higher fruit load. Consequently, the total number of shoots were higher with higher fruit load and the lowest total number of shoots was found in T1, which had 15.04 × 10⁵ shoots per hectare, and the highest in T5, which had 18.45 × 10⁵ shoots per hectare. Except for long shoots under the fruit load of 0.3 per cm2 , the growth of leafy shoots for all shoot types ceased and was not affected by fruit loads. The growth of long, medium and short shoots ceased at 60, 45 and 30 days after full bloom, respectively. The smaller the fruit load, the larger the length and the base diameter for medium and long shoots. Impacts of fruit load on leaf chlorophyll content depended on leaf position within the canopy. T4 had the lowest SPAD value than the other treatments at upper canopy and the SPAD values of T2, T3 and T4 were significantly higher than those of T1 and T5 at the bottom canopy, and were similar among treatments at the middle canopy. Leaf photosynthesis increased from full bloom and reached the highest at 90 days after full bloom and then decreased to harvest, except for T1, in which highest leaf photosynthesis rate occurred at 120 days after full bloom. During the day, the leaf photosynthesis peaked at 11:30 am. Higher fruit load could significantly improve the leaf photosynthetic capacity. The fruit fresh weight, diameter, fruit shape index, color index and soluble solid sugar were reduced, and fruit number and yield were increased with increase in fruit load. However, the income per hectare peaked at the fruit load of 4.4 No.·cm^-2 , and the corresponding yield and income were 44.38 t·hm-2 and 201 900 Yuan · hm^-2 , respectively.【Conclusion】When fruit load is extremely low, the growth of long shoots were not inhibited. Otherwise, growth of all leafy shoots will ceases after a certain period of time and is not affected by fruit load. In southern Xinjiang. The optimum fruit load for Royal Gala / M9-T337 apple tree is 4.4 fruits per cm2 of trunk cross- sectional area. This fruit load maximizes the photosynthetic capacity of leaves, and improves canopy structure, fruit yield and quality, and the accumulation of soluble sugars.