Effects of different tree-removal methods on orchard structure, solar energy utilization and fruit quality in overcrowded ‘Red Fuji' apple orchards

Abstract: 【Objective】Since early 1980 s, the close planting in apple orchards has become popular for sake of early-bearing and high yield. As the trees grew up, the canopy overlapped each other, and then the trees in those apple orchards became overcrowded. Therefore, the light conditions within the orchards deteriorated, fruiting position on the canopy moved outward, disease and pest problems got worse, fruit yield and quality decreased, and the rotten fruits increased. All of these problems have restricted the sustainable development of apple production, so the transformation of overcrowded apple orchards has become very important for sustainable apple production. Overcrowd canopies of apple trees are the important factor causing the decreased fruit quality, yield reduction and low efficiency. Tree-removal is one of effective management practices in mature overcrowded apple orchards to improve quality and increase efficiency. The aim of the trial is to study the effects of different tree-removal methods on orchard structure, solar energy utilization and fruit quality in overcrowded apple orchards.【Methods】‘Red Fuji'apple trees of 16 years old were used, which were grafted on the‘Ping Yi Tian Cha'rootstock. Three different tree-removal methods were designed for renovation of the overcrowded apple orchards: interlaced row removal (Method Ⅰ) , interlaced tree removal in a row (Method Ⅱ) , and one tree removal for every four trees in alternate rows (Method Ⅲ) . Those without any removal served as the control (CK) . After the completion of transformation, three plants with similar growth potential were selected as sample materials for each treatment. The data of the current year were used for analysis, and the other data were continuously measured for four years from 2012 to 2015. The measurement methods were as follows: the tree canopy volume, canopy coverage rate and transition rate between rows were calculated according to the tree height, crown diameter and trunk height; the branch composition of apple tree structure including spur, middle-shoot, long-shoot and extended growing shoot was calculated every year; the CI-110 canopy analyzer (made by CID Company of U.S.A.) was used to measure the canopy indicators, spacing between trees and spacing between rows of, and the relevant software and Zhang Jixiang's method (to delete the outermost ring of sensitive film of the fisheye camera) were adopted to analyze the leaf area index (LAI) , mean leaf inclination (MLA) , the transmit coefficient for defuse penetration (TCDP) , transmit coefficient for radiation penetration (TCRP) , canopy extinction coefficient (K) and leaf distribution (LD) ; the CIRAS-Ⅱ made by PP-Systems of U.S.A. was used to measure photosynthesis rate. At 10:00 in the morning of September, the net photosynthesis rate, transpiration rate and intercellular CO2 concentration of the mature leaves of peripheral shoots that were 1.5 m above the ground were measured with the photosynthetic system analyzers. Meanwhile, carboxylation efficiency and water use efficiency were calculated. Carboxylation efficiency = net photosynthesis rate/intercellular CO2 concentration; water use efficiency = net photosynthesis rate/transpiration rate; the average fruit weight, fruit firmness, soluble solid content, and quality indicators were measured in the laboratory when the fruit was naturally matured.【Results】The crowded apple orchards with different treatment methods had a great difference in appearance. Compared with CK, three different methods got different degree of optimization, such as the ratio of spurs, short shoots and midium shoots increased while long shoots and extended growing shoots decreased; in the first year, the coverage rate and shoot quantities per 666.7 m2 declined significantly and recovered year by year; the transition rate between the rows of treatments I and Ⅱ declined significantly; the order of leaf area index was Ⅰ< Ⅱ< Ⅲ< CK, the TCDP and TCRP wereⅠ> Ⅱ> Ⅲ> CK; net photosynthetic rate (Pn) and carboxylation efficiency (CE) wereⅠ>Ⅱ> Ⅲ> CK, and intercellular CO2 concentration (Ci) , stomatal conductance (Gs) and transpiration rate (Tr) were Ⅱ> Ⅰ> Ⅲ>CK. From 2012 to 2015, fruit appearance and internal quality such as the average fruit weight, color index, the rate of high quality fruits and soluble solid content with different tree-removal methods were better than those from CK. The yields of three different treatments were significantly lower than CK per 666.7 m2 in the first year, but the yield recovered in the second year, and the yields of method I and Ⅱ were significantly higher than CK in the third and fourth year.【Conclusion】In this study, three kinds of transformation treatments could completely improve the micro-environment of orchards and the fruit quality. However, the method I could increase the yield per 666.7 m2 and improve the fruit quality remarkably, and the lasting effect was longer than other treatments. Therefore, it was recommended that the method Ⅰwas better than others.