Abstract: 【Objective】Thinning is one of effective management practices in mature canopy overcrowd apple orchards with standard rootstocks. The aim of the study is to study a better thinning system.【Method】A 18-year-old ‘Fuji’ apple orchard (3×4 m) with standard rootstocks thinned in 2004 was used in 2008 when it became canopy overcrowd again to evaluate the light penetration into trees canopies, leaf characteristics and quantity and quality of fruits with different treatments.【Result】Conditions of apple trees were improved significantly with thinning (Method I). The relative light intensity outer and inner of tree canopies of apple trees with thinning (Method I) was bigger than non-thinned apple trees (Method II and III). The relative light intensity inner trees with thinning (Method I), pruning large branches(Method II) and not pruning large branches（Method III）were 26.4%, 20.8% and 14.3%, respectively. Compared to trees with thinning, the average quantity of shoots was 4.56% and 15.91% more than those of trees with Method II and Method III, respectively. Moreover, the average leaf area of trees with thinning was 0.46% and 7.28% bigger than those of trees with Method II and Method III. The soluble solids and coloring of fruits of trees were increased in a similar trend, while fruit firmness of trees with Method II was better than those of trees with thinning and Method III.【Conclusion】The results showed that thinning treatment for apple trees with overcrowd canopies significantly increased light penetration into tree canopies as well as yield of apples. The quality of leaves and fruits were significantly improved, especially the leaf area index were increased up to 3.26. Based on this study, thinning is proved to be convenient and efficient means to improve the light penetration into tree canopies of apple orchards with standard rootstocks in Weibei region of China.

Key words: apple, thinning, high-density orchard, canopy light, leaf, yield and quality