Distribution of branch composition, fruit quality and yield within the tree canopy of Regal Gala/Malus hupehensis Rehd. by top grafting

Abstract：【Objective】The stock-scion combination of Regal Gala with rootstock of Malus hupehensis Rehd. served as the experimental samples, which were top grafted. The spatial distribution characteristics of branch composition, fruit yield and fruit quality were investigated in different canopy layers, so as to provide a reference for the technical optimization and improvement of fruit quality with this cultivation mode in cold regions of Liaoning province.【Methods】Three types of samples, including Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd., Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting), and Regal Gala on the rootstock of M. hupehensis Rehd. (low grafting), were used as experimental materials. In 2020, the basic development status of the test trees was investigated, and indicators included trunk diameter of variety and rootstock, tree height, canopy width(east-west and south-north), coverage rate, etc. The experimental tree was divided into 4 layers according to the canopy height, and the number and proportion of branch types (long shoots, medium shoots and spurs) at different canopy heights were investigated. The differences in fruit yield and quality at different canopy heights among the tested materials were compared. Surveyed indicators included fruit mass, fruit shape index, firmness, soluble solids, titratable acidity, TSS/TA ratio, color difference value (L, a, b), etc.【Results】The growth rate of tree height, canopy width and coverage rate of Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) was higher than that of Regal Gala on the interstock of GM256 and rootstock on M. hupehensis Rehd.. There was difference in the growth rates between the rootstock of M. hupehensis Rehd. and the interstock of GM256. The results showed obvious differences in the variety/rootstock ratio of the apple tree. Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) showed a‘small foot’(The scion overgrows the stock), but a‘big foot’in Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd.. The number of branches and the proportion of spurs with Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) were higher than those of Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd., and Regal Gala on the rootstock of M. hupehensis Rehd. (low grafting). The numbers of branches of Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting), Regal Gala on interstock of GM256 and rootstock of M. hupehensis Rehd., and Regal Gala on the rootstock of M. hupehensis Rehd. (low grafting) were 96.88 × 104 No.·hm^-2 , 78.11× 104 No.·hm^-2 and 86.51 × 104 No.·hm^-2, respectively. The branches of Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting), or on the rootstock of M. hupehensis Rehd. (low grafting) were mainly distributed within a canopy height of 2.00 to 3.00 m or greater than 3.0 m, accounting for 43.51% and 45.37% of the total number of branches, respectively. The height from 1.00 to 2.00 m was the main distribution area of Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd., accounting for 47.96% of the total number of branches. There were differences in the spatial distribution of spurs within the tree crown between Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) and Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd.. The spurs of Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) were mainly distributed in the canopy height of more than 2.00 m, accounting for 56.77% of the total number of branches. The spurs of Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd. were mainly distributed in the canopy height of 1.00 to 3.00 m, accounting for 59.31% of the total number of branches. Fruit mass, fruit index, firmness, titratable acidity, L and a values of color difference with Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) were higher than those of Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd. within the same height of the canopy. The fruit quality within different canopy heights were compared, and it was found that the fruit firmness and titratable acidity of Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting), the rootstock of M. hupehensis Rehd. (low grafting), or interstock of GM256 and rootstock of M. hupehensis Rehd. decreased with the increasing of canopy height, while TSS/TA value and L, a, b values of color difference showed an upward trend. At the canopy height from 2.00 to 3.00 m, the fruit mass, fruit index, and soluble solid content with Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) were the highest. Fruit mass and fruit index of Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd. were the highest at the canopy height of 1.00 to 2.00 m, soluble solid content was the highest at the canopy height of 2.00 to 3.00 m. Fruit mass of Regal Gala on the rootstock of M. hupehensis Rehd. (low grafting) was the highest when the canopy height was from 2.00 to 3.00 m, and soluble solid content was the highest at the canopy height of morethan 3.00 m. There were significant differences in the distribution of fruit yield at different canopy heights. The fruit yield of Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) or (low grafting) were mainly concentrated in the spatial areas of more than 2.00 m, accounting for 69.79% and 75.55% of the total yield of the tree. The fruit yield of Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd. was concentrated from 1.00 to 3.00 m, accounting for 74.59% of the total production. Therefore, it can be seen that the concentrated distribution of fruit yield with Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting) or (low grafting) were in the upper and middle parts of the tree canopy, while Regal Gala on the interstock of GM256 and rootstock of M. hupehensis Rehd. was in the middle part of the tree canopy.【Conclusion】Using M. hupehensis Rehd. as the rootstock and adopting top grafting method, the Regal Gala apple tree had larger coverage rate, more total branches, higher proportion of spurs, and better exterior fruit quality. The canopy height from 2.00 to 3.00 m was the concentrated area for the distribution of branches and fruit yield in the tree of Regal Gala on the rootstock of M. hupehensis Rehd. (top grafting). The characteristics of fruits produced in this canopy area were that fruit mass was higher, fruit index was better and soluble solids content was higher. The Regal Gala on the rootstock of M. hupehensis Rehd., in which top grafting was adopted was suitable for application and promotion in the apple orchards in cold regions of Liaoning province. They were not only easy to manage, but also conducive to increasing yield and income.