Formation characteristics of tree structure and fruit quality in Regal Gala apple on Malus hupehensis Rehd. with top grafting

【Objective】Apple stion combination of Regal Gala on Malus hupehensis Rehd. by top-grafting was used as the test material, and the formation patterns of tree structure and fruit yield from young stage to fruiting stage was investigated, so as to provide a theoretical basis for the application and popularization of this cultivation mode in high-density planting orchards in cold regions of Liaoning province.【Methods】The Regal Gala on M. hupehensis Rehd., and Regal Gala with the interstock of GM256 and understock of M. hupehensis Rehd. were used as the test materials, and the annual growth dynamics of tree, composition proportion of branches and fruit yield were investigated from 2011 to 2019. In 2019, the test trees were divided into 4 levels according to the canopy heights, and the differ-ences in fruit yield at different canopy heights were compared. The differences in fruit quality between the test materials were compared. Measurement indicators included fruit mass, fruit shape index, firmness, soluble solids content, titratable acidity, TSS/TA ratio, color difference value, etc.【Results】The tree growth of Regal Gala could be affected by the rootstock. The growth rate of tree height, coverage rate, scaffold limb length and trunk diameter of Regal Gala/M. hupehensis Rehd. were higher than those of Regal Gala /GM256/M. hupehensis Rehd. from 2011 to 2019. The rapid growth of tree height and coverage rate was conducive to increasing a number of branches, which provided guarantee for the formation of fruit yield at the fruit bearing period. The coverage rate in Regal Gala /M. hupehensis Rehd. and Regal Gala /GM256/M. hupehensis Rehd. were 74.70% and 62.30%, respectively in 2019. By comparing the trunk diameter ratio of variety/rootstock, we found the Regal Gala/GM256/M. hupehensis Rehd. showed an overgrowth phenomenon“big foot”and the ratios of variety/rootstock were 0.90 to 0.83 from 2011 to 2019. Regal Gala/M. hupehensis Rehd. showed a“big foot”phenomenon from 2011 to 2014 and a“small foot”phenomenon from 2015 to 2019, and the ratios of variety/rootstock were 0.87 to 0.77 and 1.01 to 1.21, respectively. There was strong ground fixation of the rootstock of M. hupehensis Rehd., and the inclination and lodging phenomenon of tree has not been found in the cultivation and management. The growth rate of total branch number of Regal Gala/M. hupehensis Rehd. was significantly higher than that of Regal Gala/GM256/M. hupehensis Rehd. from 2011 to 2019. A number of branches of Regal Gala/M. hupehensis Rehd. increased rapidly and the numbers were 37.39 × 104 per hm2 to 96.70 × 104 per hm2 from 2015 to 2017, a number of branches tended to be stable and the numbers were 96.70 × 104 per hm2 , 95.10 × 104 per hm2 and 93.05 × 104 per hm2 from 2017 to 2019. Compared to the composition of tree branches, the proportion of long shoots of Regal Gala/M. hupehensis Rehd. was higher than that of Regal Gala/GM256/M. hupehensis Rehd. from 2011 to 2014, while the proportion of spurs of Regal Gala/M. hupehensis Rehd. was higher than Regal Gala/GM256/M. hupehensis Rehd. from 2015 to 2019. From 2014 to 2016, the fruit yield showed a trend of rapid increase year by year, and the fruit yield tended to be stable from 2017 to 2019. The growth rate of fruit yield of Regal Gala/M. hupehensis Rehd. was higher than that of Regal Gala/GM256/M. hupehensis Rehd. The cumulative yields of Regal Gala /M. hupehensis Rehd. and Regal Gala/GM256/M. hupehensis Rehd. were 216 686.25 kg·hm-2 and 158 802.99 kg·hm-2 , respectively from 2013 to 2019, and there was a significant difference between them. By investigating the distribution proportion of fruit yield at different canopy heights, it was found there was significant difference in the distribution of fruit yield in different canopy between Regal Gala/M. hupehensis Rehd. and Regal Gala/GM256/M. hupehensis Rehd. The fruit yield of Regal Gala/M. hupehensis Rehd. was mainly concentrated in the canopy area height greater than and equal to 2.00 m, while the fruit yield of Regal Gala/M. hupehensis Rehd. and Regal Gala/ GM256/M. hupehensis Rehd. was mainly concentrated in the heights from 1.00 to 3.00 m. They accounted for 70.86% and 76.11% of the total yield, respectively. The lowest proportion of fruit yield distribution in the canopy of Regal Gala /M. hupehensis Rehd. and Regal Gala/GM256/M. hupehensis Rehd. was at the canopy heights less than or equal to 1.00 m, greater than and equal to 3.00 m respectively. By comparing with the differences in fruit quality, it was found the fruit mass, fruit shape index, firmness, fruit surface brightness and red degree of Regal Gala/M. hupehensis Rehd. were better than those of Regal Gala/GM256/M. hupehensis Rehd., while the soluble solids content and TSS/TA ratio were lower than Regal Gala/GM256/M. hupehensis Rehd. Tree crown of Regal Gala/M. hupehensis Rehd. was higher, the fruit concentrated distribution area was the upper middle crown, the branches had a low degree of shelter to the fruit, the lighting conditions were much better and the fruit appearance qualitywas better.【Conclusion】Taking M. hupehensis Rehd. as the rootstock and adopting the top grafting method, Regal Gala/M. hupehensis Rehd. showed that growth potential was stronger, coverage rate was larger, the number of total branches was larger, the proportion of spur was higher, exterior quality of fruit was superior and the yield was higher, which was suitable for popularization and application in high-density planting cultivation in cold regions of Liaoning province.