Effects of two tree shapes on fruit quality of Gala apple

【Objective】This study aimed to clarify the effects of different training systems on the fruit quality of Gala apples, so as to provide a scientific basis for producing high-quality fruits in Gala apple cultivation.【Methods】The experiment utilized Gala apples grafted onto the dwarfing rootstock M9- T337 as plant materials. Two distinct training systems—the vertical spindle and the V-shape—were established under consistent horticultural management. Fruit sampling was conducted during the commercial maturity period across two consecutive growing seasons (2023 and 2024). Each year, from each training system, 50 representative fruits were randomly collected. A comprehensive physicochemical analysis was performed, measuring fundamental metrics (single fruit weight, soluble solids content, and flesh firmness), nutritional components (vitamin C), and detailed compositional profiles. The latter included sugar components (fructose, sucrose, glucose, sorbitol), organic acid components (malic acid, citric acid, quinic acid, shikimic acid), volatile aroma compounds, and phenolic substances. To objectively integrate these multi- dimensional data, a comprehensive evaluation model was employed. Firstly, the coefficient of variation method was used to assign objective weights to each of the 16 measured quality indicators, reflecting their relative discriminatory importance. Subsequently, subordinate function values were calculated for each indicator to normalize the data. Finally, the differences in overall fruit quality between the two training systems were quantitatively evaluated and compared using the comprehensive subordinate function evaluation method.【Results】The results indicated that significant differences in fruit quality of Gala apples were observed between the two training systems, with notable variations also present across years. These differences were primarily manifested in the following aspects: (1) In 2023, the single fruit mass of the spindle- shaped system was significantly higher than that of the Vshaped system, but this difference was not significant in 2024. The soluble solids content, flesh firmness, and vitamin C content of spindle- shaped Gala apples were significantly higher than those of Vtrained trees over two growing seasons. (2) Fructose was the predominant sugar component in Gala apples, contributing more than 51.0% of the total sugar content, followed in descending order by sucrose, glucose, and sorbitol. In 2023, the contents of total sugars, fructose, glucose, and sorbitol in apples grown under the spindle- shaped training system were 19.36% , 20.97% , 35.65% , and 40.48% higher than those in the V-shaped training system, respectively. (3) Malic acid was the major organic acid, comprising over 93.65% of the total, followed by quinic acid, citric acid, and shikimic acid. The contents of total organic acids and malic acid were significantly lower in the spindle-shaped system. The citric acid content in the spindle-shaped system was 22.2% lower than that in the V-shaped system in 2023. (4) A total of 46 volatile aroma compounds were detected, including 13 alcohols, 14 esters, 9 aldehydes, 6 ketones, and 4 others, with 37 compounds common to both systems. Alcohols and esters accounted for 37.81%-47.38% and 28.85%-38.92% of the total volatiles, respectively. The influence of different tree training systems on the volatile compound content of apple fruits exhibited notable variation. Overall, the total volatile aroma content in Gala apples was higher in spindle-shaped trees compared to V-trained trees. Specifically, the concentrations of alcohols and ketones were significantly greater in fruits from spindle-shaped trees, while no significant differences were observed in aldehydes and other volatile constituents between the two training systems. Furthermore, across multiple growing seasons, the contents of 1-octanol, trans-2-nonenal, (E)-2-octenal, hexyl butyrate and butyl hexanoate were consistently and significantly higher in spindle- shaped Gala apples than in those from V-trained trees. (5) Thirty- nine phenolic compounds were detected in the spindle-shaped system and 38 in the V-shaped system. Phenolic acids and flavanols were the major phenolic subgroups, accounting for 32.70%-34.38% and 37.5%- 46.02% of the total phenolic content, respectively. The contents of specific phenolics such as cryptochlorogenic acid, phloretin pentose-hexose (isomers), 3-hydroxyphloretin 2'-xylglucoside and procyanidin B2 were significantly higher in the spindle-shaped system. (6) Based on 16 quality indices including single fruit weight, soluble solids content, and flesh firmness, weights were assigned using the coefficient of variation method, and subordinate function values were calculated for each index. The comprehensive evaluation using the subordinate function method concluded that the fruit quality of the spindleshaped system was superior to that of the V-shaped system.【Conclusion】In summary, the fruit quality of Gala apples cultivated under the spindle-shaped training system was superior to that of those grown under the V- shaped system. Specifically, relative to the V- shaped system, the spindle- shaped system yielded fruits with higher contents of soluble solids, flesh firmness, vitamin C, and ketones, along with lower contents of organic acids and malic acid. Therefore, the spindle-shaped training system is recommended for promotion in high- density, dwarf- rootstock Gala apple orchards, which will facilitate the production of high- quality fruits and satisfy the requirements of both domestic and international markets. Future research should focus on exploring the variations in fruit quality across different fruiting positions within the canopy under distinct training systems, as well as evaluating key indicators such as the premium fruit rate, so as to further improve the economic benefits of apple cultivation.