Effects of different dwarfing interstocks on fruit quality of Ruixue apple

Abstract:【Objective】Dwarf and dense planting is the development trend of modern apple cultivation. The selection and utilization of dwarfing rootstocks are the key links to achieve dwarfness. More than 80% of the existing dwarf cultivation areas in China use interstocks. The Ruixue apple has a good fruit shape, crispy flesh, and sweet and sour taste, with strong aroma. It has an outstanding comprehensive performance in the Longdong production area of Gansu province. At present, most studies related to this cultivar mainly focused on the effects of different regions, harvest periods, and fruit bagging on the fruit quality of Ruixue, and there have been few studies on the effects of different dwarfing interstocks on the fruit quality of this variety. Therefore, this study explored the effect of different dwarfing interstocks on the fruit quality of Ruixue apple with an aim to provide reference for the screening and utilization of dwarfing interstocks in the Longdong production area of Gansu.【Methods】The experiment was carried out in 2020 and 2021 at the Qingcheng Apple Experiment Station of Northwest A & F University in Gansu. Different dwarfing interstocks (M9-T337, SH6, SH38 and SC1) were grafted with Ruixue apple (Malus domestica Borkh. Ruixue), and the most widely used M26 interstock was used as the control.One-year-old Xinjiang wild apple tissue cultured seedlings were planted in the spring of 2011. In August of the same year, the interstock scions of M9-T337, SH6, SH38 and SC1 and M26 were collected for budding at 5 cm above the ground. The grafted seedlings were transplanted in the spring of 2012. In the spring of 2017, Ruixue scions were grafted onto the grafted seedlings. In 2020 and 2021, when the apple trees were 4 to 5-year-old, the differences in fruit appearance quality (color, fruit shape index, fruit mass) and internal quality (firmness, sugar and acid content, aroma substances, and functional nutrients) of each stock-scion combination were compared. The effects of dwarfing interstocks on fruit quality were evaluated based on membership values of functional ingredients calculated by the method of membership function, and all the collected data were analyzed using SPSS 22.0. The data obtained were processed and graphed using Microsoft Office Excel 2013 (Microsoft, Redmond, USA) and Origin V9 (OriginLab Corp., Northampton, MA, USA) software.【Results】The results showed that the average fruit mass of apples with different interstocks Ruixue was in the order of M9-T337＞M26＞SH38＞ SC1＞SH6. There was no significant difference in fruit shape index, which was above 0.97. The uniformity of fruit was good, and the fruit shape was high pile. The L* value is a parameter reflecting the glossiness and brightness of the fruit; the a* value represents the green saturation and b* reflects the yellow saturation. The L* and b* of SH6, SH38 and SC1 were significantly higher than those of the control (M26), and the fruit surface was smooth and the appearance quality was excellent. The SH6, SH38 and SC1 were better than M26 and M9-T337 in the soluble solids content, fructose and soluble solid/acid ratio, especially the total soluble sugar and sweetness value, among which SH6 and SC1 were the most outstanding, with soluble solid to acid ratio of 52, a total sugar content above 135 mg · g^-1 , a fructose content above 67 mg·g^-1 , and a sweetness value above 178. Ruixue apple belongs to the green-flavored apple with high contents of esters and olefins. The contents of esters and olefins in SC1 (65.63 μg · kg^{- 1} ) and SH38 (68.45 μg · kg^{- 1} ) were significantly higher than those in the control (65.63 μg · kg^{- 1} ). The content of olefins in SH6 was the highest, which was more than 3 times that of the control. The aldehyde substances were higher in SC1, SH6 and SH38. It was shown that SH series was conducive to the accumulation of fragrant substances in Ruixue fruit, and the fragrance was strong. The contents of flavonoids and Vc in SH6, SH38 and SC1 were significantly higher than those in M26, and were the lowest in M9-T337. The total phenolic content in SH6 was significantly higher than that of the other dwarfing interstocks. 【Conclusion】Grafting Ruixue apple with different dwarfing interstocks can significantly affect the internal and external quality of the fruit. The comprehensive evaluation based on membership function analysis showed that, the SH6, SH38 and SC1 fruits had smooth fruit surface, good internal flavor, strong aroma and high functional nutrient content, and excellent fruit quality, of which SH6 was more prominent. However, after years of investigation on the growth of apples grafted on several dwarfing interstocks in rain orchards in Longdong area of Gansu province, we found that Ruixue apples grafted on SH6 interstock showed weak tree vigor and low yield. Therefore, the candidate rootstocks for cultivation still need to be comprehensively evaluated taking the tree growth in consideration in Longdong of Gansu.