Influence of different rootstocks on tree growth and fruit quality of Morus multicaulis ‘Zijing’

Abstract:【Objective】The study aimed to evaluate the effect of different rootstocks on tree growth and fruit quality of mulberry. Mulberry (Morus L., Moraceae) has become an important crop throughout the world because its fruit have been industrially exploited for various commercially valuable products. Many studies on mulberry related to genetic diversity, fruit quality, and breeding programs have been carried out, but little information on mulberry rootstocks is available, especially the possibility of applying rootstocks to improve fruit quality.【Methods】In this study, purple-colored mulberry fruit of eight rootstock-scion combinations (Zijing / Sangteyou 2, Zijing / 8632 seedling, Zijing / Guisangyou 62, Zijing /Luza 1, Zijing/Zheza 1, Zijing / Zheza 3, Zijing / Fengchisang and Zijing/Qinsang 1) were used as the materials. These seedlings were budded with Zijing in July 2019, and all the rootstocks were com-patible with the scion. The survived budded plants were transplanted at a spacing of 0.5 m× 2.0 m. Measurements of 18 indexes including stock girth, scion girth, scion/stock girth ratio, total soluble solids content (TSS), soluble sugar content (SSC), reducing sugar content (RSC), titratable acidity (TA), anthocyanin content (AC), etc. were carried out in the third year (2021) after planting.【Results】Rootstocks significantly affected tree growth, fruit physical attributes, sugar and acid content, soluble protein content (SPC) and antioxidant activity of mulberry Zijing. Zijing / Zheza 3 had the highest stock girth (167.67 mm). Zijing / Qinsang 1 exhibited the largest scion girth, branch length and branch girth, which were 145.00 mm, 101.10 mm and 34.79 mm, respectively. In contrast, the lowest stock girth, scion girth, branch length and branch girth were observed in Zijing / Sangteyou 2. Moreover, Zijing / Guisangyou 62 had the largest scion / stock girth ratio (0.95) and differred significantly from the other rootstock-scion combinations. Its ratio was closest to 1, indicating the best grafting compatibility. However, there is no significant difference in number of branches per tree. The fruit weights of the 8 rootstock-scion combinations ranged from 2.17 to 3.03 g with an average of 2.75 g. Of these, Zijing / Luza 1 had the highest fruit weight, which was not significantly different from that of Zijing / 8632 seedling, Zijing / Zheza 1 and Zijing / Zheza 3. There was no significant difference between the fruit vertical and horizontal diameters among the above rootstock- scion combinations. Different rootstocks had little effect on the fruit shape index, which ranged from 1.65 to 1.77. Different rootstocks significantly affected TSS of mulberry fruits, as well as sugar and acid contents. TSS of the 8 rootstocks of mulberry ranged from 11.46% to 16.10%. Zijing/ Zheza 3 exhibited the highest TSS, whereas Zijing / Sangteyou 2 had the lowest and differed significantly from the other rootstock- scion combinations. Zijing / Guisangyou 62 and Zijing/Qinsang 1 had the highest SSC and RSC, respectively. Conversely, the lowest SSC and RSC were observed in Zijing / Zheza 1. Moreover, Zijing / Luza 1 exhibited the maximum value of TA (1.58% ), while Zijing / Sangteyou 2 showed the minimum value (1.08% ). Significant differences in SSC/TA were discovered among the mulberry rootstocks. The highest SSC / TA was detected in Zijing/ Sangteyou 2 (7.42), which had no significant difference with Zijing / 8632 seedling, Zijing/Guisangyou 62, Zijing/Zheza 3, Zijing / Fengchisang and Zijing / Qinsang 1. In contrast, Zijing/Luza 1 exhibited the lowest values of SSC / TA (5.20). Significant difference was found in SPC, vitamin C content (VC) and AC among the different rootstock- scion combinations. Zijing / Sangteyou 2 showed the highest SPC (0.35 mg·g^-1 ), whereas Zijing / Fengchisang and Zijing/ Luza 1 exhibited the lowest and similar values of SPC (0.27 mg·g^-1 ). In addition, the VC of the fruit varied from 0.30 to 0.50 mg·g^-1 . The highest value of VC was detected in Zijing / 8632 seedling, which was significantly different from the other rootstock-scion combinations, and Zijing / Qinsang 1 showed the lowest level of VC. AC ranged from 2 010.62 nmol·g^-1 (Zijing / Guisangyou 62) to 2 477.46 nmol· g^-1 (Zijing / Sangteyou 2). The AC of Zijing/Zheza 1, Zijing/Zheza 3 and Zijing / Fengchisang showed statistically similar levels with that of Zijing / Sangteyou 2. These fruit quality traits were further evaluated by correlation analysis and principal component analysis (PCA), five components were extracted. The accumulative contribution rate of first five principal components was 94.286%, and the rest could be ignored. Particularly, PC1 explained 42.811% of the total variance. According to the comprehensive score of PCA, Zijing / 8632 seedling was with the top comprehensive fruit quality, followed by Zijing/Zheza 3, and Zijing /Sangteyou 2 ranked the last. 【Conclusion】8632 seedling could be considered as a suitable rootstock for Zijing mulberry due to the strong plant growth, high FW, fruit size, TSS, RSC, SSC/TA ratio and AC, and low TA. Furthermore, the results of PCA showed that the quality of 8632 seedling ranked the first, followed by Zheza 3.