Effect of new rootstocks on the fruit quality in Jinmei kiwifruit

【Objective】Kiwifruit is a deciduous fruit tree crop whose fruits are rich in nutrients and very popular with consumers. With its rapid development over the past two decades, kiwifruit has become the world’s most dominant fruit. However, the current world kiwifruit industry relies on a few rootstock varieties from seedlings of the Actinidia chinensis var. delicious. The kiwifruit industry is potentially at risk due to the narrow genetic diversity of rootstocks, and it is of great economic value to explore new rootstocks. Actinidia (kiwifruit) species are native to China, which provides rich germplasm resources for rootstock selections. Here, we used kiwifruit species from the National Actinidia Germplasm Repository in China to evaluate rootstock's effect on the fruit quality, so as to screen out new rootstock varieties.【Methods】In this study, two assays were designed to screen new rootstock varieties. Firstly, grafting experiments were conducted, applying six candidate Actinidia species with a vigorous root system, including A. macrosperma var. mumoides, A. glaucophylla var. rotunda, A. melanandra, A. chinensis Planch, A. valvata Dunn and A. macrosperma C. F. Liang. Secondly, ten kiwifruit genotypes were further selected based on the results of grafting experiments to determine their effect on the fruit quality of the scion cultivar (Jinmei). To obtain unified rootstock seedlings, tissue culture propagation was adopted to ensure the consistency of experimental materials. The compatibility between rootstock and scion was evaluated by measuring the stem diameter of rootstock as well as scion and grafting union after five years of grafting. Various quality indexes were measured among grafting lines to ana-lyze the impact of rootstocks on scion cultivar, including mean fruit weight, soluble solids content, dry matter content, vitamin C content and fruit firmness. In all experiments, more than three replicates were conducted, with each measuring more than 60 fruits.【Results】A total of four conclusions were drawn. (1) The rootstock of different kiwifruit species affected the exterior quality of Jinmei kiwifruit. Jinmei grafted on different rootstocks showed different fruit sizes. The fruit weight exceeded over 100 grams when grafted on A. mumoides, A. rotunda, A. macrosperma and A. valvata rootstocks, while it was only 96.69 grams on A. chinensis rootstock. The analysis of the fruit shape index showed that stion combinations of Jinmei with A. valvata and A. chinensis produced long oval and neat fruit. In contrast, it had partially deviated fruit when grafted on A. macrosperma and A. melanandra rootstocks. (2) Intrinsic quality of Jinmei kiwifruits was affected by different rootstocks. The DFR (Days taken for fruit to ripen at room temperature) were 19- 20 days and firmness of Jinmei kiwifruit was between 0.244- 0.449 N among different stion combinations. In addition, the contents of soluble solids and dry matter in fruits varied. The Jinmei kiwifruit grafted on A. macrosperma, A. melanandra and A. valvata rootstocks had the highest soluble solids content (16.07%-16.71% ) and dry matter content (17.39%-18.18% ), which was significantly higher than that on A.chinensis rootstocks (15.57%, 16.98%). (3) The different compatibility existed between various stion combinations. The average stem diameter of both A. macrosperma and A. valvata rootstocks and scions was more than 6.3 cm, showing vigorous growth between these combinations. However, the grafting union of kiwifruit with A. mumoides appeared to be an apparent enlargement, while those on A. chinensis rootstock were the smallest. Further analysis showed that the ratio of scion to rootstock diameter was more than 1, suggesting an obvious grafting incompatibility (commonly called‘small foot’) between the“Jinmei”scion, and A. mumoides and A. rotunda rootstocks. (4) The different genotypes of rootstock significantly influenced the quality characteristics of scion varieties. The fruit firmness of Jinmei at the ripening stage was relatively average among various rootstock genotypes, while the fruit on A. macrosperma 629 rootstock had the highest firmness. The contents of soluble solids, vitamin C and soluble sugar of Jinmei grafted on different genotypes of rootstocks were significantly different. Taking different rootstock genotypes of A. macrosperma as an example, the dry matter content of genotype A. macrosperma 621 (17.08%) was over 10% higher than that on A. macrosperma 627 (15.36%). Genotype A. macrosperma 626 had 25.53% vitamin C content (942.21 mg · kg-1 ) higher than genotype A. macrosperma 621 (701.57 mg·kg-1 ). In terms of flavor quality, the fruit on A.deliciosa ML1, A. macrosperma 621 and A. macrosperma 619 had the highest soluble sugar content, which was significantly higher than that on A. macrosperma 629.【Conclusion】This study examined the impact of different rootstocks of kiwifruit species and genotypes on scion varieties. Compared with traditional A. delicious rootstocks, A. mumoides, A. rotunda, A. macrosperma and A. valvata rootstocks could improve the plant vegetative growth and fruit size of Jinmei, indicating that the new rootstocks had greater potential to increase the yield of kiwifruit. In addition, the scion cultivar Jinmei had good compatibility with the A. macrosperma rootstocks. Finally, a significant increase in dry matter, vitamin C and soluble sugar contents was observed in Jinmei kiwifruit when various genotypes of A.macrosperma rootstock were utilized. New rootstocks, such as A. macrosperma 629 and A. valvata 625 were identified, which will be used for the commercialization of Jinmei to increase its yields and fruit quality. This research will be used to develop new rootstocks for the future production of kiwifruit.