Research advances in the evaluation and utilization of apple rootstock germplasm

Apple (Malus × domestica Borkh.) is one of the major cultivated fruit crops in China, and its cultivation area and yield rank first globally. The selection of excellent rootstock varieties is the basis and guarantee for high quality and high yield of apple. Rootstock affects not only its stress resistance and adaptability, but also the quality and yield of apple. In recent years, abiotic stressors, such as drought, soil salinity, heat and cold, are major limiting factors, which result in a decline in fruit quality and yield of apple, and seriously affect the healthy and sustainable development of the apple industry. These threats are likely to become even more significant under climate change and the pressures of an ever-growing human population. The stress resistance of apple trees mainly depends on the rootstock. Therefore, it is of great significance for the development of apple industry to identify and evaluate the resources of apple rootstock and screen the excellent rootstocks with strong stress resistance. China is one of the world's largest apple origin centers, abundant in germplasm resources, and holds great potential for resource innovation and utilization. It is of utmost significance for the development of the apple industry to select outstanding rootstocks with strong stress resistance and dwarfing traits. In this paper, the research and utilization situation of apple rootstock resources both at home and abroad was reviewed to provide a basis for the breeding and wide application of apple rootstocks. The main results are as follows: (1) Apple belongs to Rosaceae family, which comprises 36 species in the world, of which 25 are native to China, including more than 100 variant types. Among these different species and types, there are abundant resources of apple rootstocks with utilization and potential utilization value, which have excellent germplasm such as drought resistance, cold resistance, flood resistance, salt toler-ance, iron deficiency resistance and apomixis. Some of these rootstocks are directly used in production. For a long time, M. sieversii, M. robusta, M. hupehensis and M. baccata have been used as rootstocks in most apple producing areas in China. (2) Apple often suffers from adverse stresses, such as drought, low temperature, salt and alkali, which affects its growth and development and reduces its yield and fruit quality. It has been found that the drought tolerance of M. sieversii, M. prunifolia and M. toringoides are strong. The salt tolerance of apple rootstock varies greatly among different species, ecotypes and even among different individuals, and the salt tolerance of M. xiaojinensis and M. prunifolia were strong. Cold damage often occurs in many apple producing areas in north China, which threatens the development of apple production, so the evaluation on cold resistance of apple rootstock, breeding of cold resistant rootstock and the study on cold resistance mechanism have attracted much attention. In terms of cold resistance evaluation, membrane permeability, relative water content and malondialdehyde are considered to be closely related to cold resistance. GM256 and CX3 are two apple rootstocks that have been reported to be cold-resistant in China. (3) Dwarf cultivation is the development trend of apple production in the world, and dwarf rootstock is one of the main ways to achieve dwarf density- planting. Some dwarfing rootstocks, including SH series, Liaozhen, GM256 and Qingzhen, were bred by hybridization with apple resources in many agricultural institutions in our country. At present, dwarfing rootstocks used in apple production in China mainly include M9T337, M26, MMl06, B9, SH, GM256 and so on. (4) In the past two decades, the cultivation mode of apple in the world has undergone fundamental changes, the tree shape is developing from standard planting to dwarf dense planting, and management is developing from time-consuming and labor-intensive to labor-saving. At present, in the United States, Italy, New Zealand and other countries, the dwarf cultivation has accounted for more than 70% of the total area of apple cultivation However, 80% of apple trees in China are still applying standard planting. Standard- planting apple trees have large crowns, poor light reception, inferior fruit quality, and the cultivation and management costs are high, in which the labor cost has accounted for more than 60% of the total cost of apple production. The stress resistance and tree shape of the apple depend mainly on the rootstock. Under the new situation, how to cultivate and create apple rootstock that is resistant to stress and suitable for labor-saving cultivation and management has become a key scientific problem to be solved urgently in apple cultivation and management in China. (5) Although some apple germplasm resources such as M. sieversii, M. prunifolia and so on have strong drought resistance, they are vigorous. M. hupehensis has the characteristics of apomixis, but its drought resistance is poor and it is vigorous. In addition, the scion varieties on the seedling rootstock have some problems such as delaying fruit maturity. At present, there is a lack of apple rootstock varieties with good comprehensive traits like strong resistance and dwarfing. How to scientifically develop, breed and utilize these excellent germplasm resources is an important topic in the present research. (6) With the rapid development of molecular biology and genomics of fruit crops, people are shifting the research focus of apple rootstock from physiological traits to genetic mechanism. The completion of whole genome sequencing of apple provides a good genomic data platform for further analysis of important agronomic traits of apple rootstocks. At the same time, it will lay a solid foundation for breeding new varieties of apple rootstock with excellent characteristics such as strong resistance, dwarfing and apomixis. Therefore, accelerating the evaluation, identification, development and utilization of apple wild resources and strengthening the selection and breeding of new apple rootstocks will be of great significance to the healthy, stable and sustainable development of the apple industry in China and even on the world.