Research progress in walnut variety breeding at home and abroad

Abstract: Walnut is one of the most important nut crops in the world, and ranks first in all kinds of nut species in terms of growing area and total yield. Walnut with high nutritional value has occupied the first place in the world in both growing area and yield in China, and has significantly social, economic and ecological benefits. In addition, walnut also has significant environmental protection and ecological value, and plays an important role in national strategy of returning farmland to forest in China. Walnut varieties with excellent comprehensive characteristics are the basis of the walnut industry. For a long time, the walnut yield per unit in China is lower and the quality is poor. One of the main reasons is the low popularity of excellent varieties, which seriously restrict the further improvement of walnut yield and quality. So, this article reviews the progress in walnut variety breeding in recent years, in order to provide reference for walnut scientific research, production and application in the future. High yield, early fruiting, as well as good quality and stress resistance are the main objectives of walnut breeding, which are affected by the density of fruiting branches, the number of female flowers, fruit setting rate and nut size. China is rich in wild walnut resources, and cross breeding is an important way to improve walnut yield and quality. As there are the same chromosome number and similar genetic background among various walnut species, different walnut varieties had been used in cross breeding to develop more walnut varieties. Walnut cross breeding in China began in the middle and late 1960s. In 1990, the Ministry of Forestry approved 16 new early-fruiting walnut cultivars, which have become the principal  varieties to be popularized in China. In the breeding of walnut, the comprehensive scoring method is the most commonly used. Its main focus is to weight-score the main economic traits such as kernel yield, nut weight and high yield that affect the quality of walnut, and select the best one according to the com- prehensive score, but the weight of each trait is indeed very subjective. After years of development, prin- cipal component analysis and the comprehensive index method have also been widely used in the selec-tion of improved varieties of walnut. As early as the early 1950s, Wuhan Botanical Garden introduced many walnut varieties and types from the United States, the former Soviet Union and some Eastern Eu- ropean countries. Since then, major walnut research and teaching units in China have widely carried out the introduction of exceptional varieties, and successively introduced excellent walnut varieties from abroad. In recent years, with the rapid development of China’s walnut industry, excellent walnut variet-ies have been introduced and planted frequently among provinces, which have greatly promoted the popularization of excellent varieties. China has made some achievements in walnut breeding. All locali- ties have made full use of local walnut germplasm resources and cultivated many excellent walnut vari-eties. However, how to further cultivate new varieties with early bearing, high yield, strong stress resis-tance and better nut quality is still an important goal of walnut breeding in the future. Compared with developed countries, there are still three problems in walnut breeding in China. 1) The breeding goals need to be adjusted to change the previous standard of only pursuing kernel yield and reducing shell thickness, and the breeding of medium thick shell varieties is the mainstream of industrial development in the future. 2) Strengthen the breeding of disease-resistant varieties by molecular biology. At present, existing walnut varieties have no resistance to major diseases, resulting in a significant reduction in yield in disease epidemic years and the benefits of walnut industry. 3) Development and utilization of characteristic resources are far from meeting the needs of industrial development, and there is an urgent need to carry out in-depth work. With the improvement of people's living standards, the deepening un-derstanding of walnut nutritional and health value and medical efficacy, as well as the development and utilization of walnut series products, the demand for walnuts will continue to rise, and higher require-ments are put forward for the quality of walnuts. Looking forward to the future, the field of walnut vari-ety breeding will show the following trends. 1) Medium thick shell varieties are popular for the require-ments of industrial mechanization. 2) The development and utilization of resources should be deepened, so as to cultivate dual-purpose varieties to serve industrial development, and cultivate characteristic va-rieties to meet the diversified requirements of consumers. 3) Molecular biology should be applied to as-sist in the breeding of new varieties and help traditional technologies to improve breeding efficiency. 4) More attention should be paid to the research on germplasm resources and a good foundation should be laid for the breeding of new varieties. Therefore, we should attach great importance to the scientific research on the development and utilization of germplasm resources when cultivating and selecting im-proved varieties, so as to cultivate excellent varieties that adapt to the future trend of simplified tech-niques for walnut production, and promote the sustainable development of China’s walnut industry.