Establishment and application of large-scale hybrid creation system in Chinese jujube

Abstract: Hybridization is the most important way to create new varieties of fruit crops. However, there are some serious problems in cross breeding in terms to Chinese jujube, including difficulty in manual emasculation due to small flower, low fruit setting rate and serious embryo abortion. Only ap- proximate 0.01% of pollinated flowers can produce fruits with hybrid seeds in conventional manual hy- bridization, which definitely hinders the progress of hybridization. It is significant to improve the breed- ing technology and promote the development of jujube industry by establishing a system of large-scale hybrid creation in Chinese jujube and breeding new artificial hybrid varieties to meet the urgent need for hybrid varieties in production. On the basis of large-scale hybrid creation system practice, a system- atic summary is made for the key technical links in this article, including high-affinity hybrid combina- tions, bee-and-net-aided controlled cross, hybrid culture and identification with SSR, add-generation evaluation with soilless culture technology in greenhouse and hybrid preliminary evaluation. At last, a large-scale and high-efficiency controlled cross breeding system was established. In terms to selecting the parents, the combinations of‘JMS2’×wild jujub‘e Xing16’‘,JMS2’בJiao5’‘,Dongzao’בChenguang’,‘Dongzao’בWuhefeng’and‘Dongzao’בFengmiguan’had better affinity, which realized the large-scale fruit and seed setting, and were suitable for large-scale hybrid breeding of jujube. The triploid hybrid offspring of jujube from‘Dongzao’בChenguang’was obtained for the first time, which opened up a new way of sexual polyploid hybridization breeding. It was convenient and effective to use bees auxiliary pollination technique by covering net, which was to replace artificial pollination and greatly reduced the cost of hybrid. The 50 mesh fabric net and bigger net room were good for bee pollination. Generally, it covered an area of 20-70 m2 by putting one box beehive including two combs that can meet the demand for pollination. It was effective to place newly hatched young bees and bee pupae in the hive of netting chamber during the first flowering period. The number of pollination by honeybees showed two obvious peak curves between 11:00—14:00 and 16:00—20:00 in a day, with 18:00 (equivalent to 16:00 in the inland) as the highest peak of pollination activity in Xinjiang. Several factors that may affect hybrid formation were investigated, and the controlled cross technology was optimized. The proper allocation ratios of female‘JMS2’to male parent were 1:1-3:1, while that of female‘Dong- zao’to male parent were 1:1. Girdling may improve fruit setting and seed acquisition in‘Dongzao’. Sowing seed directly in matrix showed much simpler and higher seedling rate than test-tube culture, which was suitable for large scale hybrid culture. It was easy and convenient to put seeds in 4 °C cold refrigerator instead of stratification, to soak seeds until emergence instead of direct seeding seeds and to accelerate bud and use matrix seeding method, which was helpful to cultivate hybrid seeds. Hybrid seed- lings emerged evenly and orderly, the average seedling rate of hybrid seedlings was about 50%, and at last a large number of seedlings were obtained. Male sterility‘JMS2’had the characteristics of self-fer- tility, and the obtained seeds were real hybrids. The offspring that used‘JMS2’as female parents in control cross breeding was real hybrids, which can keep their molecular identification. The greenhouse planting combined with the sand culture trough by soilless culture technology could realize that the 2 years old hybrid plants started flowering and fruiting and bore twice per year, which can be used for the preliminary identification of the traits of the offspring. Based on the study, a large-scale and high-effi- ciency controlled cross breeding system was established by integrating the utilization of male sterile germplasm, high-affinity hybrid combinations, bee-and-net-aided controlled cross, hybrid identification with SSR and add-generation evaluation in greenhouse. With this system, the breeding efficiency in- creased from 0.01% to 11.90% compared with that of conventional artificial pollination. The breeding cycle from controlled crossing to offspring preliminary identification was condensed into 30 months, which effectively shorted the breeding cycle about 11 months compared with the 41 months of the conventional breeding cycle. Using this system, more than 60 000 fruits, 7138 seeds and 2 089 seedlings were obtained in 3 years, with seed yield of 11.90% and seedling yield of 29.27%, and seven groups of hybrid population were obtained. The establishment of these hybrid populations provided materials for the genetic variation of male sterility, polyploidy, non-nuclear traits and fruit characteristics, and it also laid a material foundation for the research on QTL mapping, map-based cloning, and molecular assisted selective breeding for important traits in jujube trees. Therefore, a breakthrough in artificial control cross breeding of jujube was made.