Retrospect, problematical issues and the prospect of plum breeding in China

Astract:Plum is a major stone fruit, belonging to the genus Prunus of Rosaceae family. Plum gives agreater range of flavor, aroma, texture color, form and size, the qualities that gratify the senses and makefruits desirable, than any other of our orchard fruits. From this great diversity, only two species predomi-nate in modern commercial production: the hexaploid, European plum (Prunus domestica L.) and the dip-loid Chinese plum (P. salicina Lindl. and hybrids) , which are grown mainly in temperate zones. China hasa centuries-old history of plum cultivation with rich germplasm resources and wide-ranging distribution.Almost all production in China is of Chinese plums. Chinese plum (P. salicina L.) , originated in China, isone commercial specie in mondial cultivation mainly for eaten fresh and also one remunerative and ecolog-ical crop, with more than half global annual yield. The native plum landraces are diverse, nice palate andfull-flavored to be highly praised and appreciated by consumers, which have been acclimatized and localcharacteristic under a long-standing cultivated historical selection by man and nature. Those reservedlandraces played an affirmatively promotional role in both Chinese plum industry and local economic de-velopment. For sufficing the market and fruit planting, plum breeding had been developed in the nineteenfifties and sixties mainly on Japanese plum toward the target of high quality and yield, ornamental, andwinter-hardy. The active breeding programmes in China were located in cold regions where plum is themain tree fruit crop. The National Germplasm Repository at Liaoning provides germplasm and resourcesfor the programmes. The main objectives were to increase fruit size, flesh firmness and eating quality, withadaptation to each cultivated region. Based on an incomplete survey, until 2017, China had a total of 64 bred cultivars of plum, which were independent breed, authorized and published, and 57.81% of themwere from scientific research units in Liaoning, Jilin, and Heilongjiang provinces. A general observationshowed that most bred cultivars were gained through selection from landrace seedlings (e.g.‘Suili #3'‘Kui-feng'‘Kuili'and‘Changli #7') and cross breeding between elites of Chinese landraces (e.g.‘Liuhaoli'‘Pingguoli'and‘Guangongli') , and minor sprout mutation (e.g.‘Hongjingli'‘Jinshuaili', ‘Qiuxiangli'and‘Longyuanmili') . In recent years, there appeared a few fine quality plum varieties through hybridiza-tion between native landraces and external modern cultivars (e.g.‘Suili #5'and‘Guomei') . Herein, wemade a pedigree chart of plum cultivars over the years and revealed that most parents in breeding were di-rectly or indirectly depending on characteristic plums from the standpoint of genetic, such as‘Formosa'‘Liuhaoli'‘Suilinghong'and‘Suili #3', which played the part of diaphyseal parent and an important rolein plum breeding for table and ornamental usage. Concurrently, we also presented a certain closed kinshipamong these native bred cultivars and revealed the existing plight in Chinese plum breeding, a fact of limit-ed shared parents with a narrow genetic base. The causes for this plight were multilateral, but mainly attrib-uted to restrictively climatic environment factors. The climate in northeast China was extremely cold in win-ter with a short frost-free period. Plum breeders, particular in Jilin and Heilongjiang province, usually in-troduced few cold-resistant landraces into breeding programmes, such as‘Hongganhe'‘Huangganhe'‘Xiaohuangli'and‘Bayanhongpaoli'. Furthermore, the consumers (market) in different regions had partic-ular recognition and demand on the taste of fruit. For instance, consumers in Jilin province preferred theplums flavour like‘Huangganhe'and‘Lizimei'; in Guizhou they were fond of‘Jianghuang'plum; whilein Sichuan and Chongqing province, people were partial towards plum types as‘Qingcuili'. Furthermore, plum breeding in China also faced nonnegligible issues such as 1) Insufficiency in inheritance of substan-tial fruit traits. A clear understand of genetic predisposition and characteristics of fruit target traits in theoffspring, is the important basis for scientific matching parents in plum breeding program. It is suggestedthat plum materials with good target and comprehensive traits should be selected as parent. The subse-quent family combination (positive and negative) and semi-sibling lines should be studied and verified. 2) Grave study on rootstock. Rootstock breeding receives less attention than scion breeding, and requireslarge populations and commitment of resources for years. Despite these obstacles, new rootstocks are beingdeveloped to provide improved economic performance and wider adaptation. However to this day, the de-velopment of plum rootstock breeding in China is far beyond. Finally, 3) Disease-resistant aim. There areover 29 major diseases that attack plums, including 4 bacteria (e.g. Xanthomonas campestris pv. pruni) , 19 fungi (e.g. Monilinia laxa) and 6 viruses (e.g. Plum pox virus) . Especially in Sharka disease, it is caused byplum pox virus, and is spreading around the world where host plants including European plum, Chineseplum, peach and apricot are cultivated. It has devastated European plum industries in the past and will bepotentially tremendous threaten to Chinese plums. In this case, our related study on Sharka disease shouldbe accelerated to develop in depth. As mentioned above, in this review, we addressed retrospect, problem-atical issues and the prospect of plum breeding in China, which we can draw lesson from.