Research progress in kumquat

Citrus is the largest fruit crop in China. Kumquat, a small citrus fruit with important economic and scientific research value, is widely distributed across the Guangxi Zhuang Autonomous Region and Jiangxi, Hunan, Fujian and Zhejiang provinces of China. However, for a long time, kumquat has not received sufficient attention from researchers. Research on kumquat is generally less extensive compared to sweet orange [Citrus sinensis (L.) Osbeck], tangerine (C. reticulata Blanco) and other citrus species. The objective of this article is to review and summarize the research advance in kumquat based on published literature both domestically and internationally, systematically clarify the research achievements on kumquat and to figure out the strategic development directions for the kumquat industry. Advanced searches were conducted on China National Knowledge Infrastructure [Topic = (kumquat OR Fortunella OR Jinju) OR Title = Jindou) AND (Document Type = (Academic Journal or Dissertation)], followed by English topic searches on Web of Science (Title=Fortunella or kumquat, covering the period from 1983 to the present. Duplicate, non-accessible, or irrelevant documents were removed from the search results, which were then supplemented with additional literature from outside the databases, including previously reviewed works and those published in previous years. A total of 2524 Chinese documents and 130 English documents related to kumquat were retrieved. After screening for relevance, over 1000 documents (combining both languages) were retained. The following results were obtained through subsequent organization and analysis of these documents: (1) China is the original center of the Fortunella. Large-scale commercial kumquat plantations are concentrated in such regions as Rong’an and Yangshuo (Guangxi), Suichuan (Jiangxi), Liuyang (Hunan), and Youxi (Fujian). Additionally, wild kumquat populations persist in the mountainous areas of Southern Fujian, Southern Jiangxi, and Southern Hunan; (2) In China’s major kumquat-growing regions, the cultivars from Rong’an of Guangxi and Liuyang of Hunan both originated from Suichuan of Jiangxi. Japanese cultivated varieties trace their lineage to Ningbo of Zhejiang. The European introduction occurred through British plant hunter Robert Fortune, who collected specimens in Fujian during the 19th century, subsequently facilitating the fruit's global dissemination; (3) The genus Fortunella comprises of six recognized species: Hong Kong kumquat (F. hindsii Swingle), Luofu kumquat (F. margarita Swingle; syn. golden jujube), Luowen kumquat [F. japonica (Thunb.) Swingle; syn. round kumquat], Meiwa kumquat (F. crassifolia Swingle), longleaf kumquat (F. polyandra Tanaka), and longevity kumquat (F. obovata Tanaka). Of these, the Meiwa kumquat series dominates commercial cultivation, whereas F. hindsii represents a basal lineage within the genus. Notably, F. hindsii has become a model system for citrus transgenic studies due to its diploid (2n=18) genome and short juvenility period; (4) Research on kumquat cultivation mainly focuses on flower and fruit protection, prevention of fruit splitting, and extension of shelf life. Additionally, significant research is conducted on germplasm innovation strategies, including hybrid breeding. New varieties such as Hua-pi, Jinqiuzao, Guijingan No. 1, Guijingan No. 2 and Cui-mi kumquat (Fortunella crassifolia) have been developed through the exploration of bud-sport and chance seedling variation; (5) The kumquat (Fortunella spp.) is a plant with dual applications in both culinary and medicinal contexts. It has been traditionally utilized in herbal medicine for managing conditions. Being rich in nutrients, kumquats are also widely used as key ingredients in various processed foods, including preserves, candied products, and functional beverages; (6) The taxonomic status of the Fortunella remains under debate; however, an increasing number of studies support its inclusion within the Citrus, and the classification system of the latter is gradually becoming clearer; (7) The earliest documented study on kumquat (Fortunella spp.) in vitro tissue culture can date to 1982. Researchers successfully cultured young stem segments of the kumquat on MS medium, achieving seedling regeneration. Concurrently, studies revealed the high embryogenic potential of the Hong Kong kumquat (F. hindsii Swingle), with its explants readily forming embryogenic callus. Building on these findings, Huazhong Agricultural University acquired a unique monoembryonic F. hindsii accession and subsequently developed its stable genetic transformation protocol. By utilizing kumquat materials with a short juvenile period, researchers assembled the kumquat genome and identified key genes involved in citrus polyembryony (FhRWP), self-incompatibility (FhS2-Rnase), and oil gland formation (CsLMI1). The past decade has witnessed transformative developments in kumquat science, with breakthroughs in cultivation techniques, processing technologies, genomic studies, and breeding innovation. The China Modern Agricultural Citrus Industry Technology System have been studying the genetic traits of Fortunella (kumquat) resources for over a decade. By leveraging the small tree size, short juvenile period, and unique oil gland-free mutant materials of kumquats, combined with multi-omics technologies, they successfully identified CsLMI1, the key gene controlling oil gland formation. This breakthrough has elevated citrus research to a new level. However, research on kumquats still faces significant challenges. Future studies should leverage modern biotechnology and digital technologies to focus on exploring key areas including genetic enhancement, environmentally- friendly cultivation techniques, functional component development, and sustainable processing methods.