''Citrus halimii'', or mountain citron, is a

citrus ''Citrus'' is a genus of flowering trees and shrubs in the family Rutaceae. Plants in the genus produce citrus fruits, including important crops such as oranges, mandarins, lemons, grapefruits, pomelos, and limes. ''Citrus'' is nativ ...

with sour fruit. Historically placed within the

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

grouping of papedas, it has since been determined to be a wild species most closely related to the kumquats, and is not related to the true

citron The citron (''Citrus medica''), historically cedrate, is a large fragrant citrus fruit with a thick Peel (fruit), rind. It is said to resemble a 'huge, rough lemon'. It is one of the Citrus taxonomy#Citrons, original citrus fruits from which al ...

. It was first discovered and catalogued in 1973.

Distribution

''Citrus halimii'' is quite rare and poorly studied. After its initial 1973 discovery and description, it has been observed in only a handful of locations scattered across

Southeast Asia Southeast Asia is the geographical United Nations geoscheme for Asia#South-eastern Asia, southeastern region of Asia, consisting of the regions that are situated south of China, east of the Indian subcontinent, and northwest of the Mainland Au ...

Thailand Thailand, officially the Kingdom of Thailand and historically known as Siam (the official name until 1939), is a country in Southeast Asia on the Mainland Southeast Asia, Indochinese Peninsula. With a population of almost 66 million, it spa ...

and

Malaysia Malaysia is a country in Southeast Asia. Featuring the Tanjung Piai, southernmost point of continental Eurasia, it is a federation, federal constitutional monarchy consisting of States and federal territories of Malaysia, 13 states and thre ...

(the locations of the first discoveries), along with isolated stands in

Indonesia Indonesia, officially the Republic of Indonesia, is a country in Southeast Asia and Oceania, between the Indian Ocean, Indian and Pacific Ocean, Pacific oceans. Comprising over List of islands of Indonesia, 17,000 islands, including Sumatra, ...

Description

''Citrus halimii'' is a midsized

evergreen In botany, an evergreen is a plant which has Leaf, foliage that remains green and functional throughout the year. This contrasts with deciduous plants, which lose their foliage completely during the winter or dry season. Consisting of many diffe ...

tree, with a mature height of ; it is somewhat less thorny than other citrus. Like other papedas, the halimii has relatively large leaves, with a long, winged petiole. The fruits of ''C. halimii'' are edible, but sour. They are round and small, measuring about in diameter. The rather thick rind eventually ripens to yellow or orange-yellow; internally the rind is tightly bound to the flesh. The yellow-green segments are filled with a number of large seeds, and a small quantity of juice.

Taxonomy

Historically, limited characterization made appropriate taxonomic placement of the mountain citron unclear, with particular uncertainty over whether it was a natural hybrid or a pure wild species. Some phylogenies placed the mountain citron within

Walter Tennyson Swingle Walter Tennyson Swingle (January 8, 1871 – January 19, 1952) was an American agricultural Botany, botanist who contributed greatly to the classification and citrus taxonomy, taxonomy of citrus. Biography Swingle was born in South Canaan, Cana ...

's grouping of inedible citrus, the subgenus '' Papeda'', but this grouping proved to be polyphyletic. A 2022 study of the genomics of the mountain papeda revealed it to have low levels of hetero

zygosity Zygosity (the noun, zygote, is from the Greek "yoked," from "yoke") () is the degree to which both copies of a chromosome or gene have the same genetic sequence. In other words, it is the degree of similarity of the alleles in an organism. Mos ...

, consistent with it being a true citrus species and not a hybrid variety. Its genetic markers, along with the profile of its

essential oil An essential oil is a concentrated hydrophobic liquid containing volatile (easily evaporated at normal temperatures) chemical compounds from plants. Essential oils are also known as volatile oils, ethereal oils, aetheroleum, or simply as the ...

s, showed it to be most closely related to

kumquat Kumquats ( ), or cumquats in Australian English, are a group of small, angiosperm, fruit-bearing trees in the family Rutaceae. Their taxonomy is disputed. They were previously classified as forming the now-historical genus ''Fortunella'' or plac ...

References

halimii Fruits originating in Asia Trees of Thailand Trees of Peninsular Malaysia Trees of Borneo Plants described in 1973 {{Rutaceae-stub