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

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

with sour fruit. Historically placed within the

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

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 rind. It is said to resemble a 'huge, rough lemon'. It is one of the original citrus fruits from which all other citrus types developed throu ...

. 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, also spelled South East Asia and South-East Asia, and also known as Southeastern Asia, South-eastern Asia or SEA, is the geographical south-eastern region of Asia, consisting of the regions that are situated south of mainlan ...

Thailand Thailand ( ), historically known as Siam () and officially the Kingdom of Thailand, is a country in Southeast Asia, located at the centre of the Indochinese Peninsula, spanning , with a population of almost 70 million. The country is b ...

and

Malaysia Malaysia ( ; ) is a country in Southeast Asia. The federation, federal constitutional monarchy consists of States and federal territories of Malaysia, thirteen states and three federal territories, separated by the South China Sea into two r ...

(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 and Pacific oceans. It consists of over 17,000 islands, including Sumatra, Java, Sulawesi, and parts of Borneo and New Guine ...

Description

''Citrus halimii'' is a midsized

evergreen In botany, an evergreen is a plant which has foliage that remains green and functional through more than one growing season. This also pertains to plants that retain their foliage only in warm climates, and contrasts with deciduous plants, whic ...

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 Swingle's grouping of inedible citrus, the subgenus '' Papeda'', but this grouping proved to be polyphiletic. 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. Mo ...

, consistent with it being a true citrus species and not a hybrid variety. Its genetic markers, along with the profile of its essential oils, showed it to be most closely related to

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

References

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