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Citrus taxonomy refers to the botanical classification of the species, varieties, cultivars, and graft hybrids within the genus Citrus, subgenus Papeda, and related genera, found in cultivation and in the wild.
Citrus taxonomy is complex. Cultivated citrus are derived from various citrus species found in the wild. Some are only selections of the original wild types, while others are hybrids between two or more ancestors. Citrus plants hybridize easily between species with completely different morphologies, and similar-looking citrus fruits may have quite different ancestries. Conversely, different-looking varieties may be nearly genetically identical, and differ only by a bud mutation.
Most hybrids express different ancestral traits when planted from seeds (F2 hybrids) and can continue a stable lineage only through vegetative propagation. Others do reproduce true to type via nucellar seeds in a process called apomixis. Some differ only in disease resistance.
Clear genetic lineages are very important for breeding improved cultivars. Only two citrus cultivars have had their full genomes sequenced, and both are hybrids (sweet oranges and clementines). Many different phylogenies for the non-hybrid citrus have been proposed, and taxonomic terminology is not yet settled.
Interbreeding seems possible between all citrus plants, and between citrus plants and some plants which may or may not be categorized as citrus. The ability of citrus hybrids to self-pollinate and to reproduce sexually also helps create new varieties.
These taxa all interbreed freely, despite being quite genetically distinct. They probably arose through allopatric speciation, with citrons evolving in northern Indochina, pummelos in the Malay Archipelago, and mandarines in Vietnam, southern China, and Japan.
The hybrids of these four taxa include familiar citrus fruits like oranges, grapefruit, lemons, limes, and some tangerines. In many cases, these crops are propagated asexually, and lose their characteristic traits if bred. However, some of these hybrids have interbred with one another and with the original taxa, making the citrus family tree a complicated network.
There are also groups that interbreed with the four core taxa, but which have not historically been categorized as citrus. The trifoliate orange and kumquats do not naturally interbreed with the four undisputed citrus taxa due to different flowering times, but hybrids (such as the citrange and calamondin) exist. Australian limes and Clymenia are native to Australia and Papua New Guinea, so they did not naturally interbreed with the four core taxa; but they have been crossbred with mandarins and calamondins by modern breeders.
Humans have deliberately bred new citrus fruits by propagating wild-found seedlings (e.g. clementines), creating or selecting mutations of hybrids, (e.g. Meyer lemon), and crossing different varieties (e.g. 'Australian Sunrise', a finger lime and calamondin cross).
Genetic analysis is starting to make sense of this complex phylogeny. Two citrus fruits have had their full genomes sequenced (sweet orange and clementines). Partial sequences of other cultivars may give contradictory results, as some citrus hybrids are not at all well-mixed. Many different phylogenies for the non-hybrid citrus have been proposed, and taxonomic terminology is not yet settled.
Citrus naming systemsEdit
Because citrus taxonomy is still understudied, there are many systems of classification that very often contradict each other. There are two main complete systems, that of Tanaka and that of Swingle; however, many citrus types were identified and named by individual taxonomists.
The basis of the "Tanaka system" is to provide a separate species name for each cultivar, regardless of whether it is pure or a hybrid of two or more species or varieties. Tanaka is therefore considered to have been a taxonomic "splitter".
The "Swingle system" works differently, in that it first divides the genus Citrus into species, secondly divides it further into varieties, and lastly divides it again into cultivars or hybrids. The Swingle system is generally followed today with much modification; however, there are still huge differences in nomenclature between countries and even individual scientists.
Common name confusionsEdit
The same common names may be given to different citrus hybrids or mutations. Fruit with similar ancestry may be quite different in name and traits (e.g. grapefruit, common oranges, and ponkans, all pommelo-mandarin hybrids).
Mandarin oranges (tangerines, satsumas) are one of the basic species. Even pure-bred ones are genetically diverse. Many varieties commercially called mandarins are actually hybrids. Most are hybrids with sweet oranges, but some are hybrids with lemons. Some are hybrids with pomelos (and thus like sweet oranges).
Many varieties of true (non-hybrid) Citron have distinctly different forms. Some fingered citron varieties are used in buddhist offerings, and some more common varieties are used as the etrog in the Jewish harvest festival of Sukkot. There is also a specific variety of citron called etrog.
In Modern Hebrew, etrog indicates any variety of citron, whether or not it is appropriate for Sukkot use. But when etrog is used as an English term of art, it applies only to those varieties and specimens used as one of the Four Species in the festival. The various Jewish ethno-cultural groups use different varieties, according to their traditions or the decisions of their respective Jewish Law scholars. The Mountain citron, however, has no relation to the true citron and belongs to the citrus subg. Papeda.
Many citrons are not hybrids but varieties of the original species. The citron is usually fertilized by self-pollination, and is therefore generally a male parent of any citrus hybrid, rather than a female one. Many citron varieties were proven to be non-hybrids despite their morphological differences; however, the florentine citron is probably of hybrid origin.
Kumquats are a separate species, with few hybrids, none of which is commonly called "kumquat".
Australian and New Guinean speciesEdit
The trifoliate orange is a cold hardy plant distinguishable by its leaves with three leaflets. It is the only member of the Poncirus genus (disputed), but is close enough to the Citrus genus to be used as a rootstock. The Indian wild orange is, according to some research, one of the ancestors of today's cultivated citrus fruits. The bergamot orange is likely to be a hybrid of Citrus limetta and Citrus aurantium.
Citrus hybrids include many varieties and species that have been selected by plant breeders, usually for the useful characteristics of the fruit. Some citrus hybrids occurred naturally, and others have been deliberately created, either by cross pollination and selection among the progeny, or (rarely, and only recently) as somatic hybrids. The aim of plant breeding of hybrids is to use two or more different citrus varieties or species, in order to get intermediate traits, or the most desirable traits of the parents. In some cases, particularly with the natural hybrids, hybrid speciation has occurred, so the new plants are considered a different species from any of their parents. Citrus hybrid names are usually marked with a multiplication sign after the word "Citrus", for example Citrus × aurantifolia.
Labelling of hybridsEdit
Citrus fruit taxonomy is still poorly understood, and even modern hybrids of known parentage are sold under general names that give little information about their ancestry, or technically incorrect information.
This can be a problem for those who can eat only some citrus varieties. Drug interactions with chemicals found in some citrus, including grapefruit and Seville oranges, make the ancestry of citrus fruit of interest; many commonly sold citrus varieties are grapefruit hybrids or pummello-descended grapefruit relatives. One medical review has advised patients on medication to avoid all citrus juice, although some citrus fruits contain no furanocoumarins.
Major citrus hybridsEdit
- Grapefruit: Grapefruits are more akin to the pommelo ancestor.
- Lemon and Lime: Most lemons are from one common ancestor, and diverged by mutation. The ancestor was a hybrid with citron, pummelo and mandarin ancestry; citrons contribute the largest share of the genome.
- Orange: Not all the fruits that are called by the name "orange" share much genetic affiliation. The common sweet orange and sour orange genetics are like those of the grapefruit, a cross between a pommelo and a mandarin orange. They are all intermediate between the two ancestors in size, flavor and shape. The above-mentioned oranges have the orange color of the mandarin orange in their outer peels and segments, and are easier to peel than the grapefruits.
Sweet lemons and sweet limesEdit
Sweet lemons, sweet limes, and rough lemons are hybrids similar to non-sweet lemons and limes, but with less citron parentage.
- The lumia from the Mediterranean basin, probably Italy, is a big dry citron-like citrus that is pear shaped and not necessarily sweet.
- The Persian sweet lime, also known as the limetta, is small and round like a common lime, with sweet juice.
- The Palestinian sweet lime – Citrus × latifolia – from India is mainly used as a rootstock.
Sweet limes and lemons are not sharply separated:
The sweet lime, Citrus limettioides Tan. (syn. C. lumia Risso et Poit.), is often confused with the sweet lemon, C. limetta Tan., (q.v. under LEMON) which, in certain areas, is referred to as "sweet lime". In some of the literature, it is impossible to tell which fruit is under discussion.
The same plant may also be known by different names:
The Indian sweet lime is the mitha nimbu (numerous modifications and other local names) of India, the limûn helou or succari of Egypt, and the Palestine sweet lime (to distinguish it from the Millsweet and Tunisian limettas, commonly called sweet limes).
Other minor citrus hybrids (partial list)Edit
- Shikwasa, Hirami lemon – Citrus × depressa
- Kaffir lime – Citrus × hystrix
- Rangpur lime – Citrus × limonia
- Sudachi – Citrus sudachi
- Yuzu – Citrus ichangensis × reticulata
- Ponderosa – Citrus limon × medica
- Rhobs al-arsa – Citrus limon
- Florentine citron – Citrus limonimedica
- Oroblanco, oro blanco (white gold) or sweetie and Melogold – (Citrus grandis Osbeck × Citrus Paradisi Macf.)
- Pixie mandarin – a cross between King tangor and Dancy mandarin with possible unknown pollen donor
- Ugli fruit – Citrus reticulata × Citrus paradisi
- Lemonade fruit – a cross between navel orange and lemon
Graft-chimaeras, also called graft hybrids, can occur in Citrus. The cells are not somatically fused but rather mix the tissues from scion and rootstock after grafting, a popular example the Bizzaria orange. In formal usage, these are marked with a plus sign "+" instead with an "x".
Citrofortunella according to the Swingle system, is a hybrid genus, containing intergeneric hybrids between members of the genus Citrus and the closely related Fortunella. It is named after its two parent genera. Such hybrids often combine the cold hardiness of the Fortunella, such as the Kumquat, with some edibility properties of the citrus species. Citrofortunellas, which are all hybrids, are marked with the multiplication sign before the word "Citrofortunella", for example × Citrofortunella microcarpa or × Citrofortunella mitis which refer to the same plant.
Carl Peter Thunberg originally classified the kumquats as Citrus japonica in his 1784 book, Flora Japonica. In 1915, Walter T. Swingle reclassified them in a segregate genus, Fortunella, named in honor of Robert Fortune. Seven species of Fortunella have generally been recognized—F. japonica, F. margarita, F. crassifolia, F. hindsii, F. obovata and F. polyandra, as well as the recently described F. bawangica. Since the kumquat is a cold hardy species, there are many hybrids between common citrus members and the kumquat (most popularly the calamondin that occurred naturally). Those hybrids are not citrus hybrids, according to Swingle, but reside in a separate hybrid genus which he called × Citrofortunella.
Subsequent study of the many commercial lineages revealed such complexity that the genera could not be separated. Consequently, in accordance with the International Code of Nomenclature for algae, fungi, and plants, the correct genus name reverted to Citrus. The Flora of China returns the kumquat to Citrus and combines the species into the single species as Citrus japonica, and today the Fortunella and Citrofortunella are nothing else but regular citrus.
These plants are hardier and more compact than most citrus plants, often referred to as cold hardy citrus. They produce small acidic fruit and make good ornamental plants. Citrofortunella hybrids include:
- Calamondin – (tangerine crossed with kumquat)
- Citrangequat – (citrange crossed with kumquat)
- Limequat – (Citrofortunella floridana) – (Key lime crossed with kumquat)
- Orangequat – Citrofortunella nippon – (satsuma mandarin crossed with kumquat)
- Procimequat – (Citrofortunella floridana) – (limequat crossed with kumquat)
- Sunquat – Citrus limon × japonica – (lemon crossed with kumquat)
- Yuzuquat – Citrus ichangensis × reticulata – (yuzu crossed with kumquat)
Citrocirus also according to the Swingle system, is a hybrid genus, containing hybrids between members of the genus Citrus and the closely related Poncirus, which includes the trifoliate orange, a cold hardy plant that is commonly used as a citrus rootstock. Citrocirus commonly refers to the citranges which are hybrids between the trifoliate and sweet oranges. However a molecular investigation suggested that Fortunella, Citrofortunella, Poncirus and Citrocirus should all be equivocally included in the genus Citrus.
According to the Swingle system, the trifoliate orange, a cold hardy plant that is commonly used as a citrus rootstock, is not included in the genus Citrus, but in a related genus, Poncirus. Therefore, the citrange, which is a hybrid between the trifoliate and the sweet orange, is placed into a hybrid genus called Citrocirus (not a valid botanical name). However, molecular investigation suggests Poncirus should be equivocally included in the genus Citrus.
Another issue of controversy is whether the citrus-related genera are to be included in the genus or not.
Reclassification of Microcitrus and EremocitrusEdit
The edible Australian limes are starting to be domesticated and commercially grown. Recent molecular research suggests that although Swingle placed them in the separate genera Microcitrus and Eremocitrus, they should also be included in the genus Citrus.
Triphasia and ClymeniaEdit
Two additional genera, Triphasia and Clymenia, are likewise very closely related to the Citrus genus and bear hesperidium fruits, but they are not usually considered part of it. At least one, Clymenia, will hybridize with kumquats and some limes.
The wild lime—native to southern Florida and Texas in the United States, Mexico, Central America, the Caribbean, and South America as far south as Paraguay—does not belong in the Citrus genus. It is part of another genus in the Rutaceae family, Zanthoxylum (which also includes Sichuan pepper), and was classified as Zanthoxylum fagara by Charles Sprague Sargent.
The International Association for Plant Taxonomy was founded in 1950 to resolve the taxonomic problem of all the plants. They hold meetings every five years to review the updated information and try to reach consensus. The current president is David Mabberley.
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- USDA Citrus Genome Database
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- A classification for edible Citrus (Rutaceae)[permanent dead link]
- Citrus Pages A comprehensive article on citrus taxonomy
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- Fruits of warm climates
- Fortunella crassifolia Swingle - Fruits and Seeds Flavon's Wild herb and Alpine plants
- International Code of Botanical Nomenclature (Tokyo Code)
- GRIN database for Species of Citrus