| Trifoliate orange | |
|---|---|
 | |
| A fruiting tree inJardin des Plantes,Paris | |
Scientific classification | |
| Kingdom: | Plantae |
| Clade: | Tracheophytes |
| Clade: | Angiosperms |
| Clade: | Eudicots |
| Clade: | Rosids |
| Order: | Sapindales |
| Family: | Rutaceae |
| Genus: | Citrus |
| Species: | C. trifoliata |
| Binomial name | |
| Citrus trifoliata | |
| Synonyms[1] | |
| |
Thetrifoliate orange,Citrus trifoliata (syn.Poncirus trifoliata), is a member of the familyRutaceae. Whether the trifoliate oranges should be considered to belong to their own genus,Poncirus, or be included in the genusCitrus is debated. The species is unusual among citrus for havingdeciduous,compound leaves and pubescent (downy) fruit.[2][3]
It is native to northernChina andKorea, and is also known as theJapanese bitter-orange (karatachi),[4]hardy orange[5] orChinese bitter orange.
The plant is a fairlycold-hardy citrus (USDA zone 6) and will tolerate moderate frost and snow, making a largeshrub or small tree 4–8 m (13–26 ft) tall. Because of its relative hardiness, citrus grafted ontoCitrus trifoliata are usually hardier than when grown on their own roots.[6]
The trifoliate orange is recognizable by the large 3–5 cm (1.2–2.0 in) thorns on the shoots, and its deciduous leaves with three (or rarely, five) leaflets, typically with the middle leaflet 3–5 cm (1.2–2.0 in) long, and the two side leaflets 2–3 cm (0.79–1.18 in) long. The flowers are white, with pink stamens, 3–5 cm (1.2–2.0 in) in diameter, larger than those of true citrus but otherwise closely resembling them, except that the scent is much less pronounced than with true citrus. As with true citrus, the leaves give off a spicy smell when crushed.
The fruits are green, ripening to yellow, and 3–4 cm (1.2–1.6 in) in diameter similar in size to alime and resembling a smallorange, but with a finely downy surface and having a fuzzy texture similar to apeach. The fruits also have distinctive smell from other citrus varieties and often contain a high concentration of seeds.
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Thecultivar 'Flying Dragon' is dwarfed in size, has highly twisted, contorted stems, and has even stronger thorns than the type. It makes an excellent barrierhedge due to its density and strong curved thorns. Such hedges have been grown for over 50 years atOklahoma State University inStillwater, and are highly student-proof.[7] The plant is also highly deer-resistant.[8] Incentral London, mature specimens of the trifoliate orange can be seen in the gardens ofSt Paul's Cathedral.[9]
Trifoliate orange and various of its hybrids with otherCitrus are widely used ascitrus rootstock, which are valued for their resistance to cold, thetristeza virus, and the oomycetePhytophthora parasitica (root rot).[10]
Recent studies have revealed that the trifoliate orange containsauraptene at a high concentration, which is one of the functional components having immunity againstcitrus tristeza virus (CTV).[11]
Trifoliate orange fruits are very bitter, due in part to theirponcirin content. Most people consider them inedible fresh, but they can be made intomarmalade.[8] When dried and powdered, they can be used as acondiment.[12]
The fruits of the trifoliate orange are widely used inmedical traditions of East Asia as a treatment for allergic inflammation.[13]
The trifoliate orange was considered a member of thegenusCitrus untilWalter Swingle (1871 – 1952) moved it in 1943 to its own separate genus,Poncirus, based on its deciduous trifoliate leaves differing from otherCitrus and as part of a larger reclassification that split the historicalCitrus into seven genera. More recently,David Mabberley and Dianxiang Zhang reunited all of Swingle's novel genera back intoCitrus in 2008.[14] Early phylogenetic analysis of trifoliate orangeplastids nestedPoncirus within the citrus, consistent with a single genus,[15][16][17][18] but the sequencing of the nuclear genome by Wu,et al. showed its genome to be most divergent, different enough to justify retention ofPoncirus as a separate genus.[19] To explain the conflict between the plastid and nuclear genomic analysis, it was speculated that the trifoliate orange is likely either the progeny of an ancient hybridization between a core citrus and an unidentified more distant relative, or at some time in its history it acquired anintrogressed cpDNA genome from another species.[20] Ollitrault, Curk and Krueger indicate that the majority of data are consistent with the enlargedCitrus that includes the trifoliate orange, though they recognize that many botanists still follow Swingle.[14]
A second species of trifoliate orange native toYunnan (China) has been reported and namedPoncirus polyandra.[21] WerePoncirus to be subsumed intoCitrus, whereC. polyandra is unavailable, the nameCitrus polytrifolia has been suggested.[22] Zhang and Mabberley concluded this Yunnan cultivar is likely a hybrid between the trifoliate orange and anotherCitrus,[2] but recent genomic analysis ofP. polyandra showed low levels ofheterozygosity,[23] the opposite of what one would expect for a hybrid. This analysis dated its divergence fromP. trifoliata about 2.82 million years ago.[23]
The trifoliate orange does not naturally interbreed with coreCitrus taxa due to different flowering times,[24] but hybrids have been produced artificially between the trifoliate orange and other citrus. In the Swingle system, where the trifoliate orange is placed inPoncirus, a hybrid genus name has been coined for these intra-generic crosses, "× Citroncirus". The most notable of these are thecitrange, a cross between the trifoliate andsweet oranges, and thecitrumelo, a hybrid of trifoliate orange and 'Duncan'grapefruit. Placing the trifoliate orange inCitrus would mean these hybrids would no longer be intergeneric, but instead hybrids withinCitrus. Genomic analysis of a number of these hybrids showed them all to derive fromP. trifoliata and notP. polyandra.[23]
