Trifoliate orange

The trifoliate orange, Citrus trifoliata (syn. Poncirus trifoliata), is a member of the family Rutaceae. Whether the trifoliate oranges should be considered to belong to their own genus, Poncirus, or be included in the genus Citrus is debated. The species is unusual among citrus for having deciduous, compound leaves and pubescent (downy) fruit.<ref name=FOC1753>Template:Citation</ref><ref>Template:Citation</ref>

It is native to northern China and Korea, and is also known as the Japanese bitter-orange (karatachi),<ref name=BSBI07>Template:BSBI 2007</ref> hardy orange<ref>Template:PLANTS</ref> or Chinese bitter orange.

The plant is a fairly cold-hardy citrus (USDA zone 6) and will tolerate moderate frost and snow, making a large shrub or small tree Template:Cvt tall. Because of its relative hardiness, citrus grafted onto Citrus trifoliata are usually hardier than when grown on their own roots.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Description

The trifoliate orange is recognizable by the large Template:Convert thorns on the shoots, and its deciduous leaves with three (or rarely, five) leaflets, typically with the middle leaflet Template:Convert long, and the two side leaflets Template:Convert long. The flowers are white, with pink stamens, Template:Convert 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 Template:Convert in diameter similar in size to a lime and resembling a small orange, but with a finely downy surface and having a fuzzy texture similar to a peach. The fruits also have distinctive smell from other citrus varieties and often contain a high concentration of seeds.

• Poncirus trifoliata0.jpg

  Flower

• Poncirus trifoliata R.H (3).JPG

  Leaves and thorns

• 20130817Citrus trifoliata3.jpg

  Green fruits

• Poncirus trifoliata (cut).JPG

  Bisected fruit

Uses

Cultivation

The cultivar 'Flying Dragon' is dwarfed in size, has highly twisted, contorted stems, and has even stronger thorns than the type. It makes an excellent barrier hedge due to its density and strong curved thorns. Such hedges have been grown for over 50 years at Oklahoma State University in Stillwater, and are highly student-proof.<ref name=klingaman>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> The plant is also highly deer-resistant.<ref name=EatTtheWeeds>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> In central London, mature specimens of the trifoliate orange can be seen in the gardens of St Paul's Cathedral.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Trifoliate orange and various of its hybrids with other Citrus are widely used as citrus rootstock, which are valued for their resistance to cold, the tristeza virus, and the oomycete Phytophthora parasitica (root rot).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Recent studies have revealed that the trifoliate orange contains auraptene at a high concentration, which is one of the functional components having immunity against citrus tristeza virus (CTV).<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

As food

Trifoliate orange fruits are very bitter, due in part to their poncirin content. Most people consider them inedible fresh, but they can be made into marmalade.<ref name=EatTtheWeeds/> When dried and powdered, they can be used as a condiment.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

Medicine

Traditional medicine

The fruits of the trifoliate orange are widely used in medical traditions of East Asia as a treatment for allergic inflammation.<ref>Template:Citation</ref>

Taxonomy

The trifoliate orange was considered a member of the genus Citrus until Walter Swingle (1871 – 1952) moved it in 1943 to its own separate genus, Poncirus, based on its deciduous trifoliate leaves differing from other Citrus and as part of a larger reclassification that split the historical Citrus into seven genera. More recently, David Mabberley and Dianxiang Zhang reunited all of Swingle's novel genera back into Citrus in 2008.<ref name="citrus taxonomy">Template:Cite book</ref> Early phylogenetic analysis of trifoliate orange plastids nested Poncirus within the citrus, consistent with a single genus,<ref name="Nicolosi 2000">Template:Cite journal</ref><ref>Template:Cite journal</ref><ref name="bayer">Template:Cite journal</ref><ref name="nuclear phylogenetic">Template:Cite journal</ref> but the sequencing of the nuclear genome by Wu, et al. showed its genome to be most divergent, different enough to justify retention of Poncirus as a separate genus.<ref name="Talon">Template:Cite journal and Supplement</ref> 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 an introgressed cpDNA genome from another species.<ref name="origin citrus">Template:Cite book</ref> Ollitrault, Curk and Krueger indicate that the majority of data are consistent with the enlarged Citrus that includes the trifoliate orange, though they recognize that many botanists still follow Swingle.<ref name="citrus taxonomy" />

A second species of trifoliate orange native to Yunnan (China) has been reported and named Poncirus polyandra.<ref>Template:Cite journal</ref> Were Poncirus to be subsumed into Citrus, where C. polyandra is unavailable, the name Citrus polytrifolia has been suggested.<ref>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref> Zhang and Mabberley concluded this Yunnan cultivar is likely a hybrid between the trifoliate orange and another Citrus,<ref name=FOC1753 /> but recent genomic analysis of P. polyandra showed low levels of heterozygosity,<ref name=Ze>Template:Cite journal</ref> the opposite of what one would expect for a hybrid. This analysis dated its divergence from P. trifoliata about 2.82 million years ago.<ref name=Ze />

The trifoliate orange does not naturally interbreed with core Citrus taxa due to different flowering times,<ref name="Froelicher">Template:Cite journal</ref> but hybrids have been produced artificially between the trifoliate orange and other citrus. In the Swingle system, where the trifoliate orange is placed in Poncirus, a hybrid genus name has been coined for these intra-generic crosses, "× Citroncirus". The most notable of these are the citrange, a cross between the trifoliate and sweet oranges, and the citrumelo, a hybrid of trifoliate orange and 'Duncan' grapefruit. Placing the trifoliate orange in Citrus would mean these hybrids would no longer be intergeneric, but instead hybrids within Citrus. Genomic analysis of a number of these hybrids showed them all to derive from P. trifoliata and not P. polyandra.<ref name=Ze />