A classification for edible Citrus (Rutaceae)

Mabberley, D.J. (Rijksherbarium, University of Leiden, Netherlands and Royal Botanic Gardens, Sydney NSW 2000, Australia) 1997. A classification for edible Citrus (Rutaceae). Telopea 7(2): 167–172. A workable Linnaean classification, taking into account recent chemotaxonomic advances, is proposed for edible citrus fruits currently referred to the genus Citrus. They are accommodated in three species and four hybrid groups. Four names are lectotypified. The species problem in Citrus subg. Citrus The taxonomy of the citrus fruits is complicated by hybridity and apomixis, with many stable hybrid lines being accorded species status, so that the number of edible species recognised in the genus Citrus L. (type: C. medica L., the citron) varies from 1 to 162, though Swingle (1944) is widely followed. He recognises 12 species with edible fruits (subg. Citrus) and six without (subg. Papeda (Hassk.) Swingle). The latter subgenus is characterised by its free as opposed to the basally connate stamens seen in subg. Citrus and juice vesicles with acrid oil droplets: in Australia it is familiar in cooking, particularly Thai cuisine, in the ‘lime-leaves’, kaffir or makrut lime, Citrus hystrix DC., a species with widely flanged petioles and probably native in tropical Asia. Intensive chemotaxonomic studies (see Scora & Kumamoto 1983, Scora 1989) indicate that subg. Citrus comprises merely four known allopatric wild species – the tropical (see Jones 1995) C. halimii B.C. Stone (Malay Peninsula and Borneo) and C. maxima (Burm.) Merr. (pomelo, pummelo, SE Asia) and the subtropical C. medica L. (citron) and C. reticulata Blanco (mandarin, tangerine). From enzymatic work it has been shown that from the last three of these and two unknown plants, all commercial edible citrus fruits presently referred to the genus Citrus have been derived through selection and hybridisation (usually unintentional), followed by further selection of ‘agamic complexes’. The major importance of these plants in commercial horticulture (cf. Walters 1961) inflated the taxonomic rank of the complexes, resulting in an increasingly complicated technical nomenclature as new hybrid lines were introduced. This prevailed up until the beginning of the eighteenth century with Tournefort according oranges, citrons and lemons distinct generic rank as Aurantium, Citreum and Limon respectively in 1700. But the morphological distinctions are slight and much of the commercially significant striking degustatory distinction rests on a subtlety, the presence and relative proportions of the two stereoisomers of limonene, one of which is bitter (as in lemon), the other sweet (as in mandarin), resulting in the differing tastes of the flesh and juice. The origins of the principal commercial citrus The wild species are indigenous to the Subhimalayan tract, China and western Malesia but most of our knowledge of them and their hybrids comes from cultivated plants introduced to much of the rest of the world via Europe, where they were first given modern Latin names, which it is therefore necessary to typify. The citron 167 http://dx.doi.org/10.7751/telopea19971007