Aurantium Tournefort ex Miller; Citreum Tournefort ex Miller; ×Citrofortunella J. Ingram & H. E. Moore; ×Citroncirus J. Ingram & H. E. Moore; +Citroponcirus H. Wu et al.; Fortunella Swingle; Limon Tournefort ex Miller; Papeda Hasskarl; Poncirus Rafinesque; Pseudaegle Miquel; Sarcodactilis C. F. Gaertner.

Shrubs or small trees, evergreen, rarely deciduous. Young branches often flat and angled, usually with solitary (rarely paired) spines at axils. Leaves 1-foliolate, rarely 3-foliolate or simple; petiole usually articulated with base of leaf blade, usually conspicuously winged; leaf blade subleathery to leathery, with dense pellucid fragrant oil dots, margin crenulate or rarely entire. Flowers axillary, hermaphrodite or male, solitary or in small fascicles, fragrant. Calyx cup-shaped; lobes 3-5, subglabrous. Petals (3 or)4 or 5(-8), white or outside pinkish red, imbricate, thick. Stamens usually 4(-10) × as many as petals, free or basally coherent. Disk annular or short, with nectary glands. Ovary (3-)5-14(-18)-loculed, each locule with 2-8 or more ovules; stigma large. Fruit a berry (hesperidium) with sarcocarp segments of pulp vesicles and adaxially attached seeds. Seed coat smooth or ridged; embryo(s) 1 to many, like cotyledons milky white, green, or rarely yellowish, germination hypogeous.

In China, many early hybridizations appear to have taken place, and many cultivated taxa have become naturalized (these are listed at the end of the generic account). For a discussion of the status of several taxa formerly considered species see Nicolosi et al. (Theor. Appl. Genetics 100: 1155-1166. 2000) and Mabberley (Blumea 49: 481-498. 2004). Hybrids readily form between species, and as apomixis is common, such hybrids can be fixed as cultivars, with occasional outcrossings leading to yet more. Because these hybrids can thereby span, through backcrossing, the spectrum of variation between the original, probably geographically isolated, species, it is impossible to provide a key to cover all plants that may be found. The key here therefore covers the apparently wild species and some of the most common cultivar groups now referred to as hybrid taxa. Doubtful taxa are not included in the key, but descriptions of two are included at the end of the treatment.

Because of the enormous worldwide economic importance of the genus, Citrus is treated more fully, particularly with regard to synonymy, than is the norm in this flora. The following treatment is the first floristic one to take account of current advances in the understanding of the genus.

Additional Hybrid Taxa Cultivated to a Limited Extent in China

Citrus ×microcarpa Bunge, Enum. Pl. China Bor. 10. 1833.

×Citrofortunella microcarpa (Bunge) Wijnands; ×C. mitis (Blanco) J. Ingram & H. E. Moore; Citrus ×mitis Blanco.

Calamondin or calamansi is hybrid between Citrus reticulata and C. japonica.

Citrus ×latifolia (Tanaka ex Yu. Tanaka) Tanaka, Kwaju Bunruigaku [Systematic Pomology], 140. 1951.

Citrus ×aurantiifolia (Christmann) Swingle var. latifolia Tanaka ex Yu. Tanaka, Iconogr. Jap. Citrus Fruits 1: 57. 1946; C. ×aurantiifolia subsp. latifolia (Tanaka ex Yu. Tanaka) S. Ríos et al.

Seedless lime is a hybrid between Citrus ×aurantiifolia and possibly C. ×limon.

Citrus ×floridana (J. Ingram & H. E. Moore) Mabberley, Telopea 7: 337. 1998.

×Citrofortunella floridana J. W. Ingram & H. E. Moore, Baileya 19: 170. 1975.

Limequat is a hybrid between Citrus japonica and C. ×aurantiifolia.

Citrus ×georgiana Mabberley, Blumea 49: 490. 2004.

Citrangequat is a hybrid between Citrus ×insitorum and C. japonica.

Citrus ×insitorum Mabberley, Gard. Bull. Singapore 54: 193. 2002.

×Citroncirus webberi J. Ingram & H. E. Moore, Baileya 19: 171. 1975, not Citrus ×webberi Wester (1915).

Citrange, a hybrid between Citrus ×aurantium and C. trifoliata, is widely grown in China as a rootstock.

Doubtful Taxa

Citrus ×polytrifolia Govaerts, World Checkl. Seed Pl. 3(1): 15. 1999.

富民枳 fu min zhi

Poncirus ×polyandra S. Q. Ding et al., Acta Bot. Yunnan. 6: 292. 1984, not Citrus polyandra Tanaka (1928).

Trees to 2.5 m tall, evergreen. Young branches green, triangular but becoming cylindric with age. Leaves palmately 3-foliolate; petiole 1-2 cm, narrowly winged; lateral leaflet blades 2.7-3.8 × 0.7-1.7 cm; central leaflet blade 3.5-5 × 0.9-1.4 cm, dark green, base cuneate, margin sinuate crenulate, apex mucronate. Flower solitary, 6.4-7 cm in diam. Pedicel 3-7 cm, ca. 2 mm in diam. Calyx lobes 5, broadly ovate, ca. 7 × 5 mm. Petals white, 5-9, 3.2-3.4 × 1.6-1.9 cm, broadly elliptic, lanuginous with more trichomes especially at margins. Stamens 35-43; filaments ca. 4 mm, distinct; anthers yellow with milky white subpellucid dots. Ovary oblate, ca. 6 mm in diam., lanuginous, 10-loculed; style ca. 2 mm; stigma green, clavate, ca. 2 mm, apex emarginate. Fruit green, oblate, lanuginous when young. Fl. Mar-Apr, fr. Aug-Sep.

● Forests on mountain slopes; ca. 2400 m. SE Yunnan (Funing).

This appears to be a Citrus trifoliata hybrid with another Citrus species. Such hybrids are commonly found where the parents are grown together.

Fortunella bawangica C. C. Huang, Guihaia 11: 8. 1991.

霸王金橘 ba wang jin ju

Trees to 4 m tall. Young branches flat; spines ca. 4 cm. Petiole 3-5(-17) mm; leaf blade elliptic to ovate, (2-)4-7(-10) × (1-)2-3 cm, base rounded to obtuse, margin conspicuously crenulate on basal half, apex rounded. Flowers solitary. Pedicel ca. 5 mm but ca. 1 cm in fruit. Calyx lobes ca. 1 mm. Petals elliptic to lanceolate, ca. 7 mm. Stamens 20-25; filaments cohering into bundles; anthers mostly fertile. Ovary ovoid, 5-7-loculed, with 1 or 2 ovules per locule; style short; stigma clavate. Fruit pyriform, 2.2-2.5 × 1.8-2.2 cm, 1- or 2-seeded; carpopodium ca. 2 mm thick. Seeds ovoid, base rounded, apex acute; seed coat smooth; embryo solitary; cotyledons green.

● Scrub; ca. 1200 m. Hainan.

Recent field studies show that populations morphologically in accordance with Fortunella bawangica are widely distributed in Hainan Island. The only character that can be used to distinguish this entity from typical wild populations of Citrus japonica is its pear-shaped fruit. Further study is needed to ascertain whether F. bawangica is a separate species or only a geographical race of C. japonica.

Between 20 and 25 species: E, S, and SE Asia, Australia, SW Pacific islands, with many cultivated taxa widely naturalized in warm countries; 11 species and hybrid species (three endemic) native, naturalized, or extensively cultivated in China, plus five hybrid species cultivated to a limited extent.

(Authors: Zhang Dianxiang (张奠湘); David J. Mabberley)