Plants annual or perennial, monoecious, trailing or climbing; stems annual, villous or pustulate-scabrous to pustulate-hispid and sparsely hirsute; roots fibrous or fleshy to somewhat woody, tuberous; tendrils unbranched or 2–3[–4]-branched. Leaves: blade ovate or elongate-ovate to lanceolate-ovate or ovate-triangular, deeply palmately 3–5(–7)-lobed, lobes oblong to ovate or triangular, each pinnately lobed to shallowly sinuate-lobulate, margins remotely serrate to dentate or denticulate, surfaces eglandular. Inflorescences: staminate and pistillate flowers solitary, from different axils; bracts caducous, linear. Flowers: hypanthium campanulate; sepals 5, [oblong-ovate to deltate] lanceolate to linear-lanceolate; petals 5, connate in proximal 1/2, yellow, obovate-oblong to widely oblanceolate [oblong-lanceolate or oblong-ovate], 6–16 mm, pubescent to glabrate, corolla rotate to campanulate. Staminate flowers: stamens 3; filaments inserted at hypanthium base, distinct; thecae distinct, replicate, forming a head, connective broad; pistillodes present. Pistillate flowers: ovary 6-locular, ellipsoid to obovoid or subglobose; ovules 50–100+ per locule; styles 1, short-columnar, deeply cleft; stigmas 3, 2-lobed; staminodes present. Fruits pepos, green, yellow, or variegated green and whitish or green and yellow, ± longitudinally striped, globose or subglobose to ellipsoid, smooth, glabrous, indehiscent. Seeds 50–300+ (or many more in some cultivated forms), usually ovoid to ellipsoid or oblong, compressed, not arillate, margins not differentiated, surface smooth or slightly verrucose. x = 11.

Distribution

Introduced; Asia, Africa, introduced widely.

Discussion

Species 5 (3 in the flora).

In Citrullus the rind (hypanthium/pericarp) may be firm and tough or thin and fragile; it may be durable (persisting intact after maturity) or not. The mesocarp can be fleshy or dry and it varies in color from red or pink to orange, light yellowish orange, yellow, greenish, or whitish.

Citrullus lanatus is commonly established in the flora area as a waif but rarely, if ever, persists more than one season. Citrullus afer is a poorly documented transient and C. colocynthis apparently grows as a weed in crop fields.

The five species of Citrullus are C. lanatus, the domesticated watermelon; C. afer (C. lanatus var. citroides), the preserving melon or citron; C. colocynthis, the bitter apple; and C. ecirrhosus Cogniaux and C. rehmii DeWinter, wild species endemic to desert regions of Namibia. Citrullus afer, C. lanatus, and C. rehmii are monoecious annuals; C. colocynthis and C. ecirrhosus are perennials. All are native to dry habitats of northern Africa (C. colocynthis) or southern Africa (C. afer, C. ecirrhosus, C. lanatus, and C. rehmii), and all are cross-compatible with each other.

The monotypic Praecitrullus Pangalo [P. fistulosus (Stocks) Pangalo, Indian round gourd, apple gourd, Indian baby pumpkin], a monoecious annual from Afghanistan, India, and Pakistan, was segregated from Citrullus and has proved to be more closely related to Benincasa (F. Dane and P. Lang 2004; A. Levi and C. E. Thomas 2005; Levi et al. 2010).

Discovery of 5000-year-old Citrullus lanatus seeds in Libya (K. Wasylikowa and M. Van der Veen 2004) indicates that its domestication might have occurred in northern Africa. Remains also have been found in tombs and temples in Egypt (ca. 1330 bce) and Sudan (1500 bce). Seeds of C. colocynthis appear in early Egyptian, Libyan, and near-eastern sites, and it was probably used prior to its domestication (F. Dane and P. Lang 2004; Dane and J. Liu 2007). Watermelons are mentioned in European herbals from the 1500s and by 1625 were widely planted as a minor crop in European gardens. They arrived early in North America with European colonists, appearing in Florida by 1567 and in Massachusetts by 1629.

Selected References

Aquino Assis, J. G. de et al. 2000. Implications of the introgression between Citrullus colocynthis and C. lanatus characters in the taxonomy, evolutionary dynamics and breeding of watermelon. Pl. Genet. Resources Newslett. 121: 15–19.

Dane, F. and P. Lang. 2004. Sequence variation at cpDNA regions of watermelon and related wild species: Implications for the evolution of Citrullus haplotypes. Amer. J. Bot. 91: 1922–1929.

Dane F., P. Lang, and R. Bakhtiyarova. 2004. Comparative analysis of chloroplast DNA variability in wild and cultivated Citrullus species. Theor. Appl. Genet. 108: 958–966.

Dane F. and J. Liu. 2007. Diversity and origin of cultivated and citron type watermelon (Citrullus lanatus). Genet. Resources Crop Evol. 54: 1255–1265.

Fursa, T. B. 1972. K sistematike roda Citrullus Schrad. (On the taxonomy of genus Citrullus Schrad.) Bot. Zhurn. (Moscow & Leningrad) 57: 31–41.

Fursa, T. B. 1981. Intraspecific classification of water-melon under cultivation. Kulturpflanze 29: 297–300.

Jarret, R. L. and M. Newman. 2000. Phylogenetic relationships among species of Citrullus and the placement of C. rehmii De Winter as determined by internal transcribed spacer (ITS) sequence heterogeneity. Genet. Resources Crop Evol. 47: 215–222.

Levi, A. and C. E. Thomas. 2005. Polymorphisms among chloroplast and mitochondrial genomes of Citrullus species and subspecies. Genet. Resources Crop Evol. 52: 609–617.

Lower Taxa

Citrullus afer, Citrullus colocynthis, Citrullus lanatus

Key

1	Plants perennial; stems pustulate-scabrous to pustulate-hispid; tendrils usually unbranched, rarely 2-branched; pepos 4–7(–10) cm diam.	Citrullus colocynthis
1	Plants annual; stems villous; tendrils 2–3-branched; pepos 12–35+ cm diam	> 2
2	Leaf blades ovate to lanceolate-ovate or ovate-triangular, mostly 8–20 cm; pepos globose to oblong-ellipsoid, 12–35+ cm diam., rind tough, not durable, mesocarp juicy, red to orange, yellow, or greenish, sweet.	Citrullus lanatus
2	Leaf blades ovate, 3–8 cm; pepos globose to globose-ovoid, 15–25 cm diam., rind tough, durable, mesocarp dry, whitish, bitter.	Citrullus afer