Solanum incanum (PROTA)
|Geographic coverage Africa|
|Geographic coverage World|
|Dye / tannin|
|Forage / feed|
- Protologue: Sp. pl. 1: 188 (1753).
- Family: Solanaceae
- Chromosome number: 2n = 24, 48
- Solanum campylacanthum Hochst. ex A.Rich. (1851),
- Solanum bojeri Dunal (1852),
- Solanum delagoense Dunal (1852),
- Solanum panduriforme E.Mey. ex Dunal (1852).
- Thorn apple, grey bitter apple, bitter tomato, Sodom’s apple (En).
- Aubergine sauvage, pomme de Sodome (Fr).
- Mtunguja, mtunguja-mwitu, mnyanya-mwitu, mnyanya-porini, mtua, mtula (Sw).
Origin and geographic distribution
Solanum incanum is distributed throughout continental Africa, including South Africa. It is also found growing wild in Madagascar and Mauritius but is probably a recent introduction, as the fruits are considered edible there. It furthermore occurs from the Middle East east to India.
Many of the medicinal uses of Solanum incanum are based on its analgesic properties. Throughout tropical Africa a sore throat, angina, stomach-ache, colic, headache, painful menstruation, liver pain and pain caused by onchocerciasis, pleurisy, pneumonia and rheumatism are treated with Solanum incanum. For these purposes, leaf, root and fruit decoctions are gargled or drunk, roots are chewed and sap swallowed, leaf paste, root infusions and pounded fruits are applied externally or rubbed into scarifications, leaf sap is used for washing painful areas, and ash of burnt plants is mixed with fat and applied externally. For relief of toothache a root infusion is used as mouth wash, fruit or root is rubbed on the gums or smoke of burning seeds is inhaled. Hiccups are suppressed by licking a mixture of the ash of burned leaves and salt.
Another widespread use of Solanum incanum is in the treatment of venereal diseases. Root powder is mixed with food or rubbed into scarifications, root infusions or decoctions are drunk, roasted pulverized roots are taken in water, leaf decoctions and fruit sap are drunk, and fruit sap is applied externally. Different plant parts are also widely used in the treatment of skin problems, including skin infections, whitlow, ringworm, burns, sores, rashes, wounds, warts, carbuncles, ulcers and benign tumours. In Senegal a maceration of the leaves is used as an eye bath to cure ophthalmia; in Malawi fruit sap is rubbed into scarifications around the eye to treat conjunctivitis. In Senegal pounded seeds are mixed with pulped fruits to massage aching ears. In Uganda, Tanzania and South Africa extracts of leaves or flowers are used as ear drops to cure inflammations.
In Senegal, Kenya, Uganda and Zimbabwe different plant parts are used to treat snakebites: a decoction of the roots is drunk, roots are chewed and sap is swallowed, and young chewed leaves or pulped fresh roots are applied to the bite wound.
In Niger, Sudan, Rwanda and Namibia the fruits are used as an ingredient of arrow poison and in Mozambique of fish poison. In Ethiopia fruit sap is mixed with butter and applied to cattle to control ticks.
Selection for larger, less toxic fruits and leaves has taken place over time and whereas the wild types produce small, bitter and toxic fruits, cultivated types are available, especially in West Africa, of which the fruits and leaves can be safely eaten in soup and as a vegetable. Leaves are added to soup to improve the flavour. The large variation in toxicity makes it dangerous to transfer specific uses from one region to another.
The fruit and the seed are used in Africa and Asia to curdle milk and to make cheese. In Ethiopia the boiled fruits are used as soap and in tanning leather.
Solanum incanum contains saponin steroids, in particular glycoalkaloids, which are found in all parts of the plant, but in highest concentrations in the fruit. The main glycoalkaloid is solasonine. Other compounds isolated from the fruits include the alkaloids solasodine and solamargine, and the steroidal sapogenins diosgenin and yamogenin. The fruits also contain dimethylnitrosamine, a potent carcinogen that is thought to cause the high incidence of oesophageal cancer in areas of Africa where the fruit sap is used to curdle milk. Alkaloids such as solasodine are used commercially as precursors for the production of steroidal compounds for medicinal use, mainly as contraceptives. Flavonoids and chlorogenic acid, a phenolic derivative, have also been isolated. Solamargine has shown promise for treatment of liver, lung and breast cancer.
In in-vitro efficacy tests on ticks (Boophilus decoloratus), solamargine resulted in 30–100% mortality. An extract of the fruits has caused skin cancer in animals.
A methanol extract of the fruits showed broad-spectrum antifungal activities. A strong effect was observed against the fungus that causes athlete’s foot, Trichophyton mentogrophytes. The water extract of the fruit showed antibacterial activity against Bacillus subtilis, Micrococcus flavus and Pseudomonas aeruginosa and the methanol extract against Staphylococcus aureus and Micrococcus flavus. The cytotoxicity of the methanol extract against human foetal liver cells was IC50 = 35 μg/ml. Extracts of the fruit and of the roots, administered daily to diabetic rats, lowered blood glucose levels, but the fruit extract also reduced food intake, whereas the root extract increased food intake and caused diarrhoea. Chlorogenic acid has antioxidant and insect repellent properties and when ingested by insects decreased growth and development.
Adulterations and substitutes
The alkaloids found in Solanum incanum occur in many species of subgenus Leptostemonum. Diosgenin is mainly obtained from Dioscorea spp., but it is also present in most Costus spp. and in the seeds of fenugreek (Trigonella foenum-graecum L.).
Erect or spreading shrub up to 3 m tall, occasionally a small tree; stems and leaves with stellate hairs and pale yellow to brown prickles, up to 1 cm long. Leaves alternate, simple; stipules absent; petiole 0.5–8.5 cm long; blade almost round to lanceolate, 1–30 cm × 1–17 cm, base rounded, truncate or cordate, often unequal, apex acute or obtuse, margin entire to pinnately lobed, densely hairy. Inflorescence a 2–15(–26)-flowered cyme, inserted above the leaf axil. Flowers bisexual or functionally male, nodding or pendent, regular, (4–)5–7(–9)-merous; pedicel 0.5–4 cm long; calyx campanulate, lobes up to 1.5 cm long, enlarging and splitting in fruit; corolla campanulate to rotate, 1–4.5 cm in diameter, with ovate or broadly triangular lobes, blue, pink, purple or violet, rarely white; stamens inserted near the base of the corolla tube and alternating with corolla lobes, filaments short, anthers slender; ovary superior, 2(–4)-celled, style up to 15 mm long, densely hairy. Fruit a globose or depressed globose, occasionally ovoid-ellipsoid berry 2.5–3.5 cm × 2–3 cm, yellow, orange or brown when ripe, many-seeded. Seeds lentil-shaped to almost kidney-shaped, up to 3.5 mm × 3 mm, pale yellow to brown. Seedling with epigeal germination.
Other botanical information
Solanum comprises over 1000 species and has a cosmopolitan distribution, except in boreal, alpine and aquatic habitats. At least 100 species are found in tropical Africa. The principal centre of diversity is located in Central and South America, with secondary centres in Africa and Australia. Solanum has been subdivided into 7 subgenera and numerous sections and series. Solanum incanum belongs to subgenus Leptostemonum section Melongena.
Solanum incanum is considered here as a single polymorphic species, whereas some authors distinguish 4 groups within the species and others consider each of these groups a different species. Solanum incanum is considered the ancestor of eggplant (Solanum melongena L.) and the 2 species are therefore considered by some as a single species. In the ethnobotanical literature the distinction of the 4 groups or species is not made and therefore can not be used here.
From Taiwan research findings have been published on Solanum incanum, but a misapplication of the name is apparent. Results refer to the indigenous species Solanum undatum Lam. (synonym: Solanum cumingii Dunal), a species that also is placed in section Melongena, hybridizes freely with the eggplant, and also contains solamargine.
Several other tropical African species of section Melongena have medicinal uses.
Solanum arundo Mattei (synonym: Solanum diplacanthum Dammer) occurs in the coastal lowlands of Somalia, Kenya and Tanzania. In Somalia, the filtrate of the crushed fruits is massaged into the skin to treat itch and scabies. Seeds are soaked in hot oil, which after cooling is applied to aching teeth as an analgesic. In Kenya the roots are pounded and soaked in water or chewed as a cure for fever. The Rendille people of northern Kenya apply fruit juice to wounds on the feet of camels. Sheep and goats browse the plants.
Solanum linnaeanum Hepper & P.M.L.Jaeger, the true ‘Sodom apple’, is a weed that originates from the Mediterranean region and is now widespread throughout tropical Africa. It is used in South Africa for its analgesic properties, to treat wound infections and to counteract bloating.
In Ethiopia the pounded roots of Solanum marginatum L.f. are applied to wounds. The roots are chewed and the sap swallowed to cure syphilis. Roots and leaves are steeped in water and the water is applied to scabies; root pulp is given to dogs as a cure for rabies.
In Kenya the root of Solanum sessilistellatum Bitter is baked and chewed as a cure for whitlow.
Growth and development
All parts of Solanum incanum contain solasodine, but the levels vary widely. The smallest leaves have the highest alkaloid concentration. The amount of alkaloids in the leaves increases during leaf development. Both leaves and roots showed alkaloid accumulation with time, although in experiments alkaloid levels in the root fell after 20 weeks.
Solanum incanum is common as a weed, around houses, in overgrazed grassland and in roadsides. It is also found at forest edges and in bushland and grassland, from sea-level up to 2500 m altitude. It is considered an indicator for low-fertility soils.
Propagation and planting
A cultivation method commonly used for eggplant can be adopted for Solanum incanum as well: seeds are sown in trays or seedbeds, and seedlings are transplanted to small pots or bags (8–10 cm diameter) 2–3 weeks later when the first leaf appears. The seedlings are kept in the nursery till they have developed 5–7 leaves and are then planted in the open or in a greenhouse at a spacing of 50 cm between plants and 1 m between rows, preferably in light and well-prepared soil.
No details on cultivation of Solanum incanum have been published but husbandry as practised for eggplant, African eggplant (Solanum aethiopicum L.) and gboma eggplant (Solanum macrocarpon L.) can be applied selectively.
Diseases and pests
As a crop Solanum incanum is unimportant and observations on its diseases and pests are lacking. It is a potential host for a wide range of pests and diseases of Solanaceous crops (especially eggplant, tomato and Capsicum pepper). It is a major host of the papaya fruit fly (Bactrocera papayae).
For medicinal use, harvesting of Solanum incanum is done from the wild whenever the need arises and fruits are available.
The genetic variation of Solanum incanum, which is of interest for breeding and selection, is not well covered in germplasm collections; the number of accessions in genebanks is modest. National research institutes in Kenya (e.g. National Genebank of Kenya, KARI, Kikuyu) maintain some germplasm as do the University of Nijmegen (Netherlands) and the Millennium Seedbank (Ardingly, United Kingdom). As Solanum incanum is widespread in tropical Africa, there is no danger of genetic erosion.
For breeding purposes of the closely related Solanum melongena, Solanum incanum is an important source of genes that can be introduced through breeding. Wild species of section Melongena offer an extensive genepool, especially for resistance to pests and diseases. On the other hand the productivity and the fruit size of the eggplant offer opportunities for breeding for commercial production of steroidal alkaloids of Solanum incanum.
An unambiguous taxonomic classification of the polymorphic Solanum incanum and closely related species is needed to be able to start detailed ethnobotanical and pharmacological studies. Although there seems to be potential for commercial production of steroidal alkaloids with Solanum incanum, more research is needed.
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Sources of illustration
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- E.N. Matu, CTMDR/KEMRI, P.O. Box 54840–00200, Nairobi, Kenya
Correct citation of this article
Matu, E.N., 2008. Solanum incanum L. In: Schmelzer, G.H. & Gurib-Fakim, A. (Editors). PROTA (Plant Resources of Tropical Africa / Ressources végétales de l’Afrique tropicale), Wageningen, Netherlands. Accessed 12 November 2020.
- See the Prota4U database.