Coleus esculentus (N.E.Br.) G.Taylor

J. R. Timberlake, E. S. Martins (2013). Flora Zambesiaca, Vol 8: part 8. Royal Botanic Gardens, Kew

Type

Type: South Africa, plant cultivated at K from material sent by J. Medley Wood from KwaZulu-Natal, 1893 (K lectotype, BOL), lectotypified by Codd (1975).

Morphology General Habit

Perennial herb or woody herb 0.6–2 m tall, often with a single stem arising from a small rootstock with several elongate, finger-like tubers, aromatic- Stems ascending or scrambling, more rarely procumbent, leafless in flower, quadrangular or rounded-quadrangular, sometimes reddish, sparsely branched below, with leathery wrinkled epidermis, pubescent to densely so with short retrorse hairs and longer hairs at nodes; inflorescence axis with short spreading hairs and pale brownish or reddish sessile glands; vegetative shoots sometimes with axilliary bulbils (cf. P. kapatensis, P. melleri)

Morphology Leaves

Leaves oppositely or ternately arranged, ascending, shortly petiolate to subsessile; blade elliptic, 30–80 × 10–30 mm, serrate, apex round to obtuse, base cuneate, pubescent with brownish or pale sessile glands; petiole 0–2 mm long

Morphology Reproductive morphology Inflorescences

Inflorescence axillary, lax or condensed, branching or unbranched, 80–200 mm long, with 2-flowered verticils (1)2–10 mm apart; bract opposite or subopposite subtending a single flower, ovate to obovate, slightly cucullate, 1–5 mm long; pedicel (2)3–19 mm long

Morphology Reproductive morphology Flowers Calyx

Calyx 4–5 mm long, pubescent with pale, brown or reddish sessile glands; fruiting calyx 6–9 mm long, tubular, pedicel attached eccentrically behind posterior lip; throat truncate; posterior lip obovate to elliptic, mucronate to obtuse at apex, slightly curved upwards, decurrent; lobes of anterior lip lanceolate, median lobes longer than lateral, curving upwards with sinus separating the two median lobes similar in size to that separating lateral and median lobes

Morphology Reproductive morphology Flowers Corolla

Corolla yellow, 10–18 m long, sparsely pubescent, with pale, brown or reddish sessile glands; tube 6–10 mm long, shallowly sigmoid; posterior lip usually shorter than anterior; anterior lip 4–8 mm long, horizontal or spreading

Morphology Reproductive morphology Flowers Androecium Stamens Filaments

Staminal filaments free

Morphology Reproductive morphology Fruits

Nutlets brown with darker speckles, broadly ovate, slightly flattened, 1–1.5 mm long; producing copious speckled mucilage when wet.

Distribution

Botswana, Zambia, Zimbabwe, Malawi, Mozambique. Also in Mali, Nigeria, Cameroon, Uganda, Tanzania, Angola and South Africa.

Ecology

Grassland, open Isoberlinia, Brachystegia or Uapaca woodland, also cultivated; 800– 2200 m.

Conservation

Widespread; Least Concern.

Recognition

Plectranthus esculentus is perhaps closest to other yellow flowered species such as P. melleri (P. luteus) and P. tetragonus. Bulbils similar to those of P. melleri are only found in Zimbabwean material of P. esculentus, but not much purely vegetative material of that species has been seen.

Labiatae, J. K. Morton. Flora of West Tropical Africa 2. 1963

Morphology General Habit

Perennial with herbaceous erect stems

Morphology General Indumentum

Rather coarsely pilose with whitish hairs

Morphology Reproductive morphology Flowers

Flowers yellow 1/2-2/3 in. long

Morphology Roots

Root elongated-tuberous

Note

Often cultivated.

Kew Species Profiles

General Description

Despite the common names suggesting this species is a type of potato (Livingstone potato), Plectranthus esculentus is actually a member of the mint family (Lamiaceae) and is not in the same family (Solanaceae) as the potato we commonly eat. Other names found in older literature for this species are Coleus dazo, Coleus esculentus, and Plectranthus floribundus.

Livingstone potato is cultivated for food in parts of Africa. It is thought that cultivation began through selection of wild populations in the Upper Niger valley of the Hausaland (Nigeria) and in the Central African Republic.

Species Profile

Geography and distribution

Plectranthus esculentus occurs in dry wooded areas in equatorial Africa, and southwards to Angola, the eastern Transvaal and KwaZulu-Natal coast of South Africa, and Swaziland.

Description

This sparsely branched perennial herb grows up to 2 m tall and produces a cluster of small, branching, edible tubers (swollen underground stems) at the base of the stem. Flowers are produced before the leaves and the stems are leafless while the plant is in flower. The yellow flowers are two-lipped, and carried on short, densely crowded branches. The leaves have toothed edges and brown gland-dots on the undersurface.

Threats and conservation

This species is not considered threatened, although it has been reported that its cultivation as a traditional crop plant has declined in some parts of Africa as a result of the social stigma attached to native plants, and a growing preference for exotic crops. However, it is being developed for the commercial market in southern Africa.

Uses

Livingstone potato is cultivated in Africa for its edible tubers, each of which may weigh up to 1.8 kg. The tubers persist underground even when the plant lacks leaves. They are dug up and usually boiled or roasted, and are often eaten as a substitute for sweet potato ( Ipomeas batatas ) or potato ( Solanum tuberosum ). Researchers in South Africa are continuing centuries of selection for its edible tubers so as to optimise the crop for local farmers.

Preliminary biochemical analysis of the species suggests it has high nutritional value, being rich in carbohydrates, vitamin A and minerals, and hence is valuable in times of food shortage. This is especially true as the plant is well suited to local environmental conditions in areas where it has a long tradition of cultivation.

Plectranthus esculentus is one of the most frequently used of the 21 species of Plectranthus employed in the treatment of disorders of the digestive system. It is also used in eastern and southern Africa as an antihelmintic to treat intestinal worms. It has cytotoxic and anti-tumour activity.

Further research is needed to determine whether this species is of wider medicinal value. The related species Plectranthus barbatus contains the compound coleonol, which is a potent stimulant that has potential for the treatment of hypertension, glaucoma, asthma and certain cancers.

Millennium Seed Bank: Seed storage

Kew's Millennium Seed Bank Partnership aims to save plant life world wide, focusing on plants under threat and those of most use in the future. Seeds are dried, packaged and stored at a sub-zero temperature in our seed bank vault.

Number of seed collections stored in the Millennium Seed Bank: Six

Germination testing: Successful

Cultivation

Livingstone potato can be propagated from tubers and small axillary bulbils. Tubers should be planted in the spring for harvest the following autumn (tubers can be harvested 180 to 200 days after planting). The crop is reportedly very hardy, but the optimal conditions for growth are a temperature of 15-28 ˚C, annual rainfall of 700 to 1,100 mm and a soil pH of 6.5 to 7. Yields of 2 - 6 tonnes per hectare have been recorded.

Livingstone potato at Kew

Dried and alcohol-preserved specimens of Plectranthus esculentus are stored in the Herbarium (one of the behind-the-scenes areas of Kew), where they are made available to researchers from around the world by appointment. The details of 15 of these specimens, including some images, can be seen in the Herbarium Catalogue.

Livingstone potato tubers are also held in Kew's Economic Botany Collection. Detailed information on this specimen can be viewed using ePIC(the electronic Plant Information Centre).

Research on Plectranthus at Kew

The genus Plectranthus has been the subject of much recent research at Kew, including a survey of its ethnobotanical uses, carried out in collaboration with Dr Catherine Lukhoba of Nairobi University, Kenya.

Three species of Plectranthus are used for their edible tubers in tropical Africa. It is likely that a number of African tuber-producing species were formerly extensively cultivated and have come to be displaced by the most successful, or culturally popular, ones.

Many of the native crops that sustained local populations and early civilisations for centuries are now neglected and unstudied. Some of these crops, including Plectranthus esculentus , have contributed significantly to rural diets and are well adapted to different environmental conditions. Further research on this subject could therefore be of great benefit.

Distribution

Angola, South Africa

Ecology

Grassland, open Isoberlinia, Brachystegia and Uapaca woodland, often in rocky areas. Also found in cultivation.

Conservation

Not threatened.

Hazards

None known.

Lamiaceae (Labiatae), A.J. Paton, G. Bramley, O. Ryding, R.M. Polhill, Y.B. Harvey, M. Iwarsson, F. Willis, P.B. Phillipson, K. Balkwill, C.W. Lukhoba, D.F. Otieno, & R.M. Harley. Flora of Tropical East Africa. 2009

Type

Lectotype, chosen by Codd (1975), see note: cultivated at K from material sent by J. Medley Wood from KwaZulu-Natal, 1893 (K!, lecto.; BOL, isolecto.)

Morphology General Habit

Aromatic perennial herb or woody herb 0.6–2 m tall, often with a single stem arising from a small rootstock with several elongate, finger-like tubers

Morphology Stem

Stems ascending or scrambling, more rarely procumbent, leafless in flower, quadrangular or roundedquadrangular, sometimes reddish, sparsely branched below and with a leathery wrinkled epidermis, pubescent to densely so with short retrorse hairs and longer hairs at nodes; inflorescence axis with short patent hairs and pale, brownish or reddish sessile glands; vegetative shoots sometimes with axillary bulbils (cf P kapatensis,P melleri)

Morphology Leaves

Leaves oppositely or ternately arranged, ascending, shortly petiolate to subsessile; blades elliptic, 3–8 × 1–3 cm, serrate, apex round to obtuse, base cuneate, pubescent, with brownish or pale sessile glands; petiole 0–2 mm long

Morphology Reproductive morphology Inflorescences

Inflorescence axillary, lax or condensed, branching or unbranched, 80–200 mm long, with 2flowered verticils (1–)2–10 mm apart; bracts opposite or subopposite, subtending a single flower, ovate to obovate, slightly cucullate, 1–5 mm long; pedicel (2–)3–19 mm long

Morphology Reproductive morphology Flowers Calyx

Calyx 4–5 mm long, pubescent, with pale, brown or reddish sessile glands; fruiting calyx 6–9 mm long, tubular with pedicel attached excentrically behind the posterior lip; throat truncate; posterior lip obovate to elliptic, mucronate to obtuse at apex, slightly curving upwards, decurrent; lobes of anterior lip lanceolate, median lobes longer than lateral, curving upwards with the sinus separating the two median lobes similar in size to that separating the lateral and median lobes

Morphology Reproductive morphology Flowers Corolla

Corolla yellow, 10–18 mm long, sparsely pubescent with pale, brown or reddish pubescence, with pale, brown or reddish sessile glands; tube 6–10 mm long, shallowly sigmoid; posterior lip usually shorter than anterior; anterior lip 4–8 mm long, horizontal or spreading

Morphology Reproductive morphology Flowers Androecium Stamens

Staminal filaments free

Morphology Reproductive morphology Fruits

Nutlets brown with darker speckles, broadly ovoid, slightly flattened, 1–1.5 mm long, producing copious speckled mucilage when wet

Ecology

Grassland, open Isoberlinia, Brachystegia or Uapaca woodland, also cultivated; 1000–1900 m

Conservation

Least concern; widely distributed

Note

Apart from its obvious relationship to the following species, P. densus, P. esculentus is perhaps closest to other yellow-flowered species such as P. luteus and P. tetragonus. Bulbils similar to those of P. luteus are only found in Zimbabwean material of P. esculentus, but not much purely vegetative material of that species has been seen. Codd (1985) cites Medley Wood 3633 at K as the holotype. However, this specimen is not present at K, though a specimen bearing this number is present in BOL. The protologue cites Medley Wood 3633 and mentions specimens cultivated at Kew. There is one specimen at K of cultivated material sent from Natal by Medley Wood in 1893, though the specimen has no number, it is possible that it was grown from numbered material sent by Medley Wood. As the description is based on cultivated material and a cited specimen, Codd’s typification of the material held at K should be regarded as a lectotypification.

Distribution

Range: Mali, Nigeria, Cameroon, Central African Republic, Angola, Zambia, Malawi, Mozambique, Zimbabwe, Botswana and South Africa, often cultivated Flora districts: U2 T4 T7 T8