Plectranthus rotundifolius (PROSEA)

Plant Resources of South-East Asia Introduction

List of species

Plectranthus rotundifolius (Poiret) Sprengel

Protologue: Syst. veg. 2: 690 (1825).

Family: Labiatae

Chromosome number: 2n= 64, 84

Synonyms

• Coleus tuberosus (Blume) Bentham (1832),
• C. parviflorus Bentham (1848),
• Solenostemon rotundifolius (Poiret) J.K. Morton (1962).

Vernacular names

• Madagascar potato, Hausa potato, country potato (En).
• Pomme de terre de Madagascar (Fr)
• Indonesia: kentang Jawa (Indonesian), kentang ireng (Javanese), huwi kentang (Sundanese)
• Malaysia: ubi kembili, ubi keling
• Thailand: man-khinu (southern), man-nu (southern).

Origin and geographic distribution

The origin of P. rotundifolius is thought to lie in Madagascar or tropical Africa, but at present it is only known from cultivation. It is frequently cultivated in Madagascar, tropical and southern Africa, Sri Lanka, India, throughout continental Asia to Peninsular Malaysia, Sumatra, Java, the Moluccas and possibly in the Philippines.

Uses

The aromatic tubers are eaten as a delicacy, cooked or steamed, often with rice, but sometimes raw. Young tubers (white) are often used in soups or in vegetable dishes. A meal made from adult tubers is used in Indonesia as a substitute for Irish potatoes in the preparation of minced-meat balls, or cooked with palm sugar and coconut milk. Cooked leaves are eaten as a vegetable.

In Africa, the tubers are regarded as a most suitable substitute for Irish potatoes. There they are also used against dysentery and certain eye diseases.

Production and international trade

P. rotundifolius is primarily a home garden crop which is only traded locally if at all. No production or trade statistics are available.

Properties

The approximate composition of 100 g fresh tubers is: water 73-78 g, protein 1.3-1.5 g, fat 0.3-0.6 g, carbohydrates 20-23 g, fibre 0.4-0.9 g, ash 1 g, Ca 17 mg, Fe 6 mg, thiamine 0.05 mg, riboflavin 0.02 mg, niacin 1 mg, ascorbic acid 1 mg.

The starch has potential as a binding or disintegration agent in tablet formulations (medicines).

Botany

• Perennial, aromatic, semi-succulent herb, up to 1 m tall, cultivated as an annual. Roots producing ovoid to oblongoid sessile potato-like tubers, up to 4.5 cm × 1.5 cm, blackish, brownish, reddish-grey or whitish, with rough skin.
• Stem decumbent or ascending, quadrangular, densely pubescent on the angles, rooting at the nodes.
• Leaves rather thick, juicy, faintly aromatic when bruised, opposite; petiole 1-3(-5) cm long, puberulous; blade ovate to suborbicular, 2-6 cm × 1.5-4 cm, cuneate at base, margin coarsely crenate, apex puberulous, dotted with glands below.
• Inflorescence a terminal false spike, 5-15 cm long, with verticillasters of 4-6 flowers.
• Pedicel 1-2 mm long; bracts small; calyx campanulate, 1.5-3 mm long, glandular-hispid, 5-toothed, upper tooth oblong, acute, median teeth very short with rounded apex, lower teeth highly connate forming an almost truncate apex but ending abruptly in 2 widely apart acute tips; corolla tubular and 2-lipped, 7-12 mm long, light to dark violet, pubescent and dotted with glands, tube strongly curved, upper lip very short and 4-lobed, lower lip boat-shaped; stamens 4, curved within the lower lip, up to 2.5 mm long, shortly united at base and enveloping the style; style 2-fid, slightly exceeding the stamens.
• Fruit unknown.

Flowering occurs between February and August. Normally the crop reaches maturity in 5-8 months.

P. rotundifolius is a variable species and several varieties have been distinguished, based mainly on tuber colour (e.g. var. javanicaminum A. Chev. with blackish tubers). However, as a cultivated species, subclassification into cultivar groups and cultivars seems more appropriate. In the literature some confusion exists about the identity of species belonging to the so-called Coleus-Plectranthus complex. Here the view of Flora Malesiana is followed, in which only one large genus Plectranthus L'Hérit. is recognized. Synonymous genera involved are: Germanea Lamk, Coleus Lour., Solenostemon Schumacher, Rabdosia Hassk., Majana Kuntze, Isodon (Bentham) Kudo. Distinguishing characteristics are mainly based on the stamens and the calyx, but some intermediate species exist, making distinction doubtful.

Other tuber-bearing species that are sometimes confused with P. rotundifolius are:

• Plectranthus esculentus N.E. Brown (synonyms: Plectranthus floribundus N.E. Brown, Coleus dazo A. Chev., C. esculentus (N.E. Brown) G. Taylor): the kafir potato, occurring wild and cultivated in tropical and southern Africa, with yellow flowers and corolla 14-16 mm long.

• Plectranthus edulis (Vatke) Agnew (synonyms: Coleus tuberosus A. Rich., C. edulis Vatke): the Galla potato, occurring wild and cultivated in East and Central Africa.

• Plectranthus barbatus Andr. (synonyms: Coleus barbatus (Andr.) Benth., Plectranthus comosus Sims): occurring wild and cultivated in East and Central Africa and India, occasionally also cultivated elsewhere.

Ecology

P. rotundifolius is preferably grown in tropical high rainfall lowlands, rarely up to 1000 m altitude. Evenly distributed rainfall and low night temperature favour tuber development. The crop cannot stand waterlogging. Well-drained sandy loams are preferred, but heavy clays are unsuitable. In South-East Asia it is often planted on dry rice fields after rice.

Agronomy

P. rotundifolius is propagated by stem cuttings. The cuttings are usually raised by planting selected tubers in well watered nursery beds. The tubers start sprouting in 10-15 days and the cuttings can be taken after about 3 months. Cuttings 15-30 cm long are usually planted out on ridges. Coiling the lower part of the cuttings before planting stimulates rooting. Planting distance is about 20 cm between cuttings and 90 cm between ridges. Weed control is done about 3 weeks and 7 weeks after planting. NPK fertilizer (125 kg/ha) and cattle manure (25 t/ha) are recommended. To promote tuber production the rooted cuttings are earthed up about 2 months after planting if not planted on ridges.

P. rotundifolius has no serious diseases and pests. Caterpillars occasionally attack the leaves.

Tubers can be harvested when the leaves begin to wither. Harvesting should not be delayed long because mature tubers deteriorate rapidly if left in the soil. They can be stored for some time in dry sand in a cool well-ventilated room. Yield of tubers normally ranges from 7.5-15 t/ha, but may attain 18-20 t/ha.

Genetic resources and breeding

No comprehensive P. rotundifolius germplasm collections and breeding programmes are known.

Prospects

P. rotundifolius deserves more scientific attention. It is now subordinate to other starchy food crops such as cassava and sweet potato, but its easy propagation and its disease resistance make it an interesting tuber crop for the lowland tropics. The properties of the starch make its possible use in pharmacy worth investigating. Germplasm collection is urgently needed to start breeding for high-yielding cultivars.

Literature

• Busson, F., 1965. Plantes alimentaires de l'ouest Africain, étude botanique, biologique et chimique [Food plants of West Africa, botanical, biological and chemical study]. Leconte, Marseille, France. pp. 402-406.
• Codd, L.E., 1975. Plectranthus (Labiatae) and allied genera in southern Africa. Bothalia 11(4): 371-442.
• Kay, D.E., 1973. Crop and product digest No 2. Root crops. The Tropical Products Institute, London, United Kingdom. pp. 76-80.
• Keng, H., 1978. Labiatae. Plectranthus. In: van Steenis, C.G.G.J. (Editor): Flora Malesiana. Vol. 8. Sythoff & Noordhoff International Publishers, Alphen aan den Rijn, the Netherlands. pp. 382-393.
• Ochse, J.J. & Bakhuizen van den Brink, R.C., 1980. Vegetables of the Dutch East Indies. 3rd English edition (translation of "Indische Groenten", 1931). Asher & Co., Amsterdam, the Netherlands. pp. 351-352.

Authors

P.C.M. Jansen