3226

9: Plants yielding non-seed carbohydrates

Dioscorea L.

Sp. pl.: 1032 (1753); Gen. pl. ed. 5: 456 (1754).

DIOSCOREACEAE

x = 10 (Old World), x = 9 (New World); 2n = 40, 60, 80, 100 (Dioscorea bulbifera); 2n = 40, 80 (Dioscorea pentaphylla)

Major species and synonyms
— Dioscorea alata L. — see separate article.
— Dioscorea bulbifera L., Sp. pl.: 1033 (1753), synonyms: Dioscorea sativa Thunb. (1784) et auct., non L. (1753), Dioscorea crispata Roxb. (1832), Dioscorea heterophylla Roxb. (1832).
— Dioscorea esculenta (Lour.) Burkill — see separate article.
— Dioscorea hispida Dennst. — see separate article.
— Dioscorea nummularia Lamk, Encycl. méth. 3: 231 (1789), synonyms: Dioscorea glabra Koorders (1898), non Roxb. (1832), Dioscorea seemannii Prain & Burkill (1914), Dioscorea palauensis R. Knuth (1924).
— Dioscorea pentaphylla L., Sp. pl.: 1032 (1753), synonyms: Dioscorea triphylla L. (1753), Dioscorea kleiniana Kunth (1850).
— Other Dioscorea species — see chapter on minor species yielding non-seed carbohydrates.

General: Yam (En). Igname (Fr). Indonesia: uwi (Javanese), huwi (Sundanese). Malaysia: ubi. Papua New Guinea: yam. Philippines: yam, ubi. Cambodia: dâmlô:ng. Laos: man. Thailand: man. Vietnam: c[ur] n[aa]u, c[ur] t[uwf].
— Dioscorea bulbifera. Aerial yam, potato yam, bulbil-bearing yam (En). Igname bulbifère (Fr). Indonesia: huwi buah (Sundanese), jebubug basu (Javanese), singal (Timor). Malaysia: ubi atas, ubi china, ubi kestali. Papua New Guinea: lapma (Yatmei). Philippines: ubi-ubingan (Tagalog), aribukbuk (Ilokano), pulugan (Bikol). Cambodia: dâmlô:ng dü:hs, dâmlô:ng sdâm préi. Laos: man pauz (general), hwà 'i: mu:z (northern). Thailand: wan-prachim (central), hampao (northern), man-soen (Nakhon Si Thammaraf). Vietnam: c[ur] d[aj]i, khoai ch[oof]i, khoai d[as]i.
— Dioscorea nummularia. Indonesia: huwi upas (Sundanese), uwi in tuwa (Sulawesi), tatopo (Halmahera), singgo (Kem, Irian Jaya). Papua New Guinea: boku. Philippines: pakit (Tagalog), baen di ipay an napagatan (Ifugao). Vietnam: t[uwf] tr[of]n, c[ur] t[uwf] tr[of]n.
— Dioscorea pentaphylla. Sand yam, fibrous yam, ginger yam (En). Indonesia: ubi pasir, huwi sawut, huwi jahe. Malaysia: ubi pasir, jabet, ubi katak. Philippines: lima-lima (Tagalog), sapang (Bisaya), kasi (Igorot). Cambodia: dâmlô:ng tük. Laos: hwà 'i: mu:z (general), hwà 'è:ng mu: (northern). Thailand: man-makmu (Chiang Mai), man-khankhao (Suphan Buri), man-sue (Nakhon Si Thammarat). Vietnam: c[ur] v[awf]n.

Dioscorea probably originated in the Far East but spread in early times in the Old and New World. It then evolved independently, as no overlap occurs. Only one species is naturally common to the continents of Africa and Asia. About 600 species are known, mainly occurring in the tropics and subtropics, and about 60 species are gathered or cultivated for their edible tubers. In South-East Asia about 60 species occur, about 20 of which produce edible tubers or bulbils.
— Dioscorea bulbifera. The most prolific and widespread of all Dioscorea spp., occurring from the Atlantic coast of Africa to the remotest islands of the Pacific. It is the only major edible yam native to two continents. It occurs wild and is cultivated all over South-East Asia.
— Dioscorea nummularia. A common wild and cultivated yam in eastern Malesia (Philippines, Sabah, northern Sulawesi, northern Moluccas, northern New Guinea) extending to Pacific islands as far as Tahiti.
— Dioscorea pentaphylla. Wild and cultivated, distributed from upper India and southern China throughout South-East Asia to the remoter islands of the Pacific.
The most important cultivated species in other continents are Dioscorea cayenensis Lamk (yellow Guinea yam) and Dioscorea rotundata Poiret (white Guinea yam) in West Africa, Dioscorea trifida L. (cush-cush yam) in tropical America, and Dioscorea batatas Decaisne (Chinese yam, syn. Dioscorea opposita Thunb.) in subtropical eastern Asia. The most widely used wild species in times of food scarcity in Africa is Dioscorea dumetorum (Kunth) Pax (African bitter or cluster yam), which also occurs in cultivation.

Yams are collected or cultivated for their edible tubers, providing a staple carbohydrate food in many tropical countries. In some cases yam plants also produce edible bulbils in the axils of the leaves. As a staple food, yams are most important in West Africa. Normally, the tubers are peeled and eaten boiled, roasted or fried.
Dioscorea species are widely used in traditional medicine. Tubers of some wild species are commercially exploited because of the steroidal sapogenins they contain, from which contraceptives, sex hormones and cortisone are manufactured.
— Dioscorea bulbifera. The bulbils are especially used for food, but their edibility varies greatly. Bulbils from wild plants are peeled, sliced, washed, boiled for a long time and sometimes still have to be soaked in water to remove toxic substances before they can be eaten. In Java, bulbils from cultivated plants (called 'gedebug', 'katak' or 'katigubug' in Javanese) are edible after peeling and cooking. The cooked bulbils are greenish-yellow, pappy and taste bitter. Old bulbils are the most tasty. Underground tubers, when produced, are also used for food. They are edible in cultivars, but nauseous and poisonous in wild plants. In Peninsular Malaysia, the Semang prepare a dough by grating the tubers and kneading them with lime in a coconut shell. The dough is then wrapped in a plantain leaf and roasted, or buried in the earth and allowed to ferment. The fermented paste will keep for a week. The dough is called 'kleb' when roasted and 'koyi' when fermented. The tubers of wild plants become increasingly unpalatable as the time of new growth approaches. They are bitter and acrid and, when prepared, they are cooked sliced after adding lime and wood ashes. Both bulbils and tubers are occasionally used to produce starch and flour.
— Dioscorea nummularia. The tubers are used for food and the tough stems are used as cordage. The tubers descend deep into the soil and grow slowly, and are therefore not harvestable until about 3 years after planting. Although it is common in its natural area of distribution, its use for food seems restricted to times of scarcity. The tubers are never poisonous but are unpleasant to eat because of the saponins they contain.
— Dioscorea pentaphylla. The tubers are used for food, eaten roasted or boiled by themselves, or mixed in vegetable dishes as a flavouring. The cultivated forms (often grown in garden hedges) are tastier than the tubers collected from wild plants, which are often nauseous but non-toxic. Some forms are indicated as priest's yams ('huwi mantri'), princess's yam ('huwi putri') or sacred yam ('huwi dewata') in Java, perhaps referring to a time when the Hindu religion imposed fasts during which inferior food, such as meal from inferior yams, might be eaten.

World production of yam is estimated at about 25 million t from about 2.5 million ha. The main producers are West Africa (95%, especially Nigeria and Ivory Coast, with predominantly Dioscorea cayenensis and Dioscorea rotundata), the West Indies (1%) and East Africa (1%). Most yam is consumed in the country of production and very small amounts enter international trade. In South-East Asia, Dioscorea bulbifera, Dioscorea nummularia and Dioscorea pentaphylla are cultivated to an appreciable extent. Papua New Guinea and the Philippines are the major producers and consumers.

The approximate composition of yam per 100 g edible portion is: water 65—75 g, protein 1—2.5 g, fat 0.05—0.2 g, carbohydrates (mainly starch) 15—25 g, fibre 0.5—1.5 g, ash 0.7—2 g, ascorbic acid 8—10 mg. The loss on peeling is usually 5—15%. The ascorbic acid is retained during cooking, hence yams contain sufficient vitamin C to be important in nutrition.
Some wild yam species contain toxic alkaloids like dioscorine or dihydrodioscorine, which cause general paralysis of the central nervous system if not removed before eating. Some wild species contain 2—5% sapogenins, the most important of which is diosgenin.
— Dioscorea bulbifera. Per 100 g edible portion the bulbil contains: water 63—67 g, protein 1.12—1.50 g, fat 0.04 g, carbohydrates 27—33 g, fibre 0.70—0.73 g and ash 1.08—1.51 g. Starch granules are triangular and 5—45 µm in diameter, with a gelatinization temperature of 72—80°C. Starch prepared from the bulbils contains 13.5% moisture, 15.0% amylose, 1.49% protein, 0.29% ash, pH of 4.4, and iodine absorption of 3.9. Viscosity at 95°C is 25 Barbender units, and the gel is not strong.
— Dioscorea nummularia. Per 100 g edible portion the tuber contains: water 71.9 g, protein 2.0 g, fat 0.06 g, carbohydrates 23.62 g (starch 23.4 g, sugar 0.22 g), fibre 1.84 g and ash 0.95 g. Lysine is the limiting amino acid.
— Dioscorea pentaphylla. Per 100 g edible portion the tuber contains: water 82.5 g, protein 1.65 g, fat 0.03 g, carbohydrates 14.02 g (starch 13.9 g, sugar 0.12 g), fibre 0.66 g and ash 0.76 g.

Dioecious perennial plants, in cultivation usually grown as annuals, with underground rhizome, or more often a woody corm giving off well-defined parenchymatous tubers, sometimes at the end of long stalks; tubers perennial and lignified, or renewed annually as in most edible yams, varying greatly in number, size and form. Roots thick, fleshy and unbranched, or thin, fibrous and branching, sometimes spiny. Stem climbing and twining, 2—12(—40) m long, tough, often woody at the base and winged or with longitudinal ridges; the direction of twining, to the right (dextrorse) with clockwise circumnutation or to the left (sinistrorse) with anticlockwise circumnutation, is constant per species and characteristic of whole sections of the genus. Leaves usually alternate, simple, cordate, acuminate (apex pointing downwards), palmately veined or palmately compound with 3—5 lobes or leaflets pinnately veined; petiole with a pulvinus at each end. Axillary buds often more than one per axil, arranged vertically with the youngest lowest, sometimes developing into a bulbil serving as a storage organ. Inflorescence unisexual, racemose or spike-like; male flowers usually small and green, opening only slightly at anthesis, perianth in 2 whorls of 3 segments, stamens 6, all fertile or 3 staminodal; female flowers usually produced in much smaller numbers, perianth lobes more fleshy, opening more widely at anthesis, ovary 3-locular, style columnar, stigmas 3. Fruit a dehiscent trilocular capsule, 3-winged or strongly angled, 1—3 cm long. Seed flattened, partly or completely winged at its margin.
— Dioscorea bulbifera. Tuber produced superficially in the soil, swelling downwards from a rather thick attachment, replaced annually, solitary, globose to pyriform, usually densely covered with rough short roots. Stem twining to the left, climbing up to 6 m tall, usually without hairs, wings or spines. Leaves alternate, simple, broad or long ovate-cordate, up to 20(—32) cm 20(—32) cm, acuminate; secondary veins conspicuously ladder-like; upper surface shiny, slightly bullate between veins, bluish bloom; petiole half as long to as long as the blade, sometimes winged; auricles usually absent but when present subfoliaceous, partly embracing the stem. Bulbils numerous, usually in leaf axils but sometimes displacing male flowers at the base of flowering axes, small ones usually warty, large ones also smooth or angled, often kidney-shaped, 2.5—5 cm in diameter, flattened, grey to brown, usually weighing 0.5 kg but up to 2 kg; flesh usually pale yellow tinted with violet, when cut, oxidizing to orange, very mucilaginous. Male inflorescence pendulous, 1—4 from the axil of a bract, up to 1 m long with up to 100 flowers; flowers pinky green to white. Female inflorescence pendulous, 1—2 per leaf axil, with about 40 flowers; flowers pediceled, with tepals free. Fruit long and elliptical, 20—22 mm 8—9 mm, reflexed, winged, shiny brown. Seed winged.
— Dioscorea nummularia. Tuber spindle-shaped, produced deep in the soil, diameter increasing gradually towards the tip, up to 1 m long and 6 cm in diameter, solitary or sometimes 2 or more; flesh white. Stem twining to the right, cylindrical, usually longer than 3 m, densely spiny at base, less so higher up, glabrous. Leaves opposite or alternate, simple, hastate, up to 11 cm 9 cm, glabrous, 5—7-veined, red-brown; petiole up to 7 cm long; auricles rounded. Bulbils absent. Male inflorescence 1—4 together, aggregated on downward-pointing leafless branches to 4 cm long with about 50 flowers. Female inflorescence 1—2 together, up to 15 cm long. Fruit red-brown, wings 20 mm 22 mm.
— Dioscorea pentaphylla. Tubers elongated and buried deeply, or globose, pyriform or lobed and produced superficially in the soil; flesh white or lemon yellow, sometimes purple flecked. Stem twining to the left, up to 7 mm in diameter, up to 10 m long, usually abundantly prickly in lowest part, glabrescent. Leaves alternate, 3—5-palmately compound, deep rusty red or dirty white pubescent; middle leaflet broadly oblanceolate to obovate, up to 15 cm 4.5 cm. Bulbils numerous, globose to shortly ellipsoid, brown. Male inflorescence on long leafless branches, bearing 50 flowers each on 3 cm long pedicels. Female inflorescence pendulous, 1—3 together per leaf axil, up to 25 cm long, red pubescent. Fruit blackish, wings up to 20 mm 6 mm, pubescent.

Four phases can be distinguished in a generalized growth cycle of the yam plant (7—9 months form planting to harvesting). In the first phase a profuse root system and a vine develop, the plant depending on nutrients stored in the parent set. In Dioscorea rotundata this phase lasts about 6 weeks. In the second phase the foliage develops. In Dioscorea rotundata the canopy is fully formed between weeks 6 and 10 after planting. Root growth continues until week 10, after which root mass remains constant. Vine growth continues until week 13. During the second phase the new plant becomes independent from the parent set. Tuber initiation occurs at week 10—11 and for species that flower, flowering begins approximately at this time. The third phase is characterized by an increase in tuber bulk, which continues until the end of the season. In this phase the fruit also develops. In the fourth phase the shoot senesces and dies back, and the tuber enters a dormant period lasting 2—3 months. The exact duration of each phase varies with the species.
Bulbil production begins when the plant has matured to a certain extent. Bulbils develop in the axils of new leaves and this continues until the end of the season, with old bulbils maturing and dropping off while new ones form closer to the shoot tip. New bulbils arise from the bulbil primordia in the leaf axils. Dioscorea bulbifera flowers profusely (north of the equator in September, south in May), and produces seed abundantly.

The taxonomy of the genus Dioscorea has not stabilized yet because essential information is still missing. Important characteristics are (in order of importance): underground parts, direction of twining, the seeds (wings, colour, shape), the position of the capsules, the shape of the torus of the flower, the degree of segregation of male and female flowers, anatomical features (hairs, glands, etc.).
Most species with edible tubers or bulbils are rather variable. Many cultivars are known.
— For Dioscorea bulbifera 4 botanical varieties have been distinguished in South-East Asia: var. bulbifera (wild plants, tubers and bulbils acrid and nauseous, leaves shortly cordate, the most common variety); var. heterophylla (Roxb.) Prain & Burkill (as var. bulbifera but with elongated cordate leaves, occurring in Peninsular Malaysia); var. suavior Prain & Burkill (cultivated or semicultivated plants, tubers and bulbils may be slightly acrid and nauseous, bulbils dark grey-brown, warty, occurring in Java, Madura, Buru, Halmahera, south-eastern New Guinea and the islands eastward); var. sativa Prain (as var. suavior but with large, smooth, pale yellow bulbils, occurring in Peninsular Malaysia, Singapore, Java, New Guinea, Pacific islands, India ('otaheite potato'), Japan).
Plants from Africa (with small hairs and curiously angled bulbils) have been collectively called var. anthropophagorum (Chev.) Prain & Burkill.
Well-known cultivars from Dioscorea bulbifera include: 'Thuma' (New Caledonia), 'Poison' (Hawaii), 'Smooth Angled' (Puerto Rico).
— Dioscorea nummularia resembles Dioscorea cayenensis from West Africa but its tubers are formed deep in the soil.
— Dioscorea pentaphylla is very variable. In South-East Asia the following varieties have been distinguished: var. pentaphylla (the most common, general form, perhaps including 'huwi sawut' from Java with small, cylindrical, unbranched tubers); var. malaica Prain & Burkill (more than twice their diameter elongated tubers, abundantly red pubescent, leaflets narrow, middle one 4 times as long as broad, occurring in Peninsular Malaysia, northern Borneo, the Philippines); var. papuana Burkill (tubers not elongated, usually lobed, not flattened, covered with roots, large vigorous plants, leaflets to 20 cm long, red pubescent, distal leaves usually simple, flowers large, occurring in New Guinea); var. javanica Burkill (as var. papuana but plant smaller, leaflets to 10 cm long, flowers smaller, occurring in Java ('huwi jahe'), Sulawesi, Lesser Sunda Islands); var. palmata Burkill (tubers flattened, smooth-skinned, leaflets to 20 cm long, silvery pubescent, flowers relatively large, occurring in Timor, the Philippines); var. sacerdotalis Burkill (as var. palmata but leaflets to 14 cm long and flowers and tubers smaller, occurring in Java ('huwi mantri', 'huwi putri', 'huwi dewata'), Kangean Islands).

Most yam species are essentially tropical plants, not tolerating any frost. Optimum rainfall is 1500 mm/year or more, well distributed over the growing season. Areas with more than 3—4 dry months per year are unsuitable. Although yam is relatively tolerant of dry conditions for survival, without ample moisture (rainfall or irrigation) tuber production is poor. Optimum average temperatures during growth are 25—30°C. Short days tend to favour tuber formation and high light intensities are favourable. Yams need fertile soils (more than e.g. cassava or sweet potato), rich in organic matter, preferably loamy, and well drained, since they do not tolerate waterlogging.
Dioscorea bulbifera will grow well in areas with 1000 mm rainfall per year. It prefers low elevations, but has been found growing in the Himalayas up to 1800 m and in Yunnan to 2700 m altitude.
The ecological requirements of Dioscorea pentaphylla are mostly like those of Dioscorea bulbifera.

Dioscorea is usually propagated by tubers. Dioscorea bulbifera is propagated by bulbils or by tubers. Bulbil dormancy is most pronounced immediately after harvest, and declines steadily with time in storage. The dormancy has been ascribed to batatasins (phenolic compounds) in the outer tissues of the bulbil, and abscissic acid distributed throughout the bulbil. Bulbil sprouting is promoted by exogenous application of benzyl adenine or indole-acetic acid, and inhibited by exogenous application of gibberellic acid. In Dioscorea bulbifera the planted bulbil sprouts from the end near its previous point of attachment to the parent plant. The larger the bulbil or bulbil piece used for planting, the more robust the resulting plant.
Dioscorea nummularia and Dioscorea pentaphylla are propagated by means of tubers. They are planted on mounds, ridges, or on the flat. Intercropping with other food crops is common.

Plants are staked soon after emergence. Weeding is done repeatedly throughout the season. Fertilizer or pesticide use is rare. In general, yam is the first crop in the rotation after fallow.

Fungal leaf spots afflict most of the species in the field. Dioscorea bulbifera is moderately susceptible to attack by the yam nematode (Scutellonema bradys).

In Dioscorea bulbifera older bulbils mature as new ones are produced towards the apex. Repetitive harvesting is therefore necessary to remove mature bulbils. The main tuber harvest occurs 9—24 months after planting, when the entire plant senesces. For the other species, tubers are harvested from wild or cultivated plants, using manual digging implements. Some plants may be deliberately left to grow for 2—3 years before harvesting. Harvesting is made more difficult in Dioscorea nummularia by the dense spines at the base of the stem, and the deep location of the tuber in the soil.

The greatest bulbil yields of Dioscorea bulbifera in trials in Puerto Rico were 19.5 t/ha from cultivar 'Thuma', while the greatest tuber yields were 22.1 t/ha for cultivar 'Poison', which also had the highest combined yield (32.7 t/ha) of tuber plus bulbil.

Yam collections are held by the Malaysian Agricultural Research and Development Institute (MARDI), Serdang, Selangor, Malaysia, and the United States Department of Agriculture (USDA) Agricultural Experiment Station, Mayaguez, Puerto Rico.

For food production, breeding should aim at achieving a higher harvest index so that more of the dry matter is stored in the economic part (bulbil or tuber), at decreasing the bitter, poisonous or acrid principles (mainly alkaloids, saponins, and phenols), at reducing the time taken to mature, at increasing the ease of harvest (less spininess and shallower tubers), and at increasing overall yield. For diosgenin production, an increased diosgenin content would be a desirable breeding objective.

For food, Dioscorea bulbifera, Dioscorea nummularia and Dioscorea pentaphylla are certainly minor yams which can hardly compete with the agronomically well-established yams, i.e. Dioscorea alata and Dioscorea esculenta. They are even less likely to be able to compete with the African yams (especially Dioscorea rotundata) on a global scale. Their main role will remain a reserve food in times of scarcity, primarily in subsistence farming systems. For industrial purposes, research should focus on further elucidation of the composition of these tubers and bulbils, and whether any of their contents (e.g. starches, sapogenins) have any particularly advantageous characteristics that will warrant economic exploitation.

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I.C. Onwueme

Onwueme, I.C., 1996. Dioscorea L.. In: Flach, M. & Rumawas, F. (Editors): Plant Resources of South-East Asia No 9: Plants yielding non-seed carbohydrates. PROSEA Foundation, Bogor, Indonesia. Database record: prota4u.org/prosea