Yam

Uses

In many respects and in many countries, yam is the potato of the tropics, where it is an important staple food, especially in many countries of Africa and the Caribbean. Yam is also important in regions of Asia, Oceania, and tropical America. Compared with other carbohydrate-rich root crops, such as sweet potato and cassava, yam is generally preferred and accordingly sells at higher prices. As a rule, yam is consumed locally where it is produced, and because of its value and large volume, only a small volume enters international trade. However, owing to recent migration of people to temperate regions, yam is increasingly in demand in ethnic markets in the United States and Europe.

Yam is prepared for consumption by boiling, frying, or baking. In Central Africa, Dioscorea rotundata, in particular, is used to prepare fufu, which is made by mashing starchy tissues into a sticky dough. Other uses include dried products such as flour, flakes, or chips. Yam is generally too expensive for animal feed or conversion into fuel alcohol. Unfortunately, a significant amount, 10–20% of production, is lost to rotting, damage, or poor storage. In addition, not all tubers are edible; preparation losses amount to 5–15% because of unsuitable thick skin and hard proximal parts, and also because tubers of some species are irregularly shaped. Sometimes the shoots of some wild species are boiled and eaten as greens.

The composition of yam varies considerably among species. In general, tubers have a high starch content (25% fresh weight), mainly in the form of amylopectin. Provitamin A content is low, and vitamin C content ranges from 5 to 15 mg/100 g. Protein content on a fresh-weight basis is about 2%, and sulfur-containing amino acids are low. There are species differences in protein content, but the differences are not pronounced. However, when daily consumption volume is considered, yam can make a significant contribution to some diets.

The mucilaginous substance released from the surface of cut tubers consists mainly of glycoproteins. The acrid taste of yam is explained by the presence of tannins. Some species contain the alkaloid dioscorine (C₁₃H₁₉O₂N), which is water-soluble and removed by soaking and boiling. The dioscorine content in cultivated species is usually negligible and does not present a problem with proper preparation. Tissues of some wild species contain a large amount of dioscorine and may be poisonous; however, after leaching they can be used as a source of starch. Some species contain diosgenin, a steroidal sapogenin compound that is used to produce cortisone and other medicines.

History and Origin

The genus Dioscorea (Dioscorea), the largest in the family Dioscoreaceae (Dioscoreaceae), includes more than 500 species. Members of Dioscoreaceae are monocotyledonous plants, but they exhibit characteristics of both monocotyledons and dicotyledons, suggesting that they may have been among the early angiosperms.

Some taxonomists believe that early in the evolution of Dioscorea, a split occurred into Old World and New World species. This is reflected in cytological differences, where Old World species (Africa and Southeast Asia) have a basic chromosome number X = 10. New World species have a basic chromosome number X = 9, and most of them are tetraploid or hexaploid. High ploidy also occurs in Old World yams, with chromosome numbers ranging from 50 to 140.

Yam cultivation apparently developed independently in West Africa and Southeast Asia, possibly around 3000 BCE or earlier. West African artifacts indicate that yam was used as food for thousands of years. Probable centers of domestication are Indochina, southern China, West Africa, and the Caribbean/South America.

Production

More than 95% of world yam production is in Africa, and more than 72% of this volume is produced in Nigeria. In recent years, world production has increased somewhat, mainly through expansion of planted area. The leading non-African yam-producing countries are Papua New Guinea, Brazil, Haiti, Japan, and Jamaica, which produced 224, 215, 212, 180, and 156 thousand tons, respectively. Although on a global scale yam production in Central America and the northern countries of South America is insignificant, it has recently been growing. In Japan, yam is grown as an expensive specialty crop that is also believed to have some beneficial properties.

Taxonomy

The taxonomy of the genus Dioscorea (Dioscorea) is sometimes confusing. For some species, morphology is not sufficiently distinctive for adequate identification, and wild forms still occur for most cultivated species. Although spontaneous hybridization probably contributed to the origin of some yam species, reproduction has been entirely vegetative; each “cultivated” species represents a single clone.

About 60 species of Dioscorea have edible tubers. The following species are commonly grown as crop plants:

• Dioscorea alata L. — winged yam, or greater yam. Other names: water yam, winged yam, and Ubi yam. It originated in Southeast Asia but is currently the most widely distributed species. Vines twine to the right (right-hand twining rule: right thumb up, fingers clasping the support point in the direction of vine growth). Bulbils are sometimes formed. Leaves are ovate and oppositely arranged. Tubers are single or numerous, vary in size, shape, and color, and have a long dormant period.
• Dioscorea bulbifera L. — air potato, potato yam, or aerial yam. African and Asian origin. Vines twine to the right. Stems are hairless and thornless. Leaves are large and simple. Aerial tubers (bulbils) form in leaf axils; their average weight is 600–700 g, and their width and length are 4 and 8 cm, respectively. Aerial tubers, although juicy and edible, may require detoxification before consumption. Underground tubers are either absent or usually smaller than those of other Dioscorea spp.; they are hard, bitter, and unpalatable without processing.
• Dioscorea cayenensis Lam. — Cayenne yam, or yellow yam. West African origin. Vines twine to the right. Vines are prickly; leaves are simple, oppositely arranged, heart-shaped with pointed tips, hairless. Bulbils are not formed. Tubers vary considerably in size and shape and have a short dormant and storage life. It requires a long growing season; a truly tropical species, it performs best under warm temperatures with a long rainy season and does not tolerate drought.
• Dioscorea dumetorum (Kunth) Pax. — bush yam, cluster yam, bitter yam, or three-leaved yam. African origin. Stems are hairy and prickly; leaves are three-angled; bulbils are rare. Tubers are medium to large, sometimes fused into a cluster. Tubers often have a high alkaloid content. After cooking, tubers harden quickly.
• Dioscorea esculenta (Lour.) Burk. — lesser yam, or Asiatic yam. Other names: potato yam, Asian yam, and Chinese yam. It originated in Indochina but occurs in all Asian regions. Twining to the left (left-hand twining rule: left thumb up, fingers clasping the support point in the direction of vine growth). Slender pubescent stems are cylindrical with numerous prickles. Leaves are few, simple, heart-shaped, alternate; bulbils are absent. Tubers are numerous and small, thin-skinned, mainly cylindrical, formed near the surface and easily harvested; some forms produce single large tubers. Dormant period is short.
• Dioscorea hispida Dennstedt — Asiatic bitter yam, intoxicating yam, or starch yam. It originates from India and Southeast Asia. Twining to the left; bulbils are not formed. Tubers formed near the surface are often lobed, sometimes elongated. Some clones require detoxification before consumption.
• Dioscorea japonica Thunb. — Japanese yam, or Jinneho yam. Native to Japan, found wild in open fields and mountains. Twining to the right. Despite similar cold hardiness, it differs from Dioscorea opposita in chromosome number, vegetative characteristics, tuber shape, and chemical composition.
• Dioscorea opposita Thunb. (syn. Dioscorea oppositifolia) — opposite-leaved yam, Chinese yam, or cinnamon yam. Synonym Dioscorea batatas. Originates from China. Twining to the right; bulbils are sometimes formed. Leaves are simple, oppositely arranged. Tuber shape is diverse, from thin cylindrical with a tendency to grow vertically downward, and some are also palmate to spherical. Tubers do not contain sapogenins. Among cultivated yam species, Chinese yam has the best temperature adaptation and is grown in many temperate regions; in Japan, more than 100,000 tons of this species are produced annually. In Japan there are three strains of Dioscorea opposita: naga imo — cold-hardy, produces long club-shaped tubers; icho imo — fan-shaped tubers; yamato imo — round tubers, some cultivars not cold-hardy.
• Dioscorea rotundata Poir. — white yam or Eboe yam. West African origin and the main cultivated species. Some consider it a subspecies of Dioscorea cayenensis. Vines are prickly and twine from right to left. Leaves are simple, oppositely arranged, heart-shaped with pointed tips, hairless. Bulbils are rarely formed. It produces one usually spherical or cylindrical tuber, usually after 8–10 months. The best species for preparing fufu.
• Dioscorea trifida L.f. — cush-cush, yampi, or aja yam. The only important cultivated species originating in tropical South America. Twining to the left; stems are spineless, rectangular in cross-section with flattened wings; bulbils are not formed. Leaves are simple with 3–5 lobes. It produces a large number of small tubers, often resembling sweet potato with varying flesh color. It is adapted to warm temperate regions.

Some cultivated and wild species are also used for medicinal purposes.

Botanical Description

Yam is a tropical perennial plant, usually cultivated as an annual because of its underground or aerial tubers. Plants exhibit a strong annual cycle of growth and dormancy, with the tuber as the dormant organ. The stem is usually formed annually when the rainy season begins. Initially erect, the stem climbs and twines around trees or supports; the stems are not self-supporting. Stems are prickly or winged. The direction of stem twining, right or left, is constant for each species. Species differ in the shape of the stem cross-section, presence of prickles, hairs, coloration of the leaf surface, and wing-like appendages of the stem. Vines usually die back at the end of the rainy season or under low temperatures.

Yam has a fibrous root system that extends mainly horizontally, with most roots located relatively shallowly. Adventitious roots form from the worm-like base of the stem (primary node) or directly from the tubers.

Leaves, borne on long petioles, are usually simple and heart-shaped, but in some species they are lobed or palmate. A characteristic feature of the genus is that the main veins of the leaf blade are arcuate. Leaves vary in intensity of green color; some sometimes produce anthocyanin pigments. Hairs are usually absent. Petioles can bend to maximize illumination of the leaf blade. Plants are usually dioecious, producing separate small greenish inconspicuous flowers on long peduncles. Flowers, although rarely, are fragrant and pollinated by insects, often thrips. The fruit is a dehiscent capsule containing up to six seeds. In some species, seed set is rare. Seeds are small, light, and winged, allowing them to be dispersed by wind. Poor flowering, seed production, and frequent sterility are probable evolutionary consequences of a long history of vegetative propagation.

All edible species form one or more tubers, which are renewed annually. In some inedible species, the tuber is a perennial organ that increases in size from year to year and contains fibers.

Some species produce aerial tubers known as bulbils. They form in the axils of newly formed leaves after the plants have reached sufficient growth and usually continue to form until the end of the growing season. Bulbils are small but otherwise similar to tubers and have a wide variety of shapes and colors.

Yam is considered a tuber because it originates from the stem and is more stem-like than root-like. However, there are some atypical stem features, such as the absence of leaf scars, buds, or nodes on or near the surface, and the absence of a terminal bud. In addition, most species exhibit strong geotropic growth. Evidence that the tuber originates from hypocotyl tissue is convincing; the first meristematic activity for root, shoot, and tuber growth occurs in the region of the worm-like structure at the base of the vine. The primary meristem is located at the distal end. As growth continues, a thin layer of meristematic cells remains beneath the tuber skin, which, with continued activity, forms parenchyma tissues. This layer is also capable of initiating buds that develop into shoots. The main part of the tuber consists of starch-containing parenchyma tissues, which are responsible for the increase in girth. Vascular bundles run through the center of the tuber, which is characteristic of monocots. The corky suberized skin layer protecting the tuber is formed from cork cambium.

Cultivated species produce annual underground tubers; in some species the tubers are perennial. Depending on the species, single or multiple tubers are formed. Tuber shape also depends on the species and environmental conditions and may be cylindrical, branched, or lobed.

Tuber weight depends on the species and environmental conditions. Some can weigh less than 1 kg, while others exceed 50 kg. Tubers weighing more than 100 kg have been reported, but most tubers grown for food weigh 2–10 kg. Growth cracks caused by tuber expansion often appear in the corky layer. The internal flesh is white or light yellow; some yams contain anthocyanins that give them colors ranging from pink to purple.

Tuber dormancy begins with the cessation of growth, which usually coincides with the beginning of the dry season. By the beginning of the wet season, the dormant period ends and new shoots appear.

Ecological Requirements

Most yams are grown in tropical and subtropical regions and are intolerant of frost. Some, such as Dioscorea opposita, tolerate low temperatures and are grown in warm temperate regions. Optimal growth of most species occurs at about 30 °C; at average temperatures below 20 °C, growth is poor. Temperatures above 35 °C are detrimental to growth. Soil mulching is often practiced to protect against excessive heat and drying. Mulching improves soil structure, aeration, and fertility. Dried grasses or straw with soil cover are often used as mulch.

Yam is usually grown in regions with relatively distinct wet and dry seasons. Average rainfall of about 1500 mm is characteristic of these yam-growing areas. Yields decline if rainfall is less than 600 mm, especially if the plant experiences moisture deficit during tuber bulking. Vine weight and tuber yield are strongly correlated with available moisture. Tuber growth is slowed on waterlogged soils. Species differ in their response to drought; some are tolerant of low moisture.

Loose, deep, well-drained fertile soils are necessary for high tuber yields. The yam root system is usually shallow and sparse, and productivity on poor soils is limited. They are often the first crop grown in slash-and-burn, or because of residual nitrogen they are likely to follow leguminous crops. Compared with other root crops, yam requires the most intensive management and a high level of soil fertility to obtain high yields of good-quality tubers. Nutrient use is similar to white potato; yam is responsive to nitrogen and potassium. Best results are achieved with a uniform supply of nitrogen throughout the growing period. Fertilizer application is most beneficial if nutrients are available at the growth stage when the plant is no longer dependent on the seed tuber piece. However, because most yam production is by subsistence farmers, additional fertilizers and crop care are often not provided. To obtain good yields, this crop requires significant labor input, which is the main reason it is a relatively expensive commodity.

Yam is often grown together with maize, where the stems serve a secondary function of supporting the twining vines. However, such intercropping often reduces yield, which is the reason for the decline in this practice. Stakes or poles 2–5 m long, as well as various types of trellises, provide support for the vine; small trees are also used to support the vine. Potential yield can decline sharply for some species if the vines are not supported for maximum illumination. Until vine growth is established, weeds can easily outgrow the crop; therefore they must be controlled, otherwise yield will decline.

Propagation and Planting

Small intact tubers, bulbils, or pieces of tubers (setts) are used for propagation. Seed tubers weighing 100 to 150 g are usually used whole. Heads (crowns) weighing 250 g are considered ideal propagation material because of rapid uniform sprouting and high productivity. Bulbils usually produce smaller tubers and often require a longer growing period before harvest. Bulbils are commonly used to propagate Dioscorea bulbifera, but bulbils of other species are used less often. Cut seed pieces of tubers weighing 200–300 g are used when small whole tubers are unavailable. Preference is given to the proximal end, since it gives the best vine growth; the middle section is next, and the poorest growth is observed when the distal end is used. To overcome the low multiplication rate of propagation material, minisetts can be used. Minisetts are small disc-shaped sections cut from the surface of the tuber. With this procedure, about 40,000 sprouts can be obtained from 1 ton of tubers.

In some areas of West Africa, large seed tubers are preferred because a larger food reserve is believed to ensure vigorous rooting, vine growth, and high yield of individual tubers. However, studies have shown that seed tubers larger than 300 g give practically no yield advantage, while a large amount of edible product is used. A significant part of the harvest must be reserved for propagation; up to 3 tons of propagation material are used per hectare. Unfortunately, many farmers sell their best yams and use low-quality tubers for propagation.

Seed tubers are planted in a dormant state or are pre-sprouted. Once dormancy is broken, sprouting occurs rapidly under high humidity at 25–30 °C. High humidity is necessary for tuber formation. Onwueme (1978) showed that sprouting begins in the meristematic region in the parenchyma tissues about 1 cm below the corky surface of the tuber, usually at the proximal (head) end of the tuber. Cells in the meristematic region produce a mass of undifferentiated cells that become organized for development of the shoot apex. The shoot apex breaks through the tuber skin as it elongates. Soon roots form at the junction between the base of the shoot apex and the tuber. Sprout initiation at the proximal end occurs faster than at the middle or distal (tail) end. After the tuber is cut, buds often form easily because apical dominance has been removed.

As a rule, vine cuttings are not used for propagation because production is delayed, although this is a method of increasing the amount of propagation material without using tubers. Meristem culture can be used to obtain virus-free propagations, but because of the very slow development of plantlets, this procedure is limited. Except for breeding, the use of true seed, which is available in very few cultivars, is limited for propagation. This is due to seed dormancy, variability, poor tuber quality, and low yield.

In developing countries, traditional slash-and-burn agriculture is used to a large extent for yam production. The land is cleared, but some small trees may be left to support the vine. Farmers practice minimum tillage, although traditional tillage and cultivation may be practiced by commercial producers.

Planting schedules depend on regional climate and are usually set at the end of the dry season. Most rely on the onset of rainfall to establish new plantings. However, planting can also be done in the dry season, since this gives seed tubers time to overcome dormancy, and sprouts appear soon after rain. Plantings may be made on mounds, ridges, or raised beds about 50 cm high to improve drainage and facilitate harvesting. These methods are less labor-intensive, as they can be mechanized, although they usually lead to lower yields.

Unlike sweet potato or white potato, in which roots or stolons penetrate the soil and later enlarge, in yam tuber enlargement begins early, but enlargement (bulking) is a continuous process whose rate depends on the species. Unless the soil is very loose, penetration into it is difficult because of the enlarging blunt distal part of the tuber. Therefore, deep and loose soil provided by tillage or mounding is an advantage for deep penetration before tuber enlargement begins. Mulching is useful for protecting tubers from injury during weeding. Propagation material is planted at a depth of 10–15 cm, usually at a spacing of about 1 × 1 m, but spacing depends on the species and size of the propagation material. Populations usually average about 10,000 plants per hectare. High planting density increases total yield, but tuber size remains small. However, for some species this is a desirable practice.

Growth and Development

The growth of the yam plant is strongly influenced by moisture availability. Since irrigation is usually not practiced in yam-growing countries, moisture deficit strongly limits growth. This is most pronounced during tuber enlargement (bulking). Day length also has an influence: long days favor vine growth, while short days favor tuber initiation. In some species, older plants form tubers earlier than young plants when exposed to short days. Since day length in tropical regions is relatively constant, photoperiod is usually not a significant factor.

Seed tubers stored for a long time sprout earlier and form tubers earlier than tubers stored for a short time. Prior direct exposure of seed tubers to light does not affect subsequent tuber formation. Early tuber formation, before sufficient canopy has formed, is undesirable and leads to small tubers and low yield.

Using nutrients and moisture contained in the seed tubers, initial shoot growth occurs very rapidly, elongating up to 15 cm per day. Within a month or two the plant becomes self-sufficient, and in most species the original seed tuber eventually shrivels and/or rots. In some species the propagating tuber continues to grow or transfers its nutrients to the new tuber(s).

Stem and leaf growth occur rapidly, and the plant reaches full canopy about the time of tuber initiation. Leaf growth slows during tuber enlargement. New tuber formation initially occurs slowly and follows a sigmoid curve, with tuber size increasing rapidly over time. Carbohydrates accumulate as long as growth continues. With leaf senescence, further tuber enlargement ceases, and the tubers become dormant.

Harvest

In most tropical regions, harvest usually occurs after the end of the rainy season, when the foliage begins to senesce. Harvest is usually delayed as long as the vines remain functional. In some situations tubers are harvested as early as 7–9 months, but usually this occurs at 10–11 months. In some areas the crop can be left until needed; this period may vary depending on species and conditions. In normal harvesting, all tubers are removed from the soil.

In partial harvesting, it is usually carried out about the middle of the growth cycle. In this case, the soil is carefully removed, and some tubers or parts of large tubers are cut below the worm-like attachment to the stem. The soil is replaced, the remaining tuber heals, and growth resumes. In addition, new tubers may form that can be harvested at the end of the growing season. Bulbils are harvested when considered sufficiently large, simply by picking them from the vine or collecting them after they fall to the ground.

Harvesting is done carefully to avoid damaging the fragile tubers. In traditional agriculture, wooden spades or digging sticks are used, making harvesting labor-intensive and slow. It is very important to avoid damage during handling and storage, since damage leads to losses from rotting. Such variable factors as tuber size, shape, and depth in the soil hinder effective mechanization of harvesting. Nevertheless, in nontraditional large-scale production, equipment is sometimes used.

Because of the large number of yam species, as well as cultural and environmental variables in yam-producing regions, there is great variability in yield. Average world yield is about 12 t/ha, but under intensive cultivation this level is easily exceeded, and yields exceed 40 t/ha.

Storage

The simplest storage is to leave dormant tubers in the ground. Some species and cultivars have a long dormant period, allowing tubers to be left unharvested. However, this period must be short to avoid sprouting of tubers once they are no longer dormant. Yam is also placed in a trench or pit and either covered with soil or mulch and shaded. As with field storage in situ, this method is not suitable for long-term storage.

In other storage methods, tubers are dried on the surface after lifting. Sometimes they are cured by holding them for several hours in dry air in the shade to heal wounds, and then carefully placed in well-ventilated weatherproof structures or on shaded platforms. Single-layer storage without direct tuber-to-tuber contact is preferable to heaps. Another method involves hanging tubers on twine between poles for ventilation and protection from rodents in structures called yam barns. Adequate ventilation is very important to prevent high humidity around the tubers and to remove respiratory heat. Weight loss due to respiration and drying can average up to 5% per month. Optimal storage temperature is 15–16 °C and 70% relative humidity, best achieved using cold rooms and humidity control. At 10 °C or below, chilling injury may occur. Under favorable conditions, some species can be stored for several months.

At first harvest, tubers are usually dormant. In this state, respiration is low and, consequently, dry matter losses (shrinkage) are insignificant. The dormant period depends on the species and may last 1–3 months, sometimes longer. After sprouting begins, respiration and moisture loss increase. Sprouting during storage can become a problem, although sprouts can be rubbed off, as is sometimes done with white potato. Sprout-inhibiting chemicals such as maleic hydrazide (MH) are partially effective in prolonging the dormant period when applied to the growing plant; soaking tubers in MH is effective in suppressing sprouts without deteriorating tuber quality. Gamma irradiation, although not a common treatment, can also reduce sprouting and weight loss.

Diseases and Pests

Bacterial diseases of yam:

• Agrobacterium tumefaciens (English Crown gall);
• Corynebacterium spp. (English Tuber dry rot).

Fungal diseases:

• Glomerella cingulata (English Anthracnose);
• Thanatephorus cucumeris (English Leaf blight);
• Cercospera spp., also Colletotrichum capsici and Gloeosporum pestis (English Leaf spots);
• Corticium rolfsii (English Neck rot);
• Goplana dioscoreae (English Rust);
• Fusarium oxysporum, Fusarium solani, Rhifzopus nodosus, and species of Armillaria, Penicillium, Armillaria, Rosellinia, and Sphaerostilbe (English Tuber rots);
• Botryodiplodia theobromae (English Tuber storage soft rot);
• Phylleutypa dioscoreae (English Witches broom).

Mosaic is the most common and most serious viral disease. Internal brown spotting virus (Internal brown spotting virus, IBSV) damages tubers during storage.

Major pests:

• Gymnogryllus lucens (English Crickets);
• Heteroligus meles (English Greater yam beetle);
• Loxura atymnus, Tagiades gana, Theretra nessus (English Lepidoptera pests);
• Heteroligus appius (English Lesser yam beetle);
• Planococcus citri, Geococcus coffeae (English Mealybug);
• Aspidiella hartii (English Yam scale);
• Crioceris livida (English Yam shoot beetle);
• Palaeopus dioscorae (English Yam weevil).

Other important insect pests include wireworms, termites, and grasshoppers.

Nematode pests include the yam nematode (Scutellonema bradys), root-knot nematode species (Meloidogyne), and lesion nematodes (Pratylenchus spp.). Rodents are chronic and serious pests, eating tubers both in the soil and in storage.

Yam competes poorly with weeds until a good leaf canopy is formed. Unfortunately, full leaf canopy is achieved slowly, and the main means of weed control is hand hoeing.

Crop Improvement

Overall, the carbohydrate yield of yam is lower than that of other starchy tuber and root crops, but yam, being a very tasty and valued food with strong ethnic and traditional use, is likely to remain a staple food, especially in many African countries. Nevertheless, yam production needs improvement to remain competitive with cassava, sweet potato, and other less expensive staple foods. There is a clear shift toward greater commercialization of this crop, which should lead to lower production costs.

Constraints on expanding yam production include high labor requirements, low net yield, diseases, cultural problems, and harvesting and postharvest control problems. Some obvious needs are breeding and selection to obtain high-yielding, pest-resistant cultivars with improved nutritional value. Improving quality and reducing the amount of propagation material used are additional goals. Alternatives for improving trellising, introducing greater mechanization, and using processed product are further goals. At the International Institute of Tropical Agriculture (IITA) in Nigeria and other international and national research organizations, research efforts are directed toward achieving these and other crop improvement goals.

References

1. Rubatzky, Vincent E., Yamaguchi, Mas. (1997). World vegetables: principles, production, and nutritive values. 2nd ed.