Yam

? Dioscorea (Yam)
Scientific classification
Kingdom: Plantae Division: Magnoliophyta Class: Liliopsida Order: Dioscoreales Family: Dioscoreaceae Genus: Dioscorea L.

Yams are members of the flowering plant genus Dioscorea. They are monocots, related to palms, grasses, and orchids. There are about 600 species of yams found around the world, mostly in the tropics. Some species of yam are cultivated for their edible tubers, for medicinal use, and for other uses. Yams remain an important food crop, especially in Africa.

In the United States sweet potatoes, Ipomoea batatas, are sometimes called "yams", although they are not closely related.

The yam plant

Most yam species grow in the tropics and sub-tropics in areas with fairly heavy total annual rainfall but with a definate dry season. During the rainy season they produce one or more underground tubers to store food and water through the dry season. The tubers are thickened stems. At the end of the dry season they send out shoots which grow into vines which grow up into nearby trees and bushes. Many species grow bulbils, small tuber like growths, at the bases of their leaves. These can fall to the ground and produce new plants. Most yam plants have small flowers with one plant having only male or female flowers (Kay, 1987).

Cultivation

Tongan farmer showing off his prize yams

The tubers of most species of yam are poisonous to humans. A few are edible and some others can be made edible by various methods including soaking, boiling, and drying. People started to cultivate yams, rather than just digging up wild ones, as long as 10,000 years ago in both Africa and Asia; and some time later in the New World.

The cultivation of yams is very labor intensive. Cultivated yams mostly do not produce seeds and so tubers or pieces of tuber must be planted in prepared soil, most often in mounds, to grow new plants. Some kind of framework or trellis must be provided to support the vines, unless they are grown next to trees or in fields previously planted with corn so the old stalks can provide support. At the beginning of the dry season the vines die away and the tubers are ready to be harvested. They must be dug out by hand very carefully; if they are damaged they could spoil soon (Kay 1987).

Yams are a nutritious food providing carbohydrates, some protein, and minerals like phosphorus and potassium. Tubers can be stored as long a six months without refrigeration (CGAIR 2006).

For hundreds of years yams were the most important food in many parts of Africa and the Pacific islands. There are many traditions associated with yams including ceremonies and festivals which show their importance in traditional society.

Today West Africa produces over 90 percent of the world's yam crop, with Nigeria the largest grower. South America, the West Indies, the Pacific islands, and some parts of Asia also grow some yams. Most are grown for local consumption. There is also a small international trade and some are grown for medicinal and other uses (CGAIR 2006).

Cultivated yam species

Dioscorea rotundata and D. cayenensis

Dioscorea rotunda, the white yam, and D. cayenensis, the yellow yam, are native to Africa. They are the most important cultivated yams. In the past they were considered two species but most taxonomists now regard them as the same species. There are over 200 cultivated varieties between them. They are large plants; the vines can be as long as 10 to 12 meters (35 to 40 feet). The tubers most often weigh about 2.5 to 5 kg (6 to 12 lbs) each but can weigh as much as 25 kg (60 lbs). After 7 to 12 months growth the tubers are harvested. In Africa most are used to make the traditional dish fufu (Kay 1987).

D. alata

A Piece of Cake Made With Ube (water yam).

Dioscorea alata, called water yam, winged yam, and purple yam, was first cultivated somewhere in Southeast Asia. Although it is not grown in the same quantities as the African yams it has the largest distribution world-wide of any cultivated yam, being grown in Asia, the Pacific islands, Africa, and the West Indies (Mignouna 2003). In the United States it has become an invasive species in some Southern states.

In the Philippines it is known as ube (or ubi) and is used as an ingredient in many sweet desserts. In India, it is known as ratalu or violet yam or the Moraga Surprise.

D. opposita

Segment of a Dioscorea opposita tuber

Dioscorea opposita, Chinese yam, is native to China. It is tolerant to frost and can be grown in much cooler conditions than other yams. It is now grown in China, Korea, and Japan. It was introduced to Europe in the 1800's when the potato crop there was falling victim to disease. It is still grown in France for the Asian food market.

The Chinese yam plant is somewhat smaller than the African yam, with the vines about 3 meters (10 feet) long. The tubers are harvested after about 6 months of growth. Some are eaten right after harvesting and some are used as ingredients for other dishes, including noodles, and for traditional medicines (Kay 1987).

D. bulbifera

File:Dioscorea bulbifera.jpg

Air potato

Dioscorea bulbifera, the air potato, is found in both Africa and Asia with slight differences between those found in the two places. It is a large vine 6 meters (20 ft) or more in length. It produces tubers; however the bulbils which grow at the base of its leaves are the more important food product. They are about the size of potatoes (hence the name air potato) weighing from 0.5 to 2 kg (1 to 5 lbs). Some varieties can be eaten raw while some require soaking or boiling for detoxification before eating. It is not grown much commercially since the flavor of other yams is preferred by most people. However it is popular in home vegetable gardens because it produces a crop after only 4 months of growth and continues for the life of the vine, as long as two years. Also the bulbils are easy to harvest and cook (Kay 1987).

References

ISBN links support NWE through referral fees

• Consultative Group on International Agricultural Research (CGIAR), 1994, "A Breakthrough in Yam Breeding"[1]
• Consultative Group on International Agricultural Research (CGIAR), 2006, "Yam"[2]
• Kay, D.E., 1987, Root Crops, Tropical Development and Research Institute : London[3]
• Mignouna, H.D., Abang, M.M., & Asiedu, R., 2003, "Harnessing modern biotechnology for tropical tuber crop improvement: Yam (Dioscorea spp.) molecular breeding"[4]
• O'Hair, S.K., 1990, "Tropical Root and Tuber Crops", p. 424-428. In: J. Janick and J.E. Simon (eds.), Advances in New Crops. Timber Press : Portland, OR.[5]
• Schultz, G.E., 1993, "Element Stewardship Abstract for Dioscorea bulbifera, Air potato", The Nature Conservancy[6]
• University of Connecticut(UC), 2006, "Dioscoreaceae"[7]
• White, L.D., 2003, Canoe Plants of Ancient Hawai'i, "Uhi"[8]

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