The sweet potato ( Ipomoea batatas ) is a dicotyledonous plant that belongs to the family Convolvulaceae. Its large, starchy, sweet tasting tuberous roots are an important root vegetable (Purseglove, 1991; Woolfe, 1992). The young leaves and shoots are sometimes eaten as greens. Of the approximately 50 genera and more than 1,000 species of Convolvulaceae, I. batatas is the only crop plant of major importance – some others are used locally, but many are actually poisonous.
The sweet potato is only distantly related to the potato ( Solanum tuberosum ). The softer, orange variety is commonly marketed as a "yam" in parts of North America, a practice intended to differentiate it from the firmer, white variety. The sweet potato is very distinct from the actual yam, which is native to Africa and Asia and belong to the monocot family Dioscoreaceae. To prevent confusion, the United States Department of Agriculture requires that sweet potatoes labeled as "yams" also be labeled as "sweet potatoes".
The genus Ipomoea that contains the sweet potato also includes several garden flowers called morning glories, though that term is not usually extended to Ipomoea batatas . Some cultivars of Ipomoea batatas are grown as ornamental plants; the slightly ambiguous name "tuberous morning glory" may be used in a horticultural context.
This plant is a herbaceous perennial vine, bearing alternate heart-shaped or palmately lobed leaves and medium-sized sympetalous flowers. The edible tuberous root is long and tapered, with a smooth skin whose colour ranges between red, purple, brown and white. Its flesh ranges from white through yellow, orange, and purple.
Origin, distribution and diversity
Sweet potatoes are native to the tropical parts of South America, and were domesticated there at least 5000 years ago.
Austin (1988) postulated that the centre of origin of I. batatas was between the Yucatán Peninsula of Mexico and the mouth of the Orinoco River in Venezuela. The 'cultigen' had most likely been spread by local people to the Caribbean and South America by 2500 BC. Zhang et al. (1998) provided strong supporting evidence that the geographical zone postulated by Austin is the primary centre of diversity. The much lower molecular diversity found in Peru-Ecuador suggests that this region be considered as secondary centre of sweet potato diversity.
The sweet potato was also grown before western exploration in Polynesia, where it is known as the kumara . Sweet potato has been radiocarbon-dated in the Cook Islands to 1000 AD, and current thinking is that it was brought to central Polynesia circa 700 AD, possibly by Polynesians who had traveled to South America and back, and spread across Polynesia to Hawaii and New Zealand from there. It is possible however, that South Americans brought it to the Pacific. The theory that the plant could spread by floating seeds across the ocean is not supported by evidence. Another point is that the sweet potato in Polynesia is the cultivated Ipomoea batatas , which is generally spread by vine cuttings, and not by seeds.
Sweet potatoes are now cultivated throughout tropical and warm temperate regions wherever there is sufficient water to support their growth.
According to the Food and Agriculture Organization (FAO) statistics, world production in 2004 was 127,000,000 tonnes. The majority comes from China, with a production of 105,000,000 tonnes from 49,000 km 2 . About half of the Chinese crop is used for livestock feed.
Per-capita production is greatest in countries where sweet potatoes are a staple of human consumption, led by Papua New Guinea at 550 kg per person per year, the Solomon Islands at 160 kg, Burundi and Rwanda at 130 kg and Uganda at 100 kg.
About 20,000 tonnes of sweet potatoes are produced annually in New Zealand, where sweet potato is known by its Māori name, kūmara . It was a staple food for Māori before European contact.
In the U.S., North Carolina, the leading state in sweet potato production, provided 38.5% of the 2007 U.S. production of sweet potatoes. California, Louisiana, and Mississippi compete closely with each other in production. Louisiana has been a long-time major producer, once second only to North Carolina, and closely followed by California, until the latter began surpassing it in 2002. In 2007, California produced 23%, Louisiana 15.9%, and Mississippi 19% of the U.S. total.
The town of Opelousas, Louisiana's "Yambilee" has been celebrated every October since 1946. The Frenchmen who established the first settlement at Opelousas in 1760 discovered the native Attakapas, Alabama, Choctaw, and Opelousas tribes eating sweet potatoes. The sweet potato became a favourite food item of the French and Spanish settlers, and thus continued a long history of cultivation in Louisiana.
Mississippi has about 150 farmers presently growing sweet potatoes on about 8,200 acres (33 km 2 ), contributing $19 million dollars to the state's economy. Mississippi's top five sweet potato producing counties are Calhoun, Chickasaw, Pontotoc, Yalobusha, and Panola. The National Sweet Potato Festival is held annually the entire first week in November in Vardaman (Calhoun County), which proclaims itself as "The Sweet Potato Capital".
The town of Benton, Kentucky celebrates the sweet potato annually with its Tater Day Festival on the first Monday of April. The town of Gleason, Tennessee celebrates the sweet potato on Labor Day weekend with its Tater Town Special.
Cultivation
The plant does not tolerate frost. It grows best at an average temperature of 24 °C (75 °F), abundant sunshine and warm nights. Annual rainfalls of 750–1000 mm are considered most suitable, with a minimum of 500 mm in the growing season. The crop is sensitive to drought at the tuber initiation stage 50–60 days after planting and is not tolerant to water-logging, as it may cause tuber rots and reduce growth of storage roots if aeration is poor (Ahn, 1993).
Depending on the cultivar and conditions, tuberous roots mature in two to nine months. With care, early-maturing cultivars can be grown as an annual summer crop in temperate areas, such as the northern United States. Sweet potatoes rarely flower when the daylight is longer than 11 hours, as is normal outside of the tropics. They are mostly propagated by stem or root cuttings or by adventitious roots called "slips" that grow out from the tuberous roots during storage. True seeds are used for breeding only.
Under optimal conditions of 85 to 90% relative humidity at 13 to 16 °C (55 to 61 °F), sweet potatoes can keep for six months. Colder temperatures injure the roots.
They grow well in many farming conditions and have few natural enemies; pesticides are rarely needed. Sweet potatoes are grown on a variety of soils, but well-drained light and medium textured soils with a pH range of 4.5-7.0 are more favourable for the plant (Woolfe, 1992; Ahn, 1993). They can be grown in poor soils with little fertilizer. However, sweet potatoes are very sensitive to aluminium toxicity and will die about 6 weeks after planting if lime is not applied at planting in this type of soil (Woolfe, 1992). Because they are sown by vine cuttings rather than seeds, sweet potatoes are relatively easy to plant. Because the rapidly growing vines shade out weeds, little weeding is needed, and farmers can devote time to other crops. In the tropics, the crop can be maintained in the ground and harvested as needed for market or home consumption. In temperate regions, sweet potatoes are most often grown on larger farms and are harvested before first frosts.
China is the largest grower of sweet potatoes, providing about 80% of the world's supply; 130 million tons were produced in one year (in 1990; about half that of common potatoes). Historically, most of China's sweet potatoes were grown for human consumption, but now most (60%) are grown to feed pigs. The rest are grown for human food and for other products. Some are grown for export, mainly to Japan. China grows over 100 varieties of sweet potato.
Sweet potatoes very early became popular in the islands of the Pacific Ocean, spreading from Polynesia to Japan and the Philippines. One reason is that they were a reliable crop in cases of crop failure of other staple foods due to typhoon flooding. They are featured in many favorite dishes in Japan, Taiwan, the Philippines, and other island nations. Indonesia, Vietnam, India, and some other Asian countries are also large sweet potato growers. Uganda (the third largest grower after Indonesia), Rwanda, and some other African countries also grow a large crop which is an important part of their peoples' diets. North and South America, the original home of the sweet potato, together grow less than three percent of the world's supply. Europe has only a very small sw
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