Sweet potato, kumara, uwi, ubi, or U.S. yam (Ipomoea batatas)

CONVOLVULACEAE, Morning Glory Family

batata. Batatas, an Arawak name, which are now called sweet potatoes, were used during pre-Columbian times in Central and lowland South America, as well as in the West Indies.

It is though that Ipomoea batatas originated from an unremarkable trailing climber (vine) on the mainland, probably in Central America. Workers have looked at species with similar lobed leaves and growth habits, such as I. trifida or I. tiliacea, or possibly from some hybridization event involving these species. Wild species lack swollen below-ground structures, whereas the cultivated species, which is a perennial herb, produces "root tubers" on adventitious roots, which arise from the nodes of the trailing stems. The cultivated sweet potato is a hexaploid (= 90 chromosomes) and must be propagated from stem cuttings or the root tubers, because the plants seldom produce fruits or seeds.

Sweet potato was already an important crop on Pacific islands when Europeans landed on the islands: Easter Island (Roggeveen, 1722), New Zealand (Cook, 1769), and the Hawaiian Islands (Cook, 1778). Sweet potato, known as kumara, was a staple in the diet of the Maoris of New Zealand; interestingly, the name kumar is used for this plant in Peru! Rongo ma-Tane, a Maori god, protects this plant; a buried tuber is powerful enough to cause enemy to go mad and run away.

Given that the species does not generally reproduce from seed, the spread of this plant must have been by the transport of the root tubers. Therefore, the pre-Columbian occurrence of sweet potato in southern and eastern Polynesia, as well as in New Zealand, needs an explanation. Certainly the plant originated in the Americas and was carried westward into areas where Asian root crops had not yet arrived. Two equally plausible hypotheses have been proposed: (1) Polynesians raided the Peruvian coastline and took sweet potatoes back with them; or (2) early Peruvians, who used balsa rafts, transported sweet potatoes to Polynesia on their oceanic forays (as tested by Thor Heyerdahl's Kon-Tiki experiment in 1947). Long-distance dispersal by ocean currents has been ruled out, because the tubers spoil in seawater. Experts agree that Polynesians carried sweet potatoes eastward from the Society Islands to the Hawaiian Islands, along with other economic plants, such as the root crop taro (Colocasia esculenta, for poi), the candlenut tree (Aleurites; for torches, oily fruits), and the ti plant (Cordyline terminalis, for grass skirts and luau materials). Spaniards introduced sweet potato into the Philippines and at an early date into Spain. From Spain, sweet potatoes were carried eastward through the Mediterranean region to the Far East, where the sweet potato became a vital starchy food crop. In Japan and Taiwan, this food has been dried and stored as "typhoon insurance."

Sweet potato has many cultivated forms, but in the United States two forms are common: (1) the dry, mealy, yellow sweet potato, and (2) the watery, orange "yam," which is not, of course, a true yam (Dioscorea). Yellow and orange pigments are carotenoids, and therefore sweet potato is a rich source of vitamin A. The root tubers also have fairly high concentrations of calcium and iron but little protein. Over one-fourth of the tuber is carbohydrate, mostly starch; but the sweetness of the tuber tells us that glucose is also present, especially in the watery yams, in which 3-6% by weight is glucose. In sweet potato there are 50% more calories per unit weight than in Irish potato (Solanum tuberosum) but less protein.

Japan leads in production of sweet potatoes, and throughout the Orient this tuber is a common table vegetable. This root crop is generally eaten fresh, but it can be canned, desiccated, and made into flour or syrup, and the carbohydrate can also be converted into alcohol via fermentation. The green foliage is used for livestock fodder, along with the below-ground parts.

Growing sweet potatoes requires much labor and often requires hand cultivation. Therefore, sweet potatoes are generally grown year-round in warm humid regions, where labor is inexpensive. High soil nitrogen for this crop is very bad, because it leads to shoot growth but weak development of tubers. The root tubers spoil easily, and almost one-third of the annual crop may be lost by spoilage. By careful handling, up to 20 tons of sweet potatoes per acre can be obtained. This crop needs a relatively long growing season (especially >120 days) in a climate that does not dip below 13 degrees C. After harvested, sweet potatoes are cured best where both humidity and temperature are high, so that the skin is well developed, and then the tubers should be stored at around 15 degrees C.

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