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Although the vast majority of people depend on wheat, corn and rice for food, the non-grain potato ranks fourth as a major food source. Columbus is often credited with bringing the white or Irish potato (Solanum tuberosum), a member of the nightshade family (Solanaceae), back to Spain, although most scholars argue that his reference to potatoes or "batatas" indicated sweet potatoes (Ipomoea batatas) rather than the Irish potato. Sweet potatoes belong to the morning-glory family (Convolvulaceae). Other Spanish explorers found true potatoes (called "papas") being grown by Indians all along the Andes from Colombia to Chile, and potatoes were known in Spain after 1565. Later, potatoes were introduced into the British Isles where they were grown extensively by the early 1800s. In fact, potatoes were so popular in Ireland that they became known as Irish potatoes even though their origin was South America. Ireland developed a monoculture of potatoes until the mid 1840s when their entire crop was devastated by the potato blight fungus (Phytophthora infestans). It has been estimated that during a five year period at least a million people died of starvation. The potato famine set into motion a wave of emigration that reduced Ireland's population by 50 percent between 1846 and 1900. The Irish constituted 35 percent of the immigrants to the United States during this period and became 15-18 percent of the population.

The common potato (Solanum tuberosum) is an autotetraploid species with a sporophyte chromosome number of 48 (4n=48). The normal sporophyte diploid number for the genus Solanum is 24. Eggplant (S. melongena) belongs to the genus Solanum and tomatoes (Lycopersicon esculentum) are closely related to the potato. Common potatoes (S. tuberosum) consist of two subspecies or groups, ssp. tuberosum ( "Chilotanum Group") and ssp. andigena ("Andigenum Group"). Older references state that potatoes taken to Europe in the sixteenth century all belonged to the andigena subspecies or "Andigenum Group" which was widely cultivated in the Andean highlands of Bolivia, Peru and northern Argentina. The origin of this tetraploid subspecies is controversial, but some authorities believe it resulted from hybridization between a diploid Andean species (S. stenotomum) and another diploid species (possibly S. sparsipilum), followed by autopolyploidy. The precise origin of the subspecies tuberosum or "Chilotanum Group" native to Chile is even more controversial, possibly derived from the subspecies andigena or the hybridization of subspecies andigena with yet another unknown Andean species. According to Mercedes Ames and David Spooner, the single Andigenum origin of the modern "European" potato (with cultivars grown worldwide) is not supported by plastid DNA studies from historical herbarium specimens. Their research points to a Chilotanum origin traced to a landrace indigenous to Chiloé, largest island in the Chiloé Archipelago off the coast of Chile. In addition, this research shows the critical importance of herbarium specimens in investigating historical origins of crop plants.

  Juzepczuk, S.W., and S.M. Bukasov. 1929. "A Contribution to the Question of the Origin of the Potato." TrVses Azeda Genet Selek 3: 593-611 (in Russian, English summary).

  Ames, Mercedes and David D. Spooner. 2008. "DNA From Herbarium Specimens Settles a Controversy About Origins of the European Potato." American Journal of Botany 95 (2): 252-257.

Whatever the exact origin of our common potatoes, they have become a major food source in the Old and New World temperate regions. Potatoes are an excellent low sodium source of complex carbohydrates. One average-sized baked potato is only about 100 dieter's calories (kilocalories), unless of course it is piled with butter and sour cream. Potatoes are used for all sorts of delicious foods, including French fries, mashed potatoes, potato pancakes, hash browns and potato chips. In Poland, a vodka that is almost 200 proof (nearly 100 percent ethanol) is made from potatoes. It should be noted here that potatoes have a high glycemic index and probably should be eaten in moderation or avoided by diabetics and people trying to lose weight.

The thin-walled parenchyma cells of a potato tuber are filled with membrane-bound, starch-storage organelles called amyloplasts. They are also referred to as "starch grains" in most general biology textbooks. Since iodine stain (gram's iodine) makes starch turn purplish-black, the amyloplasts can easily be viewed with a compound microscope (400X). Insoluble starch (amylopectin) is deposited in concentric layers within the amlyloplasts. Unlike the long, coiled molecules of soluble starch (amylose), the molecules of amylopectin are much shorter, with only 40-60 glucose subunits. Amylopectin molecules consist of highly branched chains that do not coil. Starch grains of different plant species have characteristic shapes, such as maize (corn), oats, bananas, potatoes and wheat. For example, banana starch grains are more elongate than potato starch grains. Starch is hydrolyzed (broken down) by amylase enzymes (including B-amylase and maltase). During hydrolysis a water molecule is inserted between each glucose subunit. Starch is typically stored in underground organs, including storage roots, rhizomes, tubers, corms and bulbs.

Because sweet potatoes (Ipomoea batatas) thrive in a hot, moist climate, while potatoes (Solanum tuberosum) require a cool climate, they have never become as popular as the Irish potato in Europe. Sweet potatoes have become an important root crop in warm subtropical and tropical countries, second only to cassava or manioc (Manihot esculenta), a member of the euphorbia family (Euphorbiaceae) and the source of tapioca. Archaeological evidence shows that sweet potatoes were cultivated in South America by 2400 B.C. and fossilized sweet potatoes from the Andes have been dated at 8,000 to 10,000 years old. Although the sweet potato is clearly native to South America, it was also cultivated in Polynesia as early as 1200 A.D. In fact, the sweet potato had already become the principle food of the Maoris in New Zealand by the time of Captain Cook's historic voyage to that part of the world in 1769. It is interesting to note that the sweet potato is known as "kumar" or "kumal" in the Lima region of Coastal Peru, and it is called "kumara" by the Maoris of New Zealand. In his book, Sea Routes to Polynesia (1968), Thor Heyerdahl postulated that sweet potatoes were carried across the Pacific by Peruvian Indians before Europeans began to sail the world's oceans. He tested his hypothesis in 1947 by sailing a balsa wood raft, the kon-tiki, fashioned after the reed rafts of the Oru Indians living on Lake Titicaca in Bolivia. Although Heyerdahl's hypothesis about the transoceanic exchange of sweet potatoes by skillful pre-columbian sailors remains an enigma (at least to some skeptics), his New World origin for the coconut (Cocos nucifera) has been rejected by most botanists. Widespread sea dispersal of wild-type coconuts, the remarkable dispersal of coconut crab larvae riding on coconuts, and fossil evidence of Miocene coconuts in the South Pacific indicate an Indo-Pacific origin, long before the voyages of ancient mariners. Most authorities now agree that the coconut was introduced to the New World by Portuguese and Spanish traders.

Unlike the stem tubers of Irish potatoes, sweet potatoes are fascicled (clustered) storage roots. Irish potatoes are propagated by planting sections of the tuber containing an "eye" or bud. Potatoes used for propagation are called "seed potatoes." Sweet potatoes do not have "eyes" and cannot be propagated by planting pieces of the root in the ground, although they will sprout in a glass of water. There are a number of cultivated varieties of Irish potatoes, including 'Russet,' 'White,' and 'Red Rose.'

The Jerusalem artichoke (Helianthus tuberosus) is a large, perennial sunflower native to the Great Plains of North America. It is closely related to the common sunflower (H. annuus) that grows along roadsides and vacant fields throughout the western United States. It has been cultivated by native Americans for centuries, and was introduced into Europe in the early 1600s. Like true potatoes, it produces edible tubers at the ends of underground stems called rhizomes. The tubers contain "eyes" or buds and are technically modified stems rather than roots. [The original name of "sunchoke" was applied to a hybrid between the Jerusalem artichoke and the common sunflower.] The common name may have been corrupted from the Italian name for the plant "girasole articiocco." "Girasole" refers to the way the flowers turn to face the sun, and "articiocco" refers to artichoke. Some people say the tubers taste like artichoke hearts if they are steamed with the peel on. The peel imparts the artichoke flavor to this vegetable. The tubers are eaten raw in salads, steamed, fried, baked and mashed. Raw tubers are very crisp and sweet, with a taste more like water chestnuts than potatoes. Jerusalem artichokes provide an abundant source of nutritious, tasty tubers in poor soils with very little care.

The tubers of Jerusalem artichoke contain fewer calories than potatoes, and they are especially high in vitamin A, the B-complex, potassium and phosphorus. They contain the polysaccharide inulin instead of starch, which is a nutritious food for diabetics and hypoglycemics. The hydrolysis of inulin yields fruit sugar (D-fructose), while true insoluble starch (amylopectin) yields D-glucose. [Starch (amylum) is actually composed of two polymers, soluble starch (amylose) and insoluble amylopectin. Starch polymers are packed into membrane-bound starch grains or amyloplasts within plant cells.] Sugars from the digestion of inulin do not elicit rapid insulin production compared with other starchy foods.

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