Tobacco, Nicotiana tabacum (family Solanaceae), is grown in over one hundred countries around the world, in both temperate and tropical climates. It is a stout, rapidly growing annual, 1 to 2 meters tall. It has large, ovate to oblong leaves and produces numerous white-pinkish flowers with corollas about 2 centimeters long. Tobacco seeds are minute, so in commercial production seedlings are generally produced in plant beds or in greenhouses and transferred to the field. Production and harvesting methods differ widely depending on the type of tobacco being produced, but most tobacco types require significant inputs of time, labor, and pest management. Both un-derfertilization and overfertilization may cause inferior quality leaves. Commonly, whole plants of air-cured tobaccos are cut off just above the ground and hung in barns for several months until cured. Leaves of bright, flue-cured tobaccos are typically harvested individually as they ripen. These leaves are cured by heating them up slowly through yellowing, drying, and stem-drying steps. Piles of cured tobacco leaves are generally sold at auction in large, well-lighted warehouses.

Tobacco is believed to have originated in northwestern Argentina and adjacent Bolivia. Native peoples undoubtedly used it for centuries before Europeans colonized the Americas. Christopher Columbus was introduced to tobacco by the Arawaks on October 11, 1492, when he first visited the Caribbean islands. Tobacco smoking spread throughout Europe in the second half of the sixteenth century. Tobacco soon became the most important commercial crop in Colonial America, and the tobacco trade directly contributed to the success of the first permanent English settlement at Jamestown, Virginia.

Differences in cultural practices and diverse climatic and soil conditions produce several different types of tobacco that are used in various smoking and chewing products. The major types of tobacco are bright (flue-cured), light air-cured (burley), dark air-cured, fire-cured, oriental, cigar wrapper, and cigar filler. Burley and flue-cured tobaccos are the primary tobacco types

A Zimbabwean man working in a tobacco field. During harvest, tobacco is cut and placed on sticks and later taken to cure in drying barns.

alkaloids bitter secondary plant compounds, often used for defense quantitative numerical, especially as derived from measurement physiology the biochemical processes carried out by an organism used in the manufacture of cigarettes, and they account for most of the U.S. production. Over 90 percent of the tobacco grown in the United States is from North Carolina and Kentucky, but Maryland, South Carolina, Virginia, Georgia, Florida, Ohio, and Tennessee also produce substantial amounts of this crop.

Tobacco leaves are covered with trichomes (hairs) that have multicellular glands on their tips. These glandular trichomes produce a sticky resinous material that contains many of the flavor and aroma components. Tobacco also produces many internal, secondary components, including pyridine alkaloids. The most important alkaloid is nicotine, which acts as a stimulant to the user and is addictive. Nicotine is quite toxic, and products containing nicotine were used as early insecticides. The adverse health effects of smoking, including nicotine addiction and the increased risks of cancer, emphysema, and heart attack, are well documented.

Tobacco has been extensively used as a model system in many basic scientific studies. Pioneering work in quantitative genetics, tissue culture techniques, plant physiology, and genetic engineering have utilized the unique characteristics of tobacco, which has been referred to as "the white rat of the plant world." see also Alkaloids; Economic Importance of Plants; Poisonous Plants; Psychoactive Plants; Solanaceae.

D. Michael Jackson

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