Tobacco - biology.

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Tobacco - biology.

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Publi le : 11/5/2013 -Format: Document en format HTML protg

Tobacco - biology.
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Tobacco - biology.
I

INTRODUCTION

Tobacco, plant grown commercially for its leaves and stems, which are rolled into cigars, shredded for use in cigarettes and pipes, processed for chewing, or ground into
snuff, a fine powder that is inhaled through the nose. Tobacco is the source of nicotine, an addictive drug that is also the basis for many insecticides (see Smoking).
Tobacco is a member of the nightshade family. There are more than 70 species of tobacco, of which 45 are native to the Americas. The two cultivated species, common
tobacco and wild tobacco, are annuals--they live only one growing season. Common tobacco is 1 to 3 m (3 to 10 ft) tall and has a thick, woody stem with few side
branches. One plant typically produces 10 to 20 broad leaves that branch alternately from the central stalk. The leaf size depends on the strain. The narrow, trumpetshaped flowers are dark pink to almost white. Wild tobacco is about 0.6 m (2 ft) tall and has a stem that is more slender and less woody than common tobacco. The
leaves have a short stalk that attaches to the stem. The flowers are pale yellow with five separate lobes.

II

GROWING AND HARVESTING

Tobacco grows in tropical and temperate regions, and it can be grown as far north as Canada and Norway. It thrives best in areas with a frost-free growing season of
120 to 170 days, depending on the type of tobacco. Good-quality tobacco requires fertile, well-drained, moist soil and warm temperatures. Most types of tobacco are
grown in full sun. Environmental factors influence the plant's characteristics. Soil, for example, can affect leaf size, texture, and color. Sandy soils tend to produce a
relatively large leaf that is light in color and body, fine in texture, and burns with a weak aroma. Heavier soils, which contain silt and clay, tend to produce a small, dark
leaf with a heavy body and a strong aroma when burned.
Several strains of common tobacco are grown for use primarily in different tobacco products. In the United States, Virginia tobacco is the main tobacco used in
cigarettes; most of it is grown in North Carolina, Georgia, and Virginia. Burley tobacco, which is grown mostly in Kentucky and Tennessee, is used in cigarettes and
pipes. Several countries, including the United States, Turkey, and Cuba, grow cigar tobacco.
Tobacco plants are susceptible to attack from a wide range of insects and bacterial, fungal, and viral diseases. To counteract these problems, tobacco farmers grow
strains of tobacco that resist diseases and insects. By rotating crops (planting tobacco one year and a different crop in the same field the next year), farmers keep the
population of tobacco pests in check by depriving them of tobacco plants on alternate years. Before planting, farmers may work a fungicide into the soil to control fungal
diseases, such as blue mold and damping-off. They may also fumigate the soil to control nematodes--microscopic worms that infest the roots. Growers also use
herbicides to control weeds and insecticides to control insects.
The annual tobacco cultivation cycle begins with the planting of seeds. In the United States, seed planting begins in March in southern states and June in northern
states. Tobacco seeds are extremely small: one million seeds (the potential yield of a single mature plant) weigh about 80 g (about 3 oz). Tobacco seeds are so tiny that
they need special care to keep them from drying out once they begin sprouting. To keep young plants watered and weeded, growers sow the seeds in specially
prepared seedbeds of fertile, loose soil, rather than directly in the field.
One to two months after planting, the growers transplant the seedlings into the field--a labor-intensive process called setting the tobacco. As flowers form on the plants,
growers remove them in a process called topping, which encourages more leaf growth.
Tobacco is harvested 70 to 130 days after setting. The harvesting method used depends on the type of tobacco. For some tobaccos, farmers cut whole plants off at the
ground and spear them onto a stick about 1 m (3 ft) long, called a tobacco stick. Each stick holds about six plants. For other tobaccos, farmers remove the mature
leaves and string them on wires, leaving the rest of the plant to continue growing.

III

CURING AND AGING

After tobacco is harvested, it is cured (dried), and then aged to improve its flavor. There are four common methods of curing tobacco: air curing, fire curing, flue curing,
and sun curing. The curing method used depends on the type of tobacco and its intended use.
Air-cured tobacco is sheltered from wind and sun in a well-ventilated barn, where it air dries for six to eight weeks. Air-cured tobacco is low in sugar, which gives the
tobacco smoke a light, sweet flavor, and high in nicotine. Cigar and burley tobaccos are air cured.
In fire curing, smoke from a low-burning fire on the barn floor permeates the leaves. This gives the leaves a distinctive smoky aroma and flavor. Fire curing takes three
to ten weeks and produces a tobacco low in sugar and high in nicotine. Pipe tobacco, chewing tobacco, and snuff are fire cured.
Flue-cured tobacco is kept in an enclosed barn heated by flues (pipes) of hot air, but the tobacco is not directly exposed to smoke. This method produces cigarette
tobacco that is...


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