Hormones

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Hormones are small molecules that are released by one part of a plant to influence another part. The principal plant growth hormones are the auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Plants use these hormones to cause cells to elongate, divide, become specialized, and separate from each other, and help coordinate the development of the entire plant. Not only are the plant hormones small in molecular weight, they are also active in the plant in very small amounts, a fact that made their isolation and identification difficult.

The first plant growth hormones discovered were the auxins. (The term auxin is derived from a Greek word meaning "to grow.") The best known and most widely distributed hormone in this class is indole-3-acetic acid. Fritz W. Went, whose pioneering and ingenious research in 1928 opened the field of plant hormones, reported that auxins were involved in the control of the growth movements that orient shoots toward the light, and that they had the additional, striking quality of moving only from the shoot tip toward the shoot base. This polarity of auxin movement was an inherent property of the plant tissue, only slightly influenced by gravity. Other less-investigated auxins include phenyl-acetic acid and indole-butyric acid, the latter long used as a synthetic auxin but found to exist in plants only in 1985.

The gibberellins are a family of more than seventy related chemicals, some active as growth hormones and many inactive. They are designated by number (e.g., GAi and GAL20). GA 3 (also called gibberellic acid) is one of the most active gibberellins when added to plants. Slight modifications in the basic structure are associated with an increase, decrease, or cessation of biological activity: each such modified chemical is considered a different gibberellin.

Cytokinins are a class of chemical compounds derived from adenine that cause cells to divide when an auxin is also present. Of the cytokinins found in plants, zeatin is one of the most active.

Abscisic acid helps protect the plant from too much loss of water by closing the small holes (stomata) in the surfaces of leaves when wilting begins.

compound a substance formed from two or more elements

PLANT HORMONES AND THEIR FUNCTIONS

Hormone

Functions

Auxins (indoleacetic acid; IAA)

Stimulates shoot and root growth; involved in tropisms; prevents abscission; controls differentiation of xylem cells and, with other hormones, controls sieve-tube cells and fibers

Gibberellins

Stimulates stem elongation, seed germination, and enzyme production in seeds

Cytokinins

Stimulates bud development; delays senescence; increases cell division

Abscisic acid

Speeds abscission; counters leaf wilting by closing stomates; prevents premature germination of seeds; decreases IAA movement

Ethylene (gas)

Produced in response to stresses and by many ripening fruits; speeds seed germination and the ripening of fruit, senescence, and abscission; decreases IAA movement

lateral away from the center apical at the tip vascular related to transport of nutrients

The only known gas that functions as a plant growth hormone is the small C2H2 molecule called ethylene. Various stresses, such as wounding or waterlogging, lead to ethylene production.

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