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Organic compounds other than nutrients that regulate plant development and growth. Plant hormones, which are active in very low concentrations, are produced in certain parts of the plants and are usually transported to other parts where they elicit specific biochemical, physiological, or morphological responses. They are also active in tissues where they are produced. Each plant hormone evokes many different responses. Also, the effects of different hormones overlap and may be stimulatory or inhibitory. The commonly recognized classes of plant hormones are the auxins, gibberellins, cytokinins, abscisic acid, and ethylene. Circumstantial evidence suggests that flower initiation is controlled by hypothetical hormones called florigens, but these substances remain to be identified. A number of natural or synthetic substances such as brassin, morphactin, and other growth regulators not considered to be hormones nevertheless influence plant growth and development. Each hormone performs its specific functions; however, nearly all of the measurable responses of plants to heredity or environment are controlled by interaction between two or more hormones. Such interactions may occur at various levels, including the synthesis of hormones, hormone receptors, and second messengers, as well as at the level of ultimate hormone action. Furthermore, hormonal interactions may be cooperative, antagonistic, or in balance.
The term plant growth regulator is usually used to denote a synthetic plant hormone, but most of the synthetic compounds with structures similar to those of the natural hormones have also been called hormones. For instance, the synthetic cytokinin kinetin is considered a hormone. See Abscission
There are a number of applications of plant hormones in agriculture, horticulture, and biotechnology. Synthetic auxins are used as weed killers. Auxins are also used to counteract the effects of hormones that promote the dropping of fruit from trees. Gibberellins are used extensively to increase the size of seedless grapes: when applied at the appropriate time and with the proper concentration, gibberellins cause fruits to elongate so that they are less tightly packed and less susceptible to fungal infections. Gibberellins are also used by some breweries to increase the rate of malting because they enhance starch digestion. They have also been sprayed on fruits and leaves of navel orange trees to prevent several rind disorders that appear during storage. They are used commercially to increase sugarcane growth and sugar yields. Cytokinins and auxins are used in plant cell culture, particularly in cultivating genetically engineered plants. The ability of cytokinins to retard senescence also applies to certain cut flowers and fresh vegetables. Ethylene has been used widely in promoting pineapple flowering; flowering occurs more rapidly and mature fruits appear uniformly, so that a one-harvest mechanical operation is possible. Because carbon dioxide in high concentrations inhibits ethylene production, it is often used to prevent overripening of picked fruits. Ethylene is also used for accelerating fruit ripening. See Hormone, Plant growth, Plant physiology