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growth regulator[′grōth ¦reg·yə‚lād·ər]
an organic compound that stimulates or inhibits plant growth and development. Both natural substances and synthetic preparations are used to regulate the growth of crops.
Natural growth regulators occur as plant hormones, growth inhibitors, and vitamins. Plant hormones include auxins, gib-berellins, and cytokinins. Auxins stimulate the growth of stems, leaves, and roots, thereby triggering tropistic responses. They also stimulate the formation of roots in cuttings. The discovery of auxins in plants helped to establish the internal causes of several growth processes. However, the mechanisms regulating many forms of growth, the flowering of rosette plants, interruption of dormancy, and the greening of leaves—were not elucidated until gibberellins and cytokinins were discovered.
Gibberellins induce or stimulate stem growth, cause some seeds to germinate and parthenocarpic fruits to form, and interrupt dormancy in a number of plants. Cytokinins stimulate cell division (cytokinesis), vernation, and growth of stem buds both in whole plants and in undifferentiated calluses. They also prolong life and maintain normal metabolism in isolated leaves and cause secondary greening. Abscisinic acid and coumarin and its derivatives are among the known natural substances that inhibit growth when plants become dormant.
Synthetic growth regulators became available around 1935, after the Dutch plant physiologist F. Kögle synthesized auxin (indoleacetic acid). This development led to the synthesis of similar compounds with high biological activity. Indolylbutyric, naphthylacetic, and 2,4-dichlorophenylacetic acids proved to be the most promising. Kinetin (cytokinin) was synthesized in 1955. Synthetic regulators include preparations that reduce the length and increase the width of stems (retardants) and compounds that cause deformed organs and anomalies in the growing point (morphactins). Similar substances specifically retard the migration of indoleacetic acid and its derivatives in the plant. Among the substances that have a markedly inhibitory action are herbicides—compounds that destroy weeds. Synthetic inhibitors suppress the growth processes more severely than do natural ones. They are not inactivated for a long time, and their action is frequently asssociated not only with growth but with disturbance of the morphogenetic processes.
Application in agriculture. The use of growth regulators produces changes in metabolism similar to those caused by certain external conditions, such as temperature and length of the day. Such changes include accelerated formation of reproductive organs and intensification or inhibition of growth. Growth and organogenesis in cultivated plants are stimulated by the application of auxins and gibberellins and inhibited by synthetic inhibitors. Such inhibitors include defoliants, which cause leaf fall, and desiccants, which cause withering of organs or entire plants.
Synthetic stimulants of the auxin type (β-indoleacetic acid [heteroauxin], β-indolylbutyric acid, and α-naphthylacetic acid) are used to intensify root formation in scions from woody and herbaceous plants, to improve the union of tissues when transplanted and grafted, and to prevent the ovaries from falling out in fruit and berry plants. The substances are used in different concentrations (from 20 to 1,000 mg/l), which depend on the method by which they are applied to the plant. Gibberellins are used to accelerate the growth of seedless grapes, to bring potato tubers out of dormancy, to intensify the growth of hemp and flax stems, and to accelerate the fruiting of tomatoes.
Synthetic growth inhibitors are used to retard the sprouting of potato tubers during storage, to inhibit the growth of cereal stems so that their resistance to lodging is heightened (retardants), and to destroy weeds (herbicides). The mechanism of the inhibitory action of synthetic inhibitors has not been adequately studied. Most of the substances were found to inhibit growth by dissociating the processes of phosphorylation and respiration and by suppressing the synthesis of nucleic acids.
Spraying is the most common method of treating plants with growth regulators. For example, α-naphthylacetic acid and its derivatives are sprayed on fruit and berry plants to prevent the ovaries from falling out. Gibberellin is sprayed on bast plants to increase the fiber yield.
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V. I. KEFELI