In this essay we will discuss about:- 1. Meaning of Plant Growth Regulators 2. Classification of Plant Growth Regulators 3. Importance.
Essay on PGRs
Essay # 1. Meaning of Plant Growth Regulators:
Plant growth and development are controlled by extremely low concentrations of chemical substances called plant growth substances, growth hormones, phytohormones, or plant growth regulators (PGRs).
The concept that plant growth and development are regulated by a substance produced in minute quantities in one organ that elicits a response in another was first suggested by Julius von Sachs, the father of plant physiology, in the latter half of the nineteenth century.
His observations were confirmed by Charles Darwin in 1880 in his experiments on the effect of light and gravity on plant growth. Darwin observed that canary grass seedlings bent toward the light source (phototropism) unless the seedling tips were covered with an opaque foil. He concluded that the light stimulus was perceived in the shoot tip (coleoptile), but the response was in lower or more basal tissues.
The full impact of PGR manipulation on modern agriculture began with the use of auxin-type herbicides at the close of World War II. Presently PGRs are used to control a host of physiological processes in crop production, including flowering and fruiting (fruit set and parthenocarpy), partitioning of assimilate, germination, propagation, growth suppression, defoliation, and postharvest ripening. Cloning and tissue culture would not be possible without PGRs. Most of the commercial acreage of tobacco in the United States is treated with a PGR to control suckering (growth of new shoots from buds in leaf axils).
PGRs are used as herbicides on nearly all cultivated crop acreage in industrialized countries, and their production is a multibillion-dollar industry. They are used extensively in horticultural crops to control growth and development, particularly in fruit production.
Field crop plants have a relatively short breeding cycle, and it has been possible to obtain genetic control by breeding and selection for endogenous levels of hormones that produce the desired physiological responses. Tobacco is an exception (as are barley and wheat, on which a PGR is used in Europe to control tiller growth). As more effective PGRs are produced, their activities better understood, and method of delivery of needed concentrations to the response organs over time developed, the use of PGRs in field crop production may increase.
Essay # 2. Classification of Plant Growth Regulators:
The term plant growth regulator covers the broad category of organic substances (other than vitamins and microelements) that in minute amounts promote, inhibit, or otherwise modify physiological processes. Endogenous (produced internally) PGRs are referred to as plant hormones or phytohormones.
The term hormone originated in animal physiology, where it means a substance synthesized in one organ that in turn stimulates a response in another. Plant hormones are not as specific as to organ of synthesis or organ of response as are animal hormones, but they tend to follow this general pattern of behavior.
PGRs, whether endogenous or exogenous (originating externally), elicit essentially the same plant responses. For example, two synthetic PGRs —2, 4- dichlorophenoxyacetic acid (2, 4-D) and picloram, a substituted picolinic acid (Tordon) — are equally effective with in vitro tissue cultures.
A synthetic auxin is necessary because the tissue is detached from the natural auxin source. Synthetic growth promoters, in an appropriate mix, stimulate callus (formation of undifferentiated cell mass), organ differentiation, and whole plant morphogenesis from a single parenchyma cell of, for example, tobacco pith, carrot root, or potato leaf.
PGRs are currently divided into five classes:
i. Auxins
ii. Gibberellins
iii. Cytokines or kinins
iv. Growth inhibitors
v. Ethylene.
Two hormones (brassinalide, a steroid, and triacontanol, an alcohol, the latter reportedly producing striking growth stimulation), chemically do not fit in the above five categories.
Both substances have been recently isolated from rape seed (Brassica napus) and certain higher plants, respectively. These substances and others that will likely be discovered may require revision of the current classification. Numerous analogs of most hormones in the five classes have been produced synthetically, and many have important agricultural applications.
Following properties are necessary for a compound to be considered as phytohormone:
(1) The site of synthesis differs from the site of activity (e.g., synthesis is in buds and young leaves but response is in stems, roots, or other organs).
(2) Responses are produced by micro-quantities (i.e., concentrations as low as 10-9 M).
(3) Unlike vitamins and enzymes, responses may be formative and plastic (irreversible) (e.g., tropic responses).
Often the natural supply of phytohormone is suboptimal, and an exogenous source is required to produce a desired response. Supraoptimal amounts of auxins typically behave as herbicides. Generally a phytohormone acts synergistically with other hormones in eliciting responses.
Essay # 3. Importance of Plant Growth Regulators:
Chemical substances in minute concentrations, referred to as phytohormones, regulate plant growth and development and coordinate morphogenesis. Plants may be genetically deficient in a specific hormone (endogenous source) and respond to an external application (exogenous source).
There is evidence for a flowering hormone, but it has not yet been isolated or identified. Other natural substances with hormone activity (e.g., triacontanol) that do not fit the five categories have been isolated and identified. The organ of synthesis of growth hormones is generally not the same as the organ of response.
Translocation between the two is required except for ethylene, which moves by gaseous diffusion. Young leaves and apical buds are particularly high in auxin, whereas young roots are high in cytokinins and gibberellins. Fruits and seeds generally are rich in all growth hormones.
Numerous synthetic analogs of PGRs have been produced and marketed for commercial use as herbicides, particularly auxins (e.g., 2, 4-D, 2, 4, 5-T, picloram, and certain benzoic acid derivatives). A number of growth inhibitors or retardants have been produced synthetically for agricultural uses (e.g., chlormequat [CCC], daminozide [SADH] and triiodobenzoic acid [TIBA]).
Ethephon releases ethylene slowly and is used commercially. Except in a few cases, most modern agronomic crop cultivars evidently have been selected for endogenous levels of hormones high enough to show little or no beneficial response to exogenous sources of auxins, gibberellins, cytokinins, inhibitors, or ethylene. This is not the situation for many horticultural crops with long breeding cycles.
Plant organs respond differently to varying concentrations of PGRs. Shoots are promoted by auxins over a wide concentration range, whereas roots are inhibited except in a short range. Internodes of certain dwarf plant types elongate to normal height if treated with GA over a wide range. Generally hormones act synergistically to induce a response rather than acting alone.
Indoleacetic acid (IAA), GA3, abscisic acid (ABA), and ethylene are common and widely distributed plant hormones. Zeatin, isolated from maize endosperm, appears to be the most common kinin. Biological assays, such as the Avena coleoptile, barley aleurone, and tobacco tissue culture tests, have been developed to quantify the presence of auxins, gibberellins, and kinins, respectively. Most growth hormones affect a wide range of responses so that a number of bioassays are available. Electrochemical assay methods are becoming increasingly important. Hormones are present in the plant in bound or free forms, affecting their availability.
PGRs generally elicit the following characteristic responses:
(1) Auxins stimulate growth by cell elongation and cause apical dominance.
(2) Gibberellins promote growth of intercalary meristems in the internodes and leaves.
(3) Cytokinins stimulate growth by cell division.
(4) Inhibitors retard elongation and induce abscission and senescence.
(5) Ethylene promotes ripening in fruits and horizontal growth.
Generally PGRs act synergistically, rather than alone, to cause responses.
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