Growth inhibitors of multiple types have been identified in the plants. The best-characterized one is the abscisic acid that is chemically related to cytokinins. Probably, it is universally distributed in the higher plants and contains a variety of actions; for example, it promotes the abscission (which is the leaf fall), the dormancy development in buds, and the potato tuber’s formation. Abscisic acid's mechanism of action is unknown, but it is believed to involve direct inhibition of protein and RNA synthesis.
Various Growth Inhibitors
Ethylene, which is an ethylene growth inhibitor, is also a growth inhibitor that is a natural product of plants, possibly formed from methionine (an amino acid) and from linolenic acid (which is a fatty acid). Ethylene encourages senescent leaves to abscise, possibly by promoting auxin synthesis. The effects of this extend beyond inhibiting growth; in fruit, for example, ethylene can be regarded as a ripening hormone. Fruit is another factor involved in its action, perhaps auxin or the other growth-regulating hormone that influences the tissue’s ethylene sensitivity.
Example of Growth Inhibitors
In animals, the conspicuous hormonal interaction can also be found in plants (plant growth inhibitors), which is the growth inhibitor in plants; an example is the control of abscission, which needs the synthesis of enzymes at an abscission zone and at the base of the structure concerned to catalyze the reactions involving breakdown of the cell walls. Auxin that enters the abscission zone from the structure's tip facilitates abscission; auxin that enters the structure from the opposite direction inhibits abscission. However, it tends to inhibit the same process, probably with its influence on the metabolism.
Also, the other hormones are involved in abscission; ethylene stimulates the enzyme’s synthesis, and the abscisic acid accelerates associated senescence. And gibberellin tends to inhibit the abscission by promoting growth.
The other example of hormonal interaction takes place during the germination of cereal seeds. First, the embryo (or the germ) is activated by water uptake, which enables it to form gibberellin. Gibberellin can function on the living cells surrounding the food reserves (at the aleurone layer), which is the endosperm. This particular action induces the aleurone cells to form enzymes, which break down starch to sugars and releases tryptophan from the endosperm’s protein. The tryptophan migrates to the tip of the coleoptile and can be transformed into an indolyl acetic acid that in turn moves to the growth zone and also weakens the cell walls, thereby permitting water uptake.
Target tissues are likely to play a role in such sequential behaviour, and changes in their responsiveness to hormonal action, possibly associated with environmental stimuli, are likely to lead to adaptive integration. The similarities present in the hormonal mechanisms of both animals and plants, two groups, which so profoundly vary in their structure and mode of life, illustrate the fundamental biological organization’s uniformity effectively.
Plant Growth Retardants
Plant growth retardants, also called PGRs, do much more than the control plant stretch. They make for darker green plants and improve the profits as well.
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Several growers understand that Plant Growth Retardants (PGRs) are an effective tool that helps to control plant stretch, as represented in the above figure. But if we observe, there are other benefits to using certain PGRs also. The PGRs, which block the gibberellic acid (GA) pathway, control growth due to the reason GA is a plant hormone, which stimulates cell elongation in plants.
By inhibiting the GA, there is less cell elongation (it means stretch). PGRs that work in this particular manner include those, which contain chlormequat chloride, ancymidol, flurprimidol, daminozide, uniconazole, and paclobutrazol.
The above figure represents the Cosmos bipinnatus ‘Sonata Carmine’ four weeks later to PGR treatment.
And, towards the left side - the Control (with no PGR spray) and to the Right: 3,000 ppm daminozide spray at 2 quarts per 100 sq. ft.
Greener leaves are the additional benefit of using these PGR types. This occurs for two primary reasons. First, the cells are very small, so chlorophyll is concentrated more in the cell. Second, there is an increase in the production of chlorophyll due to the reason that some metabolic energy is diverted from the GA synthesis into chlorophyll production.
Also, the Plants treated with PGRs can exhibit improved water use and very less water stress. This is likely because of the reduction in leaf size that PGRs provide that needs less water than that of their larger-leafed, non-PGR-treated counterparts. Also, some of the researchers believe that the blocked GA pathway causes an increase in the production of abscisic acid that promotes stomatal closure, which reduces water loss and improves water use also.
Gibberellins (GAs) are defined as plant hormones, which regulate multiple developmental processes, including germination, stem elongation, flowering, dormancy, leaf and fruit senescence, and flower development.