Plant hormones Guide, Meaning , Facts, Information and Description
Plant hormones (or plant growth regulators (PGRs) are internally secreted chemicals in plants that are used for regulating their growth. According to a standard definition, they:- are signal molecules produced at specific locations;
- occur in low concentrations;
- cause altered processes in target cells at other locations.
Plant hormone theories
There are several theories about plant hormones. Plant hormone theory I and plant hormone theory II will be described here. The theories have elements in common but also differ fundamentally. The first theory is described in greater detail. It is that the five "classic" plant hormones, Auxin, Cytokinins (CKs), Gibberellins (GAs), Ethylene, and Abscisic Acid (ABA), plus the "new" plant hormones, Brassinosteroids (BAs), and Salicylates or Salicylic Acids (SAs) can be gathered into three groups. These are growth hormones, stress hormones and shock hormones. All three types fall within the classic definition of an intercellular hormone. They are made by a cell and are meant to affect the behavior of other cells, either in nearby tissue or at the opposite end of the plant.
The first assumption in both theories is that plants are interested in growing larger during the vegetative period of their life and this growth requires both good environmental conditions and an amount of the four basic nutrient groups that exceeds that needed to keep the plant at its current size. Basic nutrients required for growth are sugar, gases (carbon dioxide and oxygen), water, and minerals. In general growth hormones are made mostly in young and meristematic cells, and much less in mature cells. They are made in mature cells only when there is an excess of nutrients. Stress hormones, in contrast, may be made in mature cells that are faced with a scarcity of nutrients and to a much lesser extent in young and meristematic cells faced with the same scarcity. Shock hormones may be made by all cells in equal amounts faced with the same conditions.
Growth hormones (Auxins and Cytokinins) stimulate metabolism, cause growth, and balance growth. Stress hormones (GA's, Ethylene and maybe BA's) complement growth hormones. The latter group are triggered by a deficit of nutrients, when there are not enough nutrients to allow growth. They allow a plant to survive this period and redistribute resources in order to resume growth. Shock hormones are the ABAs and SAs and they allow a plant to properly respond to rapidly developing stress situations (ABA) or return to normal conditions after the stress has passed (SA).
The alternative theory says that instead of auxin and cytokinin being released when there are more than enough nutrients for peak level metabolism, they are instead released at any time when nutrients get above survivable levels. It would say also say that GA and ethylene would be released at any time nutrients fall below peak metabolism rates. The presence of high levels of GA and Ethylene and the absence of auxin and cytokinin would be an indication that senescence is warranted and survivable levels of nutrients are not indicated. This is the way most plant scientists see ABA. This theory has its limitations.
This is an Article on Plant hormones. Page Contains Information, Facts Details or Explanation Guide About Plant hormones External links