Regulation and Control of Plant Growth
The Entire Existence of a Plant is Under Hormonal Control
Jul 9, 2009
Plants possess exquisitely precise timing mechanisms and develop their physical structure with mathematical finesse. But where and what are these regulators and coordinators? One of the first scientists to become aware of the significance of these amazing abilities in plants was Charles Darwin.
Darwin demonstrated experimentally that the behavior of a young plant stem is directed by the growing tip at its end. Somehow the tip was passing instructions back to the stem below. Darwin also showed that the root tip directs the response of the young root. However, Darwin was unable to unravel the mystery of exactly what the control agent was and how it moved through the plant.
The Answer Lies in the Tips of Roots and Stems
The actual mechanism of the internal control of plant growth and development was discovered during the mid – 1920s. First found in grass seedlings, the process was later found to work the same way in the stems of plants.
As a grass seedling develops, a sturdy hollow cylinder, or coleoptile, pushes up through the soil, enclosing the delicate seedling leaves. When the cylinder reaches the soil surface, it stops growing and acts as a protective tube through which the young grass leaves can push up without being injured by large soil particles. If the tip of the coleoptile is cut off, growth slows down and, after a few hours, stops completely. If the tip is replaced on the stump, the stem starts growing again.
However, when the tip and the stump are separated by an impermeable barrier, such a piece of aluminum foil, the stump will not resume growth. If the tip is placed on an agar block for a time and then the agar block only is placed on the stump, stem growth starts again. Finally, when an agar block on which a stem tip had rested for several hours is placed on one edge of a stump, a curvature of growth results.
Clearly such experiments demonstrate that some growth substance is produced by the tip of the shoot but identification of this substance proved to be a long and tedious task. At first attempts were made to extract it from the coleoptile tips themselves but this attempt was abandoned when it became apparent that it would take 10 people working 70 hours a week 70 years to obtain a single gram of the substance. Eventually, other approaches led to the purification and crystallization of the first plant growth – regulating substance, a chemical known as indoleacetic acid (or IIA).
Plants Have Hormones?
Botanists refer to growth – regulating chemicals such as IIA as hormones. Plant hormones or phytohormones, like animal hormones, are organic compounds that are effective at very low concentrations. These hormones are usually synthesized in one part of the plant and transported to their target tissues where they cause physiological responses, such as growth or fruit ripening. Botanists now know that plant hormones are produced primarily in apical meristems, in young leaves, and in growing seeds and developing fruits.
That a tiny amount of hormone can have an amazingly powerful effect is demonstrated by the fact that one speck of IIA diluted a million-fold in water (1 molecule of IIA per 1 million molecules of water) could produce measurable growth in a coleoptile. From this it was calculated that a single ounce of IIA could theoretically produce enough plant growth, that if all that growth were laid end to end, it would circle the earth. By comparison, one ounce of glucose sugar could generate only about a fifth of a mile of growth, or only 1/100,000 as much as IIA. This shows that the IIA, unlike sugar, does not act as a building block in growth, but is truly a hormone or growth stimulator.
What are the Functions of Plant Hormones?
Hormones can either stimulate or inhibit plant growth. The same concentration of a particular hormone can have two different effects on two different target tissues. For example, the identical concentration of a hormone can stimulate growth in stem tissues but inhibit growth in root tissues.
Also, different concentrations of a particular hormone can produce different effects on the same target organ. For example, low concentrations of a hormone can cause meristematic cells to divide, but high concentrations of the same hormone may inhibit cell division. Lastly, two or more hormones can interact in a variety of ways. For example, the effect of one particular hormone may depend on whether or not another particular hormone is present. Systems of interacting hormones are especially important in regulating plant reproduction.
Today many plant hormones can be synthesized in the laboratory, increasing the quantity of hormones available for commercial applications. Botanists currently recognize five categories of hormones: auxins, gibberellins, ethylene, cytokinins, and abscisic acid.
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