The growth and development of plants are regulated by many factors. These factors may be either extrinsic, that is, external environmental factors such as light, temperature, humidity etc. or may be intrinsic that is, factors such as hormones and other such materials which are secreted in the plant tissues themselves. Many a time, the development processes are regulated by the action of many factors rather than being regulated by a single factor itself.
Plant physiologists and biochemists have isolated many chemical substances which have great influence on the growth and differentiation of plant tissues. In all plants such secretions are found and these are collectively called growth regulators or phytohormones. Phytohormones are varied chemically and they also differ in their specific biological actions. Some of them promote growth and differentiation and some others are inhibitory in their action. These hormonal substances belong to five classes of compounds, namely auxins, gibberellins, cytokinins, abscisic acid and ethylene.
These are weak organic acids with the acidic group situated at the end of the side chain attached to an unsaturated ring(s) system. At present, the term auxin is applied to indoleacetic acid (IAA) and to natural and synthetic growth regulating substances having similar structure and functions. Auxins are synthesised in shoot apical meristems, young leaves, in developing seeds and in root apices. These growth regulators stimulate stem elongation, cambial activity and growth in developing fruit tissues. Auxins when applied exogenously, stimulate growth of plant tissues and this growth depends on the concentration of the hormones applied. Many auxins are effective herbicides, and are used as weed-killers. Auxins along with one or more of the other growth substances are effective in multiple and complex regulative functions in plants.
Structure of an AuxinThe discovery and studies on the numerous physiological activities of auxins, date back to the work of Charles Darwin and his son Francis in the 1880s. Darwin was interested in the phototropic movements of plant structures that is, plant movements toward light (as in shoots) and away from light (as in roots). Much of the work of Darwin was on canary grass seedlings grown in the dark. The etiolated grass seedling has a coleoptile covering the immature shoot. To find out the light receptive region of the coleoptile, various places of the structure were covered or 'blind folded' and then illuminated. The coleoptile grew toward the light as long as its tip was exposed. Even covering the top millimeter or more of the tip of the coleoptile failed to elicit a phototrophic response of bending towards light. From these experiments it was discovered that the tip of the coleoptile acted as the photoreceptor. However, the actual bending of the coleoptile took place a few millimeters below the tip, in a growing region. Therefore, Darwin concluded that there was some kind of a 'message' travelling within the coleoptile from the tip to the growing region.
Later work by scientists demonstrated that the 'message' actually was a chemical substance which could not pass through an impermeable barrier but could pass through certain materials such as gelatin. It was also discovered that the tip of the coleoptile produced this chemical substance which moved down to the growing region.
Darwin's Experiment to Search for the Plants "Eye"
It was the Dutch botanist, Frits W. Went, who isolated the growth substance from oat coleoptiles. To isolate the substance that moved from the coleoptile tip of the region of bending, Went placed a cut tip from a coleoptile on a small block of gelatin. After several hours he discarded the tip and placed the gelatin against the end of another seedling, the tip of which had been removed earlier. When the gelatin caused the second coleoptile to bend it was reasoned that the gelatin contained a substance produced by the tip of the first coleoptile. Went named this substance auxin. It is known to be Indoleacetic acid (IAA).
Auxins play many roles in plants as we have already stated earlier. One of the functions of auxins in the plant is prevention leaf abscision (the separation of leaves from stems). If the blade of the leaf is removed, the petiole abscises more rapidly than if the leaf remained intact. If the cut surface is treated with an auxin solution, the petiole remains attached to the plant. It appears that the natural abscision of leaves is controlled by the transport of auxin, produced in the leaf-blade, through the petiole.
Went's Experiment on the Demonstration of Auxin Production in the Tip of Coleoptile
Another important role of auxins in plants is in the maintenance of apical dominance. This is the tendency in some plants to grow with a single main stem with minimal branching. In most plants the terminal bud at the apex of the shoot suppresses the development of lateral buds into branches. Removal of the apical bud causes the lateral buds to grow out vigorously. However, this growth of the lateral buds is prevented if the cut surface of the stem is treated with an auxin solution. In the experiments described for leaf abscision and the above one on stems, it is to be noted that the removal of a particular part of the plant produces an effect namely abscision and loss of apical dominance. This indicates that the part removed (the leaf blade or stem) is the source of auxin and replacement of auxin prevents the expression of the effect. These results clearly show that it is the auxin which helps to maintain apical dominance and delays the abscission of leaves, in the intact plant.
These are also acidic compounds based on gibbane carbon skeleton which is closely related to terpenes. Gibberellins have been found to occur naturally in a variety of plant tissues. The effects of gibberellins were known to Japanese over a century ago. It was in 1938 that the crystalline form of gibberellin A was isolated from the culture of the fungus, Gibberella fujikuroi. Over 100 different gibberellins are known today and one of them, gibberellic acid, or gibberellin A, is the most extensively investigated form of gibberellin. The most dramatic effect of gibberellins is on stem growth. When applied in low concentrations to dwarf plants the stems begin to grow very rapidly. The length of the internodes increases. In addition to the stimulation of growth of stems these hormones seem to be the principal stimulants of root growth. The application of gibberellins to certain plants (examples: spinach and cabbage) promotes the development of flowers. Gibberellins are found in increased quantities in these plants when they are ready to flower. Apart from stimulation of stem and root growth and flowering, gibberellins also influence various aspects of plant development such as dormancy and senescence. Apples, figs and grapes will develop, even without pollination, following a spray treatment of gibberellins. This class of growth regulators along with auxins, control fruit growth and development. In sex expression studies, it has been found that auxin treatment causes the formation of female flowers as opposed to male flowers, while gibberellin treatment results in the appearance of more male than female flowers.
Gibberellic Acid (gibberellin A3)
Another group of plant growth regulators is the cytokinins. Each of these substances contains purine and adenine, as part of their molecular structure. Cytokinins when acting together with auxins, strongly stimulate mitosis in meristematic tissues. The food reserves of some seeds contain cytokinins. These possibly provide the chemical stimulus for rapid mitosis in the developing seedling. Cytokinins also promote the differentiation of cells produced in the meristems. In addition to their growth promoting effects cytokinins have been shown to slow down the process of ageing of plant parts such as leaves and to increase the resistance of plant parts to such harmful influences as low temperature, virus infection, weed killers and radiation.
This natural phytohormone causes inhibition rather than stimulation of growth in plants. It has been shown to control the process of dormancy in plants. Besides, it also influences other developmental processes such as senescence, abscission and flower initiation. Abscisic acid interacts with growth promoting hormones such as gibberellin to influence plant development
It is a gas at all temperatures under which plants can survive. Ethylene is functional in the ripening of fruits. This effect of ethylene is utilised in citrus industry when oranges, lemons, and grape fruits are sometimes picked when they are still green, and ripened in gas chambers containing ethylene. Bananas and mangoes are also ripened similarly. Ethylene is known to be produced in plant organs other than fruits.
Chemical Structure of Phytohormone: Ethylene
Vitamins (Vita: life) are a heterogenous group of biological products of plants. They have proved to be invaluable for normal growth and development of the organisms, maintenance of health and vigour. Thus, vitamins play a very important role in the metabolism of proteins, fats and carbohydrates and their proper assimilation in the body. Some vitamins act as prosthetic groups or coenzymes, and help in controlling important biochemical reactions in both plants and animals.
Vitamins are mostly synthesized by plants and stored in their different organs. Some vitamins can also be produced commercially. Vitamins are of many types, namely vitamins A, B (B complex - B1, B2, B6, B12, etc.) C, D, E and K.