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The below mentioned article will highlight the eleven uses of auxins in the field of agriculture and horticulture.
They are: (1) Apical Dominance (2) Meristematic Activity (3) Rooting (4) Parthenocarpic or Seedless Fruits (5) Flowering in Pineapple (6) Prevention of Premature Fall of Fruits (7) Prevention of Sprouting of Potatoes (8) Weed Killing (9) Control of Lodging (10) Differentiation of Xylem and Phloem and (11) Sex Expression.
(1) Apical Dominance:
The auxins greatly influence the development of plant form and structure. It has long been known that while the main shoot of a plant is growing, its lateral buds are inhibited. If, however, the bud at the apex is cut off, the lateral buds begin to develop. It has been postulated that an inhibiting substance diffuses from the growing bud to the tissue below it.
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With the advent of synthetic growth substances this theory was corroborated. When the apical bud was removed from the shoot and a small quantity of Indoleacetic acid was applied to the stump the lateral buds did not develop. The Indoleacetic acid was found to possess the power to inhibit lateral growth. Removal of plant apices is commonly practiced in lawns and gardens to develop hedges. The principle behind such practice is to eliminate apical dominance.
(2) Meristematic Activity:
Auxin affects the meristematic activity of cells other than those involved in tumor and callus production. Auxin produced in the apical bud stimulates and regulates the activity of the cambium in woody plants. It seems probable that the resumption of cambial growth in the spring is due to auxin produced by the buds in this season. Cambial growth may also be induced by the artificial application of auxin.
(3) Rooting:
Propagation of plants by vegetative means is quite commonly practiced in horticulture. Several experiments performed on a great variety of plants showed that auxin applications are generally beneficial in bringing about the rooting of cuttings. The process, known as vegetative propagation, is extremely useful to the horticulturists, for by means of it a great many genetically identical plants may be made from a single individual, and a desired genetic pattern, as in a variety of apple, seedless orange, or a rose of a new colour, may be preserved from generation to generation.
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In practice, the ‘cutting’ is usually a twig with a few leaves on it, but sometimes leaves, pieces of stem or root, or even bulb scales may be used to start a new plant. Auxin treated cuttings generally root more rapidly than untreated ones, and the roots are more abundant and stronger. Dipping cutting into auxin solutions or powders has become a standard horticultural practice in advanced countries, and there are, in the market today dozens of preparations for the said purpose. The same ability to promote root growth is also possessed by ethylene.
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The most widely used of the synthetic auxins for this purpose is Indolebutyric acid (IBA). It is used either alone or in combination with other auxins such as Naphthalene acetic acid (NAA). Recently attention has been called to the effectiveness of some of the chlorinated phenoxy acids.
(4) Parthenocarpic or Seedless Fruits:
Another property of auxins that has grown to economic importance is their ability, when applied to the flowers of certain species, to initiate development of fruit without pollination. Because of the difficulty of obtaining satisfactory pollination in some plants as well as to improve the quality and market value of the fruits auxin treatment in the form of sprays or aerosol is used.
Fruits so induced are usually, seedless; so besides increasing yields, auxin treatments may make possible the development of new seedless varieties. Naphthalene acetic acid (NAA) and Naphthoxyacetic acid (NOXA) have been successfully used in many plants to induce parthenocarpy.
(5) Flowering in Pineapple:
In the raising of pineapples there is great difficulty in obtaining satisfactory fertilization and development of the fruit. But here the problem is to obtain flowering at the proper time. The size to which the fruit develops is directly dependent on the number of leaves on the plant at the time of flowering. J. Van Overbeek, found in Cabezena variety of pineapple, which flowers poorly when left to itself, can be made to flower at any time of the year by a single application of an auxin.
Naphthaleneacetic acid or 2, 4-Dichlorophenoxyacctic acid. We, therefore, have the interesting possibility of producing uniform fruits of a selected size by applying the auxin to each plant when it has the appropriate number of leaves.
(6) Prevention of Premature Fall of Fruits:
In U.S.A. and Russia particularly, growers of citrus, apples and pears use large amounts of auxins for the prevention of premature fall of fruits. From a fourth to half of the entire crop may be lost because the fruit falls before it has matured or developed good colour. Thus the grower must either harvest before the best quality is attained or else, risk a heavy fall.
Auxin sprays proved highly successful, and now growers can obtain reasonable assurance against loss to their apple and pear crops by using these auxins. Naphthaleneacetic acid has given satisfactory results in apples and pears where as in case of citrus, 2, 4-Dichlorophenoxyacetic acid is more successful.
(7) Prevention of Sprouting of Potatoes:
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Still another commercial application of auxins takes advantage of their growth inhibiting ability. The methylester of Napthaleneacetic acid prevents the sprouting of potatoes in storage; thus the tubers will keep longer, even at warm temperatures.
(8) Weed Killing:
2, 4-D (2, 4-Dichlorophenoxyacetic acid) has received wide acclaim as a weed-killer, for when sprayed on plants it kills the broadleaved dicotyledons, while sparing the grasses. Under favourable circumstances it can be used to keep sugar cane fields, corn and wheat fields or lawns free from most common weeds without laborious hoeing or weeding. Recent experiments indicate that in the plants affected by 2, 4-D there is a temporary sharp increase in the rate of metabolism.
The sprayed plant is not only injured where the chemical comes in contact with it, but is stimulated to burn up its reserve food supply. As a consequence it starves to death. In smaller concentrations 2, 4-D can also be used for most of the applications of auxins.
(9) Control of Lodging:
In some plants when the crop is ripe and there is heavy rain accompanied by strong winds, the plants bend as a result of which the ear (inflorescence) gets submerged in water and decays. If a dilute solution of any auxin is sprayed upon young plants, the possibility of bending of plants is reduced as the stem becomes stronger by the application of auxins.
(10) Differentiation of Xylem and Phloem:
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Experiments with intact plants and also of tissue culture have shown that differentiation of xylem and phloem is under the control of auxins.
(11) Sex Expression:
The spray of auxins increases the number of female flowers in cucurbits. In maize, application of NAA (Nepthalene Acetic Acid) during the period of inflorescence differentiation can induce formation of hermaphrodite or female flowers in a male inflorescence. Thus auxins induce femaleness in plants.
We can see now that while the activities of plant hormones parallel those in animals upto a point, the parallel is not complete. The auxins seem to be more general and versatile in their effects. Auxin may influence a plant in many different ways, at least so far as the physical effects are observable. Sometimes it stimulates growth, under other conditions it retards growth.
Sometimes it induces a tumor. Sometimes it kills the entire plant. Such observations lead with increasing clarity to the conclusion that the auxins must influence some basic general cellular process, and that the result of this influence may be expressed in a variety of ways, depending on the nature and age of the tissue, on the availability of other interacting substances, and on the external and internal conditions.
