There are various synthetic organic compounds which are applied to plants to give some positive responses. These compounds are artificial plant hormones which help in a deficient concentration (< 0.001 M) in maintaining of growth and development of the plants. These are now widely used to get a better result in the field of agriculture and horticulture. Some of the uses of natural and synthetic hormones in the above areas are given below:
Effects on Vegetative Plant Structures
Role in the Rooting of Cuttings: There are some plants like China rose (Hibiscus), rose (Rosa), etc., generally reproduce vegetatively. They are usually propagated by cutting pieces of stem. When the cut stem piece is placed in the moist stand, then adventitious roots come out from the cut end.
The rooting of cutting now a day can be hastened by pre-treating the cuttings with powders or solution containing synthetic hormones like Indole acetic acid (IAA), Indole butyric acid (IBA), Naphthalene acetic acid (NAA), etc. By the help of such cuttings, a large number of identical plants may be raised from a single individual plant for the preservation of desired genetic pattern, generation after generation.
Role in Controlling Cambial Activity: The secondary growth in thickness of stems and roots in woody plants take place due to the activity of cambium. The primary cambium (fascicular cambium) and secondary cambium (interfascicular cambium) together form the cambium ring. The activity of this is higher in spring month; then it gradually declines and become the lowest in summer. As a consequence, the formation of annual rings occurs.
This rhythmic activity of cambium is closely linked with the hormone Indole acetic acid (IAA) which stimulates meristematic activity in the cambial tissue.
Role in Callus Formation and Healing of Wounds: Cambium performs another important function by forming callus or wound tissues for the healing of wounds. There is a possibility of infection by some pathogenic organisms in any types of injuries, especially during pruning of plants.
For protection against wounds in the exposed area, some substances come out of the wounded cells causing uninjured cells to become meristematic resulting in the formation of callus or wound tissue, whereby healing of wounds takes place. For the formation of callus, hormones like auxin (Indole acetic acid) are found to be very effective.
Role in weed Control: Unwanted plants are called weeds which can be effectively controlled by the application of synthetic hormones. Weed killing hormones are known as herbicides. One of the ideal weed killers is the herbicide 2, 4-dichlorophernoxy acetic acid (2,4-D).
Other herbicides sometimes used are 2-methyl,4-chlorophenoxy acetic acid(MCPA) and 2,4,5-trichlorophemnoxy acetic acid(2,4,5-T). The most effective way of this herbicide is used in an aqueous spray on the foliage. In this case, 1% concentration for 5 gallons per 1000 sq feet is used.
Role in the Prevention of Sprouting of Potato Tubers: The modified, underground stem is the potato tuber which breaks dormancy at an early period causing quick loss of weight and decrease in the starch content. This creates a great problem for the agriculturists.
For overcome this difficulty now a day, synthetic hormones are applied. In this case, potato tubers are dipping in indole acetic acid solution, sprouting, i.e., bud formation of potatoes can be inhibited. Another synthetic hormone, such as methyl a-naphthalene acetate, is also effective in preventing but formation.
Effect on the Reproductive Structures of Plants
Control of Floral Initiation: Control of floral initiation is a process which is controlled by the hormone. In many cases, naphthalene acetic acid (NAA) is used to enhance the vegetative bud of pineapple, causing the induction of flower bud. 2, 4-D also can induce flowering. Gibberellins have also been shown to initiate flower formation in some plants like Hyoscyamus and Samolus (Lang, 1956).
Control of Fruit Development (Parthenocarpic Fruit Formation): The formation of parthenocarpic fruits such as seedless fruits can be induced by the application of IAA and IBA (Gustafson, 1936) in plants like squash, tomato, pepper, Petunia, etc. Naphthalene acetic acid has also been found suitable for the development of seedless fruits in strawberry.
Role in the Control of Abscission Layer: In most species of plants a time comes when the shedding of leaves and flowers and dropping of fruits take place from the stem. The process of their removal from the plant is called abscission which generally takes place by the formation of abscission layer,
The abscission layer in the form of a thin plate of cells is formed at the base of the leaf petiole or fruit stall. The cells of this layer become softened and so weaker than they readily break from the plants by wind or by any other mechanical means.
Abscission can be controlled by means of the hormone. It has been observed that when auxin production diminishes then the abscission layer forms. It has been shown that by application of auxin, abscission layer formation is delayed. Pre-harvest drop of fruits (apple, orange, peaches, plums, etc.) causes a severe reduction in the yield of fruits.
Synthetic hormones like 2,4-D, NAA or naphthalene acetamide is used to prevent premature drop of fruits before they are ready for picking from the plants. It is an economically important aspect of horticulture.
Role in Thinning of Blossom and Control of Fruit Production: Plant hormones are used effectively in thinning of blossom and control of fruit production. In this case, hormones reduce fruit set by removing some of the flowers.
Role in the Fruit Growth and Maturation: Plant hormones like Indole butyric acid (IBA) can be effectively applied in the control of fruit size and their maturation, for example, tomato.