Physiological effects of plant growth regulators: Plant growth regulators include auxin, gibberellin, cytokinin, ethylene and abscisic acid.
Naturally occurring auxins include Indole Acetic Acid and Indole Butyric Acid, produced at the apex of plant stems and roots. Synthetic auxins include Naphthalene Acetic Acid and 2, 4-Dichlorophenoxyacetic. Auxins are used in plant propagation as they cause rooting in stem cuttings and induce flowering, and prevent falling of young leaves and fruit and promote abscission of mature leaves and fruit. Auxins are used in parthenocarpy. They exhibit apical dominance in higher plants. Synthetic auxin 2, 4-D is used as an herbicide.
Gibberellins are extracted from fungi and higher plants. They are referred to as GA1, GA2, GA3 etc in the order of discovery. Gibberellins have varied physiological effects on plants such as an increase in the length of the plant body axis. Gibberellins also improve the fruit’s shape, they delay senescence or ageing of fruit and are hence used commercially to extend the market period by retaining fruit on trees longer, they also hasten the maturity of seeds and hence are used to obtain early seed production from juvenile conifers. Commercially, GA3 is also used in the brewing industry to increase the enzymatic activity inside grains during the malting process.
Growth-promoter cytokinin plays an important role in the cytokinesis in a plant cell. One type of cytokinin, zeatin, was isolated from corn kernel. Naturally occurring cytokinins were identified in specific regions of plants such as the root apex, shoot apex and fruit to promote growth. They promote the growth of lateral and adventitious shoots by preventing apical dominance. They also delay senescence or the ageing of leaves.
Another growth promoter, ethylene, is present in plants in gaseous form. It is synthesised in ripened fruit. Ethylene accelerates the ripening of fruit by enhancing the rate of respiration. This property is used in the artificial ripening. It promotes the abscission of leaves and flowers. It breaks seed and bud dormancy, also aids in the lengthening of internodes in rice plants. It also promotes root and root-hair formation. It initiates flowering in some plants.
Abscisic acid is a plant growth inhibitor that regulates abscission and dormancy in plants. In the case of unfavorable environmental conditions, it inhibits germination of seeds. In the case of drought conditions, it closes the stomata, thus helping the plant withstand the stress caused by lack of water.