What are Plant Growth Regulators (PGR) – Overview

Introduction

Plant growth regulators also referred to as plant growth hormones or phytohormones. Plant growth hormones are the organic substance of minute amount in smaller concentrations which involves the physiological process like growth, development and movement of the plant. The plant hormones are either natural or synthetic hormones which alter the structure and metabolic process on application. Some of which provides the beneficial effect of increasing yield and quality of the plant.

Plant growth regulators based upon its action is classified as plant growth promoters and plant growth regulators. In which plant growth regulators include Auxins, Gibberellins, Cytokinins and the plant growth inhibitors include Abscisic acid and Ethylene. Out of which ethylene may act as either promoter or inhibitor. Many of the science enthusiasts and science students may have queries regarding plant growth regulators meaning, definition, plant growth regulators notes etc. However, we have come up with this article on Plant Growth Regulators where one can clear their basic information about plant growth regulators.

Plant growth regulators and their physiological effects – Plant growth regulators list

Auxin

The first hormone to be discovered. The word auxin is derived from the Greek word, which means ‘to grow’. The auxin is mainly produced in the shoot and root portion of the plant which migrates from the apex to the zone of elongation. The presence of auxin is seen throughout the plant. hence its abundance is noticed at the coleoptiles tip, buds, root tips and leaves. Indole Acetic Acid (IAA) is the only naturally occurring auxin in plants and the synthetic auxins are Indole Butyric Acid(IBA), Naphthalene Acetic acid(NAA), Methyl ester of Naphthalene acetic acid(MENA) , 2 Methyl 4 chloro phenoxy acetic acid(MCPA),  2, 3, 5 Tri iodo benzoic acid(TIBA), 2, 4-D : 2, 4 dichloro phenoxy acetic acid and 2, 4, 5-T: 2, 4, 5 – Trichloro phenoxy acetic acid.

Physiological effects of auxin

• Cell division and cell elongation in the
• shoots and plays an important role in the secondary growth of stem and differentiation of xylem and phloem tissues.
• The apical dominance in which the apical bud dominates over the lateral buds and does not allow the lateral buds to grow.
• Root initiation by elongation of the root in terms of increasing the number of lateral roots with a higher concentration of auxin.
• Prevention of which results in the fall of leaves, fruits and flowers.
• A higher concentration of auxin induces parthenocarpic fruits which are produced without undergoing pollination and fertilization.
• Auxin increases the rate of respiration in the plant. Addition of auxin results in callus formation which is occurred due to the activity of cell division besides cell elongation.
• 2, 4- D and 2, 4, 5-T are synthetic auxins which are useful in case of weed eradication.

Gibberellins

Gibberellin is isolated from the infected rice seedling by the fungus Gibberella fujikuroi by A Japanese scientist Kurosawa.

Physiological effects of gibberellins

• Certain seeds require light to initiate germination and are not able to germinate under dark. This is overcome by gibberellin in which on its application under dark promotes seed germination. 
• In temperate regions, the buds that form in autumn season remain dormant until next spring

due to severe cold. This dormancy of buds can be broken by gibberellin treatments which show vigorous growth.

• The higher concentration of gibberellin has little effect on inhibiting the root growth.
• The genetic dwarfism is overcome by the application of gibberellins by promoting plant growth as a result of elongation of internodes.
• Stimulates bolting/flowering in response to long days
• Parthenocarpic fruit production by the application of gibberellins. The seedless tomatoes and grapes are produced by the application of gibberellin.

Cytokinins

The chemical substance 6-furfuryl aminopurine on its specific effect on cytokinesis, that is cell division and hence known as cytokinin. The cytokinins show the effect on cell division and there regulating the growth of the plant.

Physiological effects of cytokinin

• Induce cell division especially in tobacco pith callus, carrot root tissues, soybean cotyledon and alsopea callus, etc. In addition, it also induces cell enlargement.
• External application or foliar application of cytokinin promotes the growth of lateral buds and therefore counteracts the effect of apical dominance.
• Like gibberellin, it breaks seed dormancy.
• Delays senescence to several days.
• On its application induces the flowering in case of a short day plant.
• Application of high auxin and low kinetin produced only roots whereas high kinetin and low auxin could promote the formation of shoot buds.
• Promotes active accumulation of solutes.

Ethylene Plant Growth Regulators  (PGR)

Ethylene is the only natural plant growth hormone that exists in gaseous form.

Physiological effects of ethylene

• Promotes  shoot and root growth
• Induces positive and negative effects on dormancy.
• Stimulates leaf and fruit abscission and induces the femaleness in dioecious type flowers.
• Promotes flower senescence, leaf senescence, and flower opening.
• Initiates fruit ripening.

Plant Growth Regulators  (PGR) – Abscisic Acid

Abscisic acid is mainly known for its regulatory action in abiotic stresses like drought and high salinity. Desiccation tolerance induced by promoting stomatal closure which enables to adapt water stress.

Physiological effects of abscisic acid

• The closure of stomata is induced.
• Stress responses, especially to water deficiency seed and bud dormancy, is stimulated.
• Induces seed to synthesize, storage proteins shoot growth is inhibited.

Conclusion: Plant growth regulators have several activities towards yield enhancement and quality improvement and thus playing a major role in the growth and development of the plants.