Plant Growth Substances | Botany Optional for UPSC PDF Download

Introduction

Plants require various external factors like light, water, oxygen, and nutrients for their growth and development. However, it's not just these extrinsic factors that influence plant growth. Intrinsic factors, which include intracellular genes and intercellular chemicals, also play a crucial role. These intercellular chemicals are known as Plant Growth Regulators (PGRs), and this article explores them in detail.

Defining Plant Growth Regulators

Plant Growth Regulators are naturally occurring small, simple chemicals produced by plants to control their growth and development. They are alternatively referred to as plant growth substances, phytohormones, or plant hormones. These regulators encompass a diverse chemical composition, including gases like ethylene, terpenes like gibberellic acid, and carotenoid derivatives like abscisic acid.

Classification of Plant Growth Regulators

Based on their action, plant growth regulators can be broadly classified into two categories:

• Plant Growth Promoters: These regulators stimulate cell division, cell enlargement, flowering, fruiting, and seed formation. Examples of plant growth promoters include auxins, gibberellins, and cytokinins.
• Plant Growth Inhibitors: These chemicals inhibit plant growth and promote dormancy and abscission. An example is abscisic acid. It's important to note that ethylene can function as both a promoter and an inhibitor but is primarily considered a Plant Growth Inhibitor.

Now, let's delve deeper into each of these plant growth regulators:

Auxins: The First Growth Hormone

• Discovery: Auxins were the first plant growth hormone to be discovered, thanks to observations by Charles Darwin and his son, Francis Darwin. They noted the phenomenon of phototropism in canary grass coleoptiles, which led to the isolation of the first auxin by F. W. Went.
• Types: Auxin is a term applied to natural and synthetic compounds with growth-regulating properties. Natural auxins like Indole-3-acetic acid (IAA) and Indole butyric acid (IBA) are found in growing stems and roots. Synthetic auxins, such as Naphthalene acetic acid (NAA) and 2, 4-dichlorophenoxyacetic (2, 4-D), have also been developed.
• Effects: Auxins have several effects, including promoting flowering, initiating rooting in stem cuttings, preventing premature dropping of fruits and leaves, aiding in natural detachment (abscission) of older plant parts, controlling xylem differentiation, and aiding in cell division.
• Applications: Auxins find use in plant propagation, inducing parthenocarpy (fruit production without fertilization), serving as herbicides to eliminate dicotyledonous weeds, and maintaining weed-free lawns in gardening.

Gibberellins: The Growth Extender

• Discovery: Gibberellins were identified when E. Kurosawa treated rice seedlings with sterile filtrates of the fungus Gibberella fujikuroi, causing disease symptoms. Gibberellic acid was identified as the active substance responsible for the disease.
• Types: Over 100 gibberellins have been discovered, with Gibberellic acid (GA3) being the most studied.
• Effects: Gibberellins increase axis length in plants, delay fruit senescence (aging), improve fruit shape, and have applications in the brewing industry, sugarcane yield enhancement, maturation acceleration in conifers, and promoting bolting in cabbages and beets.

Cytokinins: Promoters of Growth

• Discovery: Cytokinins were discovered when F. Skoog and colleagues observed cell proliferation in tobacco plants under specific nutrient conditions, and later identified kinetin as the active substance.
• Types: Cytokinins encompass natural (e.g., zeatin) and synthetic forms. They are found in root apices and developing shoot buds.
• Effects: Cytokinins aid in leaf and chloroplast formation, promote lateral shoot growth, overcome apical dominance, facilitate nutrient mobilization, and delay leaf senescence.

Abscisic Acid: The Stress Hormone

• Discovery: Abscisic acid was identified when three independent researchers characterized different inhibitors – Inhibitor B, Abscission II, and Dormin – only to find that they were chemically identical and collectively named abscisic acid.
• Effects: Abscisic acid regulates abscission and dormancy, inhibits plant growth, metabolism, and seed germination, induces stomatal closure, increases stress tolerance, and plays a crucial role in seed development and maturation.

Ethylene: The Ripening Gas

• Discovery: Ethylene was discovered when a group of researchers observed that a gaseous substance released from ripe oranges accelerated the ripening of unripe oranges.
• Effects: Ethylene affects seedling growth, axis swelling in dicot seedlings, promotes abscission and senescence (especially in leaves and flowers), enhances respiration during fruit ripening, and increases root growth and root hair formation.
• Applications: Ethylene is widely used in agriculture for processes like breaking seed and bud dormancy, initiating germination in peanut seeds, promoting potato tuber sprouting, inducing flowering in various crops, and accelerating fruit ripening.

Conclusion

In conclusion, plant growth regulators are vital for various phases of plant growth and development. These regulators, along with genes and extrinsic factors, play essential roles in shaping the growth and characteristics of plants. Understanding their functions and applications is crucial for modern agriculture and horticulture.

Commercial uses of growth regulators

• Rooting and Plant Propagation 
  (a) Compounds like IBA, NAA, 2,4-D, 2,4,5-T are used to facilitate root formation.
  (b) IBA is particularly effective at promoting the development of a robust fibrous root system.
• Germination and Dormancy 
  (a) Gibberellin is employed to stimulate seed germination.
  (b) Abscisic acid (ABA) acts as an inhibitor of germination, counteracting the effects of Gibberellin.
  (c) ABA can also induce dormancy, while Auxins and Gibberellin can break dormancy.
• Fruit Set and Development 
  (a) 2,4,5-T is used to promote fruit setting.
  (b) Gibberellic acid is applied to increase the size of fruits, especially in crops like grapes.
  (c) Cytokinin is utilized to extend the shelf life of fruits and flowers.
  (d) A combination of Gibberellic acid and Cytokinin helps achieve desirable fruit shapes.
  (e) Parthenocarpic fruit development is induced using Gibberellins, IAA, and PAA.
• Sex Expression in Plants: Gibberellins are used to promote the development of male flowers in crops like cucumbers, while a combination of Auxins and Gibberellins can stimulate the production of female flowers in plants like maize.
• Abiscission Control: NAA and IAA are used to control abscission (the shedding of leaves and fruits), while Ethrel can induce abscission when needed.
• Morphogenesis (Development of Plant Structures): Auxins and Cytokinins play crucial roles in regulating plant growth and development.
• Weed Control: Herbicides like 2,4-D and 2,4,5-T are employed for weed control in agriculture.
• Plant Organ Size: Gibberellic acid (GA) promotes plant height, while TIBA (Triiodobenzoic acid) reduces it. Cytokinins like BAP (Benzylaminopurine) can increase tillering.
• Antitranspirants: ABA and PMA (Phenylmercury acetate) are used as antitranspirants to reduce water loss in plants.
• Papaya Flower Induction: Ethephon is applied to induce flowering in papaya plants.
• Rubber Latex Flow in Rubber Trees: Herbicides like 2,4-D and 2,4,5-T are used to stimulate rubber latex flow in rubber trees.
• Fruit Ripening: Ethrel is employed to accelerate the ripening of fruits.
• Sugarcane Ripening: Glyphosate and CCC (Chlormequat chloride) are used as ripening agents in sugarcane.

These growth regulators play critical roles in agriculture, horticulture, and plant science to achieve desired plant characteristics and crop outcomes.