Importance of Plant Bio-Regulators in Horticulture

Plant bio-regulators (PBRs) are substances—either natural or synthetic—that affect the growth and development of plants. They include plant hormones, growth inhibitors, enzymes, vitamins, organic acids, and other compounds that influence biological activity in the plant system. PBRs can either stimulate or inhibit plant processes and are used widely in horticulture to improve productivity and quality.

Types of Plant Bio-Regulators

• Plant Growth Regulators (PGRs): These are organic compounds, other than nutrients, that can promote, inhibit, or modify physiological processes in plants, even at very low concentrations.
• Plant Hormones (Phytohormones): Naturally produced compounds within the plant that regulate growth and development. These hormones move within the plant and can influence physiological processes at low concentrations. Key phytohormones include:
  • Auxins
  • Gibberellins
  • Cytokinins
  • Ethylene
  • Abscisic Acid

Other less-known hormones such as brassinosteroids, jasmonic acid, oligosaccharides, and fusicoccin also play a role in plant regulation.

Applications of Plant Growth Regulators in Fruit Production

1. Propagation:

• • Gibberellins (GA): Used to enhance seed germination, particularly for crops that require a chilling period. Gibberellin treatments (200-500 ppm) can substitute chilling requirements for seed germination.
  • Indole-3-butyric acid (IBA): Commonly applied to promote rooting in cuttings, with concentration varying based on the type of wood (softwood, semi-hardwood, hardwood).

2. Micropropagation: Auxins (IBA) and Cytokinins (BA) are used to regulate shoot and root development in tissue culture. For shoot initiation, a high cytokinin:auxin ratio (e.g., BA:IAA) is required, whereas for root induction, a high auxin:cytokinin ratio (e.g., IBA:BA) is beneficial.

3. Breaking Dormancy: Gibberellins (GA) can break seed dormancy, particularly for seeds requiring extended periods of cold stratification. Similarly, GA sprays (200-250 ppm) can terminate bud rest in temperate fruit crops, reducing chilling requirements.

4. Control of Vigour: Growth retardants like Paclobutrazol and SADH help reduce the size of fruit trees, making them more suitable for high-density orcharding. These retardants are used on crops like apple, apricot, lemon, litchi, mango, and pear.

5. Flowering Regulation: Ethylene is used for inducing flowering in pineapple. Other chemicals like acetylene, ethephon, and NAA help promote flowering in various fruit crops. For mango, Paclobutrazol (PP333) and NAA are commonly used to regulate flowering.

6. Parthenocarpic Fruit Development: Auxins such as IAA, IBA, and phenoxy acetic acid induce parthenocarpy (seedless fruit) in crops like pineapple, citrus, and banana. In apples and pears, Gibberellins (GA) are often used for this purpose.

7. Fruit Thinning: Auxins and Gibberellins (GA) are applied to control fruit set and thinning. In grapes, GA (40-60 ppm) at the post-bloom stage effectively thins excess fruit.

8. Fruit Growth and Maturity: Auxins can promote fruit enlargement. For grapes, a combination of GA (40-60 ppm) and brassinosteroids helps increase fruit size and improve shape.

9. Control of Fruit Drop: Auxins like NAA and 2,4-D at low concentrations (10-30 ppm) help prevent fruit drop by maintaining fruit retention on the tree.

10. Improvement of Fruit Quality: For grapes, applying GA at the color break stage (40-60 ppm) enhances total soluble solids (TSS), sugar content, and reduces acidity, thus improving fruit quality.

11. Fruit Ripening: Ethylene (250 ppm) is used to induce ripening in fruits. In citrus, Ethephone (1000 ppm) can cause degreening, while in Kagzi lime, Cycocel (500 ppm) accelerates harvesting.

12. Harvesting Control: Chemicals like Ethrel and Cycloheximide help control the ripening and harvesting process, ensuring timely harvests and better quality fruits.