Plant Growth Regulators

Plant Growth Regulators
Definition and Basic Characteristics

  • Plant growth regulators, also referred to as plant hormones, are organic substances that regulate various physiological processes in plants.

  • They are synthesized in one or multiple parts of the plant.

  • These hormones exhibit mobility, allowing them to move to other parts of the plant.

  • Transport Mechanisms:

    • Short distance: Through diffusion.

    • Long distance: Through mass flow.

  • Their functions are effective at low concentrations, and they can either promote or inhibit physiological responses in the plant.

Functions of Plant Growth Regulators

  • Plant growth regulators can significantly influence:

    • Germination

    • Growth

    • Flowering

    • Fruiting

    • Branching

    • Other physiological processes.

  • They can be synthetic or natural in origin.

  • The effects of these regulators depend on the amount and timing of application.

    • Example: The synthetic auxin NAA applied to apple trees during the post-bloom stage can chemically thin fruits. If applied later, it prevents preharvest fruit drop.

  • Plant hormones are synthesized from ubiquitous molecules, including amino acids and proteins.

  • They may be deactivated via several pathways, including conjugation and oxidation.

Major Groups of Plant Growth Regulators

  • Plant growth regulators can be categorized into five major groups:

    1. Auxins

    2. Cytokinins

    3. Gibberellins

    4. Ethylene

    5. Abscisic Acid (ABA)

Auxins

  • The primary natural auxin is indole-3-acetic acid (IAA).

  • Other natural auxins include indole-3-butyric acid (IBA).

  • Synthetic auxins include:

    • 1-Naphthyl acetic acid (NAA)

    • 2,4-Dichlorophenoxyacetic acid (2,4-D)

    • 2,4,5-trichlorophenoxyacetic acid (2,4,5-T)

  • Auxins are significantly synthesized in the apical meristem (e.g., actively growing shoots, growing leaves).

Roles of Auxins in Plant Growth

  • Facilitate shoot and root development.

  • Promote cell elongation.

  • Maintain apical dominance.

  • Promote tropisms in plants, such as:

    • Phototropism: Growth towards light.

    • Gravitropism/Geotropism: Growth in response to gravity.

  • Encourage root development.

Cytokinins

  • Cytokinins are plant hormones that promote cell division.

  • They are primarily synthesized in root tips; also found in other actively growing tissues and cambium.

  • Movement occurs primarily through the xylem, from root to shoot.

  • The most common naturally occurring cytokinin is Zeatin.

Roles of Cytokinins in Plant Growth

  • Promote cell division.

  • Regulate apical dominance.

  • Enhance lateral branching.

  • Improve seed germination.

  • Prevent senescence (aging) and abscission (dropping of parts).

  • Can enhance plant performance under stress conditions.

Gibberellins (Gibberellic Acid GA)

  • Over 60 gibberellins have been isolated from plants, with more than 50 found in seed plants.

  • Primarily synthesized in immature leaves, also present in young leaves, embryos, seeds, and roots.

Roles of Gibberellins in Plant Growth

  • Promote cell elongation.

  • Facilitate elongation of internodes (stem elongation).

  • Break seed dormancy.

  • Break bud dormancy.

  • Along with auxins, prevent the abscission of young fruits.

  • Improve seedless fruit set, e.g., grapes.

Parthenocarpy

  • Parthenocarpy denotes the development of fruits without fertilization.

  • This process leads to the formation of seedless fruits.

  • Seedless fruits may be smaller than regular fruits with seeds.

  • Examples: Banana, pineapple, citrus fruits.

Ethylene

  • Ethylene is a gaseous plant growth regulator.

  • The chemical Ethephon (trade name: Ethrel) releases ethylene when applied to plants.

  • Ethylene is mainly associated with processes of maturity and senescence.

Roles of Ethylene in Plant Growth

  • Stimulates flowering.

  • Breaks dormancy of buds and seeds.

  • Promotes fruit ripening.

  • Inhibits the growth of lateral buds.

  • Induces abscission of leaves, fruits, nuts, and flowers (formation of an abscission layer at the base of fruit stalks, leaf petioles, and more).

Abscisic Acid (ABA)

  • ABA is a natural growth inhibitor in plants and acts as a stress hormone.

    • For example, during water stress, ABA promotes the closing of plant stomata.

  • It works in opposition to auxins, gibberellins, and cytokinins.

Roles of Abscisic Acid in Plant Growth

  • Involved in fruit and bud senescence and abscission.

  • Maintains bud dormancy.

  • Controls seed dormancy.

  • Enhances stress tolerance.

Key Facts for Plant Hormones

  • Plant growth regulators are beneficial for plants that do not root normally from cuttings or layering.

  • Hormones like cytokinins and auxins are critical for successful micropropagation.

    • A high cytokinin:auxin ratio favors shoot initiation.

    • A high auxin:cytokinin ratio promotes root initiation.

  • Ethylene, acetylene, and NAA are responsible for flower initiation in plants.

  • Auxins and gibberellins promote fruit set in plants.

  • Cytokinins and gibberellins can break dormancy.

Additional Practical Applications

  • Fruit thinning: Auxins and ethylene are used.

  • Controlling plant size is essential for high-density planting.

  • Ripening: Ethylene is crucial.

  • Ethylene can induce abscission, facilitating mechanical harvesting.

  • Ethylene promotes female flower production in cucurbits over male flowers.

  • Application of gibberellins increases male flower production in cucurbits.