Plant Hormones | General Awareness for SSC CGL PDF Download

Functions of Plant Hormones

Plant hormones play crucial roles in regulating various aspects of plant growth and development, including cell division, enlargement, flowering, seed formation, dormancy, and abscission. They are generally classified into two types based on their actions: Plant Growth Promoters and Plant Growth Inhibitors.

Auxin Hormone

Derived from the Greek word for "to grow," auxins are widely utilized in agriculture and horticulture. These hormones are synthesized in the apices of roots and stems and then move to other parts of the plant to exert their effects.

• Natural Auxins: Indole-3-acetic acid (IAA), Indole butyric acid (IBA)
  Synthetic Auxins: 2,4-D (2,4-Dichlorophenoxyacetic acid), NAA (Naphthalene acetic acid)

Functions of Auxin:

• Promotes stem and root elongation.
• Induces apical dominance by suppressing lateral bud growth through IAA in the apical bud.
• Triggers parthenocarpy, enabling fruit development without fertilization, as seen in tomatoes.
• Prevents premature leaf, flower, and fruit drop.
• Aids in rooting for stem cuttings and grafting.
• Stimulates flowering in certain plants like pineapples.
• Used as a herbicide (e.g., 2,4-D) to eliminate dicot weeds without affecting monocots.
• Facilitates cell division and xylem differentiation.

Gibberellins Hormone

Over 100 gibberellins (GA1, GA2, GA3, etc.) are known, and they are found in higher plants and fungi.

Functions of Gibberellins:

• Promotes bolting (sudden elongation of internodes before flowering) in rosette plants like cabbage and beet.
• Delays plant senescence (aging).
• Induces parthenocarpy.
• Promotes stem elongation and reverses dwarfism.
• Encourages the development of male characteristics in certain plants like cannabis.
• Stimulates the formation of hydrolytic enzymes in the endosperm of germinating cereal grains and barley seeds.
• Breaks seed dormancy.

Cytokinins Hormone

Essential for cytokinesis, cytokinins are naturally synthesized in rapidly dividing plant parts, such as root apices, shoot buds, and young fruits, and move in a basipetal and polar manner.

• Natural Cytokinins: Zeatin (found in corn kernels and coconut milk), isopentenyladenine
• Synthetic Cytokinins: Kinetin, benzyladenine, diphenylurea, thidiazuron

Functions of Cytokinins:

• Promotes lateral and adventitious shoot growth, useful in culture initiation.
• Counters apical dominance induced by auxins.
• Stimulates chloroplast formation in leaves.
• Mobilizes nutrients and delays leaf senescence.

Abscisic Acid

Abscisic acid inhibits growth and acts antagonistically to gibberellins. Often referred to as the "stress hormone," it enhances plant tolerance to stress.

Functions of Abscisic Acid:

• Induces leaf and fruit abscission.
• Inhibits seed germination.
• Promotes leaf senescence.
• Accelerates seed dormancy, beneficial for storage.
• Stimulates stomatal closure to reduce water loss through transpiration under water stress.

Ethylene Hormone

Ethylene, a gaseous hormone, can act as both a growth promoter and inhibitor. It is synthesized in ripening fruits and senescing tissues and regulates numerous physiological processes.

Functions of Ethylene:

• Accelerates fruit ripening.
• Controls epinasty (downward bending of leaves).
• Breaks seed and bud dormancy.
• Stimulates rapid elongation of petioles and internodes.
• Promotes aging and shedding of leaves and flowers.
• Induces root growth and root hair formation, increasing the absorption surface.
• Encourages female characteristics in monoecious plants.
• Promotes the formation of the apical hook in dicot seedlings.

In addition to these five primary hormones, other hormones like brassinosteroids, salicylates, jasmonates, and strigolactones also influence plant physiological processes.