Plant hormones (also called phytohormones) are natural, chemical substances produced in small amounts that regulate plant growth, development, and responses to the environment.

1. Auxins

• Main Function: Promote cell elongation.
• Site of Production: Shoot tips, young leaves, seeds.
• Effects on Growth:
  • Stimulate cell elongation in stems.
  • Promote apical dominance (suppression of lateral buds).
  • Initiate root formation in cuttings.
  • Involved in phototropism (growth towards light) and geotropism (growth towards gravity).
  • Prevent premature fruit and leaf drop.

2.Gibberellins (GA)

• Main Function: Stimulate stem elongation.
• Site of Production: Young leaves, root tips, developing seeds.
• Effects on Growth:
  • Promote cell elongation and division in internodes → tall growth.
  • Break seed dormancy and stimulate germination.
  • Induce bolting (sudden stem elongation) in rosette plants like cabbage.
  • Promote flowering in long-day plants.
  • Increase fruit size (e.g., grapes).

3. Cytokinins

• Main Function: Promote cell division.
• Site of Production: Actively dividing tissues, especially root tips.
• Effects on Growth:
  • Stimulate cell division (cytokinesis).
  • Delay senescence (aging of leaves → used in tissue culture).
  • Promote lateral bud growth (antagonistic to auxins’ apical dominance).
  • Help in nutrient mobilization.

4. Abscisic Acid (ABA)

• Main Function: Growth inhibitor (stress hormone).
• Site of Production: Mature leaves, seeds, fruits.
• Effects on Growth:
  • Induces dormancy in seeds and buds.
  • Inhibits growth and cell division.
  • Promotes leaf fall and fruit abscission.
  • Causes stomatal closure during water stress (drought tolerance).

5. Ethylene

• Main Function: Gaseous hormone regulating ripening.
• Site of Production: Ripening fruits, nodes, senescing tissues.
• Effects on Growth:
  • Promotes fruit ripening (climacteric fruits like banana, mango, tomato).
  • Accelerates leaf and fruit abscission.
  • Stimulates flowering in pineapple and mango.
  • Causes epinasty (downward bending of leaves).
  • Breaks seed and bud dormancy.

Summary Table

Hormone	Major Action	Growth Effects
Auxins	Cell elongation	Tropisms, root initiation, apical dominance
Gibberellins	Stem elongation, germination	Bolting, fruit growth, seed germination
Cytokinins	Cell division	Delay senescence, promote lateral buds
ABA	Growth inhibitor	Dormancy, stomatal closure, abscission
Ethylene	Fruit ripening	Abscission, flowering, epinasty