🌱 Plant Growth Regulators (PGRs) - Summary Notes

Plant Growth Regulators (PGRs), or plant hormones, are chemical messengers that regulate growth and metabolism in plant cells. The five major groups are Auxins, Gibberellins, Cytokinins, Abscisic acid, and Ethylene.

1. Auxins (e.g., Indole Acetic Acid - IAA)

Where produced: Stem apex and developing parts.
Key Functions:
Promote cell enlargement in stems.
Promote cell division in cambium.
Cause apical dominance (inhibit lateral bud growth).
Promote fruit growth and can induce parthenocarpy (fruit development without fertilization).
Inhibit abscission (shedding) and delay leaf senescence in some species.
Root effect: Promote root growth at very low concentrations, but inhibit growth at higher concentrations (e.g., in geotropism).

2. Gibberellins (GAs)

Where produced: Apical portions of roots and shoots.
Key Functions:
Promote cell enlargement (with auxins) and cell division.
Break bud and seed dormancy.
Promote "bolting" (rapid stem elongation) in rosette plants.
Promote leaf and fruit growth; may induce parthenocarpy.
Promote flowering in long-day plants and inhibit it in short-day plants.
Delay leaf senescence.
3. Cytokinins
Where produced: Roots, young fruits, and seeds.
Key Functions:
Promote cytokinesis (cell division) and increase DNA/RNA/protein synthesis rates.
Promote lateral bud growth (breaking apical dominance).
Promote lateral root growth but inhibit primary root growth.
Promote stem growth via cell division in meristem and cambium.
Delay leaf senescence (aging).
Promote stomatal opening.