🌿 Plant Growth Regulators & Development

Plant Growth Regulators (PGRs) / Phytohormones are low-molecular-weight chemical signaling molecules that modulate cell division, elongation, tissue differentiation, senescence, dormancy, and environmental stress responses.

1. 🧪 Phytohormone Comparative Matrix

                          PHYTOHORMONE DUAL CLASSIFICATION
                                         │
         ┌───────────────────────────────┴───────────────────────────────┐
         ▼                                                               ▼
   [PLANT GROWTH PROMOTERS]                                        [PLANT GROWTH INHIBITORS]
   ├── Auxins (IAA, NAA, 2,4-D)                                    ├── Abscisic Acid (ABA: Stress Hormone)
   ├── Gibberellins (GA3)                                          └── Ethylene (Gaseous: Fruit Ripening / Senescence)
   └── Cytokinins (Zeatin, Kinetin)

Phytohormone	Precursor & Bioassay	Primary Physiological Functions & Agricultural Uses
Auxins (IAA, IBA, NAA, 2,4-D)	Tryptophan; Avena curvature test (Went)	Apical Dominance (inhibits lateral buds); xylem differentiation; parthenocarpy in tomatoes; root initiation in stem cuttings; 2,4-D used as selective weedicide (kills broad-leaved dicot weeds without harming monocot crops).
Gibberellins ( $G A_{3}$ )	ent-Kaurene (Terpenoids); Barley endosperm $α$ -amylase test	Bolting (internode elongation prior to flowering in rosette plants like cabbage/beet); increases sugarcane stalk length; delays senescence; malt production in brewing industry.
Cytokinins (Kinetin, Zeatin)	Adenine (Purine derivative); Tobacco pith callus test	Promotes cell division; overcomes apical dominance; delays leaf senescence (Richmond-Lang effect); stimulates nutrient mobilization. High Cytokinin:Auxin ratio triggers shoot formation in tissue culture.
Ethylene ( $C_{2} H_{4}$ )	Methionine; Triple response bioassay	Gaseous hormone; stimulates fruit ripening (respiratory climacteric); promotes abscission and senescence; triggers horizontal growth of seedlings; induces femaleness in cucumbers; used as Ethephon.
Abscisic Acid (ABA)	Carotenoid degradation; Stomatal closure test	Stress Hormone: triggers rapid stomatal closure during drought; promotes seed dormancy and inhibits germination (antagonistic to Gibberellins).

2. ☀️ Photoperiodism & Vernalization

                          PHOTOPERIODIC RESPONSES
                                     │
         ┌───────────────────────────┼───────────────────────────┐
         ▼                           ▼                           ▼
   [SHORT-DAY PLANTS (SDP)]    [LONG-DAY PLANTS (LDP)]     [DAY-NEUTRAL PLANTS (DNP)]
   ├── Require dark > Critical ├── Require light > Critical├── Flowering independent of photoperiod
   └── Tobacco, Chrysanthemum  └── Wheat, Radish, Spinach  └── Tomato, Sunflower, Maize

Photoperiodism: Response of plants to relative day/night lengths perceived by Phytochrome pigments in leaves. The hypothetical flowering stimulus is termed Florigen (migrates from leaves to shoot apex).
Vernalization: Promotion of flowering by a period of low temperature chilling ( $0^{\circ} C - 5^{\circ} C$ ); prevents precocious reproductive development late in the growing season.