Two Types of Auxins and Physiological Role of Auxins

• First plant hormone discovered was Auxin by Frits Went.
• Tryptophan is the precursor for auxin. 
• Auxin a- C₁₈ H₃₃ O₅ & Auxin b - C₁₈ H₃₀O₄ (IAA) exist in plants.
• IAA is the active principle of Auxin.

Types of Auxins: Natural auxins and synthetic Auxins

Natural Auxins	Synthetic Auxins
IAA(Indole3- Acetic Acid) Indole-3-acetaldehyde, Indole-3-pyruvic acid Indole-3-acetonitrile Indole-3-ethanol Phenyl acetic acid	Indole 3- Butyric Acid (IBA) Indole 3-propionic acid Indole Pyruvic acid NAA- Napthelene Acetic acid 2,4,D- 2,4, Dichlorophenoxy acetic acid Synthetic auxins are more stable.

Physiological role of Auxins:

1. Auxins induce elongation of plant cells, roots, buds, stems, petiole, mid rib and major            lateral veins of leaf.

2. It is responsible for apical dominance.

3. Some of the auxin derivatives stimulate seed germination.

4. Some auxins promote cambial activity.

5. Root formation is stimulated by some auxins.

6. Auxins have been used to break seed dormancy.

7. It is responsible for callus formation in tissue culture.

8. They initiate early flowering and fruiting.

9. IAA induces parthenocarpy.

10. Some auxins inhibit the leaf fall and fruit fall.

11. Auxins are used as weed killers.