• Term ‘auxein’ means to grow.
  • Weak organic acid
  • Found in the root & shoot tip.
  • Shows polar movements: i.e., shoot tip downwards & root tip upwards.
  • Charles Darwin & his son Francis Darwin: 1st reported the presence of auxin in the coleoptile of canary grass (Phalaris). They showed that the coleoptile started growing towards unilateral illumination (phototropism), while the decapitated coleoptile did not.

  •  F W Went: Performed the Agar block experiment in Avena coleoptile (oat). He decapitated the tip of Avena coleoptile and kept it on an agar block, he later replaced the tip with agar block, the coleoptile started growing towards the light.
  • Went named it as ‘Auxin’.


  • Kogl & Haagen-smith: Isolated it from human urine.
  • Natural auxins:

    Indole 3-acetic acid (IAA) & Indole butyric acid (IBA).

  • Synthetic auxins:

    2,4-dichloro phenoxy acetic acid (2,4-D), Naphthalene acetic acid (NAA) & IBA.

  • Precursor of auxin: Tryptophan.
  • Metal used in synthesis: Zinc.
  • Site of synthesis: young leaves, flowers, fruits, shoot tips, embryos & pollens.

Physiological effects:

  • Causes growth by cell enlargement:

By increasing elasticity of cell wall, increasing permeability of cell membrane & increasing the rate of respiration.

  • It has no effect on root growth, but can produce new roots:

Used in tissue culture and in plant propagation during grafting & in stem cuttings.

  • Increases cambial activity by inducing cell division in the cambium:

Used in tissue culture.

  • Apical dominance: Apical bud prevents the growth of lateral buds this phenomenon is called as Apical dominance. Auxin synthesized in the apical bud prevents the growth of lateral buds. As the apical bud is removed, the lateral buds start growing giving the plant a bushy appearance. It is known as pruning.

Used is tea plantation & in hedge making. Also, used to prevent sprouting of potato tubers kept in storage.

  • Prevents abscission of young leaves & fruits by preventing the formation of abscission layer.

Used to prevent falling of fruits in apple, orange & tomato.

  • Normally inhibits flowering but it promotes flowering in pineapple & litchi & delays flowering in Lettuce.
  • Parthenocarpy: by treating unfertilized ovary with auxin seedless fruits are formed. Application of IAA to pistil induces parthenocarpy. E.g., tomato, banana, cucumber.
  • Eradication of weeds (weedicide): Auxin is sensitive to dicots (weeds) & has no effect on mature monocots. In a crop of cereals where weeds are growing when 2,4-D is sprayed, it kills the weeds without affecting the cereals.
  • Controls xylem differentiation.



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