Auxins-Plant Growth Harmone

AUXINS:

 Darwin performed some experiments on canary grass. In his experiments, when the coleoptile of grass was unilaterally illuminated, it curved towards the source of light. The experiments done by him confirms some interations  between the tip and light and light resulting in curvature of tip.

    Went removed the tip of avena coleoptile and placed it on agar-agar block for a measured length of time and it produces a positive curvature. He named this substance as Auxin.

Isolation of Auxin

Kosland Hargen-Smith isolated Auxin-A from human urine.
Kogl, Erxlaben and Hargen- smith isolated Auxin-B from corn germ oil. It was chemically identified as Indole-3 Acetic Acid(IAA).

Biosynthesis of Auxins

Thimann 1935 observed the fungus Rhizopus Suinus can convert the amino acid Tryptophan into IAA.

Transport:

Auxins occurs in the meristems. It is transported polarly and movement is basipetal.

Physiological effects:

  • Induces cell elongation
  • Plays an important role in callus formation.
  • Induces germination.
  • At very low concentrations promotes root growth and high concentration inhibits root growth.
  • High concentration also induces lateral buds to grow.
  • Synthetic auxins like NAA, 2,4-D etc., promotes flowering in pineapple.
  • Formula of Auxins is C18H32O5.
  • Induces Parthenocarpy.
  • Synthetic auxins increases the number of fruits.
  • Application of auxin increases number of female flowers in cucurbits.
  • Induces nodule formation.
  • IAA is commonly applied on tea plantation.
  • Promotes differentiation of shoots as well as root in tissue culture.
  • NAA is widely used as Rooting harmone.
  • Auxin receptor is ABP1.
  • 'Agent Orange' the leaf defoliator used by USA in Vietnam war was 2,4 -D and 2,4,5-T.
  • Auxin content decreases when leaf fall occurs on abcission layer.
  • 2,4-D used for eradication of weeds.
  • Prevents from abcission.
  • Brings about shortening of internodes
  • Growth of plant can be measured by Auxanometer.

Bio assay:

The use of living material for testing the effect of biologically active substances is described as bioassay.
  
  Avena curvature test and split per stem curvature test are examples.

 


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