07 Plant Growth and Mineral Nutrition - part 09 - Growth Harmone - Auxins

07 Plant Growth and Mineral Nutrition - part 09 - Growth Harmone - Auxins


Auxins (Auxien = to grow): 
  • F. W. Went in 1931, used this term first. 
  • Auxin was isolated from urine of a personsuffering from Pellagra (Kogl and H. Smit 1931). 
  • In plants, it is synthesized in growing tips or meristematic regions of plants from where it is transported to other plant parts. 
  • The most common and important natural auxin is Indole-3-acetic acid (IAA). 
  • Tryptophan is the primary precursor of IAA in plants. 
  • It is the first hormone to be discovered in plants and is primarily responsible for cell elongation. 
  • It shows polar transport - Basipetal transport in stem. 
  • Now synthetic auxins like - 
  1.  IBA (Indole butric acid)
  2. NAA (Naphthalene acetic acid)
  3. 2, 4-D pichloro (Phenoxy acetic acid), etc. are used. 
Physiological effects and applications of auxin: 
  • The primary effect is cell enlargment. 
  • In most of the higher plants, growing apical bud inhibits the growth of lateral buds. This is called as apical dominance. 
  • Auxin stimulates growth of stem and root. 
  • Auxin induces multiplication of cells, hence used in tissue culture experiments to produce callus. 
  • It stimulates formation of lateral and adventitious roots. These are marketed as synthyetic herbicides. 
  • e.g. 2, 4-D (2,4 dichlorophenoxy acetic acid). 
  • It kills dicot weeds without affecting monocot crop plants. 
  • The seedless fruits like orange, lemon, grapes, banana etc. are produced by application of auxin (i.e. induced parthenocarpy). 
  • Auxins promote cell division in cambium and also cause early differentiation of xylem and phloem. 
  • It promotes root elongation in low concentration and shooting at higher concentration. 
  • It also hastens early rooting in propagation by `cutting'.
  •  Foliar spray of NAA and 2,4-D induces flowering in litchi and pineapple. 
  • Likewise, it prevents premature fruit drop inapples, pear and oranges, and also prevents formation of abscission layer. 
  • Auxins play a role in elongation of cell. It is known to increase rate of respiration.
  • Auxins break dormancy in seed and promote quick germination.

Do you know ? 
Agent orange : 
  • Mixture of two phenoxy herbicides in easter form. 
  • 2, 4-D and 2,4, 5-T (dioxin) is known as agent orange used in Vietnam war for defoliation of forests.

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