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There are five general classes of hormone they are auxins, cytokinins, gibberellins, abscisic acid and ethylene.

  1. Auxin: The auxin, Indole, 3-acetic acid is the principal natural auxin and its main function is to promote the growth. The auxins are synthesized in the meristems and enlarging tissues, in the shoot apex, leaf primordia etc. it migrate to the regions of elongation. The auxins which moves from the apex of the stem to the downwards to the root and also from the root upward to the shoot. Auxin alone or in combination with the other hormones promotes different hormonal changes. The main functions of auxins are promotes cell division in the vascular cambium, cell elongation and also root initiation so known as the root hormone.
  2. Cytokinin: Cytokinin is also called kinetin. The main sites of synthesis are the endosperm, embryos of seeds, developing fruits and root. The most naturally occurring cytokinin is Zeatin which is isolated from the maize grains. Cytokinins in association with the auxins promote cell division.
  3. Gibberellins: The gibberellins are chemically gibberellic acids. The main sites of gibberellin productions are embryos, roots and young leaves etc. It is used to induce bolting in rosette plants.
  4. Ethylene: It is the gaseous phyto hormone which is present in the air as a component of smoke. Ripening fruits, flowers and leaves are major sites for ethylene synthesis. It is used as a natural product of ripening fruits. The presence of high concentration of auxins stimulates ethylene production. It is a growth inhibiting phyto hormone.
  5. Abscisic acid: Abscisic acid is a naturally occurring growth inhibitor which is isolated from different parts of higher plants. The ABA is synthesized in leaves and is translocated into stem tips through the phloem. Abscisic acid will interact with the other growth regulators. ABA can inhibit the gibberellin stimulated growth in plants, so it is called anti-gibberellin.
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