Class 11 Biology Plant Growth Development Physiological effects of plant growth regulators

Physiological effects of plant growth regulators


  • IAA and indole butyric acid (IBA) have been isolated from plants and NAA (naphthalene acetic acid) and 2, 4-D (2, 4-dichlorophenoxyacetic) are synthetic auxins.
  • They help to initiate rooting in stem cuttings, promote flowering, prevent fruit and leaf drop at early stages, induce parthenocarpy (example- tomato).
  • In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance.
  • Auxins are widely used as herbicides.

Example- 2, 4-D is widely used to kill dicotyledonous weeds.


  • Gibberellins are denoted as GA1, GA2, GA3.
  • All gibberellins are acidic.
  • They help to increase in length of axis, elongate and improve shape of fruits, delay senescence,
  • Spraying juvenile conifers with GAs hastens the maturity period, thus leading to early seed production.
  • Gibberellins also promotes bolting, defined as internode elongation just prior to flowering, in beet, cabbages.


  • Cytokinins were discovered as kinetin from the autoclaved herring sperm DNA.
  • Natural cytokinins are synthesised in regions where rapid cell division occurs, for example, root apices.
  • Cytokinin helps to produce new leaves, chloroplasts in leaves, lateral shoot growth and adventitious shoot formation.
  • Cytokinins help overcome the apical dominance and promote nutrient mobilisation which helps in the delay of leaf senescence.


  • Ethylene is a simple gaseous plant growth regulator synthesized by tissues undergoing senescence and ripening fruits.
  • Influences of ethylene on plants include horizontal growth of seedlings, swelling of the axis and apical hook formation in dicot seedlings.
  • Ethylene promotes senescence and abscission of plant organs, fruit ripening, enhances the respiration rate during ripening of the fruits
  • The rise in rate of respiration is called respiratory climactic.
  • Ethylene breaks seed and bud dormancy, initiates germination in peanut seeds, sprouting of potato tubers, promotes rapid internode/petiole elongation in deep water rice plants.
  • Ethylene also promotes root growth and root hair formation, thus helping the plants to increase their absorption surface.
  • The most widely used compound as source of ethylene is ethephon, which hastens fruit ripening and accelerates abscission in flowers and fruits and promotes female flowers in cucumbers.

Abscisic acid

  • Abscisic acid (ABA) was discovered for its role in regulating abscission and dormancy.
  • ABA inhibits seed germination, stimulates the closure of stomata, promote seed development, maturation and dormancy.
  • ABA increases the tolerance of plants to various kinds of stresses, that is why it is also called the stress hormone.
  • ABA acts as an antagonist to gibberellins.


Fig. Apical dominance

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