Plant hormones

Plant hormones a.k.a Plant growth Regulators (PGR’s)

Hormone is an endogenous (produced within the organism of a body) compound usually synthesized in one part and transported to another part where it exerts a physiological effect in lower concentrations.

Unlike animals each plant cell has a capacity to produce different hormones.

What are PGR’s??

PGR’ s are organic compounds which are naturally synthesized by  plants in very small concentrations.

These can also be synthesized artificially!

They promote, inhibit or modify any physiological process of a plant.

Plant hormones also referred as phyto hormones and are called as ‘signaling molecules’

These hormones are produced in the plants in extremely lower concentrations

These hormones control all the aspects of growth and development in plants. Ex: Plant defense mechanism, stress tolerance etc

Basically these plant hormones are not nutrients but chemicals!!

These plant hormones promote the plant growth, development and differentiation of cells and tissues

These hormones usually synthesize in one part and differ with in the plant tissues

As it is said above there are different types of PGR’s like growth promoters, inhibitors or modifiers.

We have five major classes of PGR’s!!

Under growth promoters there are

  1. Auxins,
  2. Gibberlins
  3. Cytokinins
  4. Ethylene
  5. Abscissic acid


-Auxin helps in cell elongation of both root and shoots.

– In plants, auxin (IAA) is synthesized in growing tips.

How auxin plays a role in plant system

  • This will kill weeds (selective herbicide) when applied at higher doses (Example-2,4-D)
  • Used in tissue culture (IAA)
  • Induce rooting in plants where cuttings are used instead of seeds (Example-IBA)
  • Increase number of female flowers and inhibit male flower production in monoecious vegetables
  • Induction of parthenocarpy (seedlessness) in fruits(IAA)
  • Induces flowering and enhance fruit set
  • Controls immature falling of fruits from plants (Pre-harvest fruit drop) and increase yield

2. Gibberlins

Gibberlins are synthesized in young tissues of the shoot and also the developing seed

Role of gibberlins

  • Substitutes long day requirement of plants and help long day plants to flower in short days
  • Sex Expression – Promote maleness at higher concentration (1500ppm)


Kinetin or Cytokinin name because of its ability to promote cytokinesis(cell division)

Cytokinin is generally found in higher concentrations in growing tissues such as roots, embryos, and fruits, where cell division is occurring.

Role of cytokinin in plants

  • Stimulate cell division and cell enlargement
  • Delay leaf senescence (Aging)
  • Break dormancy of seeds and buds
  • Flower induction (In short day palnts)

Application of Cytokinins

  • Prevention of premature senescence
  • Used in Tissue culture


Ethylene is called as Ripening Harmone

Used in various fruit ripening process

Role of Ethylene

  • Induces early flowering
  • Induces formation of abscission layer
  • Causes inhibition of root growth
  • Stimulates the formation of adventitious roots
  • Stimulates fading of flowers

5.Abscissic acid (ABA)

This hormone helps plants in adapting to stress conditions (ex. Release of this hormone in the plant system will helps in closure of ‘Stomata’, through which loss of water will takes place. When these stomata are closed, there will no loss/reduced water loss, hence plant can able to adopt and grow in water stress conditions

-This ABA also helps in seed and bud dormancy (Dormancy is a state where plant is alive but there is no active growth)

Role of ABA

  • Accelerates leaf and fruit abscission and senescence
  • Induces bud and seed dormancy due to accumulation of ABA
  • Stomatal Regulation- Moisture stressed plants produce ABA which facilitates stomata closure and help in maintaining turgidity.

Application- it is used as Anti-transpirant i.e to avoid transpiration rate from plants

Here comes the list of growth retardants

  1. AMO-1618
  2. Phosphon-D
  3. Cycocel
  4. MH
  5. B-995

How to apply these PGR’s

The many methods employed are :

  • Spraying method,
  • Injection of solutions into internal tissues,
  • Root feeding method,
  • Application of powder mixture to the bases of cutting,
  • Dipping of the cuttings in PGR solution,
  • Soaking in dilute aqueous solution.


  • For Cucurbit plants one needs to provide foliar spray at 2-4 leaf stage.


Thus , PGR’s enable a better understanding about what can be best done to ensure that the plant growth and yield can be optimised.

Happy Growing!



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