Plant Growth Regulators

Plant Growth Regulators

Certain substances affect the growth quite miraculously. These were referred to Hormones. Hormone means to urge or to stimulate (Greek word).

Hormones of plants are referred as phytoHormones. Phyto Hormones are organic substances which are naturally produced in plants, control the growth or other physiological functions, at a sight remote from its place of production and active in extreme minute quantities.(Plant Growth Regulators)

Growth Hormones can also be defined as substances which are naturally produced in plants, control the growth are other physiological functions, at a sight remote form its place of production and active in extreme minute quantities.

Growth Hormones can also be defined as substances synthesized in particular cells and are transferred to other cells where in extremely small quantities influence development process.

Common Hormones –

  • Auxins
  • Cytokinin
  • Dormin
  • Gibberellins
  • Ethylene
  • Florigen


  • Characteristics Features Polar translocation
  • Apical dominance
  • Variable Behaviour or root and shoot growth
  • Root Initiation(Plant Growth Regulators)
  • Delay in abscision and differentiation of xylem elements. 

Role of Auxins –

  • Apical Dominance:Removal of apical bud stimulates lateral buds. Auxins inhibit lateral bud formation since they are synthesised in apex. This phenomena is called apical dominance. Eg: Potato tubers for apical buds forming.
  • Cell Division And Elongation: Shoot and Root growth.
  • Xylem Differentiation: Auxins helps in establishing contact between vascular tissues of the callus and that of the bud and makes it possible for the bud to grow properly in callus. By adding auxin and sugar continued growth of callus may be obtained and new shoots and even new plant can be produced.
  • Nucleic Acid Activities of IAA increases total RNA – synthesised specific enzymes lead to cell enlargement.(Plant Growth Regulators)
  • Manifold Activities Play specific role in seed germination, growth, rooting, flowering (Reproductive phase), abscission, parthenocarpy and tissue culture.

Practical Applications of Auxins-

  • Germination: IAA, IBA, NBA, 2,4-D are mostly used in soaking seed for germination- at low concentrations promotes germination but these effects are subjected to variation depending on form and species of plants.
  • Root: NAA, 10% induces 100% rooting in mango: Dashri, langra IBA+SUGAR application leads to greater number of roots-structure of roots also changed (Vascular bundles).(Plant Growth Regulators)
  • Flowering: Play floragenic role in day neutral plants IAA promotes formation of female flowers. Increased spikelet number, leaf number and weight and number of grains in wheat. NAA & IAA increases boll-set (G.hirsutum) induced more pine-apple. Fruit weight increases.
  • Parthenocarpy: IBA, NAA produces seed less/fruits – smaller sized fruits, but more in number, hence yield not affected.
  • Fruit setting: By using 2,4,5 T fruit setting and yield of ber/fruit increased. IAA, IBA, and NAA induce high percentage fruit set.
  • Prevention of pre-mature drop of fruits: 2,4,D,IAA,IBA, 2,4,5-T, are used to prevent preharvest drop of sweet oranges( 100 to 500 ppm)
  • Tissue and Organ culture: IAA & Kinetin
  • Auxins as inhibitors: High concentration of auxins inhibit the growth and exert toxic effect on plants. In normal case, self produced auxins inhibit the growth and development of lateral buds, and as a result apical buds, remains dormant.(Plant Growth Regulators)

Auxins use in Agriculture and Horticulture-

  • Propagation of plants by hormone treatment of cuttings
  • Prevention of pre harvest drops of plants.
  • Increasing parthenocarpy.
  • Increasing fruit set.
  • Prevention of sprouting by inhibiting buds.
  • Inhibition of prolonged dormancy.
  • Control of flowering.
  • Defoliation of plants
  • Prevention of leaf fall or abscission.
  • Thinning of compact fruits.
  • Selective weed killer.


  • Second important growth Hormone. More than 60 types of Gibberellins are known. They are named as GA1, GA2—— upto GA60. About 51 types are found in higher plants.
  • Mechanism of Gibberellins: GA exerts its physiological effect on altering the Auxin status of tissue. It acts at the gene level to cause depressions of specific gene.(Plant Growth Regulators)
  • The activated genes by producing new enzymes, bring about observed morphologic changes. Alerts the RNA. GA appears to involve in alteration of nucleic acid directed protein synthesis in some long term regulatory action and some other types of activation phenomena in short term regulatory action.

Role of Endogenous Gibberellins –

  • Apical bud dormancy
  • Role in sub apical meristem
  • Cell elongation
  • Fruit growth
  • Flowering
  • Metabolisation of food in seed storage cells.

Practical Applications of Gibberellins –

  • Germination: Increases length of hypocotil and cotyledenary leaf area.
  • Root Growth: Inhibits root growth(Plant Growth Regulators)
  • Leaf Expansion: Leaves become broader and enlarged (Cabbage, Sweet corn).
  • Hyponesty of leaves: GA treated leaves of chrysanthemum plants holds their leaves more erect.
  • Flowering: Induces flowering in long day plants and in plants requiring cold induction. Also promotes formation of male flowers.
  • Parthenocorpy: Brinjal, Guava(Alahabad round). Thomson seedless
  • Fruit setting: Increased fruit setting (Phalsa, Sweet lime, Grapes).
  • Fruit Drop: Not much effective.
  • Stem elongation: Chorchorus capsularis: extention of stem and increased number of internodes. However leaf area, basal diameter of stem and fibre quality are reduced.
  • Pollen Germination: Sugar cane 15 out of 34 germinated against normal conditions.
  • Breaking Dormancy: In temperate plants buds become dormant in later summer and do not sprout even when exposed to sufficient moisture, temperature and oxygen. They require low temperatures or long days or red light. GA overcomes this dormancy. Enhanced cell elongation push through the endosperm (seed coat) Potato tubers can be made to sprout in winter by GA.(Plant Growth Regulators)
  • Other uses: Sprayed on Fruits to prevent rind disorder. Thomson seedless grape bunches if sprayed with GA, causes elongation of bunch, so they are less tightly packed and less succesptible to fungi.


Characters –

  • Initiation of cell division 
  • Delay of senescence
  • Use in tissue culture
  • Counteract apical dominence.

Action and application –

  • Cell division
  • Cell enlargement
  • Morphogenesis
  • Dormancy
  • Apical dominance
  • Mobility: Immobile obstructs the movement of amino acid, phosphate and various other substances
  • Nucleic acid metobolism: Quick increase in the amount of RNA and decreases DNA
  • Protein synthesis: Increases DNA
  • Protein synthesis: Increased rate
  • Florigens: Induction of flowering in short day plants.
  • Abscisic Acid (ABA)

Role of ABA –

  • Induces bud dormancy
  • Promotes senescence
  • Accelerates leaf abscission in cotton plant
  • Induces flowering during long days in certain short day plants – ineffective in short day plants.(Plant Growth Regulators)
  • Counteracts GA


Highly useful in inducing fruit ripening. Ethylene is a natural product of ripening fruit. Ethylene is a gas at temperatures under which a plant can live.

Auxins increase ethylene level in plants and auxin actions are attributed through ethylene such as increased percentage of female flowers, apical bud dominance and leaf epinasty.

Role of Ethylene –

  • Abscission: Principle accelerator of abscission – Capable of promoting changes associated with pre – abscission and aging of leaves, petioles, flowers and fruits. Ethylene degreening is a commercial practice (5-10 ppm).
  • Degreening occurs after ethylene treated are exposed to air – accelerates maturity and induces uniform ripening (Pine apple).


It is responsible for flower initiation in plants – synthesised in older leaves and then transferred to growing region, where it innitiates floral bud.


They are synthetic growth regulators, acts in variety of ways on the natural regulation of mechanisms of plants.(Plant Growth Regulators)

The important are:

  • Phenoxyalkancarboxylic acid (synthetic auxin)
  • Substituted benzoic acid.
  • Maleic acid hydrazide
  • Flurene – carboxylic acid and their derevatives
  • Chlorflurenol
  • Chlorfluron
  • Flurenol
  • Methyle benzilate
  • Dichlorflurenol etc.
  • The action of these substances are systemic and after their uptake they are transported and distributed not polarly, but basipetally and acropetally.

Role of Morphactins –

  • Seed Germination – inhibition
  • Growth of seedlings – inhibits growth of both root and shoot this property is similar to cytokinin(Plant Growth Regulators)
  • Stem elongation: dwarfing effect.
  • Apical dominance and branching : treatment to grasses and cereals increased tillering and also increased no of laterals. Stimulates extension of lateral shoot growth.
  • Prolonged bud dormancy : Root growth and root branching: Lateral roots are inhibited and primary roots are promoted. The action of morphactin on the longitudinal growth of root system may be considered as a reverse of their action on the shoot system.
  • Flowering: Prevents flowering in short day plants, sequence of flowering, position and no. of flowers and parthenocarpy etc.

Wound Hormone

Hormone activity increases in response to wound or injury. A wound hormone called traumatic acid has been extracted, which stimulated meristematic of cork, cambium to cover wounded or injured portions.

Some Other Growth Regulators –

  • Triacontanol: Saturated primary alcohol isolated from shoots of alfaalfa. Response is very rapid in increasing growth. 20% enhanced growth in rice and maize.
  • Brassins: Steroid growth promoters isolated from pollen grain of rape causes distinct growth promoting activity.
  • Xanthoxin: Potent growth inhibitor can be converted metabolically to ABA
  • Batasins: Isolated from yam plants -causes dormancy in bulbs.
  • Vitamins: Organic compounds – occur in several plants, substances inhibit growth of certain parts of plants.(Plant Growth Regulators)
  • Vitamins: Organic compounds – Occur in natural food stuffs either as such are as utilisable precursors which are required in minute amounts for normal growth, maintenance and reproduction. They have catalytic and regulatory functions in cell metabolism.

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