Plant Nutrition - Plant Growth Regulators

By TamilNadu Agricultural University on 29 Dec 2018 | read


Plant growth regulators may be defined as any organic compounds, which are active at low concentrations (1-10 ng / nl) in promoting, inhibiting or modifying growth and development. The naturally occurring (endogenous) growth substances are commonly known as plant hormones, while the synthetic ones are called growth regulator.

Plant hormone is an organic compound synthesised in one part of the plant and translocated to another part, where in very low concentrations it causes a physiological response. The plant hormones are identified as promoters (auxins, gibberellin and cytokinin), inhibitors (abscissic acid and ethylene) and other hypothetical growth substance (florigen, flowering hormone, etc.,)


1. Auxins
2. Gibberellins
3. Cytokinenins
4. Etylene
5. Dormins  {Abscissic Acid (ABA), Phaseic Acid}
6. Flowering Hormones {Florigin, Anthesin, Vernalin}
7. Phenolic Substances {Coumarin}
8. Miscellaneous Natural Substances {Vitamins, Phytochrome Tranmatic Substances}
9. Synthetic Growth Retardants {Ccc, Amo, 1618, Phosphin - D, Morphacting, Malformis}
10. Miscellaneous Synthetic Substances {Synthetic Auxins, Synthetic Cytokinins}


  • Auxin is a greek word  which means to increase.
  • It is a generic term for chemicals that typically stimulate cell elongation, but auxins also influence a wide range of growth and development response.
  • The chemical isolations and characterization was done by Kogi et al. (1934).
  • Auxins are the first identified hormones of which IAA seems to be the major naturally  occurring, endogenous auxin  in  crops.
  • Besides IAA, plants contain three other compounds which are structurally similar and elicit many of the same response as that of IAA, 4, chloroindole acetic acid (CIAA),phenylacetic acid (PAA), indole butyric acid (IBA).
  • Synthetic compounds are classified into five major categories; indole acids, naphthalene acids, chlorophenoxy acids, benzoic acid and picolinic acid derivatives.

Role of Auxin

1Cell divisions and enlargement Eg. cambial growth in diameter.IAA + GA
2Tissue cultureShoot multiplications (IBA and BAP),callus Growth (2,4,-D), root multiplication IAA and IBA (1-2 mg)
3Breaking dormancy and Apical dominanceNAA
4Shortening internodeApple trees (NAA) (dwarf branch-fruit)
5Rooting of cuttings(10-1000 ppm - NAA, IAA, phenyl acetic acid)
6Prevent lodgingNAA- develop woody and erect stem
7Prevent abscissionPremature leaf, fruit, flower fall 
(NAA, IAA and 2,4-D)
8Parthenocarpic fruitGrapes, banana, orange - (IAA)
9Flower initiationsPine apple -uniform flowering - fruit ripening (NAA). Delay flowering (2,4-D)
10Weed eradications2,4,D and  auxin compounds


Image titlePreventing fruit abscission

Image titleTissue culture: Callus growth and Shoot multiplication


  • It is the active substance isolated from the soil borne fungus Gibberella fujikuroi the concentration of GA 3 is usually highest in mature seeds.
  • Reaching up to 18 mg / kg fresh weight in Phaseolus species, but it decreases rapidly as the seed mature.
  • In general, roots contain higher amounts of GA3  than shoots.
  • Gibberellins have also been found effective in overcoming both kinds of dormancy, buds as wells as seeds.
  • Treatments with Gibberllins has been observed and it substitute effectively for long day, low temperature or red light exposure requirements.

1. Gibberellic acid


Synthesis in leaf and induce shoot elongation  (IAA + GA3)

2. Enhance metabolic activity

Mobilisation of reserve food material promotes growth and height, increase root actively and Kinetin production in root - translocate to growing Bud (GA3).


3. Shoot elongation

GA3  spray increase height of nursery seedlings


4. Delay senescence               

Increase photosynthesis and proteins synthesis and thereby decrease abscission


5.Increase cambial growth and differentiation 

Induce flower and fruit set (IAA + GA3)


6.Dwarf plant (genetically) to    normal height



7.Promote flowering in long day 

Substitute for long day conditions and cold treatment (vernalizations)


8. Induction of parthenocarpy

 eg. Grapes


9. Breaking dormancy and leaf 


Image titleSeedling: shoot elonagation

Image titlePromote flowering

Image titleInduction of parthenocarphy


  • Neljubow (1901) is credited with having identified the active growth-regulating component of the illuminating gas as ethylene.
  • Ethylene is formed naturally in plants in amounts sufficient to bring about regulatorly effects and it might be considered as plant hormone.
  • Ethylene may be active in alleviation of secondary dormancy also (Ross,1984).
  • Synthetic chemical known as Etherel, Ethephon, Chloroethyl phosphonic acid (CEPA) has been reported to release ethylene when applied on plants.

Role of ethylene

  • Breaking dormancy                                    
  • Induce ripening of fruits
  • Induce abscission of leaves            
  • Inhibit elongation and lateral bud growth


Image title

Fruit ripenning in Banana


  • First endogenous cytokinin was isolated from maize kernels named as zeatin.
  • Germinating seeds, roots, sap streams, developing fruits and tumor tissue are rich in cytokinins.
  • Cytokinins imbibed seeds germinate better in dark than unimbibed lettuce seeds.
  • Similarly cytokinin together with gibberllin effectively breaks the photodormancy of celery (Apium graveolens) seeds.
  • Synthetic cytokinins are kinetin, benzyladenine and ethoxy ethladenine.

Role of cytokinin

  • Cell divisions, elongation and enlargement       
  • Induction of flowering, fruit development    
  • Apical dominance-overcoming                              
  • Delay senescence   
  • Tissue culture morphogenesis
  • Parthenocarpy
  • Breaking dormancy            
  • Improves N2 metabolism

Image titleCell division

Image titleAuxillary growth

Image titleDelaying of senescence


Growth retardants

  • The term growth retarding or growth retardant is that the chemical slows cell division and cell elongation of shoot tissue and regulate plant height physiologically without formative effects.          
  •   Eg. AMO 1618, phosphon-D, CCC, MC and Alar. These do not occur naturally in plants and acts in retardation of stem elongation, preventing cell division.


Suppress the growth of plants. There are phenolic inhibitors and synthetic inhibitors and Abscissic acid (ABA). Benzoic acid, Salicylic acid, Coumarin and Chlorogenic acid are examples for phenolic inhibitors, while malichydrazide (MH) and Triiodo benzoic acid (TIBA) are examples of synthetic inhibitors. Inhibitor from young leaves of Betula spp. prevents the growth of apical buds eg. ABA and Dormin.

Abscisic Acid (ABA)

  • To stop elongation
  • Induce dormancy
  • Delay germination
  • Inhibit growth process

Role of inhibitors

  • Accumulation leads to induce dormancy
  • Regulation of flowering, senescence and tuber formation
  • Induction of cold hardiness
  • Cause abscission and dehiscence of fruits
  • ABA application increase GA (Gibberellic acid) levels and may cause increase in growth
  • Suppress the formation of a amylase in the barley endosperm
  • Interfere with DNA and RNA synthesis
  • Modify the nucleic acid and protein synthesis systems.
  • Phenolic compounds inhibit stem and root growth
  • Phenolics affect almost all the metabolic system
  • Control Gibberellin -  stimulated growth

Methods of application

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