Plant Hormones

Plant Hormones

  • In 1905, the British physician E. H. Starling introduced the term hormone
  • Plants are multicellular complex organisms, to grow themselves they require coordination between cells about their activities, cells must be able to communicate with each other. So the principal means of intercellular communication within plants are the hormones.
  • Hormones are molecular signals that individually or cooperatively direct the development of individual cells or carry information between cells and thus coordinate growth and development.
  • In simple words, Natural organic substances that regulate plant growth and development said to be Plant hormones or Phytohormones and the wide variety of hormones, including auxins, gibberellins (GA), abscisic acid (ABA), cytokinins (CK), ethylene (ET).
  • To grow a living being, each living organism increases in shape, size, volume, and weight. This important character of a living organism needs nutrition and the process is said to be growth.

Growth caused due to:

Internal forces.
External forces.

1. Increase in length
2. Cell division
3. Differentiation into different types of tissues.

The non-living material like stones and crystals of copper sulphate in its solution also increases in size but this growth always takes place by the deposition of particles of these substances through external forces.

Factors affecting the growth of the plants

External factors:

1. Light
2. Temperature
3. Oxygen
4. Carbon dioxide
5. Water
5. Food material

Internal factors: include growth hormones.

  1.  Auxins
  2. Gibberellins
  3. Cytokinins
  4. Abscisic acids
  5. Ethylene
  6. Morphactins

PLANT HORMONES are of the following types:

  1. AUXINS.


Auxin (fr. G. auxein, to increase) is the first quintessential plant hormone.  Auxin has a principal role —the enlargement of plant cells and synthesized in meristematic regions and other actively growing organs such as root tips, coleoptile apices, germinating seeds, and the apical buds of growing stems.

Sites of auxin synthesis – Young leaves, rapidly growing leaves, developing inflorescences, and embryos following pollination and fertilization.

Naturally occurring compounds in auxin activity are indole-3-acetic acid (IAA) and the indole derivatives are indole-3-ethanol, indole3-acetaldehyde, and indole-3-acetonitrile. However, these compounds all serve as precursors to IAA and their activity is due to conversion to IAA in the tissue.

The function of Auxins:

1. Cell elongation
2. Apical dominance
3. Root initiation
4. Prevention of abscision
5. Parthenocarpy
6. Respiration
7. Callus formation
8. Shortening of internodes
9. Eradication of weeds

10. Dormancy controller
11. To increase cambial activity
12. Overcoming sterility


The function of gibberellin:
1. Cell elongation
2. Bolting
3. Parthenocarpy

Role of Abscisic acid and Ethylene

Abscisic acid

  • ABA is a growth inhibitor.
  • It also reduces the growth induced by gibberellins.
  • Inhibits cell division and cell elongation.
  • Checks the synthesis of an enzyme (alpha) α-amylase which results inhibition of seed germination.
  • Helps in the abscission of flowers, leaves, and fruits by abscission layer.
  • Induces dormancy in seeds and buds, thus inhibiting the growth of buds and seed germination.
  • Helps in the formation of tubers – potatoes.
  • By closing stomata it helps in reducing transpiration rates.
  • It makes yellow to the leaves due to the spoiling of chlorophyll, proteins, and nucleic acids.

Roles of Ethylene

  • Regulates the growth of cell wall.
  • It promotes the synthesis of beta-1,3-glucanase.
  • Induces do novo-synthesis of peroxides.
  • Accelerates apical dominance.
  • Reduces flowering in plants except pineapple, it increases flowering.
  • Stimulate the formation of new roots.
  • Induces growth in underground stems and seed germination but after sprouting growth rate increases in leaves and stem.
  • Helps in the ripening of fruits like mango, banana, orange etc.
  • It also called it as ripening hormone.
  • Stimulates the formation of abscission layer in leaves, flowers and fruits.
  • Inhibits linear growth of stem and increases diameter.
  • Inhibits geotropism in stems.
  • Controls epinasty.
  • Female flower Increases due to this hormone.
  • Decrease the number of male flowers.

Differences between Auxins and Gibberellins

They are mostly found in higher plants.They are mostly found in fungi and few higher plants.
They are synthesized in the merestematic regions of the plants eg. in coleoptile, buds, growing tips of roots and leaves.They are synthesized in fungal mycelia, in immature bean seeds, in pea seedlings, hypocotyls , cereal endosperm.
Chemically they are indole 3-acetic acid (IAA) or its derivatives or chemicals with identical properties like IBA, NAA, IPA etc.Chemically they are gibberellic acid which are diterpenoid acids derived from the tetracyclic diterpenoid hydrocarbon.
They promote apical dominance.They show no effect on apical dominance.
They cause growth in darf pea stem sections, but show no effect on bolting and flowering.They cause growth of intact plants, but show no effect on its stem sections
Auxins don't affect seed germination and breaking of dormancy.. They promote seed germination and breaking of dormancy.
They promote bolting and flowering in non-vernalized biennials and long dry plants.
Auxins like 2, 4-D; 2,4,5-T and Dalapon etc. are used as weed killer.Gibberellins are not used as weed killers.
They don't cause de-novo synthesis of hydrolytic enzymes.They cause de-novo synthesis of hydrolytic enzymes.
Auxins in higher concentrations inhibit growth of roots but initiate formation of new roots.Gibberellins do not effect growth of roots.
Auxins promote cell elongation, cell- division and cambial activity.They promote cell elongation by synthesizing enzymes and checking the effect of inhibitors..