An Overview of Plant Growth Regulators - Plant Hormones (Phytohormones) : Auxins, Gibberllins, Cytokinins, Ethylene and ABA

Plant hormones or Phytohormones may be defined as an "an organic substance produced naturally in plants which control growth and other physiological functions at a site away from its place of synthesis". Thimann (1948) proposed the term phytohormones. As these hormones are synthesis by plants, they are also called phytohormones.

An Overview of Plant Hormones (Phytohormones)

These are certain synthetic chemicals that are similar to phytohormones in structure and function. All these substances both synthetic and natural phytohormones which regulate the physiological activity of plants are called plant growth regulators. The different growth regulators are Auxins, Gibberellins, Cytokinins, Ethylene, and Abscisic acid (ABA).

I.Growth Promoting Hormones:

Functions of Auxins:

Cell elongation: Auxin stimulate the elongation of cells of shoots
Initiation of Roots: Higher the concentration of auxin inhibits elongation of shoots, but it initiates more lateral branches of roots.
Inhibition: Natural auxin delays abscission of young fruits and leaves.
Promotes Apical dominance: Allow shoot apex to grow
Promotes Parthenocarpy: Make unpollinated pistil to develop into endocarp

Functions of Gibberellins

It elongates the internodes, so as to increase the height of the plant.
It helps to overcome the dwarfism in plants.
It also helps in promoting bolting just prior to their flowering or reproductive phase.
It helps in overcoming the natural dormancy of buds, tubers, seeds, etc., and allows them to grow.

Functions of Cytokinin:

It induces cell division.
They promote the growth of lateral buds by breaking apical dominance.
It helps in morphogenesis
They also increase resistance of plants to high or low temperature

II. Growth Inhibiting Hormones:

Functions of Ethylene:

It is highly effective in fruit ripening
It proves to be helpful in increasing absorption surface of plants.
It promotes rapid internode petiole elongation.

The function of ABA:

It has a primary role in regulating the abscission and dormancy of buds and seeds.
It acts as a general plant growth inhibitor and also inhibits the metabolism of plants.
It also plays an important role in seed development and maturation.