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.
These are certain synthetic chemicals which 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, Gibberllins, 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 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 help to overcome the dwarfism in plants.
- It also help in promoting bolting just prior to their flowering or reproductive phase .
- It helps in overcoming 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 increases 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.
Function of ABA:
- It has a primary role in regulating abscission and dormancy of buds and seeds.
- It acts as a general plant growth inhibitor and also inhibits metabolism of plants.
- It also play an important role in seed development and maturation.