Plant growth regulators are the chemical substance which occur naturally in plants as well as synthetic other than nutrients in small amounts promote or modify process.
Minor quantity they regulate growth and development of plants. It is also called phytohormone.
Plant growth regulators
These organic substances are generally produced in meristematic tissues of the plant and translocated towards the site of action including a physiological process and can work in minutes quantities.
The plant growth regulators are mainly two types :
1.) Growth promotor
2.) Growth inhibitors
- Abscisic acid (ABA)
They are the growth hormone which promote the cell elongation of the coleoptile tissues. The primary physiological effect of auxin on growth of a plant is the elongation of cells.
The cell elongation is activated by auxin in three ways:
(i) by increasing osmotic solute
(ii) by decreasing wall pressure
(iii) by increasing permeability of cytoplasm to water.
Auxin is also called polar transport due to movement of auxins from the morphological apex towards the base of the plant.
According to the nature, auxins are divided into two category which are known as natural auxin (IAA) and artificial auxin (IBA), (NAA).
Functions of auxin
i.) Auxin promote cell elongation and growth of stem & root.
ii.) Auxin promote cell division in certain tissues like cambium.
iii) It is responsible for apical dominance. It means the terminal bud dominants over the lateral buds by inhibiting their development.
iv.) Auxin help in development of callus in plant tissue culture.
v.) It helps in root initiation on stem cutting.
vi.) 2,4-D auxin helps to kill weeds as a weedicide.
vii.) foliar spray of NAA and 2,4-D induce flowering in litchi and pineapple.
It is the plant hormones which promotes shoot growth by accelerating the cell elongation and cell division in the sub-apical meristem region.
Most common gibberellins is GA-3.
This ‘Plant growth regulators‘ produce in embryo root and young leaf near the shoot tip. And it has no effect on root growth and the activity of apical meristem of stem apex.
Function of Gibberellins
i.) It stimulate stem elongation and leaf expansion, but no effect on root.
ii.) Application of gibberellin cause elongation of genetically dwarf variety of plant such as corn and pea.
iii.) Gibberellins cause bolting.
iv.) It stimulate the production of digestive enzymes at the time of germination.
v.) Gibberellin breaks seed dormancy and act as antagonistic to abscisic acid.
vi.) Gibberellins cause parthenocarpy in apple and increase the fruit size in grapes.
vii.) It induce in formation of male flowers in cucurbit.
Cytokinin are the plant hormone which promote cytokinesis (cell division). It stimulate growth of lateral bud, and act as antagonistic to auxin in controlling apical dormancy.
Cytokinin alongwith auxin increases mitotic activity at large scale because division is promote mainly by cytokinin and auxin induces cell extension.
Function of cytokinin
i.) It promote cell division.
ii.) Cytokinin together with auxin, control cell differentiation in plant tissues culture.
iii.) It promote cytokinesis in cell of various plant organs.
iv.) It promote cell division and the related DNA and RNA synthesis.
v.) It is used in the breaking of dormancy. It also promote the seed germination.
4. Abscisic Acid (ABA)
The plant hormone which generally act as growth inhibitor. It inhibits GA-induced α-amylase synthesis thus inhibiting germination of seeds.
It push the senescence phase of growth. And this hormone is also called stress hormone.
Function of abscisic acid
i.) It induce dormancy in bud and seed during unfavorable condition.
ii.) It promote plant aging.
iii.) It promote abscission of leaf, fruit and flowers.
iv.) It cause closer of stomata under severe drought, hence act as stress hormone.
v.) It inivite cell division in plant tissue.
It is a gaseous plant hormone which is produce from ripening of fruit and act as growth inhibitor. The inhibitory effect of auxin on root elongation and buds growth is due to auxin induced production of ethylene.
Function of ethylene
i.) It is synthesized in plant from the amino acid methionine.
ii.) Ethylene increase the cell permeability due of which the fruit become soft.
iii.) It promote aging of plant organ.
iv.) Ethylene application increase the female flower number and fruit in cucumber.
v.) It induce ripening of fruit.