Growth regulators or hormones are substances other than food materials which can regulate or control growth of plants and animals. Those which play an important role in controlling the growth of plants are called PHYTOHORMONES and PLANT GROWTH REGULATORS.
PLANT GROWTH REGULATORS:
Means any organic substance which when applied in small quantities modify, regulate or alter the growth of living plant tissues.
PHYTOHORMONES:
Plant hormones are substances synthesized in plants and translocated to another part, where in small quantities, it will have specific formative effects.
Auxins, Cytokinins, Gibberrellins, Abscisic acid, Ethylene, Inhibitors, Retardents and Morphactants form the broad group of growth regulators.
Inhibitors are substances which inhibit or suppress the action of other substances. Eg. ABA or Abscisic Acid is a growth inhibitor which suppresses the action of auxins.
Retardents are substances which decrease the rate of growth in plants. Eg. Cycocel (CCC) and Alar (B-9).
AUXINS
Auxins are the fundamental plant growth regulators. Charles Darwin in the 1890s first reported the existence of these chemicals in plants. They have the capacity of cell elongation in a longitudinal manner. Auxins in low concentrations acts as growth promoters while in high concentrations they act as destroyers or plant killers.
There are both naturally occurring and synthetic auxins. IAA or Indole-3-Acetic Acid is the most common naturally occurring auxin. Indole –3-Butyric Acid (IBA) and Naphtalene Acetic Acid or NAA are other naturally occurring auxins. IBA is commonly used as rooting hormones in plant nurseries. Several chemicals which have the property of IAA has been produced. They are called synthetic auxins. Eg. 2,4-D (2,4-Dichloro Phenoxy Acetic Acid) and 2,4,5-T or(2,4,5-Trichloro Phenoxy Acetic Acid) . These are cheaper and are used in large quantities as weedicides or weed killers.
How auxins cause cell elongation and growth?
In the plant cell walls, the bonding or linkages are weak wherever a glucose molecule links with a protein molecule. IAA and other auxins in small quantities has the ability to break the hydrogen linkages between those glucose (cellulose) and protein molecules in the cell wall. As a result of these breaks in bonds, the cell wall loosens, become more flexible and elastic. Water enters these cells to make them more turgid and they enlarge in size and divide.
Tryptophan, an aminoacid present in animal cells is converted to IAA when it is absorbed inside plants. Tryptophan as such cannot promote plant growth. This is the reason why animal wastes such as cow dung when applied to plants promote growth in plants.
Effect of auxins as plant killers
In high concentrations, these auxins cause the breakage of all the hydrogen bonds between molecules in the cell walls. As a result, cell walls are destroyed and cells collapse and the plant dies. Synthetic auxins like 2,4-D are less mobile than IAA and acts on the point of application when applied externally and cause the plants to die. Hence they are widely used as weed killers.
Movement of auxins:
Auxins are synthesized in the tip or the apical part of the plant which is the growing tip. From the tip it moves downwards to other parts of the plant. This movement is affected by
1. Gravity: Auxins are produced in the plant apex and they are positively geo-tropic, which means it moves in the downward direction only from the tip towards the roots.
2. Photosensitivity: Auxins are highly sensitive to light. It always moves toward the darker side. If light is not uniformly distributed, the auxins tend to accumulate in the darker side thereby causing more cell elongation in that side, thereby making the plant bend on that side and lean towards light. This is known as Phototropism or movement of plants towards sunlight.
3. Polarity: auxins always move in one way direction only- top to bottom and never from bottom to top. If a plant is placed upside down, the auxins produced in the top is not capable of moving and gets accumulated in the tip. As a result, the tip will take a upward curve and grow upwards. From this growing tip auxins naturally move downward and accumulate again at the bend producing more upward growing branches.
Functions of growth regulators
• They are used in plant propagation
• For the prevention of pre-harvest fruit drops
• Increase parthenocarpy or production of fruits without seeds.
• For increasing fruit set
• For prevention of sprouting by inhibition of buds
• Inhibition of prolonged dormancy
• Control of flowering
• Defoliation
• Prevention of leaf fall
• Thinning of fruits
• Acts as weed killers
