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Growth Hormones

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Rural Technology

Plant growth substances play major role in growth and development of plants. To grow in length tissue must receive growth substances. Growth substances normally control plant growth, modify growth and physiological activities.

Plant hormones:-

These are organic substances synthesized by the plant which regulate, retard or modify plant physiological activities. They can move within the plant from the site of synthesis to the site of action. Diagramatic Representation of Tissue culture.


Mainly there are five categories of growth regulators:- 1 Auxins:- These are organic acid and their derivative, naturally occur in the apical bud, embryo and anthers of the plant. Natural auxin is IAA. Auxins are responsible for cell division, cell enlargement, root promotion and flower initiation. Synthetic auxins are available in the form of IBA, NAA, 2,4-D 2 Gibberellins :- Gibberellins are first time identified from the fungus gibbrella. The natural occurrence is in the immature seed. Gibberellins are responsible for cell enlargement and sex modification. These are available in the form of GA1 - GA63 but most common type is GA3. 3 Cytokinines : Natural occurrence is in the form of zeatin. Coconut water is rich source of cytokinines. Synthetic form available is kinetin. Cytokinine action is cell division and cell elongation. 4 Ethylene: - It is known as ripening hormone. Ethylene trigger the ripening but not participate in ripening. Ripe fruits are source of ethylene. The synthetic form is Ethrel or Ethephon. 5. Inhibitors: - Plants contain many inhibitory substances which inhibits the processes like seed germination and shoot growth. Abscisic Acid (ABA) is usually present in the mature and scenesing tissues. Synthetic forms are malic hydrazide (MH), cycocel.

Uses of growth hormones:

• To promote rooting of cutting. • To terminate the dormancy of seed and to increase germination percentage. • Control of flowering. • To control excessive vegetative growth. • To increase number of female flowers. • To improve fruit set and fruit development. • To induce parthenocarpy. • To control fruit drop. • To control fruit ripening process. • To impart attractive colour to fruit. • To avoid spoilage of fruits and vegetables during storage. • To control weed. • To propagate plants by tissue culture.

Methods of application of growth regulator

1. Lanolin paste:- This technique is most commonly used for laboratory experiments to obtain clue for theoretical and practical significance of a particular compound on a physiological process. Lanolin is soft fat prepared form wool and is a good solvent for growth regulators. The paste is prepared with the fat and the growth regulator in a suitable preparation, sticks firmly and growth regulator does not dry out and remain in constant contact with the treated organ. 2. Immersion into dilute or concentrated solutions:- Stock solutions are made by dissolving growth regulator in a small volume of 95% ethanol or methanol or absolute alcohol. Then it is diluted to an appropriate strength. The cuttings or seeds are soaked for 10 to 24 hours in dilute solution. Dipping in concentrated solution is recommended for rapid action. The treatments are known as prolonged soaking and quick dip method. In quick dip method cuttings are immersed for few seconds. 3. Spraying technique:- This method has been used on large scale for various purposes, as it is easy to practice. The dilute solution is sprayed on the leaves with sprayers. The effectiveness of the application depends on the ability of growth regulator to penetrate through the leaf cuticle in to the mesophyll cells. For proper response, right concentration and spray at right stage is necessary. If properly done, single spray may be sufficient. In some cases two applications are necessary. Growth regulators can also be injected into the fleshy parts with the help of hypodermic needles. The injection technique is used to study the effect of growth regulator on a particular organ. 4 Dust method (Powder method):- This is one of the simplest methods. The required quantity of chemical is dissolved in an absolute alcohol and mixed in right proportion with a carrier substance like talc, chemical or bentonite. This is considered by some growers as an easier and quicker method of application. It is useful for propagation through cutting and layering. 5 Aerosol and vapour method:- These methods are becoming popular in case of many green house plants for promoting fruit setting, parthenocarpy and control of fruit drop. The Aerosol method is based on preparation of auxin solution in a solvent of low boiling point, usually a gas under pressure in a cylinder. The solution when released into the green house from the container through nozzle the auxin is suspended in green house atmosphere as mist. The mist may persist for five hours and the entire plant is subjected to auxin action. For application of the vapour method, the active compound should be movable. It is vaporized over an electrical hot plate and the vapour is then spread to green house by electric fan. After vaporization, the green house is kept closed for 10 -15 hours. During this time the auxins are absorbed by the plants. Two successive applications at an interval of few days are found more effective than single treatment. 6 Soil application:- Considerable success has been claimed in the effectiveness of auxin as herbicides. When applied promotion of fruit setting in male sterile varieties of tomato has recorded by soil application. Dilute solutions of active compounds are added to the soil and they are absorbed by the roots and transported to the desired parts.

Intext questions

A) Fill in the blanks:- 1. are first time identified from the fungus gibberella. 2. is known as ripening hormone. 3. The organic substances synthesized by the plant which either regulate retard or modify plant physiological activities are known . 4. Coconut water is rich source of . 5. is usually present in the mature and senescing tissues.

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