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MHT-CET : Biology Entrance Exam

MHT - CET : Biology - Domestication of Plants Page 1

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Introduction, Principles and Methods of Plant-breeding

1.

Plant-breeding

 

It is a branch of applied botany which deals with improving and changing the characters of plants.

 

2.

Objectives of Plant-breeding

 

a. To improve quality and yield of crop plants.
b. To get disease-resistant, drought-resistant plants.
c. To develop plants having early maturity.

 

 

3.

Different Methods used in Plant-breeding

 

a. Plant introduction
b. Mass selection
c. Pure line selection
d. Hybridisation
e. Polyploidy
f. Mutation breeding

 

Methods of Plant-breeding

 

1.

Plant Introduction

 

It is introduction and cultivation of new plants in a locality where they did not exist before. Many plants in our country were introduced by pilgrims, traders, monks, kings, etc. For example, emperor Akbar introduced Cheenar tree into Kashmir from Persia.

 

 

2.

Mass Selection

 

It is a method of Plant-breeding which is based on external, i.e. phenotypic characters. In this method, seeds of large number of plants are mixed together to raise a next season crop. This is continued till the desirable characters are passed on.

 

 

 

3.

Pure Line Selection

 

In self-pollinated crops, pure line selection is done to produce a homozygous variety. Here, progeny of each single self-pollinated plant is grown separately and the best progeny is selected. These plants are genetically more stable.

 

 

4.

Hybridisation

 

It is a method of plant-breeding in which plants of a different species are crossed to get new varieties. Many a times, hybrids are larger than parents. This is 'hybrid vigour'.

 

 

5.

Mutation Breeding

 

Sudden change in the genetic material of an organism is called mutation breeding. These are artificially induced to get new varieties of plants. The mutagenic substances used here are certain chemicals like mustard gas, nitrous acid, ethylene oxide, etc. Radiations like b rays, (Beta rays), gamma rays and x-rays are also used. Example: Wheat, Rice, Groundnut, Pea, Cotton, etc.

 

Polyploidy

1.

Plants with two sets of chromosomes are called diploid (2n). Those plants having more than two sets of chromosomes are called polyploids. Polyploidy can produce phenotypic changes which are favourable or unfavourable.

2.

In a polyploid, if the additional set or sets of chromosomes are from different species, then it is called allopolyploid.

3.

If the additional set of chromosomes is from the same species then that polyploid is called autopolyploid.

 

Techniques of Hybridisation

Technique of hybridisation involves various steps such as:

 

1.

Selection of Parents: Here, the parents have to be appropriate with respect to certain characters like healthy, disease-free, good combining ability, well-adaptated to local climatic conditions, etc.

 

2.

Selfing of Parents: In order to obtain homozygous plants, the selected plants are subjected to self-pollination.

 

3.

Emasculation: Stamens from the flowers of female plant should be removed before they release pollen grains, so that self-pollination is prevented. This can be done by hand using forceps or by hot/cold water treatment.

 

4.

Bagging: The emasculated flowers are covered with paper or polythene bag immediately, to prevent pollination by other unwanted pollens.

 

5.

Labelling (Tagging): The labels carrying details about male and female parents, date of emasculation, date of crossing, etc. should be tied to plants.

 

6.

Crossing: Collecting pollen grains from the already bagged flowers and dusting them to the stigma of the bagged female flowers.

 

7.

Harvesting: The seeds are collected separately and kept in separate envelops with labels.

 

1. Tissue Culture

 

This is a technique of producing a complete plant from protoplast, cell, tissue or organ on an artificial medium. For this technique, an important character of cells called as 'totipotency' is put into use. Totipotency means capacity of a cell to grow, produce embryo and then an entire new plant.

 

2.

Aspects of Methodology of Tissue Culture

 

a.

Maintaining aseptic conditions throughout the process is an important step in tissue culture which can be carried out by using laminar air flow chamber, autoclaving the medium, sterilizing the plant material, etc.

 

b.

Nutrient Medium: An ideal nutrient medium containing required micro and macronutrients is used. Double-distilled water is used to prepare nutrient medium. Murashige and Skoog's medium with PH 5.6 is the most commonly used nutrient medium.

 

c.

Appropriate conditions of light, temperature (25C - 27C) and humidity (70% to 75%) are maintained.

 

d.

The small plant material used for tissue culture is called 'explant' which could be a cell, tissue or pieces of roots, stem, leaf, etc.

 

e.

After the inoculation of an explant, a mass of unorganised and undifferentiated cells is formed which is called a 'Callus'.

 

f

Organogenesis: The process of development of an organ from a callus is called organogenesis or organography.

 

g

Clonal propagation: The method of raising genetically identical plants from a tissue cultured plant by asexual reproduction is called clonal propagation.

 

h

Micropropagation: The method of tissue culture which is applied for the production of a large number of plants from a single explant is called micropropagation.

 

3.

Applications of Tissue Culture

 

a.

To produce disease-free plants.

 

b.

This micro-propagation helps to produce many plants in a short time and small space.

 

c.

Haploid plants are produced by Anther culture.

 

d.

Certain secondary products like antibiotics, alkaloids, enzymes, etc. are produced by the culture of cells. These secondary metabolites are economically important.

 

e.

Induction and Selection of Mutants: Out of millions of cells grown on nutrient medium, only the cells which are resistant to the stress factors like high temperature, high salinity, drought, etc. are isolated and entire plants showing desired trait are regenerated from them.

 

f.

Protoplast Culture is used in somatic hybridisation and in introduction of foreign cell-organelles in protoplasts.

 

g.

Somatic Hybridisation is used for the fusion of two or more isolated protoplasts belonging to different species. When there is complete fusion of cytoplasm, the fusion product is called a 'cybrid'.



 

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