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Chapter: 11 th 12th std standard Bio Botany plant tree Biology Higher secondary school College Notes

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Plant Breeding : Hybridization, Mutation breeding

Hybridization is a method in plant breeding to improve the native crops by obtaining diverse genotypes that can be used as a source material for collection of crop with desirable characters and genes obtained from many parts of the world.

Plant Breeding : 

Hybridization

 

Hybridization is a method in plant breeding to improve the native crops by obtaining diverse genotypes that can be used as a source material for collection of crop with desirable characters and genes obtained from many parts of the world. It involves crossing of two varieties or species or genera having desirable genes and breeding them together of the desirable traits into one progeny, which is called the hybrid. Hybrids are the products of first generation obtained by crossing genetically unrelated parents.

 

When two individuals of the same species are crossed, it is called inbreeding or selfing or self-pollination. This results in the increase of homozygosity. Particularly homozygous recessive alleles develop loss of vigor in plants. By careful observation of morphological features, we can remove these deleterious and harmful alleles by selection.


Protoplasmic fusion or somatic hybridization

 

A hybrid produced from fusion of protoplasts of two different species is called somatic hybridization. Naked protoplasts are obtained through dissolution of their cell walls by the macerating enzymes such as pectinase and cellulase. Fusion of protoplasts from two different varieties can be enhanced by treatment with the chemical called polyethylene glycol (PEG) in the presence of high voltage electric current on a suitable medium. By this method somatic hybrid plants with desirable changes can be obtained. This method in plant breeding is called protoplasmic fusion.

 

Heterosis

 

The superiority of the F1 hybrid in performance over its parents is called heterosis or hybrid vigour. Vigour refers to increase in growth, yield, resistance to diseases, pests and drought. F1 hybrids of maize show 25% increase in yield when compared to their own parent crop. Vegetative propagation is the best suited measure for maintaining hybrid vigour, since the desired characters are not lost and can persist over a period of time.

 

Polyploid breeding

 

The source for plant breeding is variations in plants.. Heritable and desirable variations occur in nature by mutation, polyploidy, recombination and chromosomal aberrations. A diploid plant has two sets of chromosomes but any organism in which the number of sets of chromosome is doubled is called a polyploid.

 

When chromosome number is doubled by itself in the same plant, it is called autopolyploidy. For example, three sets of chromosomes i.e. a triploid condition in sugar beats, apples and pear has resulted in the increase in vigour and fruit size, large root size, large leaves, flower, more seeds and sugar content in them. Seedless tomato, apple, watermelon and orange are autopolyploids.

 

Polyploidy can be induced by the use of colchicine to double the chromosome number. Allopolyploids are produced by multiplication of chromosome sets that are initially derived from two different species. eg. Triticum ' Secale gives Triticale.

The haploid individual plant will have only one set of chromosome. Through the technique of anther and ovary culture, haploid plants can be modified to diploid ones by doubling their chromosomes. Variations that are brought forth through plant tissue culture are called somoclonal variation. eg. disease resistant potato and rust resistant wheat. Varieties of short duration sugarcanes are produced by polyploid breeding.

 

Mutation breeding

 

Radiation induces mutation to develop new variety of crops. Now with newer and more powerful sources of radiations (UV shortwave, X-ray, Alpha, Beta, Gamma waves) and many chemicals (mutagens) eg. Caesium, ethyl methane sulfonate, nitromethyl urea), we can increase the rates of mutation eg. Triple gene dwarf wheat with increase in yield and height. Atomita 2-rice with saline tolerance and pest resistance, groundnuts with thick shells are products of breeding methods through induced mutation.


Breeding for disease resistance 

 

Many crop plants suffer from several diseases caused by pathogens such as bacteria, fungi, viruses, nematodes, protozoa and mycoplasma. In vegetatively propagated plants like potato, cassava, sugarcane and dahlia, viral pathogens are transmitted through their roots, tubers, bulbs and rhizomes. Disease free plants are obtained by shoot apical meristem culture technique. Plants raised through tissue culture are free from pathogens, which are widely cultivated.

 

Whenever, a trait that shows disease resistance in a plant is observed, the best way to transfer that trait to other useful crop is by the method of backcross. Repeated back crosses are attempted with the parent crop with more desirable characters and such a crop is known as recurrent parent.

For  example, A is a non-recurrent parent and B* is a recurrent parent with desirable trait.

 


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