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.
Radiation induces mutation to develop new variety of crops. Now
with newer and more powerful sources of radiations (UV shortwave, Xray, 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.