Monday, September 28, 2009

Plant Breeding Techniques

Plant breeding techniques are as under:

(1) Selection (2) Hybridization (3) Mutation breeding

(4) Polyploid breeding (5) Tissue culture (6) Genetic engineering

(1) SELECTION

It is the process of isolating desirable genotypes from undesirable ones. Selection is oldest method and basis of all plant breeding techniques. The genotypes most suitable for human needs are isolated. It is carried out in a number of ways depending on the mode of reproduction and types of breeding method used. Selection tends to change gene frequencies and is thus responsible for creating new gene pools. A plant breeder selects plants according to his specific needs, for example higher grain yield, sugar or potato content or early maturity. Since artificial selection tends to change gene frequencies in genotypes, changes in the genetic make up of new varieties are expected.

(2) HYBRIDIZATION

If sufficient genetic variability is not available for effective single plant or mass selection, the variability can be created artificially through hybridization. The varieties or species with desirable characteristics are selected and crossed to produce hybrids. The populations are heterozygous and plants with combined desirable features of the parents are selected for testing and further growth. The hybridization result in a single true breeding strain possessing best characteristics of the parent varieties. Large number of plants are used in these crosses because the greater the number of plants used, the greater the chances of obtaining the desired combination of characters and the easier the development of a variety with good agronomic qualities.

(3) MUTATION BREEDING

Mutation is sudden change in hereditary material of a cell. Mutation involves:

(a) A change in gene from one allele to other.

(b) Rearrangement of chromosome material.

(c) Loss or duplication of chromosome segment.

Changes in genes are often called point mutations. The use of mutation in plant breeding is popularly known as ‘mutation breeding’. Mutations are induced through physical mutagens, as x-rays, fast neutrons, thermal neutrons, ultra violet radiation and beta radiation. In addition to physical mutagens large numbers of chemical mutagens are also used to induce mutations in crop plants. Whole plant or any part of plant can be treated by radiation seeds, are most commonly used for irradiation. They offer a number of advantages. They are easily to handle and store and can be maintained for extended periods of time. When dry seeds are almost inert biologically and sever environment cause no significant biological damage.

(4) POLYPLOID BREEDING

Polyploid is a condition in which individuals have more than two chromosome sets or genomes in their somatic cells. In contrast to the normal diploid (2n) they may be triploid (3n), tetraploid (4n), pentaploid (5n), and hexaploid (6n) and so on.

Polyploid plants may arise by duplication of the chromosome sets for single species, auto polyploidy, or by combining chromosome sets from two or more species, alloploidy. Alloploidy in more common method of ploidy in nature. An alloploid in which total chromosome compliment of two other species is combined to form a fertile species hybrid is called amphiploid. Many commonly cultivated crop species have evolved in nature as polyploids for example oats, cotton, tobacco, sorghum, Brassica, common wheat, forage grasses and legumes.

(5) TISSUE CULTURE TECHNIQUE

It is commonly used to describe in vitro and aseptic cultivation of any part on a nutrient medium. The technique was first introduced by Haberlandt in 1902. It has provided an efficient selection tool for modern plant breeding.

In part, if a plant breeder wished to propagate a hybrid plant, a method of vegetative reproduction was used, such as taking stem, leaf or root cuttings. This is a relatively rapid way of reproduction in plant, but not all plants can reproduced like this Tissue culture is a method of vegetative reproduction on mass scale. Within a few years several thousand plants, mostly identical to the original plant can be produced. However some mutations may occur giving occasional variant plants. The propagation of plants by tissue culture is also used after genetic engineering.

(6) GENETIC ENGINEERING

The aim of genetic engineering is to remove a gene from one organism and transfer it into another in such a way that the gene is expressed in its new host. The host is now known as transgenic. Genetic engineering provides a way of overcoming barriers to gene transfer between species. Indeed the genes in question have often been taken by organisms in different kingdoms, such as a bacterial gene but into a plant or a human gene into a bacterium. Unlike selective breeding where whole sets of genes are transferred; genetic engineering results only in the transfer of single gene. Genetic engineering must obtain the wanted gene, clone the gene to produce many copies and insert a copy of the gene into the host DNA.

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