Crop production can be improved by breeding new varieties of crops having higher yield. The main aim of plat breeding is to produce new crops superior to the existing ones. By this methods new varieties of crops having higher yield, resistant to pests and diseases can be groomed. Hence plant breeding can be defined as a science as well as improving genetic make up of plants in relation to their economic use. The various approaches used for genetic improvement of crop plants are referred to as plant breeding methods or techniques.
Plant breeding is defined as identifying and selecting desirable traits in plants and combining these into one individual plant. Since 1900 the Mendel's' Laws have provided for scientific genetic breeding of plants. As all traits of a plant are controlled by genes located on chromosomes conventional plant breeding can be considered as the manipulation of the combination of chromosomes. In general there are four main procedures to manipulate plant chromosome combination.
• Plants of a given population which show desired traits can be selected and used for further breeding and cultivation by a process called selection.
• Desired traits found in different plant lines can be combined together to obtain plants which exhibit both traits simultaneously by a method termed hybridization. Heterosis is a phenomenon of increased vigor, and it is obtained by hybridization of inbred lines.
• Polyploidy can contribute to crop improvement.
• New genetic variability can be introduced through spontaneous or artificially induced mutations.
Selection is the most ancient and basic procedure in plant breeding. It generally involves three distinct steps.
1. A large number of selections are made from the genetically variable original population.
2. Progeny rows are grown from the individual plant selection form observational purposes.
3. After obvious elimination the selections are grown over several years to permit observation of performance under different environmental conditions for making further eliminations.
4. The selected and inbred lines are compared to existing commercial varieties in their yielding performance and other aspects of agronomic importance.
The most frequently employed plant breeding technique is hybridization. The aim of hybridization is to bring together desired traits found in different plant lines into one plant line via cross pollination. The first step is to generate homozygous inbred lines this is normally done by using self pollinating plants where pollen from male flowers pollinate female flowers from the same plants. Once a pure line is generated it is out crossed and is combined with another inbred line. Then the resulting progenies are selected for combination of the desired traits.
If a trait from a wild relative of a crops species which is resistant against a disease is to be brought into the genome of the crop. A large quantity of undesired traits like low yield, bad taste and low nutritional value are transferred to the crop as well. These unfavorable traits must be removed by time consuming back crossing which is repeated crossing with the crop parent. There are two types of hybrid plants interspecific and intergeneric hybrids. Beyond this biological boundary hybridization cannot be accomplished due to sexual incompatibility which limits the possibilities of introducing desired traits into crop plants.
Heterosis is an effect which is achieved by crossing highly inbred lines of crop plants. Inbreeding of most crops leads to a strong reduction of vigor and size in first generation. After six or seven generation no further reduction in vigor or size is found. When such highly inbred plants are crossed with other inbred varieties very vigorous, large sized, large fruited plants may result.
The process of introducing new plants from the place of their cultivation to a place with different climate is termed as plant introduction. The adjustment of such plants to this new locality is called aclimization. The new crops are introduced in the form of seeds or bulks or cuttings. This is an easy and rapid method for crop improvement.
Several plants have been successfully introduced in INDIA. They have got well adapted to the new climatic and soil conditions such as ground nut which was introduced from Philippines.
Most plants are deployed with three or more complete sets of chromosomes. They are common and refer to as polyploids. The increase of chromosome sets per cell can be artificially inducted by applying the chemical colchicines which leads to a doubling of the chromosome number. Generally the main effect of polyploidy is increased in size and genetic variability. On the other hand polyploidy plants often have a lower fertility and grow more slowly.
Instead of relying only on the introduction genetic variability from the wild species gene pool are formed from other cultivars and alternative is the introduction of mutations induced by chemicals or radiations. The mutants obtained are tested and further selected for desired traits.
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