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- Plant breeding is a technique (purposeful manipulation) of improvement of economically important crop plants and production of a new crop that are better suited for the cultivation, give better yield and the disease resistance.
- Conventional plant breeding is in practice from 9,000-11,000 years ago. Most of our major food crops are derived from the domesticated varieties in ancient times.
- Classical plant breeding involves crossing or hybridization of pure lines having desired characters, followed by artificial selection to produce plants with desirable traits of higher yield, nutrition, and resistance to diseases. With advancements in genetics, molecular biology, and tissue culture, plant breeding is now increasingly being carried out by using molecular genetic tools.
Objectives of Plant Breeding:
- The productivity of the crop per unit acre of a land is the criteria of the measurement of the yield. Majority of plant breeding programmes aims at increased yield. This is achieved by developing more efficient genotypes. Through artificial selection, cross-breeding and hybridization, high yielding varieties are developed. These varieties improve the productivity per acre of the land.
- By plant breeding, a crop variety of improved nutritional values, taste and shelf life can be developed.
- Expected Qualities From Crop:
- Rice: milling, cooking quality, aroma, and grain colour
- Wheat: milling and baking quality and gluten content.
- Pulses: Protein content and improving sulphur containing amino acids
- Oilseeds: PUFA content
Elimination of Toxic Substance:
- By plant breeding, toxic substances from plants are removed in a new variety. HCN content in jowar plants, Erucic acid in Brassicas, Cucurbitacin in cucurbits is removed by plant breeding.
Resistance against biotic and abiotic stresses:
- Biotic stress: New varieties of crops are Evolved, which are pests and diseases resistant.
- Abiotic stress: New varieties of crops are capable of surviving in environmental changes like drought, waterlogging, the salinity of soil, etc.
Change in Maturity Duration:
- By plant breeding, new varieties of early maturing varieties of crops are developed. It increases the yield and productivity of land per acre.
- Other objectives include improved agronomic characters, reducing the plant height to prevent lodging, photo-insensitivity, Non-shattering nature of grain and grams, synchronized maturity, elimination or introduction of dormancy.
Steps in Plant Breeding (Development of New Variety Of Crop)
Step – 1: Collection of Germplasm:
- Variations are the differences seen among individuals of species or population, for a particular character. Such genetic variations are heritable and are useful in selection of a plant for breeding.
- Germplasm is the sum total of all the alleles of the genes present in a crop and its related species. The germplasm of any crop species consists of the following types of materials: cultivated improved varieties, improved varieties that are no more in cultivation, old local or ‘desi’ varieties, pure lines produced by plant breeders, and wild species related to the crop species.
- The entire collection (of plants/seeds) having all the diverse alleles for all genes in a particular crop is called germplasm collection.
Step – 2: Selection and Testing of Superior Recombinants:
- The selection involves picking up the better one from the entire crop plants.
- The collected germ-plasm is now evaluated to identify the plants showing the desirable combination of characters. Such selected plants are used in the process of the next step called hybridization.
Types of Selection:
- This is the most common and old method of crop selection. This method of selection depends mainly on the selection of plants according to their phenotype and performance.
- In this selection, a large number of similarly appearing vigorous plants are selected for the desired trait and their seeds are mixed together. The seeds are used for cultivation of the next generation.
- It is useful in the cultivation of cross-pollinated plants such as maize, watermelon, radish, grapes, apples, onion, pear, etc.
- This method is used to improve the overall population by positive or negative mass selection. Mass selection is only applied to a limited degree
Advantages of Mass Selection:
- Varieties developed by this method have wide adaptability, wide genetic base and stable.
- It is rapid, simple and least expensive of all plant-breeding procedures. It doesn’t require any scientific knowledge.
- It retains considerable genetic variability.
- It is useful in meeting the need of the farmers
- it is the only method for improving the local or wild varieties.
Disadvantages of Mass Selection:
- Variation exists in variety developed by mass selection. The phenotype of these varieties is not as uniform as pure line varieties.
- The selected character of crop exhibit segregation due to natural cross-pollination. Without progeny testing selected types will show segregation in the next generation, hence a test of homozygosity of plants progeny is always required.
- There is a less improvement in the crop as compared to pure line selection. The yield depends on the environment.
- Limited use in the improvement of self-pollinated crops.
- Varieties difficult to identify in the seed certification program than pure lines.
- It is only effective for characters with high heritability.
- Only those varieties which show genetic variation can be improved by this method, as it uses variability present in the population.
- It cannot bring any new change in the genotype.
- There is no control over pollination, which causes greater heterozygosity and as a result the desirable qualities gradually diminish. Also the pollination may be both by superior and inferior pollens.
Pure Line or Single Plant Selection:
- The pure line is the progeny of single self- fertilized self-pollinated homozygous plant. In pure line selection, large numbers of plants are selected from a self-pollinated crop and are harvested individually.
- The plants with the desired trait or traits are selected out of the variable population in the field. Seeds from selected plants and sown separately to produce progeny by self-pollination.
- Wheat varieties like Kalyan – 27 and PV – 18 are developed by this method.
- This process is continued for many generations and plants with inferior characters are eliminated in each generation.
Advantages of Pure Line Selection Method:
- It is easier than hybridization and requires less skill.
- Due to extreme uniformly, it is easily identified in seed certification and is more supported by farmers and consumers.
- It achieves the maximum possible improvement over the original variety.
- The selected plant shows the desired character for several generations.
Disadvantages of Pure Line Selection Method:
- It is not useful in cross-pollinated crops because it is expensive, laborious.
- The breeder has to devote more time to pure line selection than mass selection. Thus it is time-consuming and laborious. It may take 10-12 years for development of the new variety.
- There is an upper limit on the improvement in new variety by the genetic variation present in the original population.
- The new variety doesn’t have wide adaptability and stability in production.
- A variety that is propagated vegetatively (asexual reproduction) from a single plant is called clone
- Advantages of Clonal Selection:
- It is the only method to improve the clonal crops.
- The desired clone can be obtained in less than 1 year.
- As there is no danger of deterioration due to segregation and recombination., varieties obtained are as stable as pure lines and easy to maintain.
- It helps in eliminating unproductive and undesirable types.
- This method is useful in conserving hybrid vigour for several generations.
- Disadvantages of Clonal Selection:
- This method is applicable only to vegetatively propagated crops.
- In this method, no new genetic variability can be created.
Selection of the Crop for the Next Step of Plant Breeding:
- Selection of the plant that has desired character combination by any of the method mentioned above.
- Selfing the selected plant for several generations to obtain homozygous inbred pure line.
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