In this plant breeding methods in agriculture post we have briefly explained about conventional plant breeding methods, plant introduction, selection, hybridization, and heterosis.
Plant breeding is the science of improvement of crop varieties with higher yield, better quality, resistance to diseases and shorter durations which are suitable to particular environment. In other words, it is a purposeful manipulation of plant species in order to create desired genotype and phenotype for the benefit of humans.
In early days, plant breeding activities were based mainly on skills and ability of person involved. But as the principles of genetics and cytogenetics have elucidated breeding methods such as selection, introduction, hybridization, ploidy, mutation, tissue culture and biotechnology techniques were designed to develop improved crop varieties.
Plant Breeding Methods in Agriculture
Plant Breeding Methods in Agriculture
Conventional Plant Breeding Methods
Conventional plant breeding methods resulting in hybrid varieties had a tremendous impact on agricultural productivity over the last decades. It develops new plant varieties by the process of selection and seeks to achieve expression of genetic material which is already present within the species.
Plant introduction may be defined as the introduction of genotypes from a place where it is normally grown to a new place or environment. Rice variety of IR8 introduced from Philippines and Wheat varieties of Sonora 63, Sonora 64 from Mexico.
The newly introduced plant has to adapt itself to the new environment. This adjustment or adaptation of the introduced plant in the changed environment is called acclimatization. All the introductions must be free from presence of weeds, insects and disease causing organisms. This has to be carefully examined by the process called quarantine, a strict isolation imposed to prevent the spread of disease.
Primary introduction is When the introduced variety is well adapted to the new environment without any alternation to the original genotype. Secondary introduction is wen the introduced variety is subjected to selection to isolate a superior variety and hybridized with a local variety to transfer one or a few characters to them. The botanical garden in different parts of the world also played a significant role in plant introduction.
Selection is the choice of certain individuals from a mixed population for a one or more desirable traits. Selection is the oldest and basic method of plant breeding. There are two main types of Selection.
1. Natural Selection
This is a rule in the nature and results in evolution reflected in the Darwinian principle “survival of the fittest”. It takes longer time in bringing about desired variation.
2. Artificial Selection
It is a human involved process in having better crop from a mixed population where the individuals differ in character. The following are the three main types of artificial selection.
a. Mass Selection
In mass selection a large number of plants of similar phenotype or morphological characters are selected and their seeds are mixed together to constitute a new variety. The population obtained from the selected plants would be more uniform than the original population and are not individually tested. After repeated selection for about five to six years, selected seeds are multiplied and distributed to the farmers. The only disadvantage of mass selection is that it is difficult to distinguish the hereditary variation from environmental variation.
b. Pureline selection
Johannsen in 1903 coined the word Pureline. It is a collection of plants obtained as a result of repeated self-pollination from a single homozygous individual. Hence, a variety formed by this method shows more homozygosity with respect to all genes. The disadvantage of this type is that the new genotypes are never created and they are less adaptable and less stable to the environmental fluctuations.
c. Clonal Selection
In asexually propagated crop, progenies derived from a plant resemble in genetic constitution with the parent plant as they are mitotically divided. Based on their phenotypic appearance, clonal selection is employed to select improved variety from a mixed population (clones). The selected plants are multiplied through vegetative propagation to give rise to a clone. The genotype of a clone remains unchanged for a long period of time.
Hybridization is the method of producing new crop varieties in which two or more plants of unlike genetically constitution is crossed together that result in a progeny called hybrid. Hybridization offers improvement in crop and is the only effective means of combining together the desirable characters of two or more varieties or species. The first natural hybridization was observed by Cotton Mather in maize.
Steps in Hybridization
Selection of Parents, Male and female plants of the desired characters are selected. It should be tested for their homozygosity. Emasculation, It is a process of removal of anthers to prevent self-pollination before anthesis (period of opening of a flower). Bagging, The stigma of the flower is protected against any undesirable pollen grains, by covering it with a bag. Crossing, Transfer of pollen grains from selected male flower to the stigma of the female emasculated flower. Harvesting seeds, The pollination leads to fertilization and finally seed formation takes place. The seeds are grown into new generation which are called hybrid.
Types of Hybridization
Intravarietal hybridization, The cross between the plants of same variety. Such crosses are useful only in the self-pollinated crops.
Intervarietal hybridization, The cross between the plants belonging to two different varieties of the same species and is also known as intraspecific hybridization. This technique has been the basis of improving self-pollinated as well as cross pollinated crops
Interspecific hybridization, The cross between the plants belonging to different species belonging to the same genus is also called intragenic hybridization. It is commonly used for transferring the genes of disease, insect, pest and drought resistance from one species to another.
Intergeneric hybridization, The crosses are made between the plants belonging to two different genera. The disadvantages are hybrid sterility, time consuming and expensive procedure.
Heterosis G.H. Shull was the first scientist to use the term heterosis in 1912. The superiority of the F1 hybrid in performance over its parents is called heterosis or hybrid vigour.
Vigour refers to increase in growth, yield, and greater adaptability of resistance to diseases, pest and drought. 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.
Many breeders believe that its magnitude of heterosis is directly related to the degree of genetic diversity between the two parents. Depending on the nature, origin, adaptability and reproducing ability heterosis can be classified as:
Mutational Euheterosis, Simplest type of euheterosis and results from the sheltering or eliminating of the deleterious, unfavourable oft en lethal, recessive, mutant genes by their adaptively superior dominant alleles in cross pollinated crops. Balanced Euheterosis, Well balanced gene combinations which are more adaptive to environmental conditions and agricultural usefulness.
Muller and Stadler (1927- 1928) coined the term mutation breeding. It represents a new method of conventional breeding procedures as they have the advantage of improving the defect without losing agronomic and quality character in agriculture and crop improvement.
Mutation means the sudden heritable changes in the genotype or phenotype of an organism. Gene mutations are of considerable importance in plant breeding as they provide essential inputs for evolution as well as for re-combination and selection.
It is the only method for improving seedless crops. Radiation such as UV short wave, X-ray, Alpha (α), Beta (β), Gamma waves and many chemicals such as cesium, EMS (ethyl methane sulfonate), nitromethyl, urea induces mutation to develop new variety of crops. Example: Triple gene dwarf wheat with increase in yield and height. Atomita 2 – rice with saline tolerance and pest resistance.
Majority of flowering plants are diploid (2n). The plants which possess more than two sets of chromosome are called polyploids. Polyploidy is a major force in the evolution of both wild and cultivated plants. Polyploidy often exhibit increased hybrid vigour increased heterozygosity, increase the tolerance to both biotic and abiotic stresses, buffering of deleterious mutations.
In addition, polyploidy often results in reduced fertility due to meiotic error allowing the production of seedless varieties. When chromosome number is doubled by itself in the same plant, is called autopolyploidy.
Example: A triploid condition in sugar beets, 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. It also resulted in seedless tomato, apple, watermelon and orange. 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.
Example: Triticale (Triticum durum x secale cereale) Raphanobrassica (Brassica oleraceae x Raphanus sativus).