The following points highlight the six methods of crop improvement. The methods are: 1. Plant Introduction 2. Acclimatization 3. Mutant Breeding 4. Polyploidy 5. Tissue Culture 6. Genetic Engineering.
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Crop Improvement Method # 1. Plant Introduction:
Plant introduction usually means the introduction of the plants from places outside the county, may be of same or another continent. It can be defined as the “process of introducing plants from their growing locality to a new locality.”
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The introduction of the genotypes from the place where it is grown to an entirely new area. It is the easiest or most common method of crop improvement.
Plant introduction may be of following types:
(i) Intercontinental plant introduction:
Plant material is introduced from the country of another continent e.g., ridley wheat variety from Austria in India.
(ii) Intercontinental or Inter-countries plant introduction:
Introduction of plant material from another country within same continent, for e.g., Litchi and loquat from China.
(iii) Interstate plant introduction:
Introduction of plant material from another state within the same country, for e.g., N.P. wheat varieties from Delhi.
(iv) Intrastate or Inter-district plant introduction:
Introduction of plant material into different states within the country or from another district within the same state, for e.g., distribution of state recommended varieties of different crops from one district to another within the same state for general cultivation.
Any plant material therefore brought from another place may be within or outside the country, may be termed as introduced material or method of plant introduction. These introductions are also termed as exotic collection (EC, Introductions may be from outside the country) or indigenous collections (IC, Introduction may be from other parts or other states of the same country).
On the basis of nature, plant introduction is divided into two types:
Primary introduction:
Introduced plant is released directly for cultivation as a variety, for e.g., Introduction of Wheat varieties ‘Lerma Rojo’ and ‘Sonora 64’ in our country from Mexico in early 1960s; IR8 and Jaya varieties of rice.
Secondary introduction:
Introduced material is used for selection or for hybridization with the indigenous germplasm to bring about the desired improvement to make it suitable for cultivation for e.g., wheat variety ‘kalyan sona’ and ‘sonalika.’
Crop Improvement Method # 2. Acclimatization:
Acclimatization follows the introduction and both processes go side by side. Acclimatization is the adaptation or adjustment of an individual plant or a population of plants under the changed climate for a number of generations: Thus, it is a sort of natural selection operating into the introduced plant material.
Some important facts about the acclimatization are:
i. Genetic variability must be present in the introduced material for acclimatization to occur.
ii. Due to better genetic recombination, cross pollinated species have better acclimatization capacity than self-pollinated crops.
iii. Annual crop has better acclimatization capacity than perennial crop because in annuals each generation would produce new gene recombination every year.
iv. If the period of acclimatization is very long, the mutation also plays an important role in acclimatization.
Domestication:
All cultivated plants today were wild some lay. The process of domestication started when man started superior plants for use. Domestication is the process of bringing wild species under human management.
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Domestication is the taming and training of plants to suit the human needs and putting them under cultivation.
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Domestication of plants is the change of ideotype to adopt them better to manmade environments. It is the first step in the development of cultivated plants. Most of the crops were domesticated by the prehistoric man under domestication and the crop species have changed considerably.
Domestication of wild species is still being done and is likely to continue for a long time in near future. A notable case of recent domestication is that of several members of family Euphorbiaceous producing latex. The latex of these plants may be commercially used for extraction of petroleum products including petrol and diesel.
Crop Improvement Method # 3. Mutation Breeding:
A sudden heritable change in a characteristic of an organism is called mutation; function of mutations with the aid of mutagens is called mutagenesis.
Breeding method utilizing variation created through mutagenesis is called mutation breeding. In this method, gamma rays and X-rays are the most commonly used physical mutagens, while EMS (ethyl methane sulphonate), EI (ethylene imine) and sodium azide are the most commonly used chemical mutagens.
More than 300 varieties have been developed through mutagenesis in various countries. In India, a number of varieties have been developed by mutation breeding e.g., BGM 408 (Chick pea), Hans (pea), JRO 3690 (Jute), Sharbati Sonora, (Wheat), Jagannath (Rice), Co8152 (sugarcane) etc.
Crop Improvement Method # 4. Polyploidy:
An organism or individual having more than two basic monoploid sets of chromosomes is called polyploid (Gr. Polys = many + aploos = one fold + eidos – form) and such condition is called polyploidy. Polyploidy is of two types – autopolypoidy and allopolyploidy.
Polyploids which originate by multiplication of the chromosomes of a single species are known as autopolyploids or autoploids and such situation is called allopolyploidy Autopolyploids are generally larger in size, have larger cells, stomata, pollen grains and seeds.
Some of our crops are autopolyploids e.g., potato, coffee, sweet potato, ground nut, alfa alfa, banana etc. Autopolyploids are produced spontaneously by gamma irradiation, tissue culture and with treatment of the chemicals like colchicine.
A polyploid organism, which originates by combining complete chromosomes sets from two or three species is known as alloploid and the condition is known as allopolyploidy. Some important natural allopolyploid crops are wheat, cotton, tobacco, mustard, oat etc.
Artifical polypleids have also been synthesized. Some of the examples are:
Raphanobrassica:
Synthesized by a cross between radish (Raphanus sativus, n = 9) and cabbage (Brassica oleracea n = 9).
Triticale:
Synthesized by a cross between wheat (Triticum aestivum, n = 7) and rye
(Secale cereale, n = 7).
Tobacco:
Synthesized by a cross between Nicotiana tabacum (2n = 48) and TV. glutinosa (2n = 24). The new species is called as N. digluta.
Cotton:
Varalakshma, a hybrid variety is synthesized by a cross between Gossypium. hirusutum (American cotton) and G. barbadense (Egyptian cotton).
Crop Improvement Method # 5. Tissue Culture:
The growth of tissues of living organism in a suitable culture medium (in vitro)I is called tissue, culture. Plant tissue culture refers to growth of plant tissues in a suitable culture medium. The plant part which is used in regeneration is called explants).
It has several useful applications in crop improvement for e.g., development of haploids, embryo rescue generation of variability, selection of disease resistance, selection for salinity and metal toxicity resistance, selection for drought resistance, micro propagation, preservation of germplasm, somatic hybridization etc.
Crop Improvement Method # 6. Genetic Engineering:
Isolation, introduction and expression of foreign DNA in plants and animals is called genetic engineering. It involves introduction of a gene by the techniques of recombinant DNA technology and genetic transformation.
It provides an exciting tool to improve specific traits of inbreeds provided a transgene is available for this purpose e.g., suitable version of the cry gene of Bacillus thuringiensis has been transferred in maize inbreeds and insect resistant commercial hybrids have been produced using such inbreeds.
Similarly, many transgenes have been shown to generate resistance to fungal diseases (e.g.. Vacuolar’chitinase, (β-1, 3-glucanase, groundnut stilbene synthase etc.)
The cry gene transfers have been so far the most successful. Insect resistant transgenic varieties of maize, soybean, cotton etc. expressing this gene are being cultivated in U.S. A. In India, efforts are being made to develop aphid resistant transgenic brassicas by expressing in them protease inhibitor genes from wheat etc..