11 Enhancement of Food Production - part 02 - Hybridization and its technique

 

11 Enhancement of Food Production - part 02 - Hybridization and its technique

A. Hybridization and its technique :
  • It is the cheif method that offers greater possibilities in the crop improvement than other methods. 
  • Only effective means of combining together the desirable characters of two or more varieties.
  • Can create new genetic combinations of already existing characters and new genetic variations. 
  • It also exploits and utilizes hybrid-vigour.
  • Hybridization can be - 
  1. intravarietal
  2. intervarietal (between two varieties of the same species)
  3. interspecific (between two species of the same genus) and
  4. intergeneric (between two genera of the same family). 
  • As parental plants are distantly related, such crosses are also called wide/ distant crosses. 
  • Interspecific and intergeneric hybrids are seldom to occur in the nature.
Main steps of the plant breeding program (Hybridization)  :
1. Collection of Variability :
  • Wild species and relatives of the cultivated species having desired traits, should be collected and preserved. 
  • The entire collection having all the diverse alleles (i.e. variations) for all genes in a given crop, is called germplasm collection. 
  • Variations are useful in the selection. 
  • Germplasm conservation can be done in following ways-
  1. In situ conservation : It can be done with the help of forests and Natural Reserves.
  2. Ex situ conservation : It is done through botanical gardens, seed banks, etc.
2. Evaluation and Selection of Parents :
  • Important and essential step. 
  • Collected germplasm is evaluated (screened) to identify plants with desirable characters.
  • Selected parents must be healthy, vigorous and should show desirable but complementary features. 
  • The selected parents are selfed for three to four generations to make them pure or homozygous. 
  • It is made sure that only pure lines are selected, multiplied and used in the hybridization.
3. Hybridization :

  • Variety showing maximum desirable features is selected as female (recurrent) parent and the other one as male parent (donor) which lacks good characters found in recurrent parent.
  • The pollen grains from anthers of male parent are collected and then artificially dusted over stigmas of emasculated flowers of female parent. 
  • Pollination is followed by seed and fruit formation in due course. 
  • The seed, thus obtained represents the hybrid generation.
  • The hybrid F1 progeny is selected and evaluated forthe desired combinations of characters.

4. Selection and Testing of Superior Recombinants :

  • F1 hybrid plants showing superiority over both the parents and having high hybridvigour, are selected. 
  • Such hybrids are then selfed for few generations to make them homozygous for the said desirable characters till there is a state of uniformity, so that the characters will not segeregate further.

5. Testing, Release and Commercialization of New Cultivars :
  • Newly selected lines are evaluated for the productivity and other features like disease resistance, pest resistance, quality, etc.
  • Initially, these plants are grown under controlled conditions of water, fertilizers, etc. and their performance is recorded. 
  • The selected lines are then grown for three generations at least in natural field, in different agroclimatic zones.
  • Finally variety is released as new variety for use by the farmers.
  • Many high yielding, hybrid varieties of rice, wheat, sugarcane, millets, developed through hybridization.
  • Have helped farmer community to attain record agricultural production in India since 1961. This is called green revolution.
Indian Hybrid Crops :
1. Wheat and Rice :
  • In 1960s, wheat and rice production increased tremendously. 
  • Norman E. Borlaug developed semi-dwarf varieties of wheat. 
  • Sonalika and Kalyan Sona are two of the hybrid wheat varieties, grown in India.
  • Semi-dwarf rice varieties were taken from IR−8 (International Rice Research Institute) and Taichung native−I (from Taiwan) and introduced in India. 
  • Jaya, Padma and Ratna are the better-yielding, semi-dwarf rice varieties that were developed later.
2. Sugarcane :
  • Saccharum barberi is a native of North India and S. officinarum belongs to South India.
  • S. officinarum has thicker stem and high sugar contents, but it does not grow well in North India.
  • These two varieties were crossed to get the desirable qualities of both (high sugar content, thicker stem and the ability to grow in North India). 
  • CO -419, 421, 453 are high yielding and having high sugar contents are developed in India at Coimbatore (Tamilnadu).
3. Millets :

  • Hybrid maize (Ganga-3), Jowar (CO-12), and Bajra (Niphad) have been successfully developed in India.
  • These varieties are high yielding and resistant to water stress.
  • The basic objective of breeding for disease resistance is to develop inherent quality in the plant to prevent the pathogen from causing the disease. Such varieties of plants are called disease resistant plants. 
  • The basic technique used is the same as for normal hybridization process.
B. Mutation Breeding :

  • Mutation is sudden heritable change in the genotype, caused naturally. 
  • It can also be induced by application of chemical mutagens.
  • Natural (physical) mutagens are : High temperature, high concentration of CO2, X rays, UV rays.
  • Chemical mutagens are : Nitrous acid, EMS (Ethyl- Methyl- Sulphonate), Mustard gas, Colchicine, etc.
  • Seedlings or seeds are irradiated by CO- 60, exposed to UV bulbs, X ray machines, etc.
  • Mutagens cause gene mutations and chromosomal aberrations. 
  • The treated seedlings are then screened for resistance to diseases/ pests, high yield, etc. 
  • e.g. Jagannath variety of rice, NP 836 variety of wheat (rust resistant), Indore-2 variety of cotton (resistant to bollworm), Regina-II variety of cabbage (resistant to bacterial rot), etc.
Plant Breeding for Developing a Resistance to Insect Pest:

  • Insects being herbivores, incur heavy loss in the quantity and quality of crops. 
  • Resistance in crops can be developed by following ways :
  1. Development of morphological characters like hairy leaves in cotton and wheat develop vector resistance from jassids and cereal leaf beetle, respectively.
  2. Solid stem in wheat leads to resistance to stem borers.
  3. Biochemical characters provide resistance to insects and pests. 
  4. For example, the high aspartic acid, and low nitrogen and sugar content in maize, lead to resistance against stem borers.
  5. The nectar-less cotton having smooth leaves develop resistance against bollworms.

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