Sunday, July 26, 2020

Strategies for Enhancement in Food Production - Notes | Class 12 | Part 2 : Plant Breeding



-    It is the manipulation of plant species to create desired plant types suitable for better cultivation, better yields and disease resistance.
-    Green Revolution: The development and flourishing of the agriculture. It was dependent on plant breeding.
-    Classical plant breeding involves hybridization of pure lines and artificial selection to produce desirable traits.
-    Now molecular genetic tools are used for plant breeding.
Desirable traits for plant breeding:
o  Increased crop yield and quality.
o  Increased tolerance to environmental stresses (salinity, extreme temperatures & drought).  
o  Increased resistance to insect pests and pathogens.

Steps of Plant breeding

(i) Collection of genetic variability

-    In wild relatives of many crops, pre-existing genetic variability is available.
-    Collection and preservation of wild varieties, species and relatives of the cultivated species is a pre-requisite for effective exploitation of natural genes.
-    The entire collection of plants/seeds having all the alleles for all genes in a given crop is called germplasm collection.

(ii) Evaluation and selection of parents

-    The germplasm is evaluated for identifying plants with desirable characters.
-    Selected plants are multiplied and used for hybridisation.
-    Pure lines are created wherever desirable and possible.

(iii) Cross hybridisation of the selected parents

-    In this, desired characters are genetically combined from 2 different parents to produce hybrid plant.
-    E.g. high protein quality of one parent is combined with disease resistance from another parent.
-    Limitations:
o  Very time-consuming and tedious process.
o  Hybrids may not combine the desirable characters. Usually only hundreds to a thousand crosses show the desirable combination.
(iv) Selection & testing of superior recombinants

-    It is crucial to the success of the breeding objective and requires careful scientific evaluation of the progeny.
-    It yields plants that are superior to both parents.
-    These are self-pollinated for several generations till they reach a state of uniformity (homozygosity), so that the characters will not segregate in the progeny.

(v) Testing, release & commercialization

-    The newly selected lines are evaluated for their yield and other agronomic traits of quality, disease resistance, etc.
-    This is done by growing them in research fields and recording their performance under ideal fertiliser application irrigation and other crop management practices.
-    The evaluation is followed by testing the materials in farmers’ fields, for at least 3 growing seasons at several locations in the country, representing all the agro-climatic zones. The material is evaluated in comparison to the best available local crop cultivar (a check or reference cultivar).

Wheat and Rice:

-    In India, food production has increased by the development of high yielding varieties of wheat and rice in the mid-1960s (Green Revolution).  
-  During 1960-2000, wheat production increased from 11 million tons to 75 million tons. The rice production increased from 35 million tons to 89.5 million tons.
-   Nobel laureate Norman E. Borlaug (International Centre for Wheat & Maize Improvement, Mexico) developed semi-dwarf wheat.
-    In 1963, high yielding and disease resistant wheat varieties like Sonalika & Kalyan Sona were introduced in India.
-  Semi-dwarf rice varieties were derived from IR-8, (developed at International Rice Research Institute (IRRI), Philippines) and Taichung Native-1 (from Taiwan). Later better-yielding semi dwarf varieties Jaya and Ratna were developed in India.

Sugar cane

Saccharum barberi (grown in north India, but poor sugar content & yield) was crossed with Saccharum officinarum (tropical canes in south India, thicker stems and higher sugar content but do not grow well in north India) and got a hybrid sugar cane having desirable qualities like high yield, thick stems, high sugar and ability to grow in north India.

Millets: Hybrid maize, jowar & bajra developed in India. It includes high yielding varieties resistant to water stress.

Plant Breeding for Disease Resistance

-    Plant diseases cause crop losses up to 20-30% or even total.
-    Disease-resistant cultivars enhance food production and helps to reduce the use of fungicides and bactericides.
-    Resistance of the host plant is the genetic ability to prevent the pathogens from disease.
-    Some plant diseases:
o  Fungal: Rusts. E.g. brown rust of wheat, red rot of sugarcane and late blight of potato.
o  Bacterial: Black rot of crucifers.
o  Viral: Tobacco mosaic, turnip mosaic, etc.

Methods of breeding for disease resistance

1. Conventional breeding: The steps are:

o  Screening germplasm for resistance sources.
o  Hybridisation of selected parents.
o  Selection and evaluation of the hybrids.  
o  Testing and release of new varieties.
Some crop varieties bred by Conventional method:
Resistance to
Leaf & stripe rust, hill bunt
Pusa swarnim (Karan rai)
White rust
Pusa Shubhra,
Pusa Snowball K-1
Black rot and curl blight black rot
Pusa Komal
Bacterial blight
Pusa Sadabahar
Chilly mosaic virus, Tobacco mosaic virus and leaf curl.
-    Conventional breeding is constrained by the availability of limited number of disease resistance genes.
-    Inducing mutations in plants and screening them for resistance help to identify desirable genes. Such plants can be multiplied directly or can be used in breeding.
-    Other breeding methods are selection amongst somaclonal variants and genetic engineering.

2. Mutation breeding:

Mutation (sudden genetic change) can create new desirable characters not found in the parental type.
Mutation breeding is the breeding by mutation using chemicals or radiations (e.g. gamma rays) to produce plants with desirable characters. Such plants are selected and multiplied directly or used as a source in breeding.
E.g. In mung bean, resistance to yellow mosaic virus and powdery mildew were induced by mutations.
- Resistant genes from wild species have introduced into the high-yielding cultivated varieties. E.g. In bhindi (Abelmoschus esculentus), resistance to yellow mosaic virus was transferred from a wild species. It resulted in a new variety of A. esculentus called Parbhani kranti.
-    Resistance genes can be transferred by sexual hybridisation between the target and the source plant.

Plant Breeding for Developing Resistance to Insect Pests

-    Morphological, biochemical or physiological characteristics give insect resistance in host crop plants. E.g.
o  Hairy leaves: E.g. resistance to jassids in cotton and cereal leaf beetle in wheat.
o  Solid stems in wheat lead to non-preference by the stem sawfly.
o  Smooth leaved and Nectar-less cotton varieties do not attract bollworms.
o  High aspartic acid, low nitrogen and sugar content in maize leads to resistance to maize stem borers.
-    Sources of resistance genes for breeding are cultivated varieties, germplasm collections of crop or wild relatives.
Some crop varieties bred for insect pest resistance:
Insect pests
Brassica (rapeseed mustard)
Pusa Gaurav
Flat bean
Pusa Sem 2,
Pusa Sem 3
Jassids, aphids & fruit borer
Okra (Bhindi)
Pusa Sawani,
Pusa A-4
Shoot and Fruit borer
Plant Breeding for Improved Food Quality

-    More than 840 million people in the world do not have adequate food. 3 billion people suffer from micronutrient, protein and vitamin deficiencies (‘hidden hunger’).
-    Breeding crops with higher levels of nutrients is called Biofortification. It helps to improve public health.

Objectives of breeding for improved nutritional quality:

§  To improve Protein content and quality.
§  To improve Oil content and quality.
§  To improve Vitamin content.
§  To improve Micronutrient and mineral content.

Examples for hybrids with improved nutritional quality:

o  Maize hybrids having twice the amount of amino acids, lysine & tryptophan compared to existing maize hybrids.
o  Wheat variety, Atlas 66, having high protein content.
o  Iron-fortified rice variety containing over five times as much iron as in common varieties.
o  Vitamins & mineral rich vegetable crops: Released by Indian Agricultural Research Institute, New Delhi.
§ Vitamin A enriched carrots, spinach, pumpkin.
§ Vitamin C enriched bitter gourd, bathua, mustard, tomato.
§ Iron & calcium enriched spinach & bathua.
§  Protein enriched beans (broad, lablab, French & garden peas).

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