9. STRATEGIES FOR ENHANCEMENT IN FOOD PRODUCTION
II. 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
-
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
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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