Crop Improvement Methods:
Top 6 Methods
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.
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.”
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.
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(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.
Domestication is the taming and training of plants to suit the
human needs and putting them under cultivation.
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 polyploids 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..