agriculture2 Flashcards
Plant breeding is
an art and science of improving the heredity
of plants for the benefit of mankind
n i vavilov
“Studies on the Origin
of
Cultivated Plants”
A center of origin (or centre of diversity)
is a geographical area
where a group of organisms, either domesticated or wild, first
developed its distinctive properties.
1.crop plants evolved from
wild species in the areas showing great diversity and termed
them as Primary centres of origin.
Main features of these centres are given below:
1. They have wide genetic diversity.
2.Have large number of dominant genes.
3.Mostly have wild characters.
4. Exhibit less crossing over.
5. Natural selection operates
2.crop species show considerable diversity of
forms although they did not originate there; such areas are
known as Secondary centres of origin.
E.g. Sorghum. The primary centre of origin for this crop is
Africa but India exhibits maximum diversity for this crop
main features.
1. Have lesser genetic diversity than primary centres.
2. Have large number of recessive genes.
3. Mostly have desirable characters.
4. Exhibit more crossing over
5. Both natural and artificial selections operate.
- Microcenters:J. R. Harlon made exploration to turkey and
added micro centres.
*In some case, small areas within the centres of
diversity exhibit tremendous genetic diversity of
some crop plants.
main features of micro centres are
1.They represent small areas within the centres of diversity.
2.Exhibit tremendous genetic diversity.
3.The rate of natural evolution is faster than larger areas.
4.They are important sites for the study of crop evolution.
CoO
central america - corn &chily
south america- potato &peanut
mediteran- olive and cabbae
west asia - wheat&barley
hornafrica- coffee
india- chickpea &apple
china - soybean
southeast asia- sugarcane &rice
Law of homologous series of variation.
* Vavilov also developed the concept of parallel series of
variation or Law of homologous series of variation.
- This concept states that a particular variation observed in a crop
species is also expected to be available in its another related
species
OBJECTIONS TO VAVILOV’S THEORY
- According to Vavilov whenever a crop plant exhibits
maximum diversity, that place is the centre of origin for that
crop. But this view is no longer valid. E.g. maize and tomato. - For maize the centre of diversity is Peru but
archeological evidence shows Mexico as centre of origin. - For tomato, South America is considered to be primary centre
of origin but it is Mexico as per archeological evidence. - Secondly Vavilov stated that primary centre is marked by a
high frequency of dominant genes in the centre and recessive
genes towards the periphery. But it is not so. E.g. Wheat,
maize, oil palm - Vavilov’s claim that centre of origin confined to mountainous
regions only. But this is not the case. For E.g. Maize exhibits
maximum diversity in plains - Many crops have more than one centre of origin E.g. Balsam,
Sorghum. In some crops centre of domestication cannot be
determined for want of suitable evidence. - Vavilov could not adequately cover Africa.
- Australia was not at all covered.
- These two continents have tremendous wealth of crop
genetic diversity of several crop plants
Megagene centre and Microgene centres
*Zhukovsky, in 1965, recognised 12 mega-gene centres of
crop plant diversity.
*Mega gene centres were the places where cultivated
plant species exhibit diversity
*micro gene centre is the place where wild species
occur.
The cultivated forms are believed to have first
originated in these microgene centres.
*These centres may not be the centres of origin of the
species concerned, but they are the areas of the maximum
diversity of these species.
PLANT GENETIC RESOURCES - GERMPLASM
genetic resources or
gene pool or
genetic stock
The sum total of hereditary material i.e. all the
alleles of various genes, present in a crop species
and its wild relatives is referred to as germplasm.
Components/Types of germplasm
a. Landraces (J.R.Harlan, 1975) : Primitive cultivars selected and
cultivated by farmers for many generations.
Main features
* These were not bred like modern cultivars and evolved
under subsistence agriculture
* Have high level of genetic diversity with high degree of
resistance to biotic and abiotic stresses
* Have broad genetic base with wider adaptability and
protection from epidemic pests and diseases
* They have recognizable morphology, name, nutritive value
* They are genetically diverse and balanced populations
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Components/Types of germplasm (contd.,)
b. Obsolete cultivars
*Improved varieties of recent past
*These were popular earlier and now have been replaced by new
varieties
*These have desirable characters and constitute an important part in
gene pool
*Eg. Wheat varieties K65, K68, Pb 591
*(Popular traditional tall varieties, have attractive grain colour and
chapatti making quality and good genetic resources and widely
used in wheat breeding programes for improvement of grain
quality)
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Components/Types of germplasm (contd.,)
c. Modern cultivars
*Currently cultivated high yielding varieties
*High yielding and uniformity and constitute a major part of working
collections and used as parents in breeding programmes
*However, these have narrow genetic base and low adaptability
compared to land races
d. Advanced breeding lines
*Pre-released plants which have been developed by plant breeders for
use in modern scientific plant breeding and are valuable part of gene
pool
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Components/Types of germplasm (contd.,)
e. Wild forms of cultivated species
*Have high degree of resistance to biotic and abiotic stresses
*These are utilized in breeding programs
f. Wild relatives
*Those naturally occurring plant sps. which have common
ancestry with crops and can cross with crop sps.
*These are important sources of resistance to stresses
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Components/Types of germplasm (contd.,)
g. Mutants
*Through mutation breeding, extra variability is created in
cultivated sps.
Eg. Mutant gene pool Dee-Geo-Woo-Gen in rice and Norin 10
in wheat proved to be valuable genetic resources in
development of high yielding and semi dwarf varieties in the
respective cro
Gene pool concept
Proposed by Harlan and De Wet in 1971.
Gene pool consists of all the genes and their alleles
present in all such individuals, which hybridize with
each other
Classification of gene pool on the basis
1. Area of collection
a)Indigenous (collected within the country)
b)exotic (collected from other country)
2. Domestication
a)Cultivated (germplasm of domesticated species)
b)Wild (germplasm of uncultivated species)
genepool classifi
- Duration of conservation
1.Base collection, 2. Active collection 3. Working collection
Base collection
*Plant materials which are meant for long term storage
*Regeneration is carried out after a long time dep. on viability
*Seed viability should not drop to 95% before regeneration
*Seeds with 5 ± 1% moisture content and stored at -18 to -20˚C
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Active collection
*Meant for medium term storage (10-15 years)
*Subjected
to
regeneration,
multiplication,
distribution, documentation after every 10-15 years
*Stored at zero ˚C and moisture should be around 8%
*Routine germination after every 5-10 years
Working collection
*Stored for short term (3 to 5 years)
*Regularly used in crop improvement programmes
* No need to grow such materials every year
*Stored at 5-10˚C with moisture content of 8-10% - Crossability
- Primary gene pool
- Secondary gene pool
- Tertiary gene pool
evaluation,
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Primary gene pool (GP1)
* Crossing is easy and leads to production of fertile hybrids.
* Includes plants of the same species or closely related sps.
* Genes can be exchanged b/w lines by normal crosses
Secondary gene pool (GP2)
* Leads to partial fertility on crossing with GP1
* Includes plants belonging to related sps.
* on crossing with GP1, resultant hybrids are sterile and
some are fertile
* Transfer of gene from such material to GP1 is possible
but
difficult
Tertiary gene pool (GP3)
* Leads to production of sterile hybrids on crossing with
GP1
* Includes material which can be crossed with GP1 but
hybrids are sterile
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* Transfer of genes from GP3 to GP1 is possible with help
Genetic erosion
The gradual loss of variability from cultivated species,
and their wild forms and wild relatives is called genetic
erosion.
Main causes of genetic erosion
*Replacement of genetically variable land races by the
improved genetically uniform pureline or hybrids
*Improved crop management practices have eliminated the
weedy forms of many crops
*Increasing human needs have extended farming and
grazing into forests, the habitats of most wild species
*Development activities like hydroelectric projects, roads,
industrial areas, railways, buildings, etc. have disturbed
the wild habitat
*Introduction of a weedy species may result in the
invasion of wild habitats by this species and lead to the
elimination of the native wild relatives of crops
Activities in Germplasm Maintenance
1.Exploration and collection
2.Conservation
3.Evaluation
4.Documentation
5.Distribution
6.Utilization
1.Germplasm Conservation
Conservation refers to protection of genetic diversity of
crop plants from genetic erosion.
Two important methods of germpalsm conservation
i) In-situ conservation ii) ex situ conservation.
i) In - situ conservation:
Conservation of germplasm under natural conditions is
referred to as in situ conservation. This is achieved by
protecting the area from human interference.
Such an area is often called natural park, biosphere reserve
or gene sanctuary. NBPGR, New Delhi has established gene
sanctuaries in Meghalaya for citrus, North Eastern regions
for musa, citrus, oryza and saccharum.
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Advantage and disadvantages of In - situ conservation
Merit:
The wild species and the complete natural or
semi natural ecosystems are preserved together.
Demerits:
*Each protected area will cover only very small portion of
total diversity of a crop species, hence several areas will
have to be conserved for a single species.
*The management of such areas also poses several
problems.
*This is a costly method of germplasm conservation.
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ii) Ex - situ conservation:
It refers to preservation of germplasm in gene
banks. This is the most practical method of germplasm
conservation.
Advantages
* It is possible to preserve entire genetic diversity of a
crop species at one place.
* Handling of germplasm is also easy.
* This is a cheap method of germplams conservation.
This type of conservation can be achieved in 5 ways.
a) Seed banks:
Germplam is stored as seeds of various genotypes.
Seed conservation is quite easy, relatively safe and
needs minimum space .
Seeds are classified on the basis of their storability
into two major groups.
1) Orthodox 2) Recalcitrant
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Orthodox seeds: Seeds which can be dried to low
moisture content (5%) and stored at low temperature
without losing their viability for long periods of time is
known as orthodox seeds. (eg.) Seeds of corn, wheat, rice,
carrot, papaya, pepper, chickpea, cotton, sunflower.
Recalcitrant: Seeds which show very drastic loss in
viability with a decrease in moisture content below 12 to
13% are known as recalcitrant seeds. (e.g) citrus, cocoa,
coffee, rubber, oilpalm, mango, jack fruit etc.
Seed storage:
Based on duration of storage, seed bank collections
are classified into three groups. (1) Base collections (2)
Active collections and (3) Working collection.
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b. Plant Bank: ( Field gene bank )
It is an orchard or a field in which accessions of fruit trees or
vegetatively propagated crops are grown and maintained.
Limitations: 1. Require large areas
2. Expensive to establish and maintain.
3. Prone to damage from disease, insect attacks
4. Man – made
5. Natural disasters
6. Human errors in handling
c. Cell and organ banks:
A
germplasm collection based on cryopreservation
(at – 196OC in liquid nitrogen) in the form of embryogenic cell
cultures, somatic/ zygotic embryos.
d.DNA banks:
In these banks, DNA segments from the genomes of
germplasm accessions are maintained and conserved.
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e. Shoot tip banks:
Germplasm is conserved as slow growth cultures of
shoot-tips and nodal segments.
Advantages
* Each genotype can be conserved indefinitely free from
virus or other pathogens.
* It is advantageous for vegetatively propagated crops like
potato, sweet potato, cassava etc., because seed
production in these crops is poor
* Vegetatively propagated material can be saved from
natural disasters or pathogen attack.
* Long regeneration cycle can be envisaged from meristem
cultures.
* Regeneration of meristerms is extremely easy.
* Plant species having recalcitrant seeds can be easily
conserved by meristem cultures.
Central tuber crops research
Institute(CTCRI), - Tuber crops other
than potato
Thiruvananthapuram
Central Rice Research Institute,
(CRRI)Cuttack- Rice
Central Institute for Cotton
Research (CICR), Nagpur
2 Central Plantation Crops Research
Institute(CPCRI), Kasargod
3 Central Potato Research Institute,
Shimla
IRRI
CIMMYT
ICRISAT
- NBPGR (National Bureau of Plant Genetic Resources),
New Delhi - Established by ICAR in 1976
Basic function is to conduct research and promote
collection, conservation, evaluation, documentation and
utilization of crop genetic resources in India
Main functions of NBPGR
* Sole agency in India for export and import of plant genetic
resources and helps in exchange of germplasm
*Promotes national genetic resources activities
* Five stations in India 1.Shimla,HP 2.Jodhpur,Rajastan
3. Akola,Maharastra 4. Kanyakumari 5. Shillong, Meghalaya
*Organizes national and intl. explorations to collect GP
*Provides guidance about cold storage facilities for medium
and short term conservation of germplasm
*Takes decision about setting up of gene sanctuaries for
endangered species in India
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Activities of NBPGR
*Introduces required germplasm from its other agencies in other
countries
*Explorations inside and outside the country to collect valuable
germplasm
*Testing, multiplication and maintenance of germplasm obtained
*Supplies germplasm to various institutions on request
*Supplies germplasm to its counterparts or other agencies in other
countries
*Maintains records of plant name, variety name, propagating
material, special characteristics, source, date and other
information about germplam
*Publishes its exchange and collection lists.
*Sets up the natural gene sanctuaries of plant
*Looks after improvement of certain plants like medicinal and
aromatic plants
Mechanisms promoting self-pollination
- Cleistogamy
In this case, flower does not open at all. This ensures
complete self- pollination since foreign pollen cannot reach
the stigma of a closed flower. Cleistogamy occurs in some
varieties of wheat (Triticum sp.), oats (Avena sp.) barley (H.
vulgare) and in a number of other grasses. - Chasmogamy
In some species, the flowers open, but only after
pollination has taken place. This occurs in many cereals,
such as, wheat, barley, rice and oats. Since the flower does
open, some cross-pollination may occur. - In crops like tomato (L. esculentum) and brinjal
(S. melongena) the stigmas are closely surrounded by
anther. Pollination generally occurs after the flowers open.
But the position of anthers in relation to stigmas ensures
self-pollination.
25 - In some species, flowers open but the stamens and the
sigma are hidden by other floral organs. In several
legumes, e.g., pea (P. sativum), mung (V. radiate), urd
(V.mungo), soybean (G. max) and gram (C.arietinum), the
stamens and the stigma are enclosed by the two petals
forming a keel. - In a few species, stigmas become receptive and
elongate through the staminal columns. This ensures
predominant self-pollination.
echanism promoting cross pollination
1.Dicliny or unisexality is a condition, in which the flowers are
either staminate (male) or pistillate (female)
1(a). Monoecy. Staminate and pistillate flowers occur in the
same plant, either in the same inflorescence, e.g., castor,
mango (Mangifera indica), banana(Musa sapientum) and
coconut or in separate inflorescences, e.g, maize.
1(b). Dioecy. The male and female flowers are present at
different plants, i.e., the plants in such species are either male
or female, e.g, papaya (C. papaya), datepalm, palmsugar
- Dichogamy
Stamens and pistils of hermaphrodite flowers may
mature at different times facilitating cross-pollination.
2a. Protogyny In crop species like bajra, pistils mature
before stamens. Eg. Cumbu
2b. Protandry In crop like maize and sugarbeets, stamens
mature before pistils. Eg. Maize - In Lucerne or alfalfa, stigmas are covered with a waxy film.
The stigma does not become receptive until this waxy film is
broken. The waxy membrane is broken by the visit of honey
bees, which also effect cross-pollination. - A combination of two or more of the above mechanisms
may occur in some species. This improves cross-pollination.
Eg. maize exhibits both monoecy and protandry.
- 5) Heterostyly: Different length of style and
filaments E.g Linseed. - 6) Herkogamy: Presence of physical barrier or
mechanical obstacles between the anther and
stigma ensures cross pollination. E.g.
(Calotropic gigantia).
7. Self- incompatibility. It refers to the inability of viable pollen
to fertilize the same flower or other flowers on the same plant.
Self-incompatibility is of two types: sporophytic and
gametophytic. In both the case, flowers do not set seed on
selfing. Self-incompatibility is common in several specie of
Brassica (mustard, cauliflower) some species of Nicotiana,
radish, rye and many grasses. It is highly effective in
preventing self-pollination.
8. Male Sterility. Male sterility refers to the absence of
functional pollen grains in otherwise hermaphrodite flowers.
Genetic Consequences of
Cross Pollination
1) It preserves and promotes heterozygosity in
population.
2) Cross pollinated species shows inbreeding depression
and considerable heterosis.
3) Usually hybrid and synthetics without reducing
heterozygosity
Self – Incompatibility: It refers to the failure of pollen grain to fertilize
the same flower or other flower on the same plants.
- Incompatibility may occur due to the lot of reason.
- 1) Pollen grain fails to germinate on the stigma.
2) Pollen grain germinates but the pollen tube fails to enter the stigma.
3) Sometimes pollen tube enters the style but growth is very slow to
effect fertilization.
4) Pollen tube enters the ovule but there is no fertilization due to
degeneration of egg cell.
5) Fertilization is effected but embryo degenerate at very early stage.
Main features of self incompatibility
- Self incompatibility is an important outbreeding
mechanism which prevents autogamy and promotes
allogamy - Self incompatible species do not produce seed on self
pollination but lead to normal seed set on cross
pollination - It maintains high degree of heterozygosity in a species
due to outbreeding and reduces homozygosity due to
elimination of inbreeding or selfing - Self incompatibility results due to morphological, genetic,
physiological and biochemical causes. It is not under
simple genetic control
