PLANT BREEDING: backcross breeding, heterosis and their genetic basis.

1. PLANTBREEDING
Submitted to Submitted by
Merin Alice Jacob Krishnapriya M
Assistant Professor Roll. No:10
Dept. of Botany 1st M.Sc. Botany
St. Teresa’s College, Ernakulam St. Teresa’s College, Ernakulam
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2. BACKCROSSBREEDING
• Backcross is the crossing of an F1 hybrid with one or other of it’s parents.
• But in plant breeding. Backcross method involves crossing of an F1 hybrid
with it’s superior parent whose genotype has to be transferred to local
variety.
• Backcrosses are usually used to impart to a local variety one or more
desirable characters inherent in another variety, without disturbing genetic
integrity of former.
• Here, recipient parent is used repeatedly to get all desired genes from donor
parent.
• Recipient parent is called recurrent parent and donor parent is called non-
recurrent parent.
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3. • The F1 hybrids and subsequent generations, which have acquired foreign
genes from donor, may be crossed again with recurrent parent.
• Backcrossing is most used to eliminate a defect (susceptible to disease) from
a variety which has a set of desirable characters.
• The hybrids have to be crossed with recurrent parent over and again in order
to get rid of undesirable properties of parental form.
• During the process, recurrent parent’s genotype is restored in hybrid
progenies and undesirable genes of donor get eliminated.
• So, segregation becomes more simple.
• As a rule, five or six backcrosses are sufficient for nearly complete transfer
of desirable characters.
• In some cases, ten backcrosses are used.
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4. BACKCROSSBREEDINGPROCEDURES
1. Selection of parents :
• The recurrent parent (parent A) is most popular variety of region.
• The donor parent (parent B) should possess character desired to be
transferred in genetic background of recurrent parent (A).
• It must be controlled by one or other major genes.
2. Dominant gene controlled character :
• If the character to be transferred in genetic background of recurrent parent
(A) is governed by a dominant gene.
• Then F1of cross (A×B) could be backcrossed to recurrent parent (A)
successively up to BC4-BC5 generations.
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5. • Thereafter, F2 and F3 generations should be raised and selection of new
genotype lines having restored almost 99% of agronomic traits of recurrent
parent plus one character transferred from donor parent B.
• After purification, homozygous lines should be bulked and tested in progeny
row trials for one year along with recurrent parent as check to transfer
desired character.
• There is no need to raise F2 and F3 generations in initial backcross
generation (BC1, BC2, BC3 and BC4) because in every backcross
generation, the dominant character is visible.
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6. 3. Recessive gene controlled character :
• In cases, where character desired under transfer is controlled by recessive
gene, in F1 generation it is not visible, hence breeder has to raise F2 and F3
generations after each backcross .
• Selection of desired genotypes should be done to each backcross generation.
• eg: BC1= The F1 of cross A×B.
• F1 is backcrossed to A (recurrent parent).
• The seeds of BC1 plants are selfed to produce F2 population (1500-2000
plants) in which intensive seed is done for desired character of donor parent
‘B’ combined with general characters of recurrent parent A.
• The selected plants are raised to produce F3 lines, among and within F3 lines
, selected plants recover traits of recurrent parent.
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7. HETEROSIS
• The term heterosis was first used by Shull in 1914.
• It is also called as true heterosis or euheterosis.
• It is defined as superiority of an F1 hybrid over both it’s parents in terms of
yield or some other character.
• Heterosis is manifested as an increase in vigour, size, growth rate, yield or
some other characteristics.
• But in some cases, hybrid may be inferior to weaker parent. This is also
regarded as heterosis.
• Often the superiority of F1 is estimated over average of two parents or mid-
parent.
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8. • If the hybrid is superior to mid-parent, it is known as average or relative
heterosis.
• If the hybrid is superior over superior parent =heterobeltiosis.
• Powers suggested that term heterosis should be used only when hybrid is
either superior or inferior to both parents.
• A/C to this, heterosis is the superiority of F1 hybrid over mid-parent and
heterobeltiosis is the superiority of F1 hybrid over superior parent.
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9. MANIFESTATIONSOFHETEROSIS
a) Increased Yield:
• Heterosis is expressed as increase in yield of hybrids.
• Higher yields are most important objective of plant breeding.
• The yield may be measured in terms of grain, fruit, seed, leaf, tubers or the
whole plant.
b) Increased Reproductive Ability:
• The hybrids exhibiting heterosis show an increase in fertility or reproductive
ability.
• Higher yield of seeds or fruits or other propagules, stem in sugarcane etc.
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10. c) Increase in Size and General Vigour:
• Hybrids are generally more vigorous ie, healthier and faster growing and
larger in size than their parents.
• The increase in size is usually a result of increase in number and size of cells
in various plant parts.
Eg: increased size in tomato, head size in cabbage, cob size in maize etc.
d) Better Quality:
• In many cases, hybrids show improved quality.
• This may or may not be accompanied by higher yields.
Eg: many hybrids in onion show better keeping quality but not yield, than open
pollinated varieties.
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11. e) Earlier Flowering and Maturity:
• In many cases, hybrids are earlier in flowering and maturity than parents.
• This may associate with lower total plant weight.
• But earliness is highly desirable in many cases, particularly vegetables.
• Many tomato hybrids are earlier than their parents.
f) Greater Resistance to Diseases and Pests:
• Some hybrids are known to exhibit a greater resistance to insects or diseases
than their parents.
g) Greater Adaptability:
• Hybrids are more adapted to environmental changes than inbreds.
• Variance of hybrids is smaller than inbreds.
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12. h) Faster Growth Rate:
• Hybrids show faster growth rate than their parents.
• But total plant size of hybrids may be comparable to that of parents.
• Faster growth rate is not associated with larger size.
i) Increase in Number of a Plant Part:
• There is an increase in number of nodes, leaves and other plant parts.
• But total plant size may not be larger.
Eg: hybrids in beans.
j) Manifestation at Molecular Level:
• Increased rate of DNA duplication, genetic transcription, genetic translation
and enzyme activity.
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13. k) Manifestation at Metabolic Level:
• Increased and effective co-ordination and regulation of metabolic processes
and morphogenetic events.
l) Manifestation at Cellular Level:
• Increased rate of cell proliferation.
m) Manifestation at Organismal Level:
• High rate of cellular growth and differentiation, increased synthesis,
accumulation and utilization of substances etc.
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14. GENETICBASISOFHETEROSISANDINBREEDINGDEPRESSION
1. DOMINANCE HYPOTHESIS
• First proposed by Davenport in 1908.
• Later expanded by Bruce, Keeble and Pellew in 1910.
• Hypothesis suggest that at each locus the dominant allele has a favourable
effect, while the recessive allele has an unfavourable effect.
• In heterozygous state, deleterious effects of recessive alleles are masked by
their dominant alleles.
• Thus, heterosis results from masking of harmful effects of recessive alleles
by their dominant alleles.
• Inbreeding depression – due to harmful effects of recessive alleles, which
become homozygous due to inbreeding.
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15. A/c to this,
• Heterosis is not a result of heterozygosity.
• It is the result of prevention of expression of harmful recessives by their
dominant alleles.
• Inbreeding depression does not result from homozygosity but from
homozygosity of recessive alleles, which have harmful effects.
eg: In open-pollinated populations, plants are highly heterozygous.
• So, they do not show harmful effects of large number of deleterious
recessive alleles present in population.
• Inbreeding increases homozygosity. So, many recessive alleles become
homozygous.
• Lethal recessive alleles are eliminated by natural selection. But recessive
alleles with smaller harmful effects survive in homozygous condition.
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16. • Such alleles reduce vigour and fertility of inbred lines that carry them in the
homozygous state.
• Inbred lines are nearly homozygous and different inbred lines would receive
different proportions of dominant and recessive alleles.
• Different inbred lines vary in vigour and yield.
• It should be possible to isolate such inbreds that have all dominant alleles
present in population.
• Such inbreds will be vigorous as open pollinated varieties.
• But such inbreds have not been isolated yet.
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17. OBJECTIONS
a) Failure in the Isolation of Inbreds as Vigorous as Hybrids :
• A/c to this hypothesis, it should be possible to isolate inbreds with all
dominant genes.
• Such inbreds would be vigorous as F1 hybrids.
• But such inbreds are not isolated yet.
b) Symmetrical Distribution in F2 :
• In F2, dominant and recessive characters segregate in ratio of 3:1.
• A/c to this, quantitative characters should not show symmetrical distribution
in F2.
• This is because dominant and recessive phenotypes would segregate in
proportion (3/4 +1/4)^n.
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18. • n is number of genes segregating.
c) Magnitude of Heterosis :
• The inbred lines have improved in terms of per sec performance over
decades.
• If dominance were main cause of heterosis, magnitude of heterosis generated
by such inbreds should have declined.
d) Progressive Heterosis in Tetraploids :
• In autotetraploids, hybrids between two inbred lines show heterosis.
• Such hybrids have genotype A1A1A2A2 at a given locus.
• But tetraploid hybrids have genotypes A1A2A2A3, A1A1A2A3 etc (three
different alleles) and A1A2A3A4 ( 4 different alleles) show greater heterosis
than A1A1A2A2 (2 different alleles).
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19. 2. OVERDOMINANCE HYPOTHESIS
• Proposed by East and Shull in 1908.
• Also known as single gene heterosis, super dominance, cumulative action of
divergent alleles and stimulation of divergent alleles.
• The idea of super dominance, ie, heterozygote superiority was put forth by
Fisher in 1903 and was later elaborated by East and Shull.
• A/c to this, in heterozygotes at least some of loci are superior to both
relevant homozygotes.
• Thus heterozygote Aa would be superior to both homozygotes AA and aa.
• Heterozygosity is essential for and is cause of heterosis.
• Homozygosity resulting from inbreeding produces inbreeding depression.
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20. • In 1936, East proposed that at each locus showing over dominance, there are
several alleles ie, a1, a2, a3, a4….etc with increasingly different functions.
• He also proposed that heterozygotes for more divergent alleles would be
more heterotic than those involving less divergent ones.
Eg: a1a4 would be superior to a1a2, a2a3 or a3a4.
Eg: In case of maize, gene ma affects maturity. The heterozygote Ma ma is
more vigorous than homozygotes Ma Ma and ma ma.
Eg: Gustafsson reported two chlorophyll mutants in barley that produce
larger and more number of seeds in heterozygous state than do their normal
homozygotes.
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21. PHYSIOLOGICALHYPOTHESIS
a) Nucleo-cytoplasmic interaction hypothesis :
• Proposed independently by Shull, Michalis and others.
• States that heterosis results from nucleo-cytoplasmic interactions which
involves effect of changed nucleus on unchanged cytoplasm and vice-versa.
b) Greater initial capital hypothesis :
• Put forward by Ashby in 1930.
• Holds that heterosis is due to greater initial size of embryo.
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22. INBREEDING
• Inbreeding or consanguinous mating is mating between individuals related
by descent or ancestry or it is the form of controlled breeding between
genetically related individuals of species.
• When individuals are closely related, eg: in brother-sister mating or sib-
mating, degree of inbreeding is high.
• The highest degree of inbreeding is achieved by selfing.
• The chief effect of inbreeding is an increase in homozygosity in progeny,
which is proportionate to degree of inbreeding.
• The measure of degree of inbreeding is provided by degree of homozygosity
in progeny.
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23. For example:
• Selfing reduces heterozygosity by a factor of ½ in each generation.
• The degree of inbreeding increases in the same proportion.
• Degree of inbreeding in any generation is equal to degree of homozygosity in
that generation.
• Inbreeding differs from outbreeding : Outbreeding promotes heterozygosity
and introduce new genes into the population.
• Most genes of undesirable or harmful traits are recessive and are only
expressed in homozygous condition.
• Thus, inbreeding promotes expression of harmful traits in progeny.
• This enables to sort out and exclude individuals with undesirable characters
from breeding practices. Thus careful selection is carried out.
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24. • The degree of inbreeding of an individual is expressed as inbreeding
coefficient (F).
• The value of F for an individual = probability of two alleles of a gene present
in that individual to have been derived from a single allele of a common
ancestor.
eg: an ancestor that occurs in the pedigree of both maternal and paternal
parents of this individual.
• In a random mating population, the value of F for any individual is 0.
• While, that for an individual produced by selfing of a plant from a random
mating population is ½.
• The value of F is cumulative over generations.
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25. INBREEDINGDEPRESSION
• It is defined as reduction or loss in vigour and fertility as a result of
inbreeding.
• Or continued inbreeding in regular succession may lead to progressive
decrease in growth, size, vigour, fitness and fertility of offspring.
• The degree of inbreeding depression depends on plant species concerned.
• But within a species, extent of depression is related to value of F.
• Inbreeding depression is common in case of such traits that form an
important component of fitness, while those that contribute little to fitness
usually show little or no inbreeding depression.
• The extent of inbreeding depression is not same in all lines produced by
inbreeding.
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26. • After several generations of inbreeding, a stage may be reached beyond
which no further inbreeding depression occurs.
• This stage is called inbreeding minimum.
• The crossing of inbred lines, which have reached inbreeding minimum, often
results in heterosis.
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27. DEGREE OF INBREEDING DEPRESSION
a) High Inbreeding Depression
• Several plant species eg: alfafa (Medicago sativa), carrot ( Daucus carota) etc
show very high inbreeding depression.
• A large proportion of plants produced by selfing show lethal characteristics
and do not survive.
• The loss in vigour and fertility is so great that very few lines can be
maintained after 3-4 generations of inbreeding.
• The lines that do not survive show greatly reduced yields, generally less than
25% of yield of open-pollinated varieties.
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28. b) Moderate Inbreeding Depression
• Many crop species, such as maize (Zea mays ), jowar, bajra etc show
moderate inbreeding depression.
• Selfing of progeny result in 2 types of individual i) normal types and ii)
weak, sublethal or lethal types.
• There is appreciable reduction in fertility and many lines reproduce so
poorly that they are lost. Elimination of 2nd category maintain population.
• However, large number of inbred lines can be obtained, which yield up to
50% of open-pollinated varieties.
• Production and maintenance of inbred lines are relatively easier in these
species than in those showing a high degree of inbreeding.
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29. c) Low Inbreeding Depression
• Several crop plants eg: onion, many cucurbits, rye, sunflower etc show only
a small degree of inbreeding depression.
• Only a small proportion of plants show lethal characteristics.
• The loss in vigour and fertility is small, rarely a line cannot be maintained
due to poor fertility.
• The reduction in yield due to inbreeding is small or absent.
• Some of inbred lines may yield as much as open pollinated varieties from
which they were developed.
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30. d) Zero Inbreeding Depression
• Self-pollinates progeny does not exhibit any effect of inbreeding depression.
• But shows some degree of heterosis.
• It is because these species reproduce by self fertilization and as a result, have
developed homozygous balance.
• In contrast, cross pollinated species exhibit heterozygous balance.
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31. IDEOTYPEBREEDING
• The term was introduced by Donald (1968).
• It is defined as a biological model, which is expected to perform or behave in
a particular manner within a defined environment.
• A/c to him, ‘ a crop ideotype is a plant model, which is expected to yield a
greater quantity or quality of grain, oil or other useful product when
developed as a cultivar’.
• It is also known as model plant type, ideal model plant type and ideal plant
type.
• In general terms, an ideotype is a conceptual model plant, which has all such
characteristics that are considered ideal for given environment.
• A model plant is optimally equipped for maximum yield under defined
environment.
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32. TYPESOFIDEOTYPE
a) Isolation Ideotype
• It is the model plant that performs best when plants are space-planted.
• In cereals, isolation ideotype is lax, free-tillering, leafy, spreading plant ie,
able to explore environment as fully as possible.
• It is unlikely to perform well at crop densities.
b) Competition Ideotype
• This ideotype performs well in genetically heterogenous poipulations, such
as segregating generation of crosses.
• In case of cereals, competition ideotype is tall, leafy, free-tillering plant ie,
able to shade it’s less aggressive neighbours and gain larger share of
nutrients and water.
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33. c) Crop Ideotype
• This ideotype performs best at commercial crop densities because it is a poor
competitor.
• It performs well when it is surrounded by plants of same forms.
• But it performs less well when it is surrounded by plants of other forms.
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34. FEATURESOFIDEOTYPEBREEDING
1. Emphasis on Individual Trait:
• Emphasis is given on individual morphological and physiological trait which
enhances yield.
• The value of each character is specified before initiating breeding work.
2. Includes Yield Enhancing Traits:
• Various plant characters to be included in ideotype are identified through
correlation analysis.
• Only those characters which exhibit positive association with yield are
included in the model.
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35. 3. Exploits Physiological Variation:
• Genetic difference exists for various physiological characters such as
photosynthetic efficiency, photo respiration, nutrient uptake etc.
• Ideotype breeding makes use of genetically controlled physiological
variation in increasing crop yields, besides various agronomic traits.
4. Slow Progress:
• Ideotype breeding is a slow method of cultivar development because
incorporation of various desirable characters from different sources into a
single genotype takes long time.
• More over, sometimes undesirable linkage affects progress adversely.
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36. 5. Selection:
• In ideotype breeding selection is focused on individual plant character which
enhances yields.
6. Designing of Model:
• Here, the phenotypes of new variety to be developed is specified in terms of
morphological and physiological traits in advance.
7. Interdisciplinary Approach:
• Ideotype breeding is in true sense an interdisciplinary approach.
• It involves scientist from disciplines of genetics, breeding, physiology,
pathology, entomology etc.
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37. 8. A Continuous Process:
• It is a continuous process, because new ideotype have to be developed to
meet changing and increasing demands.
• Thus development of ideotype is a moving target.
• Ideotype breeding differs from traditional breeding in the sense that values
for individual traits are specified in case of ideotype breeding.
• Whereas such values are not fixed and then efforts are made to achieve such
model.
• In traditional breeding, such models are not developed before initiation of
breeding programmes.
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38. METHODSOFIDEOTYPEBREEDING
Ideotype breeding consists of four important steps
1. Development of conceptual theoretical model.
2. Selection of base material.
3. Incorporation of desirable characters into single genotype.
4. Selection of ideal or model plant type.
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39. 1.Development of Conceptual Model:
• Ideotype consists of various morphological and physiological traits.
• The values of various morphological and physiological traits are specified to
develop a conceptual theoretical model.
Eg: Value for plant height, maturity duration, leaf size, leaf number, angle of
leaf, photosynthetic rate etc are specified.
2. Selection of Base Material:
• Selection of base material is an important step after development of
conceptual model of ideotype.
• Genotype to be used in devising a model plant type should have broad
genetic base and wider adaptability.
• So that new plant type can be successfully grown over a wide range of
environmental condition with stable yield.
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40. • Genotypes for plant stature, mature duration, leaf size and angles are selected
from global gene pool of concerned crop species.
• Genotypes resistant or tolerant to drought, soil salinity, alkalinity, disease and
insects are selected from gene pool with cooperation of physiologist, soil
scientist, pathologist and entomologist.
3. Incorporation of Desirable Traits:
• The next important step is combining of various morphological and
physiological traits from different selected genotypes into single genotype.
• Knowledge of association between various characters is essential before
starting hybridization programme, because it help in combining of various
characters.
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41. • Linkage between procedures, viz single cross, three way cross, multiple
cross, composite crossing, backcross.
Eg: Mutation breeding, heterosis breeding etc. are used for development of
ideal plant types in majority of field crops.
• Backcross technique is commonly used for transfer of oligogenic traits from
selected germplasm lines into background of an adapted genotype.
4. Selection of Ideal Plant Type:
• Plant combining desirable morphological and physiological traits are
selected in segregating population and intermated to achieve desired plant
type.
• Morphological features are judged through visual observation and
physiological parameters are recorded with help of sophisticated instruments.
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42. • Screening for resistance to drought, soil salinity, alkalinity, disease and
insects is done under controlled conditions.
• This task is completed with help of scientist from disciplines of physiology,
soil science, pathology and entomology.
• Finally, genotypes combining traits specified in conceptual model are
selected, multiplied, tested over several locations and released for
commercial cultivation.
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43. APPLICATIONSOFIDEOTYPEBREEDING
1. WHEAT
• A short strong stem. It imparts lodging resistance and reduces losses due to
lodging.
• Erect leaves. Such leaves provide better arrangement for proper light
distribution resulting in high photosynthesis or co2 fixation.
• Few small leaves. Leaves are important sites of photosynthesis, respiration
and transpiration. Few and small reduce water loss due to transpiration.
• Larger ear. It will produce more grains per year.
• A presence of awns. Awns contribute towards photosynthesis.
• Single culm.
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44. 2. MAIZE
• In 1975, Mock and Pearce proposed ideal plant type of maize.
• In maize, higher yields were obtained from plants consisting of
i. Low tillers.
ii. Large cobs.
iii. Angled leaves for good light interception. Planting of such type at closer
spacing resulting in higher yields.
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45. 3. COTTON
• Short stature (90-120cm).
• Compact and sympodial plant habit making pyramidal shape.
• Determinate the fruiting habit with unimodal distribution of bolling.
• Short duration (150-165 days).
• Responsive to high fertilizer dose.
• High degree of inter plant competitive ability.
• High degree of resistance to insect pests and diseases.
• High physiological efficiency.
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46. 4. CHICKPEA –RAINFED CONDITION
• Early vigour.
• 50-60cm plant height with 9-10 secondary branches.
• Tall, erect or semi-erect plant.
• More number of pods per plant.
• Podding from 10th node.
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47. 5. CHICKPEA- IRRIGATED CONDITION
• High input responsiveness.
• Tall (75-90cm) and erect habit with broom shaped branching behaviour.
• Synchronous flowering, delayed senescence and determinancy.
• Long fruiting branches and short internodes.
• Lodging resistance.
• Pod bearing from 20cm above ground.
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48. 6. PIGEON PEA
• Long and medium duration.
• Semi-dwarf plant type (1.5-1.8m) for mechanized plant protection.
• Open canopy with determinancy.
• Non-cluster pod bearing.
• Long fruiting branches for high yield.
• Middle and top bearing.
• Spreading type for intercropping in south and central india.
• Compact plant type for intercropping in northern india.
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49. REFERENCES
1. Ram, M. ( 1982 ). Plant Breeding Methods. PHI Learning Pvt.ltd, Delhi.
2. Singh, B. D. (1983). Plant Breeding. Kalyani Publishers, New Delhi.
3. Singh, B. D. ( 2000). Plant Breeding Principles and Methods. Kalyani
Publishers, New Delhi.494949
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50. THANK YOU
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