ABSTRACT
Many
agricultural crops of economic importance are vegetatively propagated. This
work was done to elaborate more on vegetatively propagated crops, importance
and developing new cultivars of vegetative propagated crops. Vegetative
propagation aids to fix favorable combinations of important traits, very
specific chemical compositions, superior genetic variance interactions and high
levels of heterozygosity. Breeding vegetative propagated crops involve few
genetic crossing and genetic combination by sexual reproduction. Vegetatively
propagated crops are best suited for maintaining hybrid vigour because once a
desired hybrid has been produced; there are fewer chances of losing it.
Breeding by mutation enables to get improved characters in plant like tolerance
to environment pressure, plant shape, changes in oil content, disease resistant
in plant and protein quality.
INTRODUCTION
Vegetative
propagation is an important part of agriculture, it refers to a process by
which new organisms arise without production of seeds or spores. Vegetative
propagated crops are very important horticulture crops, because it makes possible
to fix and multiple favorable genetic combinations, provide food and cash
crops.Vegetative
or clonal propagation is an asexual reproduction in which successive mitosis of
specialized vegetative propagules (e.g. bulb, corn, tubers, cuttings, buds and
apomictic seed) develop new plants and results in a clonal population.Clone
is defined as a genetically uniform material derived from a single individual
that is vegetative propagated either in vitro or invivo. In species cultivated
for vegetative products, as potato (Solanium
tuberosom), cassava( Manihot
esculenta), and sugar cane (Saccharum
officinarum), the flowering and fertility are reduced and less useful in a
new clone. Vegetative propagated crops are among the most important food crops.
The food and agricultural organization ranks potato as the fourth crop after
wheat, maize and rice, cassava is ranked sixth after barley. Potato is known as
the world’s major staple crop, it has protein with the highest biological
quality, sugar cane, a member of the grass family (Poaceae) is widely cultivated and propagated, providing around 70%
of the world’s sugar. Sugar cane yields the highest number of calories per unit
area of cultivated crops, is the process of changing the traits of plants in
order to produce desired characteristics.Vegetatively
propagated crops are bred by intentionally crossing of closely or distantly
related individual to produce new crop varieties or lines with desirable
traits. Breeding of vegetative crops have successfully improved quality, yield,
tolerance of crops to environmental pressure. Breeding helps in producing crops
that are resistant to viruses, fungi and bacteria and helps in longer storage
period for the harvested crop.Species
cultivated for fruit or reproductive product, like strawberry (Fragoria x ananassa), fruit yield and
quality are very important traits, as most of these crops combine sexual and
asexual reproduction, and there is wide genetic variation among species and
populations, depending upon the relative importance of sexual and asexual
reproduction (Eckert, 2002).
WHAT IS VEGETATIVE PROPAGATION?
Plant
propagation is the method of increasing the number of plants of a particular
cultivar or species. Vegetative propagation also known as asexual or clonel
vegetation is a plant propagation method that produce new plants from
vegetative parts (leaves, stems and roots) of the original plant. It is a
process by which new organisms arise without production of seeds or spores. It
can occur naturally or be induced by horticulturists.
TYPES OF VEGETATIVE PROPAGATION
i)
Natural vegetative propagation. ii) Artificial
vegetative propagation.
·
Natural
Vegetative Propagation
Natural
vegetative propagation occurs when an axillary bud grows into a lateral shoot
and forms its own roots (adventitious root). Plant structures involved in
natural vegetative propagation include Bulbs, Rhizomes, Stolons and tubers. Bulbs:
They form lateral buds from the base of the mother bulb producing smaller bulbs
or bulbels in coming years e.g.Onion. Rhizomes: They are root like stems that grows horizontally under the round. New roots and shoot form at the nodes with shoots growing upwards to form new plantlet. Examples includes iris and root ginger.
Stolons: They are also known as runners. They are horizontal stems that grows above the ground e.g. strawberries. Tiny plantlets are seen along stolons, and form roots where they touch the ground. The new plant are independent when they break from the parent plant.
Tubers: They are swollen part of an underground stem that store food so a plant can lie dormant. E.g. Potatoes. Auxiliary bud form over the surface of the tuber and produce shoots that grow into a new plant.
·
Artificial
Vegetative Propagation
These are induced form of vegetative
propagation. It involves taking a piece of one parent plant and causing it to
regenerate itself into a new plant. They methods are commonly: cuttings,
grafting and budding, tissue culture and vegetative apoximixis. Cutting:
A cutting is a piece from a plant that has been cut off from a mother plant and
then caused to grow in a whole plant. Often the stems piece are treated with
hormones(Auxins) to encourage new roots to form before planting. Tissue culture: In these method, tiny fragments of plants are treated with plant hormones in a sterile growing medium. The hormones stimulate the growth of a callus, from which a new seedling can grow. Large numbers of identical seedlings are produced using this method.
Grafting and budding: This method involves joining a stem piece or a single bud onto the stem of a plant that has roots. The stem piece is called the scion and the plant with root is called a root stock. This method is used to produce fruit trees sometimes with more than one variety of the same fruit species growing from the same
Vegetative Apomixes: This is a method that does not involve fertilization. It includes (apospory and diplospora) in flowering plants the unfertilized seeds are used or plantlets that grow instead of flower all use this method e.g. Dandelion and some citrus plants.
ADVANTAGES
OF VEGETATIVE PROPAGATION
The main advantage of vegetative
propagation is that new plants contain the genetic material of only one parent,
they are essential clones of the parent plant. Quality plant and ensure
consistency of a variety crop by commercial growers. Vegetative propagation helps plants by
passing the immature seedling phase and reach mature phase earlier. It saves
time, energy, and money for commercial production of plant.
DISADVANTAGES
OF VEGETATIVE PROPAGATION
· The main disadvantages are the potential
to impact on biodiversity of a species.
·
If a particular plant clone is
susceptible to certain disease, there is a potential to lose entire crops. E.g.
is seen in the impact of Kiwi fruit disease PSA. The only way to protect
against the disadvantages is to establish a genetic bank of seeds or plants
(germ plasma collection).
Plant Breeding?
Plant breeding is the genetic
improvement of plants in order to create desired plant types that are better
suited for cultivation, give quality yields, tolerate environment pressure and
are disease resistant. Vegetative propagated crops which are the most
horticultural crops used for food and cash crops are bred to improve human and
economic live of a nation. Most of our major crops are derived from the
domesticated varieties. Due to advancement in genetics, molecular biology and
tissue culture plant breeding is being carried out by using molecular genetics
tools.The possibility of combining sexual and
asexual reproduction systems in almost all vegetatively propagated
horticultural crops is the key strategy to develop new cultivars, because
hybridization is the best method of increasing genetic diversity and combine
traits in an individual (Hancock, 2004).In vegetative propagation breeding, the
identification of one genotype with superior hybrid combination is sufficient
because vegetative propagation fixes its heterozygosity and non- additive –
interaction and allow its multiplication, even of sterile genotypes. An
adequate breeding strategies and population size enable fully exploitation of
general and specific combination ability that result in high hybrid vigour
(Mckey et al., 2010).Therefore allowing a reasonable breeding
strategy that will exploit as much as possible the gene pool diversity.
METHODS
INVOLVED IN THE BREEDING OF VEGETATIVE PROPAGATED CROPS
·
Domestication
: This
is not majorly involved in vegetative propagated crops breeding, but is
involved in the process. It is the process of cultivating vegetative crops and
keeping them under human care and management.
·
Collection
of germ plasm: Germ plasm is the sum total of all genes
present in a crop. The entire collection of vegetative propagated crops having
all the diverse alleles for all genes in a given crop is known as Germ plasm
collection. The germ plasm of any crop species consist of the following types
of materials:
1) Cultivated
improved varieties
2) All
the wild species related to the crop species
3) Improved
varieties that are no more cultivated
4) Old
local varieties.
The collection of germ plasm from
different sources id an essential first step in any breeding work, and germ
plasm is usually stored at a low temperature, and it is cone within country or
from other countries.
·
Evaluation
and selection of parents: The parents are evaluated to
identify plants with desirable combination of characters.
·
Plant
Evaluation: This is the process of introducing
plants of germ plasms either from a foreign country or introducing plants or
germ plasm from one region to other region of the same country. After plant
introduction, an adaptation period is followed. Plant introduction is done for
the purpose of genetically improvement of economical crops and also for
studying the origin, distribution, classification and evolution of plants.
5) Hybridization:
This is the mating or crossing of two plants or lines of dissimilar genotype.
The objective of hybridization is to create genetic variation. When two
dissimilar genotypical palnts are crossed, the genes from both the parents are
brought together in F1 generation. Segregation and recombination produce many
new gene combination in F2 and further generation. The aim of hybridization may
be transfer of one or few qualitative characters, improvement of one or more
quantitative traits or use of the F1 as a hybrid variety.
STEPS
INVOLVED IN HYBRIDIZATION
A) Selection
of parents from available material possessing desired characters.
B) Selfing
of plants to obtain homozygosity in desired traits.
C) Emasculation
In this the anthers are removed before they mature and have shed their pollen.
D) Bagging
, tagging and labeling of males as well as females to be used in crossing.
E) Crossing
, in which the pollen from bagged males are spread on the bagged female plant.
HYBRIDIZATION
IN VEGETATIVE PROPAGATED CROPS
We have two types of hybridization
1) Interspecific.
2) Inter
– generic.
In interspecific hyvridization: The
plants of two different species belonging to the same genus are crossed
together. It is also known as intra- generic hybridization. Disease, insect,
drought and frost resistant varieties in wheat, tomato, sugarcane e.t.c. have
been evolved by this method.
Gene
Introgression to Cultivated Germ Plasm
Gene introgression also known as
introgressive hybridization in genetics is the movement of a gene from one
species into the gene pool of another by the repeated backcrossing of an inter
specific hybrid with one of its parent species.
Introgression
in Potato
Solanium
rybinii is a wild diploid species of potato and is
resistant to frost and virus infection. Another species Solanium tuberosum is cultivated and tetraploid species. The potato
probably has the wildest genetic diversity among related wild species than any
other cultivated plant, with the vast majority (74%) of the diploid species,
and the rest are triploid, tetraploid, pentaploid and hexaploid (Hawkes, 1990).
In most Solanium species, the germ plasm can not all be directly used for
breeding due to a combination of diploid level and endoplasm balance number
(EBN) imcompatibility (Hewkes and Jackson, 1992).
The majority of the diploid wild species
can be directly crossed with dihaploids (2n = 2x= 24) of cultivated potato.
Dihaploid occur as result of parthenogenesis (haploid pollination technique) or
another culture (Hermsen, 1994).
Dihaploid x wild species can be crossed
with cultivated potato through unilateral sexual poly ploidization (4x – 2x
crosses) using 2n gametes. Also, two dihaploid x wild species hybrids that
produce 2n pollen and 2n eggs can be crossed in a bilateral sexual polyploidization
(2x – 2x crosses) to create novel tetraploids (Hermsen, 1994; Hanneman, 1999).
The wild parents are selected for useful traits as tuberization, tuber quality
traits, tolerance to environmental stresses and resistance to diseases and
sexual fertility and 2n gamete production before crossing with dihaploids.
Importance
of Gene Introgression to Cultivated Germ Plasm
The gene introgression was use for the
introgression of late blight (Phytophthora
infestans) resistance from the wild diploid species S. microdontum (2n = 4x = 24e 2EBN) to cultivated potato (2n = 4x =
48e 4EBN). A total of 175 clones from
six accessions of Solonium microdontum
were evaluated against the use of genotype.
A Z matting type of P. infestons and 27 highly resistant ones were selected (Oouches et
al., 2001).
The quantitative trait loci conferring late
blight resistance in S. microdontum
could be followed through poliploidization and identified microsatellite used
in a marker assisted selection program to introgress the resistance gene from a
wild species to cultivated potato (Bisognin et
al., 2005). Besides , gene introgression, interspecific hybridization was
also involved in the origin of a new horticultural crop. Solanium rybinii , a wild diploid species of potato and is
resistant to frost and viral infection and Solanium
tuberosum are not crossed directly because S. tuberosum is tetraploid and the ploidy levels are different. So Solanium rybinii is first raised to
tetraploid level by auto- polyploidy one then crosses with Solanium tuberosum to introduce the characters of the wild
species.
INTER – GENERIC HYBRIDIZATION
This is a crossing beteen plants
belonging to two different genera. Inter generic hybridization is been observed
when sugar cane and sorghum are crossed. Sugar cane takes about 9 months to
ripe and so no other crop can be grown. Sorghum is a short duration crop (3 to
4 months). So early maturing sugar cane varieties have been evolved by crossing
with sorghum and since sorghum has less sugar content, the crossing results to
a low sugar content hybrid, but by repeated back crossing of the hybrid with
sugar cane early maturing varieties having normal sugar contents have been
evolved.
HETEROSIS
AND INBREEDING DEPRESSION
Heterosis also known as hybrid vigour is
the increased size, yield, general vegetative luxuriance, resistance to
diseases and to insects e.t.c. observed in the F1 generation of certain crosses
as compared to the parents. A.F Sholl, 1914 attributes vigour to the “effects
of a changed nucleus and relatively unchanged cytoplasm upon each other.
Dominance and over dominance forms the
genetic bases of heterosis. In dominance hypothesis, the superiority of hybrid
are attributed to the suppressing of undesirable recessive alleles from one
parent by dominant alleles from the other. It attributes the poor performance
of inbreed strains to the loss of genetic diversity, with the strains becoming purely homozygous at
many loci. These hypothesis was expressed in 1908 by the geneticist Charles
Davenport.
In over dominance hypothesis of
heterosis, certain combinations of alleles that can be obtained by crossing two
inbreed strains are advantageous in the heterozygote. The over dominance
hypothesis attributes the poor performance of inbreed strains to a high
percentage of these harmful recessives. It was first developed independently by
Edward M. East (1908) and George Shull (1908).
Inbreed vigour has been exploited in
commercial crops such as maize, sorghum, barja, rice, sugar- beet, tomato,
petunia etc and it is lost by inbreeding. Vegetatively propagated crops are
best suited for maintaining hybrid vigour because once a desired hybrid has
been produced there are fewer chances of losing it.
HYBRID
VIGOUR IN PLANT
Crosses between inbreeds from different
heterotic groups result in vigorous f1 hybrids with significantly more
heterosis than f1 hybrids from inbreeds within the same heterotic group.
Heterotic groups are created by plant breeders to classify inbreed lines, and can
be progressively improved by reciprocal recurrent selection.
INBREEDING
DEPRESSION
This is the reduced biological fitness
in a given population as a result of inbreeding or breeding of related
individuals. Inbreeding depression is often as a result of s population
bottleneck. The degree of inbreeding depression varies considerably from one
species to another. Vegetative propagated crops such as onion show little or no
inbreeding depression. Species like maize and barja have moderate inbreeding
depression.
MUTATION
BREEDING IN VEGETATIVE PROPAGATED CROPS
Mutation are sudden unpredictable
heritable changes without any intermediate stage in characteristics of
organism. It is the permanent and relatively rare change in the nucleotide
sequence of the concerned gene or chromosome structure and chromosome number.
Mutation was first discovered by Wright
in 1791 in male lamb with short legs, and it was later discovered and studied
in Denothera by Hugo deuries in 1900 Morgan in Drosoptilla (White eye mutant)
in 1910. The term mutation was coined by De vries.
Mutation occurring naturally are called
spontaneous mutations. They are both germinal and somatic. Useful somatic
mutation can be incorporated in crop improvement only in vegetative propagated
plants e.g. seedless grape, Naval Orange, Bhaskara banana. Vegetative
propagation is also useful in maintaining germinal variation through sexual
reproduction e.g. apple, mango, potato, sugar cane.
Thus, spontaneous mutations are the
source of all the genetic variations occurring in all living things today.
INDUCED
MUTATION
The rate of spontaneous mutation is very
low. Therefore , rate of mutation is accelerated by means of certain agents
called mutagens. Mutagens are of two types;
* Physical
* Chemical
Chemical
mutagens: They are mutagens from chemical substances such as
ethylmethane sulphonate (EMS) sodium azide that induce mutation, maleic
hydrazide and Dimethyl Nitrosoamine.
Physical
mutagens: They are different kinds of radiation like x rays,
gamma rays, ultra violet rays, etc . They induce changes in DNA and chromosomes
which produce mutations. Mutations produced in response to mutagens are known
as induced mutations.
SOMATIC
MUTATIONS IN IMPROVEMENT OF VEGETATIVELY PROPAGATED CROPS
When a mutation (plastidial, genic, chromosomal,
genomic) arises in a cell within a shoot apical of a bud. The mutated cell
propagates mitotically and produces a mutated sector (sectorial or mericlinal
chimera) (D’ AMATO, 1977).
During further vegetative growth of the
initial bud one or more wholly mutated buds (solid mutant) producing mutated plants (buds) may develop
into an independent plant through various naturally occurring modes of
vegetative peopagation (tubers, tubercles, tuberous, roots, bulbs, pseudo
bulbs, cormers, stolons, rhizomes, root crowns, stem points, inflorescence,
bulbils, adventitious embryo and apomictic seed) obviously sports can be
propagated via true camphimictic) seed when the original plant reproduces
sexually. In most cases sports differ from the parent plant for a monogenic
character. In some other cases, two or more genes or gene complexes are
involved (Whitham and Slobodckikofe, 1981; Shamel and Pomeroy, 1936).
SELECTION
AMONGST SOMACLONAL VARIATION
Genetic variation present among plant
cells during tissue culture is called somaclonal variation. It is used for the
genetic variation present in plants regenerated from a single culture. This
variation has been used to develop several useful varieties.
Some of the somaclonal variations are
stable and useful e.g. resistance to disease and pest, stress tolerance, male
sterility , early maturation, better yield, better quality e.t.c.
Thus , somaclonal variation have
produced wheat tolerance to rust and high temperature, potato tolerance to
phytophthora infestions (Late blight of potato) e.t.c. other useful variations
include high protein content of potato, short durations sugar cane and increase
of shelf life of tomato.
ACHIEVEMENT
OF MUTATION BREEDING
·
The first commercial success with
induced mutations was reported in 1934 with the release of a new tobacco
ciltiva, “Chlorina” through x ray
radiation.
·
Many varieties of barley contain
artificial mutated genes which contributed to reduction in height, increase in
yield, in sensitivity to day length and
resistance to mildew disease. Shorbati
sonara and pusa lerne are two
amber grain colour mutants of wheat produced from the red grained sonara 64 and
lerma Rojo 64A. Induced mutations have also become recently important in
developing parents, useful in hybridization programmes. 45 cultivars were been
developed by the year 1982. Either by direct radiation or by crossing with
induced mutants.
·
Many crop plants are propagated
vegetatively e.g. Potato, Tapioca and sugar cane. In these genetic improvement
is carried out using sexual reproduction but the maintenance of the improvement
varieties is by cloning.
·
Spontaneous mutation is somatic cells of
a vegetatively propagated plant are commonly called SPORT. Such desirable
sports occurring in well adapted, asexually reproducing plants may result in
quick improvements such as the colour sports in many apple and superior shrub
typed in coffee plants.
·
The characters improved through mutation
breeding include tolerance to environment pressure, plant shape, changes in oil
content, disease resistant and protein quality.
GENETIC ENGINEERING AND
BIOTECHNOLOGY IN PLANT BREEDING
Genetic
engineering also known as recombinant DNA
technology is aimed at introducing one or more genes into an organism
that normally does not possess them. This requires isolation of fragment of DNA
corresponding to a desirable character, hooking it to a vector (such as the
plasmid in a bacterium,agrobacterium tumifaciens) and transferring it to a
cell.
Genetic
transformation is also possible through co- cultivation, electro poration
(applying high electric potential for a few micro seconds to change the
porosity of protoplast to take up DNA) and by micro- injection of DNA into the
cell by fine needles.
Successful
genetic engineering involves identification of the desired genes, their
transfer to the cells of a target crop plant, their integration and expression.
SELECTION
Selection
is one of the oldest methods for crop improvement. It can either be artificial
or natural. Natural selection acts as a sieve in favour of the well adapted
strains and varieties. Local varieties of crops are the result of natural
selection. Selection pressures have arising differences between species and sub
species.
Artificial
selection involves picking out of the plants having desired characters from a
mixed population where the individuals differ in characters. There are various
methods of artificial selection:
A) Mass
selection: It is practiced in those plants which are cross pollinated like Zea,
Brassica. Plants are selected based on the phenotypic expression from the mixed
population of a crop, then the seeds are obtained, the seeds obtained are used
for the raising of next crop, this selection is continued till the plants show
uniformity in the desired characters.
B) Pure-
line selection: This method is practiced in self pollinated crops such as
wheat, barley, rice, legume. It also made based on the phenotypic expression.
It involves testing the progeny of single individual plant separately. This
method of selection from a single individual is continued till a true breeding
type is obtained. Thus a breeder by pure line selected renders a particular
type more or less homozygous and the progeny has a uniform population, but it
lacks variability.
C) Clonal
selection: This method is practiced in vegetatively propagated crops such as
banana, potato, onion, citrus e.t.c. Clones are plants propagated vegetatively
from a single individual. The genotypic constitution of plants propagated in
this way is not likely to change. Superior clones are the ones selected on the
basis of their phenotypic characters. Selections are done between clones and
never within clones, as all the individuals of a clone have the same genetic
constitution.
POLYPLOIDY
BREEDING
Polyploidy breeding in vegetative
propagated crops involves the number or sets of chromosome or genomes in a cell
of an organism. Vegetative propagated crops having polyploidy conditions are
potato, sugar cane, banana, tobacco, e.t.c. This condition arises due to
failure of chromosomes to separate at the time of anaphase either due to non-
disjunction or due to non- formation of spindle. This condition can be
artificially induced by application of colchicines depending upon the number of
genomes present in a polyploidy. It is known as: Triploid (3n), Tetraploid
(4n), Pentaploid(5n), Hexaploid (6n), Heptaploid(7n), Octaploid(8n) e.t.c.
The polyploids with odd numbers of
genomes present in an triploids, pentaploids are sexually sterile because the
odd chromosomes do not form synapsis and they are therefore propagated
vegetatively e.g. potato, sugar cane.
Polyploidy is of two types;
Autopolyploidy and Allopoly ploidy.
Auto
polyploidy: This is a type of polyploidy in which
there is a numerical increase of the same genome e.g. autriploid (AAA), auto
tetraploid (AAAA). Some of the crop and garden plants are auto polyploids e.g.
maize, rice, etc. auto ploidy induces gigas effect.
Allopoly
ploidy: It was
developed through hybridization between two species followed by doubling of
chromosomes e.g. (AABB) Allotraploid is the common type. Allopolyploids
functions as new species e.g. wheat, American cotton. Auto allopolyploidy is a
type of allopolyploidy in which one genome is more than diploid state. Common
auto allopolyploids are hexaploid (AAAABB) e.g. Helianthus tuberous.
INDUCTIONN
OF POLYPLOID
Polyploidy in plant can be induced by
colchicines treatment, colchicines is an alkaloid obtained from the corms of
colchicium autonale (Lillaceae), colchicines inhibits the formation of the
spindle in the dividing cells and hence chromosomes do not separate at
anaphase.
Thus a
restitution nucleus (A nucleus in which the chromosomes have divided but could
not divide into 2 daughter cells) is formed. The effect of colchicines is
temporary, so as a cell recovers, a new spindle is formed and restitution
nucleus undergoes normal mitosis as a polyploidy cell.
ROLE OF AUTOPOLYPLOIDS IN PLANT
BREEDING
·
It is used to increase fruit size.
·
It is used to increase and develop
seedless varieties.
ROLE OF ALLOPOLYPLOIDS IN PLANT
IMPROVEMENT
·
It is useful in interspecific and
interheneric hybridization
·
Allopolyploid are used un cultivating
vegetative propagated crops.
·
Genera raised through inter- generic
allopolyploidy include Triticale (Tritium x secale) and Ralphano brassia
(Raphanus and Brassica).
POLYPLOIDS IN VEGETATIVE PROPAGATED
CROPS
ORIGIN
OF TOBACCO
There
was evidence that Nicotiana tabacum (2A=
48) is an allotetraploid between N.
sylvestris (2n = 24) and Nicotiana
tomentosa or some allied species (2n = 24) Nicotiana tobacum is known only under cultivation and does not
occur as a natural species.
ORIGIN
OF TRITICALE
It
is a man – made cereal, an allopolyploid between Triricum (wheat) and Secour
(rye). The release varieties of Triticale
are hexaploid (2n = 42) and have been synthesized by doubling the chromosome
complements of sterile hybrids between Tritocum
turgidum (Durum wheat 2n = 28) and S.
cereal (rye, 2n = 14).
ROLE OF PLANT BREEDING IN
VEGETATIVE PROPAGATION
·
All the sugar cane varieties that are
cultivated today are interspecific hybrids.
·
Plant breeding has also given us improved
varieties of crops like Sonora- 64 of wheat.
·
Through mutation breeding more than 200
varieties of crops have been developed.
·
Disease resistance in plants have been
introduced through breeding.
CONCLUSION
Vegetatively
propagated crops which possess high uniform and hybrids vigour are very
important set of food and cash crop, promoting diversification and producing
nutritional and food security.
The
possibility of combining sexual and asexual reproduction systems in almost all
vegetatively propagated horticultural crops is very important for both broaden
of the genetic base, and developing new countries. The inter and intra specific
hybridization are the best approach to increase the genetic base of cultivated
germoplasm and the cross ability with related species and to combine traits in
one individual. Hybridization also increases the genetic diversity for traits
less or still not exploited in breeding programs and to reduce the genetic
vulnerability of the crops.
The
different breeding methods used in the improvement of the vegetatively
propagated crops have been developed to increase the effectiveness of the new
species. These species of vegetative propagated crops also depends upon high
quality propagules of their parent plants.
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