Botany : Plant Breeding
Answer the following questions
19. Differentiate primary introduction from secondary introduction.
Introduced variety is
well adapted to the new environment without any alternation to the original
Introduced variety is
subjected to selection to isolate a superior variety and hybridized with a
local variety to transfer one or a few characters to them.
Example : Tea varieties collected from China and North East India
initially grown in Botanical Garden of Kolkata and then introduced to different
parts of india.
20. How are microbial innoculants used to increase the soil fertility?
Answer: (i) Biofertilizers are defined as preparations containing
living cells or latent cells of efficient strains of microorganisms that help
crop plants uptake of nutrients by their interactions in the rhizosphere when
applied through seed or soil.
(ii) Biofertilizers are also called as microbial cultures,
bioinoculants, bacterial inoculants or bacterial fertilizers.
(i) They are efficient in fixing nitrogen, solubilising
phosphate and decomposing cellulose.
(ii) They are designed to improve the soil fertility, plant
growth, and also the number and biological activity of beneficial
microorganisms in the soil.
(iii) They are ecofriendly organic agro inputs and are more
efficient and cost effective than chemical fertilizers.
(i) Bio-fertilisers containing rhizobium bacteria are called
rhizobium bio-fertilizer culture.
(ii) They are symbiotic bacteria living inside the root nodules
and convert the atmospheric nitrogen into a bio available form to the plants.
(iii) This nitrogen fixing bacterium when applied to the soil
undergoes multiplication and fixes the atmospheric nitrogen in the soil.
(iv) Rhizobium is best suited for the paddy fields which
increase the yield by 15 - 40%.
Azolla is a
free-floating water fern that fixes the atmospheric nitrogen in association
with nitrogen fixing blue green alga Anabaena
azolla. It is used as a bio-fertilizer for wetland rice cultivation and is
known to contribute 40 - 60 kg/ha/crop.
(i) Arbuscular mycorrhizae (AM) is formed by the symbiotic
association between certain phycomycetous fungi and angiosperm roots.
(ii) They have the ability to dissolve the phosphates found in
abundance in the soil and provides necessary strength to resist disease, germs
and unfavourable weather conditions. It also assures water availability.
21. What are the different types of hybridization?
Answer: According to the relationship between plants hybridization
is divided into.
(i) Intravarietal hybridization - The cross between
the plants of same variety. Such crosses are useful only in the self-pollinated
(ii) Intervarietal hybridization - The cross between
the plants belonging to two different varieties of the same species and is also
known as intraspecific hybridization. This technique has been the basis of
improving self-pollinated as well as cross pollinated crops
hybridization - The cross between the plants belonging to different species
belonging to the same genus is also called intragenic hybridization. It is
commonly used for transferring the genes of disease, insect, pest and drought
resistance from one species to another.
Example: Gossypium hirsutum x
(iv) Intergeneric hybridization - The crosses are
made between the plants belonging to two different genera. The disadvantages
are hybrid sterility, time consuming and expensive procedure.
Example: Raphanobrassica, Triticale.
22. Explain the best suited type followed by plant breeders at present?
Answer: Conventional plant breeding methods resulting in hybrid
varieties had a tremendous impact on agricultural productivity over the last
decades. It develops new plant varieties by the process of selection and seeks
to achieve expression of genetic material which is already present within the
The following are the
types of conventional plant breeding methods :
(i) Plant Introduction method
(ii) Selction method
(iii) Hybridization method
(iv) Heterosis method
(v) Mutation Breeding method
(vi) Green Revolution method
23. Write a note on heterosis.
Answer: (i) Heterosis (hetero – different, sis – condition ) G.H.
Shull was the first scientist to use the term heterosis in 1912.
(ii) The superiority of the F1 hybrid in performance over its
parents is called heterosis or hybrid vigour.
(iii) Vigour refers to increase in growth, yield, greater
adaptability of resistance to diseases, pest and drought.
(iv) Vegetative propagation is the best suited measure for
maintaining hybrid vigour, since the desired characters are not lost and can persist
over a period of time.
Depending on the
nature, origin, adaptability and reproducing ability heterosis can be
classified as :
(i) Euheterosis : This is the true heterosis which is inherited and is
further classified as :
(a) Mutational Euheterosis - Simplest type of euheterosis and
results from the sheltering or eliminating of the deleterious, unfavourable
often lethal, recessive, mutant genes by their adaptively superior dominant
alleles in cross pollinated crops.
(b) Balanced Euheterosis - Well balanced gene combinations
which is more adaptive to environmental conditions and agricultural usefulness.
Also termed as luxuriance. Progeny possess superiority over parents in
vegetative growth but not in yield and adaptation, usually sterile or poorly
24. List out the new breeding techniques involved in developing new traits in plant breeding.
Answer: In the milestones of plant breeding methods Genetic
Engineering, Plant tissue culture, Protoplasmic fusion or somatic hybridisation,
Molecular marking and DNA finger printing are some of the modern plant breeding
tools used to improve the crop varieties.
NBT - New Breeding Techniques.
NBT are a collection
of methods that could increase and accelerate the development of new traits in
plant breeding. These techniques often involve genome editing, to modify DNA at
specific locations within the plants
to produce new traits in crop plants. The various methods of achieving these
changes in traits include the following.
(i) Cutting and modifying the genome during the repair process
by tools like CRISPR / Cas.
(ii) Genome editing to introduce changes in few base pairs using
a technique called Oligonucleotide-directed mutagenesis (ODM).
(iii) Transferring a gene from an identical or closely related
(iv) Organising processes that alter gene activity without altering the DNA itself (epigenetic methods).