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 genotype.
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 crops.
(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
(iii) Interspecific 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 species.
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.
(ii) Psuedoheterosis: Also termed as luxuriance. Progeny possess superiority over parents in vegetative growth but not in yield and adaptation, usually sterile or poorly fertile.
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 species (cisgenesis).
(iv) Organising processes that alter gene activity without altering the DNA itself (epigenetic methods).