Crop Improvement
Category : 12th Class
The development of new variety of plants possessing desirable characters from the existing ones is called plant breeding. The crop improvement depends upon favorable environment (Good irrigation, better fertilizers and precautions to avoid losses due to disease) together with superior hereditary characters. This superiority or improvement may be in following respects :
(1) Quantitative characters : Increase in yield of seeds, grains, fibres, oil etc.
(2) Qualitative characters : Increase in biochemical components as well as taste, milling, baking, cooking etc.
(3) Resistance : To diseases, insects, pests, drought, frost, cold, lodging etc.
(4) Earliness or lateness : In maturity period or change in maturity behaviour.
(5) Adaptability : To wider range of conditions etc.
Important plant breeders
(1) N.I. Vavilov famous Russian plant breeder, who gave centres of origin of cultivated plants.
(2) N.E. Borlaug famous Mexican plant breeder, who was awarded Nobel peace prize (1970) for developing high yielding dwarf varieties like Sonora-64, Lerma rojo 64 etc. He is known as father of green revolution.
(3) Dr. M.S. Swaminathan is pioneer mutation breeder. He has produced sharbati sonora variety of wheat by mutation, which is responsible for green revolution in India. Dr. Swaminathan is called father of green revolution in India.
(4) Thomas Fairchild (1717) produced first hybrid plant artificially.
(5) Cotton Mather (1761) recognised the process of natural selection in maize.
(6) Joseph Kolreuter (1760-66) produced many hybrids in tobacco.
(7) Dr. Boshisen : Famous maize breeder.
Methods of plant breeding
(1) Plant introduction
(i) Plant introduction means introducing a plant having desirable characters (e.g., genetic improvement, high yield, disease resistance and vigorous growth) form a region or a country where it grows naturally to region or a country where it did not occur earlier.
(ii) If brought from foreign country, it is called Exotic Collection (EC) but if brought from same country, then it is called Indigenous Collection (IC).
(iii) Introduced plants may be used directly for cultivation (Primary introduction) or may be used after subjecting to selection/ hybridization (Secondary introduction).
(iv) Acclimatization : The adjustment of newly introduced plant to new or changed environment is called Acclimatization.
(v) There is a definite procedure for introducing a plant material from abroad and strict plant protection and quarantine laws have been made. If the plant material is found suitable, phytosanitary certificates are issued and only then the plant material is introduced in our country.
(vi) New plants are usually introduced in the form of cuttings or seeds.
(vii) Portugese traders and East India Company were foreign agencies which introduced many plants in India.
National Bureau of Plant Genetic Resources, Delhi (Estd., 1976) helps in plant introduction in India.
(2) Selection : It is the picking up of plant having desirable characters (e.g., high yield, disease resistant and vigorous growth) from a given population of plants based on its phenotypic characters. This involves preserving of favourable characters and gradual elimination of undesirable ones. There are two main types of selection :
(i) Natural selection : This is a rule in the nature and result in evolution. Here the fittest can survive and rest wipe out. All the local varieties of crop resulted because of such selection.
(ii) Artificial selection : Artificial selection is to choose certain individual plants for the purpose of having better crop from a mixed population where individuals differ in character these are divided into following types :
(a) Mass selection : It is practised in naturally cross-pollinated crops e.g., Maize. The first step involves selection of plants, having desirable characters from a given population of plants, based on phenotypic characters. The seeds of selected plants are then mixed and sown in the same field (Mixed cropping) to allow natural cross pollination. The plants are selected from this field by eliminating the undesirable ones and saving the best. The seeds of selected plants are multiplied in large numbers and supplied to the farmers.
(i) It is the simplest, easiest and quickest method of crop improvement.
(ii) It is only method for improving the wild or local varieties to meet the immediate needs of farmers.
(b) Pure line selection : It is practised in natural self - pollinated crops e.g., Wheat.
First step involves selection of few plants each having one or more desirable characters from a genetically mixed population. Each of these selected plant is then selfed through several generation to attain homozygosity for the selected characters. The homozygous plants are then multiplied. A population of homozygous plants raised from a single homozygous plant is called pure line (Johannsen). The pure lines are now crossed to introduce several desirable characters in to a single synthetic one which is then multiplied and supplied to the farmers for cultivation. Pure line selection is method of improvement in self-pollinated plants.
Advantage : In pure line selection the selected plants retain their desirable characters for several years.
Disadvantage : No new genotype are created by pure line selection. It requires 10 - 12 years for raising the desired variety.
(c) Clonal selection : It is practised in vegetatively propagated plants e.g., sugarcane, banana, potato, onion, turnip etc. Clonal selection is the method of selection of desirable clones from the mixed population of vegetatively propagated crops. All the progenies of a single plant obtained vegetatively are known as clone.
The first step is selection of a plant from a population of a crop based on phenotypic characters. The plant is then multiplied vegetatively and supplied to the farmers for cultivation.
Advantage
(i) Varieties are stable and easy to maintain.
(ii) Hybrid vigour is easily utilized.
(iii) Only methods to improve the clonal crops.
Limitations
(i) Only applicable to vegetatively propagated crops.
(ii) Creats no new variation.
(3) Hybridization : It is the method of producing new crop varieties in which two or more plants of unlike genetical constitution are crossed together. The plants which are crossed together may belong to the same species different species or different genera. Hybridization doesn't change genetic contents of organisms but it produces new combination of genes.
According to this relationship between parental plants, the hybridization is divided into following categories :
(i) Intravarietal hybridization : The crosses are made between the plants of same variety.
(ii) Intervarietal hybridization : The crosses are made between the plants belonging to two different varieties of the same species and is also known as intraspecific hybridization.
(iii) Interspecific hybridization : The plants of two different species belonging to the same genus are crossed together. It is also known as intrageneric hybridization.
(iv) Intergeneric hybridization : The crosses are made between the plant belonging to two different genera.
(v) Introgressive hybridization : In this type of hybridization one species is completely replaced by another in nature.
(a) Hybridization procedure
Selection of parents : The first step in hybridization is to select the plants which are to be used as parents and can supply all the desired important characters which lack in a good standard variety.
Selfing of parents : This is the second step consisting in artificial self pollination of parents. It is very essential for eliminating the undesirable characters and obtaining inbreeds.
The selected inbreeds, before utilization, are tested for combining ability, both specific and general and the most suitable ones are further utilized in the hybridization technique.
Hybridization Technique
The inbreeds are grown under normal and protected conditions in the isolated plots so that they may develop properly and get full shelter against insects, pests, animals, birds and diseases.
They are sown at different dates to secure simultaneous flowering. The males and females to be crossed are marked in such a way that the dehiscence of anthers coincides the stigma receptivity. They are then carried out under the following operations :
Emasculation : "The removal of stamens from female parent before they burst and have shed their pollens".
Bagging : To avoid contamination by unwanted pollen, the female and male flowers are covered with celophane or parchment or paper bags. This process is called bagging.
Crossing : "The artificial cross - pollination between the genetically unlike plants" and after that the female cross pollinated flower is again bagged.
Labelling : The crossed flowers are properly tagged and labelled.
Harvesting hybrid seeds and raising \[{{F}_{1}}\]generation : The bags are removed and the crossed heads of desirable characters are harvested and collected with their attached labels separately in envelopes. After complete drying, they are threshed individually and preserved as such.
In coming season, these seeds are sown separately to raise the \[{{F}_{1}}\] generation. The plants of \[{{F}_{1}}\] generation are progenies of crossed seeds and called hybrids.
Hybridization methods : Handling of \[{{F}_{1}}\] and subsequent generations by different selection methods of hybridization which are different for self and cross-pollinated crops.
Self pollinated crops : (i) Pedigree method (ii) Bulk method (iii) Back cross method.
Cross pollinated crops : (i) Single cross (AxB) (ii) Three cross (AxB) x C.
Heterosis or Hybrid vigour : Heterosis or hybrid vigour is the increased vigour growth yield or function of a hybrid over the parents, resulting from the crossing of genetically unlike organisms. The term was coined by G.H. Shull (1914) though heterosis was first studied by Kolreuter (1763) followed by Darwin (1876). The heterosis normally involves two steps :
(i) The plants are selected for certain desirable characters and are selfed repeatedly through several generations to get pure lines for different characters.
(ii) The pure lines for different desirable characters are crossed to get the heterotic effect in the hybrids.
The hybrid vigour is lost after few generation. Hybrid vigour has been commercially exploited in different commerical crops like maize, sorghum, bajra, tomato, sugar beet, petunia, zinnia and cucumber.
(4) Mutation : Hugo devries was the first person who defined mutation as sudden phenotypic changes which are heritable.
But nowadays mutations are used in strict sense to cover only changes in the chemical structure of gene at the molecular level and it is specifically known as gene mutation or point mutation. Near about 1960, it was felt that mutations can be used in crop improvement.
Types on the basis of site
(i) Somatic mutation : It is mutation occurring in somatic or body cells. Somatic mutation can be used in crop improvement by vegetative propagation, e.g., colour spots in apple, bhaskar variety of banana, seedless grape navel orange, superior shrubs in coffee.
(ii) Germinal mutation : Mutation occurring in germ cells which is transferable to the progeny is called germinal mutation. It may or may not be expressed because mutation is generally recessive and recessive traits appear only in homozygous state, e.g., stiff ears in wheat, rice, spathe in maize, lint in cotton (White gold).
Mutations are spontaneous as well as induced, i.e., can be artificially induced by certain mutagens or mutagenic agents.
These mutagens are of two types
(i) Physical mutagens : It comprise mainly radiations. Radiation has been used to induce mutations for the first time by H.J. Muller (1927) on animals (Drosophila) and L.J. Stadler (1928) on plants e.g. \[X-\]rays, \[\beta -\]rays, \[\gamma -\]rays as well as UV-rays.
(ii) Chemical mutagens : e.g., nitrous acid, maleic hydrazide, hydrazine, methyl methane sulphonate (MMS), ethyl methane sulphonate (EMS) etc.
Examples of Induced Mutations
(i) Wheat : Norman Borlaugh, 1970 developed two wheat varieties – Sonora 64 and Lerma Roja 64 A. Both varieties were red coloured and rejected by Indian population. When exposed to gamma radiation, they mutated into amber coloured, Sharbati sonora and Pusa Lerma.
Sonora 64 \[\xrightarrow{\text{Gamma}\,\text{radiation}}\] Sharabati sonora
Lerma Roja 64 A \[\underset{\text{Radiation}}{\mathop{\xrightarrow{\text{Gamma}}}}\,\] Pusa Lerma
(ii) Rice : In Indonesia, a high yielding rice variety Reimei was developed by gamma radiations. Pelita-I also was mutated into Atomita-I through gamma radiations. It is high yielding variety which is resistant to brown plant hopper.
(iii) Peanut : Mutation gave rise to thick shelled groundnut variety which could be handled without cracking.
(iv) Penicillin production has been increased enormously by UV-rays treatment of Penicillium notatum and P. chrysogenum.
Limitations of mutations breeding
(i) Most induced mutation are undesirable. Some of which result in death of the organism.
(ii) The rates of mutations are very low and large number of plants are employed to select a certain desirable mutant.
(iii) Most mutations are not stable and get reverted.
(iv) Since mutations are recessive they are expressed only in recessive homozygous condition otherwise they remain undetected.
(v) In sexually reproducing plants mutations are expressed and inherited only if they occur in gametes.
(5) Polyploidy : Organism with more than two sets of chromosomes are known as polyploids. It may be triploid with three sets of chromosomes (3n) or tetraploid with four sets of chromosomes (4n) and so on. Polyploidy is of three types :
(i) Autopolyploidy : It is a type of polyploidy in which there is a numerical increase of the same genome, e.g., Autotriploid (AAA), autotetraploid (AAAA), e.g., maize, rice, gram. Autopolyploidy induces gigas effect.
(ii) Allopolyploidy : It has developed through hybridization between two species followed by doubling of chromosomes (e.g., AABB). Allopolyploids function as new species. e.g., Wheat, American cotton, Nicotiana tobacum. Two recently produced allopolyploids are Raphanobrassica and Triticale.
(iii) Autoallopolyploidy : It is a type of allopolyploidy in which one genome is in more than diploid state, commonly autoallopolyploids are hexaploids (AAAABB), e.g., Helianthus tuberoseus.
Polyploidy arises either due to fusing of one egg with two sperm or vice versa; or by failure of mitosis in somatic cells where chromosomes have duplicated in S- phase of interphase. Artificially polyploidy can be induced by using colchicine. Colchicine is an alkaloid obtained from Colchicum autumnale (Liliaceae).
Triploid condition arises by crossing a tetraploid (4n) and a diploid (2n) plant e.g., sugarbeets, apple, pear, guava, banana, water melon, pea, etc.
They are more vigorous and they have higher yield.
Triploids exhibit a large degree of sexual sterility and have, therefore, to be propagated mostly by vegetative means.
The polyploids which possess an exact multiple of the haploid set of chromosomes are called "euploids".
Some polyploids where numerical change in chromosome number of individuals is not the exact multiple of haploid genome, which are called "aneuploids".
Example : Back cross between hybrids of Saccharum officinarum X S. spontaneum with either S. spontaneum or S. officinarum.
Some major crop species of presumed polyploid origin
|
Common name |
Scientific name |
Number |
Present diploid number and ploidy level |
|
Wheat |
Triticum |
\[x=7\] |
\[2n=28,\]tetraploid |
|
(a) Durum |
T. turgidum |
|
\[2n=28,\]tetraploid |
|
(b) Club |
T. aestivum |
|
\[2n=42,\]hexaploid |
|
Sugarcane |
Saccharum officinarum (Poaceae) |
\[x=10\] |
\[2n=80,\]octaploid |
|
Tobacco |
Nicotiana tabacum (Solanaceae) |
\[x=12\] |
\[2n=48,\]tetraploid |
|
Coffee |
Coffea arabica (Rubiaceae) |
\[x=11\] |
\[2n=44,\]tetraploid |
|
Cotton |
Gossypium hirsutum (Malvaceae) |
\[x=13\] |
\[2n=52,\]tetraploid |
|
Potato |
Solanum tuberosum (Solanaceae) |
\[x=12\] |
\[2n=48,\]tetraploid |
|
Strawberry |
Fragaria ananassa (Rosaceae) |
\[x=7\] |
\[2n=56,\]octaploid |
(6) Tissue culture : Tissue culture requires separation of cells, tissues or organs of a plant and allowing them to grow in aseptic nutrient media under controlled light and temperature. The cultured parts termed explants, require energy (Usually a carbohydrate like sucrose) and salts (Both macro and micro nutrients) apart from vitamins and the amino acid glycine. When a tissue from an organ is cultured, It grows into undifferentiated tissue called "callus". The callus can be differentiated into shoot, root or complete plants by manipulating the concentration of auxin and cytokinin.
Tissue culture technique is based on totipotent nature of plant cell or phenomenon of totipotency, i.e., each and every plant cell has inherent capacity to develop into complete plant.
The concept of totipotency was given by Haberlandt (1902) and practical application of totipotency was shown by Steward (1932), when he developed a complete carrot plant from a single cell obtained from root of wild carrot. The advantages of tissue culture in the improvement of crop plants are:
Explant\[\to \]Disinfection\[\to \]Culture medium\[\to \]Callus\[\to \]Plantlet (Embryoids).
The following techniques of tissue culture are useful in crop improvement.
(i) Micropropagation : Propagation through tissue culture is called micropropagation. Production of large number of individuals in vitro in a limited space which can be employed for agriculture, horticulture and forestry. e.g., Potato, Bananas, Begonia, Carnation, Chrysanthemum and Gerbera.
(ii) Somatic embryogenesis : Somatic cells are cultured in electric shakers to obtain single cell suspension. When the number of cells has increased to a maximum depending upon the amount of medium, the culture is made stationary. Each cell starts differentiating into an independent embryo showing all the stages of embryo development such as globular heart shaped and torpedo shaped stages. They are called "embryoids". Somatic embryoids can give rise a complete plant having normal root system. Success has been achieved in carrot, celery and alfalfa.
(iii) Raising of disease free plants : The virus free clones can be obtained from a virus infected plant by tissue culture since virus is translocated through sieve tubes, the apical meristem of virus infected plant remain free of virus. The shoot apex of such plant can be cultured.
(iv) Androgenic haploids : These are haploid plants raised from pollen grains by anther culture technique. The first example of androgenic haploid was reported by Guha and Maheshwari (1964) from anther culture of Datura innoxia. Haploid plants are always pure because they are having one gene for each trait, i.e., no dominant and no recessive. If such a gene undergoes mutation, it can be easily expressed. In China, this technique has produced Jinghua-I (winter wheat) and Guan-18 (Rice variety). These two are superior, high yielding and disease resistant varieties.
(v) Rescue of hybrid embryos : The hybrid embryos produced as result of interspecific or intergeneric crosses normally collapse due to incompatibility. These embryos can be isolated from female plants and rescued by growing on synthetic medium.
(vi) Induction and selection of desirable mutants : The single cell cultures raised in electric shakers are allowed to grow in static cultures where the cells divide to form colonies. These cells are treated with chemical or physical mutagens to induce mutations. The desirable mutants are selected and multiplied.
(vii) Somaclonal variations : The spontaneous variations which appear in cells or tissues in artificial medium are known as somaclonal variations. The variants having desirable traits such as tolerance to pests, pesticides, diseases and environmental stresses are selected and exploited for agricultural purpose.
(viii) Somatic hybridization : Cells of two plants belonging to different varieties, species and even genera are first treated with pectinase and cellulase enzymes. The enzymes dissolve away the walls. The naked protoplasts of the two are made to fuse by electrofusion (high frequency alternating electric field with short current pulses) and chemofusion through sodium nitrate or polyethyleneglycol (PEG). It produces hybrid protoplasts. The latter may have a single fusion nucleus (synkaryon) or two unfused nuclei (heterokaryon). Sometimes one of the two nuclei degenerates. In that case the hybrid protoplast is called cytoplasmic hybrid or cybrid (heteroplast). Pomato is a somatic hybrid between tomato and potato and is example of intergeneric hybrid.
(7) Genetic engineering : This is the latest method of crop improvement in which instead of involving whole chromosomal set (genome), manipulation of a segment of DNA (gene) is done.
Recombinant DNA technology is connected with obtaining desired DNA sequences from different organisms with the help of restriction endonucleases, joining them together to produce a new combination, chimeric or recombinant DNA (rDNA) that is then incorporated to produce the required product. There is cutting and pasting of DNA fragments. Because of it, recombinant DNA technology is also called gene splicing.
A plant in which a specific character has been introduced is called transgenic. The first transgenic commercial crop was tobacco. It was made tolerant to certain herbicides used for removing weeds. Transgenic tomato has been made resistant to horn-worm larvae.
Many transgenic plants such as tomato, cotton, tobacco, etc., have been developed which are resistant to certain specific insects and pests.
Genetic engineering is helpful in producing tomatoes with delayed ripening, mangoes with less ethylene production and potatoes with \[2040%\] more starch content.
Prospects of genetic engineering
Genetic engineering has put us in a threshold of a new form of medicine, “gene therapy” to find cures for crippling diseases like haemophilia and phenylketonuria.
Introduction of genes coding for vitamins, hormones etc., in higher animals opens up new vistas.
Possibility of transfer of nitrogen fixing genes from bacteria or blue green algae to major food crops is bound to enhance food production.
Production of new plants and animals tailored to new characteristics is now a reality.
Thorough study of the nature and functions of the heredity material is possible because of their technique leading to location of specific genes within the chromosomes and a deeper insight with in to when and where enzymes are made.
You need to login to perform this action.
You will be redirected in
3 sec
