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In this article we will discuss about:- 1. History of Plant Breeding 2. Origin and Evolution of Crop Plants 3. Scope and Objectives.
History of Plant Breeding:
The process of plant breeding is assumed to be initiated nearly 7000 years ago with the beginning of human civilization. Bringing a wild species under human management as the source of food which can be referred to as the process of domestication. Movement of man from one place to another also helped the movement of cultivated plant species.
In this way the introduction of new plant species or varieties into new area from other parts of the world became an integral part of plant breeding today. Furthermore man has started the process of selection by selecting the best seeds or good grains from the field to be planted in future. The first artificial hybridization procedure to get the hybrid was carried out by Knight and described hybrid vigour.
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After the rediscovery of Mendelism at the beginning of 20th century, the concept of plant breeding underwent tremendous change. The cytogenetical principle behind plant breeding got sound base; different selection methods, back-cross method, and the techniques for inbred line development were established.
Polyploid breeding, mutation breeding and ultimately the alien gene transfer method came into use. The non-conventional breeding methods through plant biotechnological approach have become the major thrust of twenty first century.
Origin and Evolution of Crop Plants:
A. Centres of Origin:
N. I. Vavilov has proposed that crop plants evolved from wild species in the areas showing diversity and termed them as primary centres of origin. From these places the crops moved to other areas with the movements of man. But in some areas, certain crop species show considerable diversity of forms although they did not originate there. Such areas are known as secondary centres of origin of these species.
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Vavilov has suggested eight main centres of origin:
B. Patterns of Evolution:
There are three major lines of evolution pattern for various crops, those can be broadly classified as Mendelian variation, interspecific hybridization and polyploidy.
(A) Mendelian Variation:
Naturally occurring beneficial mutation causes genetic variability and these variability can be retained through hybridization and genetic recombination. Generally the macro-mutation produces a distinct morphological character or often affects several characters of a plant.
Cabbage, Cauliflower, Brussel’s Sprouts-all have been originated from a common wild species of Brassica oleracea through mutation, they differ from each other with respect to few major genes. Natural selection procedure has acted on variation and several important crops have evolved through Mendelian variation like barley, rice, peas, tomatoes, beans, etc.
(B) Interspecific Hybridization:
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Generally the interspecific hybridization produces sterile plants which are unable to produce any seeds. These can be repeatedly back-crossed to one of the parents which help in restoration of genotype of one parent along with few genes from other species.
This process is known as introgressive hybridization which leads to transfer of one species to another, thus playing an important role in evolution. For example, modern day maize has been developed by this kind of introgressive hybridization between primitive maize variety and a wild grass Tripsacum.
(C) Polyploidy:
Autopolyploidy (triploid or tetraploid) produces more vigour, larger flowers and fruits, e.g., cultivated banana, apple, watermelon are autotriploid. But allopolyploidy plays major role in crop evolution, it is the process of chromosome doubling of interspecific F1 hybrid.
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Many important crop plants have been evolved in this way like Triticum, Gossypium, etc. Triticale is a man made allohexaploid developed from chromosome doubling of F1 hybrid between Secale cereale and tetraploid wheat.
C. Domestication and its Effects:
Domestication is the first step of making the wild weed species to cultivated plants. It is the process of bringing wild species under human management according to their needs. In other words domestication of plants is the change of the ideo-type to adopt them better to manmade environment.
Under domestication the wild spec les get changed to cultivated species mainly due to natural selection or human selection. In nature there is continuous selection by natural forces like temperature, soil, weather, pests, diseases, etc. The genotype which is more suited to a given environment leaves behind others which are less adaptive in nature.
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Most of the characteristics of wild species have been affected under domestication which involves three processes like mutation, hybridization and gem-tic recombination under the influence of human selection or natural selection. Some characters have got changed, some have lost and many have developed during domestication.
Some of the important characters which have been affected are listed below:
1. Elimination or reduction of spattering of pods or spikes.
2. Elimination of dormancy period.
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3. Decrease in toxins or other undesirable substances.
4. Increase in size of the grains or fruits.
5. Plant type change like decrease or increase in height, more number of tillers, leaf size, branching pattern, etc.
6. Early maturity.
7. Increase in economic yields.
8. Change in photoperiodic behaviour.
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9. Mode of reproduction.
10. Pollination habit.
11. Synchrony in flowering.
12. Loss of defensive adaptation likes hairs, thorns, etc.
13. Selection of bisexual variety rather than dioecious.
14. Decrease in variability.
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15. Selection of polyploidy.
The crops like rice, wheat, barley, sugarcane, cotton, potato, tobacco, arhar have become cultivated types from the wild species through the process of domestication.
D. Origin of Crop Plants:
Origin of Rice:
Rice (Oryza sativa) had originated probably in 300 B.C. in South and South-East Asia. The cultivated forms might have been arisen from the wild species of rice like O. perennis (Fig. 1.1). From India rice moved to China and to Africa and America where during domestication three different forms has been originated: indica, japonica and javanica.
During domestication many morphological, physiological changes have occurred, starting from adaptation to habitats of normal lands and open sun to the shady swamps. Change in leaf shape and size, grain character and other plant type characters differentiated O. sativa into three forms.
Origin of Wheat:
Triticum might have been originated about 6000 years ago in Afghanistan and South-Western Himalayas and then moved with man westwards and eastwards. In total, four species of Triticum is cultivated in India: T. durum, T. aestivum, T. dicoccum and T. turgidum which are allopolyploids (Fig. 1.2). During domestication the plant type has got changed with the major changed character in non-brittle rachis.
Origin of Cotton:
Out of four species of cotton (Gossypium) under cultivation, two are diploid and others two are tetraploid. The diploid cotton have been originated during Indus civilization about 2000 B.C. From India two species moved to Africa and America and there during domestication the tetraploid species have originated.
The major character which has been developed during domestication is the corboluted lint and its spinnability. The diploid species are G. arboreum, G. herbaceum whereas the tetraploid species are G. barbadense, G. hirsutum. New world cotton is an allotetraploid (Fig. 1.3).
Origin of Tea:
This crop might have been originated in Central Asia and then differentiated into two types, small leaved China type and broad leaved Assam type. The China type (Camellia sinensis) further differentiated and domesticated in South China which has been introduced into India in the early part of 19th century and the Assam type (C. assamica) had the secondary centre of origin in North East India.
Scope and Objectives of Plant Breeding:
The different methods of plant breeding for the improvement of crop are directed in the following aspects:
(i) Higher Productive Capacity:
This is the first and foremost objective of plant breeding programme, but this higher yield is associated with many other criteria like responsiveness to fertilizer, tillering capacity, resistance to disease or adverse situation, etc.
(ii) Improved Quality:
Besides quantitative characters, the quality of food grains is considered in plant breeding programme keeping an eye to market value, feeding quality and seed quality. High protein content, vitamin content, good amino acid profile, palatability, texture, large and bold grains, good colour all are qualitative characters.
(iii) Disease Resistance:
In contrast to chemical control of diseases and pests, genetic resistance conferred by resistant crop varieties is stable, safest and cheapest which is highly desirable. The disease resistance can be categorized like insect, fungus, bacteria and virus resistant.
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(iv) Adaptive Ability:
The plant breeding programme aims at developing the resistance or tolerance property against adverse environmental situation such as drought; flood, cold, high salinity, etc. All these qualities are influenced and altered by genotype environment interaction.
(v) Agronomic Properties:
Most of the agronomic properties are often desirable as those are associated with some other characters linked with higher yield, such as dwarfness of cereal crop is associated with lodging resistance; more number of tillers is associated with more yields.
(vi) Change in Maturity Duration and Synchronous Maturity:
The duration for maturity of particular crop can be reduced which helps in multiple crops ping and permits new crop rotation. Synchronous maturity is needed mainly in food crop for good harvesting.
(vii) Physiological Properties:
Development of insensitivity against photoperiod and temperature is very much desirable in case of crops like rice, wheat, etc., so that the same variety can be cultivated in different areas and in different seasons.
Dormancy is another criterion which is desirable in case of crop plants because if there is rain at the time of maturity, the seeds get germinated in field before harvesting. But in some other cases it is desirable to remove dormancy.
(viii) Varieties for New Seasons:
The crops cultivated in India can be broadly subdivided into two categories like Rabi and Kharif. But if the crop can be adjusted to grow in new season then the same crop may be cultivated throughout the year.
(ix) Varieties for New Areas:
Newer varieties are developed by plant breeders which are adapted to wider areas and newer agro-climates. Many of the temperate crops by continued selection can be adapted to tropical or sub-tropical conditions.
(x) Some Other Desirable Properties Specific to Crop:
In case of leguminous crop non-shattering is one of the desirable characters. Elimination of toxic substances from particular crop sometimes become essential such as khesari seeds have neurotoxin chemical, Brassica seeds contain undesirable erucic acid. Removal of these substances increase the nutritional value of crop.
