In this article we will discuss about:- 1. Meaning of Ideotype Breeding 2. Features of Ideotype Breeding 3. Steps 4. Factors 5. Achievements 6. Future Research 7. Merits and Demerits.
Meaning of Ideotype Breeding:
Ideotype breeding or plant type breeding can be defined as a method of crop improvement which is used to enhance genetic yield potential through genetic manipulation of individual plant character. Each character plays a definite role in the enhancement of yield. In other words, plant characters are chosen in such a way that each character contributes towards increased economic yield.
In broad sense an ideotype is a biological model which is expected to perform or behave in a predictable manner within a defined environment. More specifically, crop ideotype is a plant model which is expected to yield greater quantity of grains, fibre, oil or other useful product when developed as a cultivar. The term ideotype was first proposed by Donald in 1968 working on wheat.
The main points about ideotype are given below:
1. Crop ideotype refers to model plants or ideal plant type for a specific environment.
2. Ideotype differs from idiotype. The former refers to a combination of various plant characters which enhance the yield of economic produce, whereas the latter refers to the morphological features of the chromosomes of a particular plant species.
3. Donald included only morphological characters to define an ideotype of wheat. Subsequently, physiological and biochemical traits were also included for broadening the concept of crop ideotype.
4. Ideal plants or model plants are expected to give higher yield than old cultivars in a defined environment.
5. Ideotype is a moving goal which changes according to climatic situations, type of cultivation, national policy, market requirement etc. In other words, ideotypes have to be redesigned depending upon above factors. Thus, development of crop ideotypes is a continuous process.
6. Ideal plant type or model plant type also varies from species to species. Moreover, this is a difficult and slow method of cultivar development because various morphological, physiological and biochemical characters have to be combined in single genotype from different sources.
Features of Ideotype Breeding:
Main features of ideotype breeding are briefly discussed below:
1. Emphasis on Individual Trait:
In ideotype breeding, emphasis is given on individual morphological and physiological trait which enhances the yield. The value of each character is specified before initiating the breeding work.
2. Includes Yield Enhancing Traits:
Various plant characters to be included in the ideotype are identified through correlation analysis. Only those characters which exhibit positive association with yield are included in the model.
3. Exploits Physiological Variation:
Genetic differences exist for various physiological characters such as photosynthetic efficiency, photo respiration, nutrient uptake, etc. Ideotype breeding makes use of genetically controlled physiological variation in increasing crop yields, besides various agronomic traits.
4. Slow Progress:
Ideotype breeding is a slow method of cultivar development, because incorporation of various desirable characters from different sources into a single genotype takes long time. Moreover, sometimes undesirable linkage affects the progress adversely.
5. Selection:
In ideotype breeding selection is focussed on individual plant character which enhances the yield.
6. Designing of Model:
In ideotype breeding, the phenotype of new variety to be developed is specified in terms of morphological and physiological traits in advance.
7. Interdisciplinary Approach:
Ideotype breeding is in true sense an interdisciplinary approach, it involves scientist from the disciplines of genetics, breeding, physiology, pathology, entomology etc.
8. A Continuous Process:
Ideotype breeding is a continuous process, because new ideotypes have to be developed to meet changing and increasing demands. Thus development of ideotype is a moving target.
Ideotype breeding differs from traditional breeding in the sense that values for individual traits are specified in case of ideotype breeding, whereas such values are not fixed in case of traditional breeding. In other words, first a conceptional plant model is fixed and then efforts are made to achieve such model. In traditional breeding, such models are not developed before initiation of breeding programme. There are several differences between traditional breeding and ideotype breeding (Table 38.1).
Morphological and Physiological Features:
The crop ideotype consists of several morphological and physiological traits which contribute for enhanced yield or higher yield than currently prevalent crop cultivars. The morphological and physiological features of crop ideotype differ from crop to crop and sometimes within the crop also depending upon whether the ideotype is required for irrigated cultivation or rainfed cultivation.
Ideal plant types or model plants have been discussed in several crops like wheat, rice maize, barley, cotton and beans.
The important features of ideotype for some crops are briefly described below:
A. Wheat:
The term ideotype was coined by Donald in 1968 working on wheat.
He proposed ideotype of wheat with following main features:
1. A Short Strong Stem:
It imparts lodging resistance and reduces the losses due to lodging.
2. Erect Leaves:
Such leaves provide better arrangement for proper light distribution resulting in high photosynthesis or CO2 fixation.
3. Few Small Leaves:
Leaves are the important sites of photosynthesis, respiration and transpiration. Few and small leaves reduce water loss due to transpiration.
4. Larger Ear:
It will produce more grains per ear.
5. An Erect Ear:
It will get light from all sides resulting in proper grain development.
6. Presence of Awns:
Awns contribute towards photosynthesis.
7. A single culm.
Thus, Donald included only morphological traits in the ideotype. However, all the traits were based on physiological considerations. Finlay (1968) doubted the utility of single culm in wheat ideotype. He considered tillering as an important feature of wheat ideotype. Some workers suggested that a wheat plant with moderately short but broad flag leaf, long flag leaf sheath, short ear extrusion with long ear, and moderately high tillering capacity should give high yield per plant. Asana proposed wheat ideotype for rainfed cultivation. Recent workers included both morphological and physiological characters in wheat ideotype.
B. Rice:
The concept of plant type was introduced in rice breeding by Jennings in 1964, though the term ideotype was coined by Donald in 1968. He suggested that in rice an ideal or model plant type consists of (1) semi dwarf stature, (2) high tillering capacity, and (3) short, erect, thick and highly angled leaves.
Jennings also included morphological traits in his model. Now emphasis is also given on physiological traits in the development of rice ideotype.
C. Maize:
In 1975, Mock and Pearce proposed ideal plant type of maize. In maize, higher yields were obtained from the plants consisting of (1) low tillers, (2) large cobs, and (3) angled leaves for good light interception. Planting of such types at closer spacings resulted in higher yields.
D. Barley:
Rasmusson (1987) reviewed the work on ideotype breeding and also suggested ideal plant type of six rowed barley. He proposed that in barley, higher yield can be obtained from a combination of (1) short stature, (2) long awns, (3) high harvest index, and (4) high biomass. Kernel weight and kernel number were found rewarding in increasing yield.
E. Cotton:
In cotton, genotypes with zero branch, short stature, compact plant, small leaves and fewer sympodia were considered to enhance yield levels. Singh et al. (1974) proposed an ideal plant type of upland cotton (Gossypium hirsutum) and tree cotton (G. arboreum) for irrigated conditions of North Indian cotton growing belt.
The proposed ideotype includes (1) short stature (90-120 cm), (2) compact and sympodial plant habit making pyramidal shape, (3) determinate in ‘ fruiting habit with unimodal distribution of boiling, (4) short duration (150-165 days), (5) responsive to high fertilizer dose, (6) high degree of inter plant competitive ability, (7) high degree of resistance to insect pests and diseases, and (8) high physiological efficiency.
Singh and Narayanan (1993) proposed an ideotype of above two species for rainfed conditions. The main features of proposed ideotype include, earliness (150-165 days), fewer small and thick leaves, compact and short stature, indeterminate habit, sparse hairiness, medium to big boll size, synchronous boiling, high response to nutrients, and resistance to insects and diseases.
F. Sorghum and Pearl-Millet:
Improvement in plant type has been achieved in Sorghum and pearl-millet through the use of dwarfing genes. In these crops dwarf F1 hybrids have been developed which have made combine harvesting possible.
Genetic improvement has been achieved through modification of plant type in several crop species. New ideotypes have been proposed for majority of crop plants. Swaminathan (1972) has listed several desirable attributes of crop ideotypes with special reference to multiple cropping in the tropics and sub tropics.
These features include:
(1) Superior population performance,
(2) High productivity per day,
(3) High photosynthetic ability,
(4) Low photo respiration,
(5) Photo and thermo-insensitivity,
(6) High response to nutrients,
(7) High productivity per unit of water,
(8) Multiple resistance to insects and diseases,
(9) Better protein quantity and quality,
(10) Crop canopies that can retain and fix a maximum of CO2 and
(11) Suitability to mechanization.
Steps Involved in Ideotype Breeding:
Ideotype breeding consists of four important steps, viz.:
(1) Development of conceptual theoretical model,
(2) Selection of base material,
(3) Incorporation of desirable characters into single genotype, and
(4) Selection of ideal or model plant type.
These steps are briefly discussed below:
i. Development of Conceptual Model:
Ideotype consists of various morphological and physiological traits. The values of various morphological and physiological traits are specified to develop a conceptual theoretical model. For example, values for plant height, maturity duration, leaf size, leaf number, angle of leaf, photosynthetic rate etc., are specified. Then efforts are made to achieve this model.
ii. Selection of Base Material:
Selection of base material is an important step after development of conceptual model of ideotype. Genotypes to be used in devising a model plant type should have broad genetic base and wider adaptability, so that the new plant type can be successfully grown over a wide range of environmental conditions with stable yield.
Genotypes for plant stature, maturity duration, leaf size and angle are selected from the global gene pool of the concerned crop species. Genotypes resistant or tolerant to drought, soil salinity, alkalinity, diseases and insects are selected from the gene pool with the cooperation of physiologist, soil scientist, pathologist and entomologist.
iii. Incorporation of Desirable Traits:
The next important step is combining of various morphological and physiological traits from different selected genotypes into single genotype. Knowledge of the association between various characters is essential before starting hybridization programme, because it helps in combining of various characters.
Linkage between desirable and undesirable traits hinders the progress of ideotype breeding. Various breeding procedures, viz. single cross, three way cross, multiple cross, backcross, composite crossing, intermating, mutation breeding, heterosis breeding, etc., are used for the development of ideal plant types in majority of field crops. Backcross technique is commonly used for transfer of oligogenic traits from selected germplasm lines into the background of an adapted genotype.
iv. Selection of Ideal Plant Type:
Plants combining desirable morphological and physiological traits are selected in segregating populations and intermated to achieve the desired plant type. Morphological features are judged through visual observations and physiological parameters are recorded with the help of sophisticated instruments.
Screening for resistance to drought, soil salinity, alkalinity, diseases and insects is done under controlled conditions. This task is completed with the help of scientist from the disciplines of physiology, soil science, pathology and entomology. Finally, genotypes combining traits specified in the conceptual model are selected, multiplied, tested over several locations, and released for commercial cultivation.
Factors Affecting Ideotypes Breeding:
There are several factors which affect development of ideal plant type. Ideotype differs based on crop species, cultivation practices, and socio-economic conditions of farmers and economic use of the plant parts.
These are briefly discussed below:
1. Crop Species:
Ideotype differs from crop to crop. The ideotype of monocots significantly differs from those of dicots. In monocots, tillering is more important whereas in dicots branching is one of the important features of ideotype.
2. Cultivation:
The ideotype also differs with regard to crop cultivation. The features of irrigated crops differ from that of rainfed crop. The rainfed crop needs drought resistance, fewer and smaller leaves to reduce water loss through transpiration. In dicots, indeterminate types are required for rainfed conditions, because indeterminate type can produce another flush of flowers if the first flush is affected by drought conditions.
3. Socio-Economic Condition of Farmers:
Socio-economic condition of farmers also determines crop ideotype. For example, dwarf Sorghum is deal for mechanical harvesting in USA, but it is not suitable for the farmers of Africa where the stalks are used for fuel or hut construction.
4. Economic Use:
The ideotype also differs according to the economic use of the crop. For example, dwarf types are useful in Sorghum and pearl millet when the crop is grown for grain purpose. But when these crops are grown for fodder purpose, tall stature is desirable one. Moreover, less leafy types are desirable for grain purpose and more Jeafy genotypes for fodder purpose. The larger leaves are also desirable in case of fodder crop.
Achievements of Ideotype Breeding:
Ideotype breeding has significantly contributed to enhanced yields in cereals (wheat and rice) and millets (Sorghum and pearl millet) through the use of dwarfing genes, resulting in green revolution. Semi-dwarf varieties of wheat and rice are highly responsive to water use and nitrogen application and have wide adaptation.
These qualities have made them popular throughout the world. Spontaneous mutations have played significant role in designing new plant types in wheat and rice. The Norin 10 in wheat and Dee-geo-Woo-gen in rice are the sources of dwarfing genes. These sources of dwarfing genes were obtained as a result of spontaneous mutations.
Several high yielding semi-dwarf varieties have been evolved in wheat and rice through the use of respective dwarf mutant. The Norin 10 dwarfing gene in wheat, the Dee-geo-Woogen dwarfing gene in rice and the genie cytoplasmic male sterile systems in Sorghum and pearl millet laid the foundation of green revolution in Asia. Thus, ideotype breeding has been more successful for yield improvement in cereals and millets than in other crops.
In rice, the improved plant type includes:
(1) Erect, short and thick leaves,
(2) Dwarf stature (short and thick straw),
(3) Light leaf sheath,
(4) High tillering capacity,
(5) Responsiveness to high levels of nitrogen, and
(6) High harvest index.
Examples of semi-dwarf varieties of rice are IR 8, IR 20, TN 1 etc. The Chinese variety Dee-geo-Woogen is the source of dwarfing gene in all these varieties.
In wheat, the improved plant type included:
(1) Short and stiff straw (culms),
(2) Insensitivity to photoperiods,
(3) High response to nitrogen application,
(4) High harvest index, and
(5) Resistance to different rusts.
The semi-dwarf Japanese variety Norin 10 was used as source of dwarfness in the development of semi dwarf Mexican varieties of wheat.
In Sorghum and pearl millet, short statured hybrids have been developed through the use of dwarfing genes. The dwarf hybrids have made machine harvesting possible in these crops. In Sorghum, combine harvesting has reduced the labour requirement by 1/8.
In cotton, as a result of high selection pressure for earliness, short stature and compactness in the past, there has been a gradual reduction in the overall plant size. The earlier varieties were late maturing, tall growing and spreading types leading to bushy appearance. The target shifted to development of varieties with medium height, medium maturity and semi spreading habit. Now major emphasis is to evolve varieties with sort duration, short stature and compact plant type.
Future Research on Ideotype Breeding:
In India, the future research on crop ideotypes should be directed towards following aspects:
1. India has achieved self-sufficiency in the production of food grains through modification of plant characters and development of high yielding varieties/hybrids. The further breakthrough in yield and quality has to be achieved through the exploitation of physiological variation. Ideotypes both for high and low input technology conditions have to be developed.
2. To further enhance the yield potential of food grain crops, ideotypes have to be evolved for straight varieties and hybrids. There is ample scope of developing hybrid ideotypes in crops like maize, Sorghum, pearl millet and rice. China has developed hybrid rice for commercial cultivation which covers more than 18 million hectares.
3. Crop ideotypes have been developed especially in cereals and millets. There is ample scope for developing ideal plants or model plants in pulses, oilseeds, cotton and several other field crops. In these crops, again ideotypes have to be evolved both for irrigated as well as rainfed cultivation. In cotton, ideotypes have to be developed for north, central and south zones which differ significantly from each other with regard to agro-climatic conditions.
4. In addition to traditional breeding approaches, biotechnological approaches, especially tissue culture and protoplast technology, have to be utilized in future for designing new plant types. Biotechnology may help in the development of insect resistant cultivars through the use of transgenic plants.
5. Development of crop ideotype is a continuous process. Ideotype is a moving goal which changes with advancement in knowledge, new requirements, change in economic policy etc.
6. Ideotypes should be developed to adverse conditions such as heat cold, salinity and drought conditions.
Merits and Demerits of Ideotype Breeding:
Merits:
1. Ideotype breeding is an effective method of enhancing yield through manipulation of various morphological and physiological crop characters. Thus it exploits both morphological and physiological variation.
2. In this method values of various morphological and physiological traits are specified and each character or trait contributes towards enhanced yield.
3. Ideotype breeding involves experts from the disciplines of plant breeding, physiology, biochemistry, entomology and plant pathology. Each specialist contributes in the development of model plants for traits related to his field.
4. Ideotype breeding is an effective method of breaking yield barriers through the use of genetically controlled physiological variation for various characters contributing towards higher yield.
5. Ideotype breeding provides solution to several problems at a time like disease, insect and lodging resistance, maturity duration, yield and quality by combining desirable genes for these traits from different sources into a single genotype.
6. It is an efficient method of developing cultivars for specific situation or environment.
Demerits:
1. Incorporation of several desirable morphological and physiological and disease resistance traits from different sources into a single genotype is a difficult task. Sometimes, combining of some characters is not possible due to tight linkage between desirable and undesirable characters. Presence of such linkage hinders the progress of ideotype breeding.
2. Ideotype breeding is a slow method of cultivar development, because combining together of various morphological and physiological features from different sources takes more time than traditional breeding where improvement is made in yield and one or two other characters.
3. Ideotype breeding is not a substitute for traditional or conventional breeding. It is a supplement to the former.
4. Ideotype is a moving object which changes with change in knowledge, new requirements, national policy, etc. Thus new ideotypes have to be evolved to meet the changing and increasing demands of economic products.
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