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Preface This book is in your hands because of the initiative of team NIPA whose leadership’s diagnostics acumen lead them to believe that “Plant Breeding: Theory and Practices” is a requirement of felt need of the student and teaching communities interested in grasping and practicing this discipline. The book which records the thoughts and experiences of some of the best teachers of this discipline has been out of print for quite some time. Revision of the book in the traditional sense of the word ‘revision’ was not possible. We have resorted therefore to ‘restructuring’ the contents by retaining the theoretical and principal operational steps of the relevant procedures of plant breeding which are at the core of understanding and practicing the discipline.

Plant breeding is essentially a technology. Any technology will stand the test of time if it is based on sound concepts. In the case of plant breeding, the concepts are provided by the science of genetics. Plant productivity in its broad sense requires consideration of not only yield but also of a number of its direct and indirect components. For example in groundnut, besides pod yield, shelling percentage, 100-kernel weight and oil content are components relevant to defining productivity. A number of physiological (like biomass, leaf area, specific leaf weight), morphological (like days to flowering, number of branches, number of mature seeds), nitrogen-fixing, (where relevant, for example, number of nodules, nodule mass, nitrogenase activity) and such other characters influence yield directly or indirectly. Yield and most yield contributing characters are quantitative in nature. A quantitative character is one which shows continuous variation. In contrast, qualitative characters like flower colour and seed coat colour show, in general, discrete variation and are controlled by fewer genes with major effects. A quantitative trait, however, is governed by many genes. The range of values of a quantitative character is described by a continuous distribution, usually assumed to be a normal distribution. The distribution is specified by the two parameters: mean and variance.

Man-directed evolution of distinct types and cultivars (cultivated populations) of self-fertilised crops had their beginning about 10,000 to 12,000 years ago. As a breeding system, self-fertilisation favoured constancy and fitness (Stebbins, 1950) of the genotypes to the immediate environment. This provided an alternative to the slash and burn migrant agriculture of the Neolithic farmers (7,000-6,000 B.C.) and to stabilisation of production. Manu Smriti (700–500 B.C.) proclaimed Subeejam Sukshetre Jayate Sampadyate meaning that good seed in a good field yields abundant produce. The knowledge that good seed is essential for a good harvest, led to conscious efforts for selecting types which provided a dependable source of food. Besides their utility as food, choice of plants and their seeds as progenitors of the next generation depended on personal fancy, observation and experience. As farmers learnt their crop plants better, they sorted them out into similar types (species). While migrating from the centres of origin they carried seeds of mixed populations. Self-pollinated cereal crops of barley, wheat, oats and important grain legumes such as lentil, peas and chickpea spread from the Near East Centre, first to far and remote areas of Asia, Europe and Africa; and subsequently in comparatively recent times, to the Americas and Australia. In new environments, the introduced mixed populations diversified into well adapted identities of land races. These land races continued to be the main basis of cereal production till early 1900. Improvement in crop plants made by some early civilisations was mainly through introduction and mass selection, based on the natural tendency of preserving the seeds of preferred colour and size from vigorous, healthy and more productive plants. It was only after the rediscovery of Mendel’s laws in 1900 that breeding procedures based on the knowledge of the genetic principles and the inheritance pattern of different characteristics were developed. This permitted the improvement of crops with objectivity and predictability.

Cross-pollinated species, or outbreeders, are highly heterozygous, heterogeneous and carry a higher mutation load than the inbreeders. Their mode of reproduction bestows upon them considerable heterozygous balance under panmitic population structure. Inbreeding such species disturbs the natural rhythm and leads to inbreeding depression. It is, therefore, essential that if selfing has created any disturbance in outbreeders, the heterozygous structure be restored. Since artificial selfing and outcrossing can be carried out in such crops at will, all breeding methodologies, even those meant for self-pollinated crops, can be used for their improvement. In fact, allogamous crops have provided opportunities for innovating breeding procedures to achieve the maximum efficiency by combining selfing and outcrossing cycles. Thus, the classical separation of breeding procedures for self- and cross-pollinated crops has become unnecessary, particularly, for outbreeders which are amenable to selfing. Exploitation of heterosis has been the major objective in cross-pollinated crops and most breeding procedures developed earlier aimed at maximising heterosis. In later years, with better understanding of genetic structure of populations and availability of efficient biometrical methods of estimating genetic parameters, newer procedures of accumulating desirable genes have been developed. The presently available breeding procedures for cross-pollinated crops can be broadly classified under two heads: (1) Hybrid development, and (2) Population improvement. Both these methodologies, though independent, are not mutually exclusive. The steps for operating the two procedures, however, have contrasting requirements. For hybrid development, fixation of genes through cycles of selfing is necessary whereas such fixation is avoided while developing populations. As can be seen from the scheme given below, in both cases the homozygosity attained through initial selfing is converted to heterozygosity at the end of breeding programme. Irrespective of whether hybrids, synthetics or composites are being developed, great emphasis is placed on the attainment of homogeneity in the end products.

1) Origin and distribution Pearl millet (Pennisetum americanum (L) Leeke, 2n-14) originated in west Africa from where it was introduced into India. Large variability is still found in Africa. Pearl millet is a dual purpose crop; its grain is used for human consumption and its fodder as cattle feed. It is cultivated mainly in Africa and Asia, primarily as a grain crop in an area of 26 million ha with a production of 13 million tonnes. As a poor man’s bread, it sustains a large population of Africa and Asia. It also contributes to the economy of countries like the U.S.A. where it is grown as a forage crop. In India, Pennisetum americanum (bajra) is cultivated on 11 to 12 million ha land with a production of about 6 million tonnes. It is grown on marginal lands and in areas of scanty rainfall. Rajasthan, Gujarat, Maharashtra, Uttar Pradesh and Haryana account for about 90 per cent of the total area under this crop.

A close relationship between the mode of reproduction of a cultivated plant and the methods to be employed for its improvement is now well accepted. Crop plants are broadly placed in two groups according to the mode of their reproduction: sexually reproduced and asexually propagated or reproduced plants. Asexual reproduction does not involve the union of gametes and/or reduction in chromosome numbers. Plants may reproduce asexually in a variety of ways. In 1980, Mayo has identified the following methods of asexual reproduction on the basis of the classification made earlier by Stebbins in 1941 and Brown in 1972.

Plant diseases, in spite of the various measures adopted to control them, continue to be a major cause of crop losses. At times these losses have been so heavy that they have caused severe famines. Diseases are caused by a wide variety of organisms such as fungi, bacteria, viruses, mycoplasma and nematodes. The diseases may be partially or completely controlled by chemical, cultural and genetic methods. When no single method is effective in controlling a disease, integrated control constituting a combination of the three methods of disease control (chemical, cultural, genetic) may be effective. If host resistance alone is sufficient to check a disease, then it is to be preferred to other methods because it is the least expensive, has no adverse environmental effects and involves no specific action on the part of the farmer, even when only partial, genetic resistance supplements chemical control and makes it more effective in comparison with chemical control on a completely susceptible cultivar. Many plant diseases have been successfully controlled by exploiting genetic resistance especially where differentiation in the pathogen with respect to resistance genes in the host is lacking. In some cases, even when pathotype differentiation occurs in the pathogen, genetic resistance in combination with other methods has given adequate control. Breeding for resistance to highly variable air-borne fungal pathogens has claimed much time and effort of plant breeders and success has often been transient. The reasons for the shortlived success have become clear from research during the last quarter of a century.

The immediate and long-term objective of modern plant breeding in all countries irrespective of the state of their advancement is increasing the availability of food to meet the calorie-protein requirement of their people. In the past, breeders have concentrated their efforts on increasing the yield of the important crop species. Better adaptation to local environmental conditions, high production and better resistance to the major pests and diseases have been among the priority goals of genetic plant improvement. Yielding ability and stability undoubtedly deserve high priority but enough attention also needs to be paid to improving nutritional characteristics of the plant produce for their maximum utilisation. Awareness of nutritional requirements and the optimal characteristics of crop products to be utilised as food and feed, therefore, becomes an important parameter of breeding effort. The main source of food in the developing world is cereals which provides 45 to 85 per cent of the total calories and 50 to 80 per cent of the total protein requirements. The consumption of animal proteins, which are nutritionally well balanced, is low and costly and beyond the reach of the low income groups. On the other hand, plant proteins are cheaper to produce and easier to store and transport. All these factors go in favour of increasing the quantity and quality of cereal proteins in order to improve the dietary and nutritional status of the masses in the developing world. Proteins are made up of about twenty different amino acids. For man, eight of these amino acids, valine, methionine, leucine, isoleucine, threonine, phenylalanine, lysine and tryptophan, are essential (EAA) for constituting a well balanced diet. In impioving the grain quality of cereals consideration should, therefore, be given apart from the content of carbohydrates, fats, minerals and vitamins in the grains, to improving the protein content as also the amino acid composition of the protein.

Most varieties of fruit plants have originated as chance seedlings or through somatic mutation and have been maintained and multiplied through vegetative propagation by fruit growers. The improvement of fruit plants through breeding has not yet attracted the serious attention of the research workers. Unlike field crops, fruit plant breeding is time consuming and laborious. Studies on the mode of inheritance of important economic characters of fruit plants are difficult because of the heterozygous nature of the plants. Moreover, varying degrees of sexual incompatibility have complicated the situation. Due to a long juvenile phase, it takes a long time to identify a useful hybrid. This factor has become a serious handicap because most research workers leave or shift from the programme before tangible results are obtained. During the last two decades, considerable useful information has been generated on breeding methodology for the improvement of some of the important fruit crops. In this chapter fruits like mango, banana and papaya have been included.

Introduction Tree improvement through the application of genetic principles is directed towards modifying the heredity of tree populations so that they are able to meet the needs of the forester and the wood based industry. The breeder of annual crop plants resorts to hybridisation, selection, testing of the improved strains, multiplication of improved seeds and finally their release for commercial planting in that logical sequence. But the tree breeder, whose career may be shorter than the life cycle of the tree species taken up for improvement, tends to carry out the above operations in a modified form and often some of the operations are carried out concurrently. Many of our tree species are wild unselected populations. They are, therefore, expected to exhibit considerable natural variability for many characters and also to respond well to selection for further improvement. Further, altitudinal and latitudinal variation in many characters between populations are expected for those tree species which have a wide geographical distribution. So, the selection of desirable seed sources or provenances is an essential step that is basic to all tree improvement programmes. The right provenance is identified through well laid out provenance trials at a number of locations in the area where the improved selection is to be grown. Further improvement is then planned and executed for exploiting the naturally available intrapopulation variation. The variation normally measured is the phenotypic variation which is the result of interaction of the genotype of the individual with the environment. However, what is essential for selection to be effective is genetic variation for the character under consideration. Genetic variation is assessed by appropriate progeny trials. Since many of the economically valuable characters of tree species are polygenically controlled and exbibit continuous variation, measurements have to be in metric units. The use of biometrical techniques for assessing and analysing this variation, therefore, becomes an essential requirement.

Forage crops are plant species used to feed domestic animals (such as cattle, sheep, goat, pig, and poultry). Both cultivated and wild plants can serve as forage. The domestic animals feed on forages either by grazing or stall feeding (cut fodder eaten in stalls). Grazing, that is pasturing of animals, is cheaper and allows production of both livestock and dairy produce at a lower cost because no expense is involved in growing, cutting, transporting or feeding. In North and South America, Australia, New Zealand, and in many other temperate countries pastures are widespread and popular because sufficient land is available. While land is cheap and plentiful, labour is scarce and expensive; hence management of natural, and in recent years sown pastures has received much attention. Until about fifty years ago, the natural grasslands in the Western countries met the minimum needs of the farmers and grassland management was operated at a level comparable to subsistence agriculture of many developing countries. In other words, emphasis was laid on exploitation of natural productivity of the grassland, using a low level of management based on minimum use of inputs like chemical fertilizers. However, it was soon realized that productivity of native grasslands will have to be greatly improved if the high demands for meat and milk were to be adequately met. In India, however, out of a total cultivable area of about 131.11 million ha less than 2 per cent is utilized for forage crops because feeding the large human population takes the first preference for land use to produce grain and cash crops. It is, therefore, necessary to intensify efforts, particularly in over-populated countries, for a more efficient production of high yielding forage crops to get maximum return out of minimum land.

Breeding methods for vegetable crops are basically similar to those adopted in others like cereals, oilseeds or pulses. Certain aspects, peculiar to vegetables, however, require specific attention: (i) In many cases, the edible part of the plant is not the seed (as cauliflower, cabbage, carrot and onion) and hence while breeding for improved plant part that is used as vegetable, its seed production capacity needs due consideration, (ii) Vegetables like tomato, garden pea and onion are consumed in different forms (fresh or processed) and the end use of the product (like processing quality) has to be taken into account in breeding programmes, (iii) Consumer preferences (as for example in eggplant) size, colour and shape of the vegetable has to be given due emphasis, (iv) Being perishable in nature, keeping quality and suitability for long distance transportation are important attributes in vegetable crops unlike grain crops where bulk handling and haulage can be mechanised, (v) Another peculiarity with vegetable crops is that they vary widely in modes of reproduction necessitating appropriate choice of breeding methods. In addition to vegetatively propagated crops like potato, sweet potato, cassava (tapioca), garlic, pointed gourd or “Parwal (Trichosanthes dioica L.) and ivy gourd or ‘Kundru’ (Coccinia cordifolia L.), there are many seed propagated vegetable crops which are auto- or allogamous or intermediate (Table 1). Within these broad groups, variations in the extent of cross- or self-pollination, namely, 15-20 per cent natural cross-pollination in chillies and 20-25 per cent natural self-pollination in muskmelon have been recorded.

Since genetic variability is essential for any crop improvement programme, the creation and management of genetic variability becomes central to crop breeding. Experimentally induced mutations provide an important source of variability. An advantage of such created variability is that the starting point can be an agronomically accepted cultivar rather than a genetic stock of no direct commercial value with many undesirable attributes. It should, however, be realized that none of the imposed treatments in a plant system has so far been able to direct a specific change. As a result, following mutagenic treatments, a mixed bag of induced variants is found and many of these may not be of any value. Also, many events take place concurrently in a treated cell to produce variation at several loci within a genome. Because of these reasons it will be unrealistic to expect miracles out of a mutation breeding programme. The desired results can be achieved only when the objectives are clearly defined and the experiments are specifically designed to achieve those objectives. When a mutant of the required type is obtained, we only reach the step of identification of a desired recombinant in a conventional breeding programme and all the succeeding steps of evaluation of the mutant are in no way different from those in the conventional breeding procedures.

Genetic resources deal with germplasm of a given species that provides the necessary genetic variation for the improvement of that species in terms of particular products, increased efficiency and higher levels of productivity. Genetic resources are, in this sense, the building blocks, fundamental not only to a crop improvement programme, but also for the very survival of the species in time and space (Swaminathan, 1983). As agriculture is becoming more and more intensive and location-specific, plant breeding objectives are becoming more and more complex. Besides high yield, a modern variety is often expected to combine specific duration, nutritional attributes, adaptation to varying soil and water regimes, and resistance to ever-multiplying pests and diseases—to mention only a few of the character combinations. To meet these objectives, a wide range of germplasm must exist, and the plant breeder should have an easy access to the genetic resources.

In conventional breeding for genetic upgradation of yield and other desirable attributes of crop plants, favourable assemblages of genes from two or more parents are made to develop a commercial variety. The important steps of this exercise are: locating the right kind of parents that possess the desired characteristics; making crosses between or among them; handling the segregating generations in an appropriate manner so as to identify the required recombinants; stabilizing the recombinants and finally evaluating their performance in a series of trials. This approach has been very successful in the sense that it has provided increments in productivity continuously. In an analysis done by the American Society of Agronomy on the genetic contributions to yield gains of five major crop plants, it was concluded that sorghum yields increased due to genetic improvements at the rate of 1.2 per cent per year between 1950 and 1980. For corn, the corresponding yield increase was 90 kg/ha/year between 1930 and 1980, which is equivalent to a rate of increase of 1 per cent per year. It was also estimated that 70 per cent of the total gains in corn yields are attributable to genetic improvement and 30 per cent of the yield gains are due to non-seed inputs like fertilizers, pesticides and cultural practices. In India also, the improvement of productivity of the major food, fibre and several cash crops has been very impressive. The success is due to intensive research in plant breeding resulting in the evolution of varieties with very high genetic yield potentials and the concurrent support from other disciplines that has provided the package of practices which help in the realization of the created potential. In all crop plants, where the breeding efforts have been intensive, the phenomenon of plateauing of yields has become apparent. Wheat provides, perhaps, the best example of the situation. Since the 1970s, when the gains from genetic restructuring of the plant type had been established, major yield increases have not occurred. Since the demands of the future will be undoubtedly much larger, and additional cultivable land will not be available, ways will have to be found for a sizable increase in productivity per unit area in the immediate future. This requirement calls for the development of strategies and procedures that will make dramatic yield gains possible. For meeting the challenge, a continuous search is being made for innovative methods which will give yield increments of a much higher order than those provided by the conventional approach. In this chapter, an attempt has been made to introduce some such promising approaches.