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Professor V.L. Chopra, R.S. Paroda
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Crop growth and productivity is limited by a large number of adverse conditions of ambient environment and soil composition. Among these, drought and soil salinity are most prevalent stresses. In view of shrinking land resources and continuing pressures of growing population in most developing countries, it has become imperative that crop yields are not allowed to suffer on any type of cultivable land. Breeding crop varieties resistant to drought and salinity stress, therefore, assumes great importance. Major limitations to exploiting genetic resistance have been the lack of (i) definition of the level of stress that is relevant to crop productivity, (ii) screening procedures that can be reliably employed for identifying genotypes possessing attributes governing stability of performance under stress conditions, and (iii) Understanding of the processes that have direct as well as indirect relevance to conferment of resistance. This book presents the much-needed information on drought and salinity stress under one cover. s have been contributed by a leading authority and practitioner of his own field of specialization. The book will be an indispensable source of information for students and researchers interested in the subject of drought and salinity stresses and breeding approaches for achieving resistance to these stresses.

0 Start Pages

Preface World agriculture today presents two contrasting production systems. On the one hand is the highly mechanised, input intensive and highly productive agriculture exemplified by the agri-business approach of North America. On the other, is the agriculture with numerous limitations as operated in less developed countries. In the former, high productivity is achieved without much regard to investment in energy; there is also no dearth of cultivable area. In the less fortunate agriculture, obtainable in countries of Asia for example, there are numerous constraints on production and productivity. In most cases the possibilities of bringing additional land under cultivation are excluded. In fact, the cultivable land resource gradually shrinks under pressure of urbanisation and population increase. The economic position of the cultivators does not allow investments in costly agricultural inputs (machinery, fertiliser, agrochemicals, irrigation). Worst still, varying proportion of land is unproductive because of hostile factors like stresses of moisture, temperature, soil texture and composition. Since land is a scarce resource, devising procedures and practices that will improve production from stressed soil assume great relevance. Some countries, of which India is a good example, have made remarkable progress in improving their agricultural productivity. The productivity increases, however, have been restricted to irrigated agriculture and have utilised the avenue of genetic upgradation of the productivity potential of crop varieties. The improved genotypes make more efficient use of the applied agricultural inputs and partition a large proportion of their photosynthetic products into seed. The elevated yield levels of the new high yielding genotypes are consistently realised only for those crops which grow under stable environment. For example, high yielding varieties have been produced in India both for wheat and rice but the translated effects have been conspicuous only in the case of wheat. The diversity of specific environment in which rice grows and the limiting influences of ‘Kharif’ environment combined with incidence of pests and pathogens has not allowed the effect of the achieved genetic upgradation of production potential of rice to become perceptible uniformly at the national level. From the scientific and sociological viewpoints the challenge is to overcome the limitations to the above-mentioned factors of productivity increase. It is imperative that the benefits of improved agricultural technology become available to all sections of the farming community. The production and economic requirements demand that agriculture, even in areas suffering from one or the other kind of stress, becomes productive and remunerative. Unfortunately, research efforts for the improvement of agriculture in areas suffering from water and soil composition stresses have so far not been as vigorous as the problem demands. One of the possible causes for this has been the anxiety to increase production in areas suited for intensive agriculture so as to buy time for mounting efforts for tackling a relatively more difficult situation. A more scientific reason is our lack of knowledge of the mechanisms by which crop plants cope with a stress situation and of the operational parameters which can be employed for identifying more productive genotypes under stress situation. Experience has shown that the assumption of a productive type under favourable conditions also being productive under stress situations is not uniformly valid. Evolution of crop varieties suitable to stress situation specifically can, therefore, no longer be ignored. For this objective to be realised, it is essential to proceed in a systematic way both to clearly and scientifically define the stress and to mount basic research for gaining an understanding of the physiological and biochemical pathways that express in plants when subjected to stress. Equally important will be to understand the genetics of resistance to the relevant stresses. Only when this information is available, it will be possible to systematically devise selection criteria that can be applied for identifying donors for resistance and develop breeding strategies and methodologies useful for making selections that combine stress resistance with other required agronomic characteristics. The present volume is an attempt to present the relevant information and the state-of-the-art for two predominantly prevalent stresses i.e. drought and salinity. It is our hope that this information will provide the spring board from which major advances will be reached in the future.

1 Salt-affected Soils: An Overview

Introduction Accumulation of excess soluble salts in the root zone of soils resulting in partial or complete loss of soil productivity is a worldwide phenomenon. The problems of soil salinity are widespread in the arid and semi-arid areas but salt-affected soils also occur extensively in the sub-humid and humid climates particularly in the coastal regions where the ingress of sea water through estuaries and rivers and through ground water movement causes large scale soil and water salinisation. Soil salinity is also a serious problem in areas where ground waters of high salt content are the only source of water available for irrigation. By far the most serious problems of salinity are being faced in the irrigated, arid and semi-arid regions of the world. Our ability to manage salt-affected soils and waters both in the irrigated and in the unirrigated regions will constitute a major effort in meeting potential food requirements. The magnitude of the problem can be appreciated from the following.

1 - 23 (23 Pages)
2 Breeding Crop Varieties for Salt-affected Soils

Expression of genetic potential for growth and yield of a crop variety is directly dependent upon the environment comprising both climate (weather) and soil components. As discussed in the preceding chapter, the term ‘salt-affected soils’ encompasses a broad group of adverse but distinct soil conditions, namely, saline, alkali (sodic) and acid soils with further variations such as coastal and inland locations as well as arid and water-logged situations (Swaminathan 1977; Bresler et al. 1982). Factors limiting growth and yield of crop plants in various categories of problem soils are different and specific. Likewise, adaptive strategies evolved by plants to cope with the prevailing edaphic stresses, such as salt stress, also vary a great deal as indicated in Table 1. It is not unexpected therefore, that even varieties within a crop often show notable differences regarding tolerance to adverse soil conditions (Epstein et al. 1980; Devine 1982; Shannon 1984; Sayed 1985). Hence, it is essential that the breeder must understand the inherent charaderistics of the problem soil with which he is concerned and should also simultaneously appreciate that basically it is the sensitivity of crop variety, rather than the soil parameters per se, that essentially determines the occurrence as well as the magnitude of actual soil problem (Rana 1977). Since the degree of tolerance (adaptation) of crop varieties to different kinds of salt affected soils is genetically controlled, it is axiomatic that it can be improved through genetic manipulations by adopting suitable breeding procedures.

24 - 55 (32 Pages)
3 Drought Resistance in Crop Plants: A Physiological and Biochemical Analysis

Introduction Drought is the most common adverse environmental factor which limits crop production in different parts of the world. Often drought is accompanied by relatively high temperatures, which promote evapotranspiration, and hence could accentuate the effects of drought and thereby further reduce crop yields. Since these events occur more frequently in tropical and semitropical regions where most of the developing countries are situated, droughts are often associated with food shortages and an overall setback to developmental activities. Therefore, raising of drought-resistant crops is common sense to achieve stability in production and to enhance the possibility of self-suficiency in food. Consequently, breeding for drought resistance is a major objective of many research programmes in the international and national institutions. This has been so for the past several decades. But, it is disappointing that, there has not been any measurable success in these programmes compared with the success achieved in breeding for yield; or breeding for disease resistance or breeding for quality attributes. In fact, this was amply expressed by Arnon (1980) when he stated “Breeding for drought resistance has been a consistent theme for as long as I remember and probably the greatest source of wasted breeding efforts in the whole field of plant breeding.” However, the fact remains that we do hear of drought resistant varieties in many crops. For example, C-306 in wheat, Lalnakanda in rice and M 35-1 in Sorghum are some of the better known examples in India. It is also claimed that a large number of cultures among the Assam Rice Collection exhibit a considerably high degree of drought tolerance. Many land races and wild relatives of several crop species are said to contain drought resistant traits, which could be profitably utilised in breeding programmes. This situation is not unique to India alone but is a common experience in different parts of the world. However, to afirm a conviction a short questionnaire to assess the current state-of-art on breeding for drought resistance was distributed to 75 distinguished plant breeders and plant physiologists all over the world. The questionnaire and a summary based on the responses is as follows:

56 - 86 (31 Pages)
4 Breeding Approaches for Drought Resistance in Crop Plants

Introduction Man’s intervention in the evolution of plants, to adapt them to suit his needs, constitutes the practice of plant breeding. Historically regarded as an art and still too empirical to be accepted universally as a science, it has recently been described as a technology (Riley 1978). Whether art, science or technology, it certainly draws on a wide range of disciplines and embodies all those principles of organisation and management that determine success in any production-oriented programme. Perhaps the biggest contribution that the plant breeder has made to increased production during the past 50 years has been in helping to alleviate the effects of natural hazards such as lodging, pests and diseases on crop production. While most of this progress has been through the manipulation of biotic stresses, much is still to be achieved under non-biotic stresses such as drought, salinity, temperature, etc. This argument is strengthened by the fact that in spite of the spectacular achievements made possible by the ‘Green Revolution’, the productivity level of most crop plants, including wheat and rice, has remained both low and static under stress conditions. Concerted efforts are needed through an approach of interdisciplinary research, since not much has been done to define criteria for stress assessment and evolving breeding approaches for incorporating stress tolerance in crop plants.

87 - 107 (21 Pages)
5 Screening Techniques for Drought Resistance in Rice

Introduction Drought is one of the primary factors responsible for depressing rice yield in chronic areas and in destabilising rice production in drought-prone areas. It is a production constraint common to all rainfed rice cultures: wetland, upland, and deepwater (O’Toole and Chang 1979). Breeding for drought resistance, a major component of the International Rice Research Institute’s (IRRI) Genetic Evaluation and Utilization (GEU) Program since the early 1970s, aims at countering the adverse effects of drought in unfavoured rice production areas. Rice germplasm has such remarkably rich diversity that a continuous spectrum of genotypes differing in the various physiological mechanisms can be found and is available for genetic manipulation (Chang et al. 1982a). However, effective evaluation methods are essential for unravelling the specific mechanism(s) involved and for devising eficient selection criteria in a breeding program. Moreover, the broad spread in growth duration among rice cultivars makes it impractical to obtain uniform evaluation of a large number of cultivars when they differ in physiological responses that are specific to certain growth stages. Methods that can accommodate large numbers are essential to breeding program.

108 - 139 (32 Pages)
6 End Pages

INDEX Acacia 34 Adaptation strategies 25 Aeroponic 96, 98, 116, 118 Agropyron 32, 34, 48 Alkali soils 6 Allopolyploids 33 Back cross method 99 Betaine 93 Bipa rental approach 98 Brassica 33 Breeding for enhanced tolerance 21 Breeding for salt resistance 42 cell culture technique 44 classical genetics 44 distant hybridization 48

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