Tài liệu 8132227239 abiotic stress physiology of horticultural crops (springer, 2016)

N.K. Srinivasa Rao K.S. Shivashankara R.H. Laxman Editors Abiotic Stress Physiology of Horticultural Crops Abiotic Stress Physiology of Horticultural Crops N.K. Srinivasa Rao • K.S. Shivashankara R.H. Laxman Editors Abiotic Stress Physiology of Horticultural Crops Editors N.K. Srinivasa Rao Division of Plant Physiology and Biochemistry ICAR-Indian Institute of Horticultural Research Bengaluru, Karnataka, India K.S. Shivashankara Division of Plant Physiology and Biochemistry ICAR-Indian Institute of Horticultural Research Bengaluru, Karnataka, India R.H. Laxman Division of Plant Physiology and Biochemistry ICAR-Indian Institute of Horticultural Research Bengaluru, Karnataka, India ISBN 978-81-322-2723-6 ISBN 978-81-322-2725-0 DOI 10.1007/978-81-322-2725-0 (eBook) Library of Congress Control Number: 2016933032 # Springer India 2016 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. Printed on acid-free paper This Springer imprint is published by Springer Nature The registered company is Springer (India) Pvt. Ltd. Preface Abiotic stress factors, mainly salinity, drought, flooding and high temperature, are the main elements which drastically limit the horticultural crop productivity globally. Abiotic stress leads to a series of morphological, physiological, biochemical and molecular changes in plants that adversely affect growth and productivity. Extreme environmental events in the era of global climatic change further aggravate the problem and remarkably restrict the plant growth and development. The mechanisms underlying endurance and adaptation to environmental stress factors have long been the focus of intense research. Plants overcome environmental stresses by the development of tolerance, resistance or avoidance mechanisms. Plant acclimation to environmental stresses is the process to adjust to a gradual change in its environment which allows the plants to maintain performance across a range of adverse environmental conditions. Stress tolerance mechanisms in horticultural crops are gaining attention because most agricultural regions are predicted to experience considerably more extreme environmental fluctuations due to global climate change. It has been estimated that salinity and drought are expected to cause serious salinisation of more than 50 % of all available productive, arable lands by the year 2050. Water availability and water use efficiency are among the important abiotic factors that have had and continue to have a decisive influence on plant evolution. Water stress in its broadest sense encompasses both drought and flooding stress. In-depth knowledge on molecular mechanisms of abiotic stress effects on plants is needed for developing tolerant genotypes. A clear understanding of environmental factors and their interaction with physiological processes is extremely important for improving horticultural practices. Horticultural crops include a wide range of commodities, such as fruits and vegetables that are highly valuable for humanity. They are extensively grown worldwide, and their production can be described as an open and highly complex system affected by many factors, among which we can count weather, soil and cropping system, as well as the interaction between these factors. Scope of this book includes chapters on tropical and subtropical species written by scientists from different fields of specialisation. The influence of environmental factors, such as temperature, water and salinity on plant physiology and on vegetative and reproductive growth, is comprehensively discussed for each crop. In this book Abiotic Stress Physiology of v vi Preface Horticultural Crops, we present a collection of 19 chapters written by many experts in the field of crop improvement, genetic engineering and abiotic stress tolerance. Various chapters included in this book provide a state-ofthe-art account of the information on (1) mechanisms of abiotic stress tolerance responses, and (2) abiotic stress tolerance in various horticulture crops – tomato, onion, capsicum, mango, grapes, banana, litchi, Arid Zone fruit crops, coconut, arecanut, cahew, cocoa, spices, oil palm and tuber crops – which is a resourceful guide suited for scholars and researchers working in the field of crop improvement, genetic engineering and abiotic stress tolerance of horticultural crops. We, the editors, would like to give special thanks to the authors for their outstanding and timely work in producing such fine chapters. We are highly thankful to Ms. K.C. Pavithra for her valuable help in formatting and incorporating editorial changes in the manuscripts. The editors and contributing authors hope that this book will include a practical update on our knowledge for plant acclimation to environmental stress and lead to new discussions and efforts to the use of various tools for the improvement of horticultural crops for abiotic stress tolerance. Bengaluru, India N.K. Srinivasa Rao K.S. Shivashankara R.H. Laxman Contents Part I 1 2 3 4 Mechanisms of Abiotic Stress Tolerance Responses Physiological and Morphological Responses of Horticultural Crops to Abiotic Stresses . . . . . . . . . . . . . . . N.K. Srinivasa Rao, R.H. Laxman, and K.S. Shivashankara 3 Role of Plant Growth Regulators in Abiotic Stress Tolerance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K.K. Upreti and Maryada Sharma 19 Antioxidant Protection Mechanism During Abiotic Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . K.S. Shivashankara, K.C. Pavithra, and G.A. Geetha 47 Molecular Mechanisms of Heat Shock Proteins and Thermotolerance in Plants . . . . . . . . . . . . . . . . . . . . . . . Vidya S. Murthy and Kundapura V. Ravishankar 71 5 Mechanisms of Heavy Metal Toxicity in Plants . . . . . . . . . . . D. Kalaivanan and A.N. Ganeshamurthy 6 Seed Priming for Abiotic Stress Tolerance: An Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 K. Bhanuprakash and H.S. Yogeesha Part II 85 Abiotic Stress Tolerance in Horticultural Crops: Vegetables 7 Tomato . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 A.T. Sadashiva, Aradhana Singh, R. Punith Kumar, V. Sowmya, and Dominic P. D’mello 8 Onion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 133 N.K. Srinivasa Rao 9 Capsicum (Hot Pepper and Bell Pepper) . . . . . . . . . . . . . . . . 151 K. Madhavi Reddy, K.S. Shivashankara, G.A. Geetha, and K.C. Pavithra vii viii Contents Part III Abiotic Stress Tolerance in Horticultural Crops: Fruit Crops 10 Mango . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . R.H. Laxman, C.J. Annapoornamma, and Geeta Biradar 11 Grapes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 183 Satisha Jogaiah 12 Abiotic Stress Tolerance in Banana . . . . . . . . . . . . . . . . . . . . 207 Iyyakutty Ravi and M. Mayil Vaganan 13 Arid Zone Fruit Crops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 223 N.K. Srinivasa Rao 14 Litchi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rajesh Kumar Part IV 169 235 Abiotic Stress Tolerance in Horticultural Crops: Plantation and Tuber Crops 15 Coconut and Areca Nut . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 269 S. Naresh Kumar, V. Rajagopal, and K.V. Kasturi Bai 16 Cocoa and Cashew . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 307 D. Balasimha 17 Black Pepper and Water Stress . . . . . . . . . . . . . . . . . . . . . . . 321 K.S. Krishnamurthy, S.J. Ankegowda, P. Umadevi, and Johnson K. George 18 Oil Palm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333 K. Suresh, R.K. Mathur, and S.K. Behera 19 Tropical Tuber Crops . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 343 O. Kumari Swadija, Atul Jayapal, and V.B. Padmanabhan Contributors S.J. Ankegowda ICAR-Indian Institute of Spices Research, Madikeri, Karnataka, India C.J. Annapoornamma Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India K.V. Kasturi Bai ICAR-Central Plantation Crops Research Institute, Kasaragod, Kerala, India D. Balasimha Central Plantation Crops Research Institute, Karnataka, India B-404, Mantri Alpyne, Banashankari 5th Stage, Bengaluru, India ICAR-Central Plantation Crops Research Institute, Kasaragod, Kerala, India S.K. Behera ICAR-Directorate of Oil Palm Research, Pedavegi, Andhra Pradesh, India K. Bhanuprakash Section of Seed Science and Technology, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Geeta Biradar Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Dominic P. D’mello Division of Vegetable Crops, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India A.N. Ganeshamurthy Division of Soil Science and Agricultural Chemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India G.A. Geetha Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Johnson K. George ICAR-Indian Institute of Spices Research, Kozhikode, Kerala, India Atul Jayapal College of Agriculture, Vellayani, Thiruvananthapuram, Kerala, India ix x Contributors D. Kalaivanan Division of Soil Science and Agricultural Chemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India K.S. Krishnamurthy ICAR-Indian Kozhikode, Kerala, India Institute of Spices Research, R. Punith Kumar Division of Vegetable Crops, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Rajesh Kumar ICAR- National Research Centre for Litchi, Muzaffarpur, Bihar, India S. Naresh Kumar Centre for Environment Science and Climate Resilient Agriculture, ICAR-Indian Agricultural Research Institute, New Delhi, India R.H. Laxman Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India R.K. Mathur ICAR-Directorate of Oil Palm Research, Pedavegi, Andhra Pradesh, India Vidya S. Murthy Division of Biotechnology, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India V.B. Padmanabhan College Thiruvananthapuram, Kerala, India of Agriculture, Vellayani, K.C. Pavithra Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India V. Rajagopal ICAR-Central Kasaragod, Kerala, India Plantation Crops Research Institute, N.K. Srinivasa Rao Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Iyyakutty Ravi ICAR-National Research Centre for Banana, Trichy, Tamil Nadu, India Kundapura V. Ravishankar Division of Biotechnology, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India K. Madhavi Reddy Division of Vegetable Crops, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India A.T. Sadashiva Division of Vegetable Crops, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Satisha Jogaiah Division of Fruit crops, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Maryada Sharma Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Contributors xi K.S. Shivashankara Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India Aradhana Singh Division of Vegetable Crops, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India V. Sowmya Division of Vegetable Crops, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India K. Suresh ICAR-Directorate of Oil Palm Research, Pedavegi, Andhra Pradesh, India O. Kumari Swadija College of Agriculture, Vellayani, Thiruvananthapuram, Kerala, India P. Umadevi ICAR-Indian Institute of Spices Research, Kozhikode, Kerala, India K.K. Upreti Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India M. Mayil Vaganan ICAR-National Research Centre for Banana, Trichy, Tamil Nadu, India H.S. Yogeesha Section of Seed Science and Technology, ICAR-Indian Institute of Horticultural Research, Bengaluru, Karnataka, India About the Editors Dr. N.K. Srinivasa Rao, Principal Scientist (Plant Physiology-Retd.) and Ex-Emeritus Scientist, Indian Institute of Horticultural Research, Bengaluru, has contributed significantly to the understanding of mechanism of abiotic stress tolerance in horticultural crops. He was associated with the release of one important tomato variety Arka Meghali, recommended for cultivation under rainfed conditions, and breeding of tomato and onion for water stress and capsicum, French bean and peas for high-temperature tolerance. Dr. Srinivasa Rao has significantly contributed in identifying genotypes tolerant to abiotic stresses, and the same has been used by the vegetable breeders for breeding varieties for water and high-temperature stress conditions. Dr. Srinivasa Rao was the Chairman of the group to discuss and prepare a work plan for South Asian countries on the management of heat, moisture and other plant stresses under SAVERNET at the Joint Planning meeting held at the Bangladesh Agricultural Research Council, Dhaka, Bangladesh, from 24 to 27 February 1992. Dr. N. K. Srinivasa Rao was the Principal Investigator of the National Initiative on Climate Resilient Agriculture (NICRA) project. He was instrumental in planning various programmes on horticultural crops under NICRA project. He was the organising Secretary of the National Dialogue on Climate Resilient Horticulture held at IIHR from 28 to 29 January 2012. He was also the PI of ICAR Network Project on Impact, Adaptation and Vulnerability of Indian Agriculture to Climate Change since the inception of the project. He has developed a good laboratory for climate change research at IIHR, Bengaluru. He has devoted his full time in planning and developing these facilities like free air temperature enhancement (FATE), climate-controlled green house (CTGC) and phenomic platform which are of national importance. As a Senior Faculty Member, he had the opportunity of visiting facilities for studies on climate resilient agriculture, CO2 enrichment and free-air temperature enrichment facilities at the Department of Horticulture, Cornell University and Brookhaven National Laboratory, Long Island, New York. He has published more than 60 scientific papers in national and international journals and he has seven book chapters to his credit. He has edited one book ClimateResilient Horticulture: Adaptation and Mitigation Strategies published by M/S. Springer. xiii xiv Dr. K.S. Shivashankara Principal Scientist in the Division of Plant Physiology and Biochemistry at Indian Institute of Horticultural Research, Bangalore. He has got more than 20 years of research experience in various fields like fruit aroma, fruit and vegetable antioxidant phytonutrients, antioxidant protection mechanism under stress, mango flowering physiology and ripening and storage disorders of fruits and effect of climate change on fruit and vegetable quality. Dr. Shivashankara has more than 40 research articles in various peer-reviewed national and international journals. He has the experience of working in international laboratories like food engineering lab of NFRI, Tsukuba, Japan, and at Lethbridge Research Centre of Agriculture and Agri-Food Canada at Lethbridge, Canada, in the area of antioxidant phytonutrients and fruit volatile flavours. He has been awarded the Fellow of International College of Nutrition, by the International College of Nutrition, Alberta, Canada. Dr. Shivashankara has been training many researchers and guiding students in the area of fruit and vegetable quality as affected by varieties, storage conditions and environmental factors. He has identified many indigenous fruits with high antioxidant capacity. He was also involved in the evaluation and selection of high antioxidant lines in many vegetables. Dr. R.H. Laxman Principal Scientist in the Division of Plant Physiology and Biochemistry at ICAR-Indian Institute of Horticultural Research, Bengaluru. He has got more than 21 years of research experience in horticulture crops. He has published about 25 research articles in various peerreviewed national and international journals and published eight book chapters. He has the experience of working in abiotic stress physiology, production physiology and climate change aspects of important horticultural crops like coconut, banana, mango and tomato. About the Editors Part I Mechanisms of Abiotic Stress Tolerance Responses 1 Physiological and Morphological Responses of Horticultural Crops to Abiotic Stresses N.K. Srinivasa Rao, R.H. Laxman, and K.S. Shivashankara Abstract The crop-environment interaction in horticultural crops is receiving increased attention in the context of changing climatic conditions. Environmental stresses can cause morpho-anatomical, physiological and biochemical changes in crops, resulting in a strong profit reduction. A clear understanding of environmental factors and their interaction with physiological processes is extremely important for improving horticultural practices. Drought, excess moisture, salinity and heat stress are amongst the most important environmental factors influencing crop growth, development and yield processes. A comprehensive understanding of the impact of these stress factors will be critical in evaluating the impact of climate change and climate variability on horticultural crop production. Environmental stresses influence an array of processes including physiology, growth, development, yield and quality of crop. A clear understanding of environmental factors and their interaction with physiological processes is extremely important for improving horticultural practices. This review presents the most recent findings about the effects of the main abiotic environmental factors (water, temperature, salinity) on whole plant physiology of horticultural crops. 1.1 N.K.S. Rao (*) • R.H. Laxman • K.S. Shivashankara Division of Plant Physiology and Biochemistry, ICAR-Indian Institute of Horticultural Research, Hessaraghatta Lake Post, Bengaluru 560089, Karnataka, India e-mail: [email protected] Introduction Consideration of abiotic stresses in crop species is of vital importance due to the widespread presence of such stresses on agricultural land, the probable increase in their severity and incidence due to global climatic change and other anthropogenic activities and the frequent deleterious effects such stresses have on crop productivity. These effects are the result of processes # Springer India 2016 N.K.S. Rao et al. (eds.), Abiotic Stress Physiology of Horticultural Crops, DOI 10.1007/978-81-322-2725-0_1 3 4 that can be observed at different levels of plant responses, i.e. morphological, physiological and biochemical/molecular changes. At the morphological level, abiotic stress can cause altered shoot, root and leaf growth, as well as developmental changes that result in altered life cycle duration and fewer and/or smaller organs. Physiological processes are also affected, such as photosynthetic rate, transpiration, respiration, partitioning of assimilates to different organs within the plant and mineral uptake. At the cellular level, cell membranes can be damaged, thylakoid structures disorganized, cell size reduced, stomatal guard cell function altered, degree of cellular hydration modified and programmed cell death promoted. And finally, at the biochemical/molecular level, the effects include enzyme inactivation, the production of reactive oxygen species, osmotic damage, changes in primary and secondary metabolite profiles, changed water and ion uptake or movement and altered hormone concentrations. Adverse environmental conditions such as drought, high soil salinity and temperature extremes are important abiotic stresses causing severe yield loss to agricultural and horticultural crops. Environmental stress is the primary cause of crop losses worldwide, reducing average yields for the major crops by more than 50 % (Bray et al. 2000). Abiotic stresses are often interrelated, either individually or in combination; they cause morphological, physiological, biochemical and molecular changes that adversely affect plant growth and productivity and ultimately yield. Horticultural crops include a wide range of commodities, such as fruits and vegetables that are highly valuable for humanity. They are extensively grown worldwide, and their production can be described as an open and highly complex system affected by many factors, amongst which we can count weather, soil and cropping system, as well as the interaction between these factors. Given that plant growth and development are directly and indirectly influenced by environmental factors (Schaffer and Andersen 1994), in order to obtain a successful production, it is essential to understand N.K.S. Rao et al. clearly how said factors affect plant physiology (Wien 1997). Being succulent in nature, most of the vegetable crops are sensitive to drought stress, particularly during flowering to seed development stage. Moreover, the legume vegetables, for instance, cowpea, vegetable pea, Indian beans, etc., grown in arid and semiarid regions are generally affected by drought at the reproductive stage. Cullis (1991) opined that a perceptive of how the interaction of physicochemical environment reduces plant development and yield will pave the ways for a combination of breeding methods for plant modification to improve tolerance against environmental stresses. 1.2 Moisture Stress Moisture stress is one of the greatest environmental factors in reducing yield in the arid and semiarid tropics. Plant experiences drought stress either when the water supply to roots becomes difficult or when the transpiration rate becomes very high. From agricultural point of view, its working definition would be the inadequacy of water availability, including precipitation and soil moisture storage capacity, in quantity and distribution during the life cycle of a crop plant that restricts the expression of full genetic potential of the plant (Sinha 1986). Drought stress modifies photosynthetic rate, relative water content, leaf water potential and stomatal conductance. Ultimately, it destabilizes the membrane structure and permeability, protein structure and function, leading to cell death (Bhardwaj and Yadav 2012). Drought stress is affected by climatic, edaphic and agronomic factors. The susceptibility of plants to drought stress varies in dependence of stress degree, different accompanying stress factors, plant species and their developmental stages (Demirevska et al. 2009). Acclimation of plants to water deficit is the result of different events, which lead to adaptive changes in plant growth and physiobiochemical processes, such as changes in plant structure, growth rate, tissue osmotic potential and antioxidant defences (Duan et al. 2007). It 1 Physiological and Morphological Responses of Horticultural Crops to Abiotic Stresses has become imperative to elucidate the responses and adaptation of crops to water deficit and take actions to improve the drought resistance ability of crop plants and to ensure higher crop yields against unfavourable environmental stresses. Water is fundamental for maintaining normal physiological activity and membrane transport processes (Jones and Tardieu 1998); therefore supplying it adequately is crucial for obtaining maximum productivity of horticultural crops. Further, water plays an important role in horticultural crops, since fruits and vegetables are usually sold on a fresh weight basis and yield is predominantly determined by water content (Marcelis et al. 1998). Drought stress occurs when there is not enough soil water content for successful growth or water supply replenishment (Larcher 2003; Lombardini 2006b). A decline in leaf relative water content (RWC) initially causes stomatal closure, which in turn leads to a decrease in the supply of CO2 to the mesophyll cells and thus reduces leaf photosynthetic rate. Likewise, drought stress also affects processes such as cell division and expansion, ABA synthesis and sugar accumulation, consequently reducing crop yield (Marsal and Girona 1997; Chartzoulakis et al. 1999; Raviv and Blom 2001; Arquero et al. 2006; Lombardini 2006b). In general, it can be said that horticultural crops require a high water supply through appropriate irrigation schedules. Nevertheless, deficit irrigation can enhance fruit quality by raising dry matter percentage and sugar content (Jones and Tardieu 1998; Spreer et al. 2007). Furthermore, controlled water deficit has been used as a technique to stimulate blossoming in crops such as guava or litchi or to substitute for adequate chilling when temperate crops such as apple are grown in the tropics (Chaikiattiyos et al. 1994). On the other hand, it is important to discuss about flooding, since plant development is affected by either too little or too much water in the root zone. Flooding is produced by storms, over irrigation, poor drainage, high water tables and dam and river overflowing (Rao and Li 2003). As it has been previously mentioned, plants induce a series of physical, chemical and biological processes in response to stress conditions. Under 5 flooding conditions, plants show similar symptoms to those they develop under heat or water stress. Plant responses to waterlogging include increased internal ethylene concentration; low stomatal conductance; decrease in leaf, root and shoot development; changes in osmotic potential and nutrient uptake; and reduced chlorophyll content and photosynthesis (Tamura et al. 1996; Ashraf and Rehman 1999; Rao and Li 2003; Issarakraisila et al. 2007). Flooding also increases the severity of certain diseases, mainly root-rotting fungi (Rao and Li 2003). The decrease of oxygen level in soils affects the bioavailability of nutrients as well as the ability of root systems to uptake and transport water and mineral nutrients (Lizaso et al. 2001). Waterlogging also causes inhibition of N uptake from the soil and reduced leaf concentrations of N, P, K, Ca and Mg in avocado (Schaffer and Andersen 1994) and pea (Rao and Li 2003). 1.2.1 Mechanisms of Drought Resistance In genetic sense, the mechanisms of drought resistance can be grouped into three categories, viz. drought escape, drought avoidance and drought tolerance. However, crop plants use more than one mechanism at a time to resist drought. 1.2.1.1 Drought Escape Drought escape is defined as the ability of a plant to complete its life cycle before serious soil and plant water deficits develop. This mechanism involves rapid phenological development (early flowering and early maturity), developmental plasticity (variation in duration of growth period depending on the extent of water deficit) and remobilization of pre-anthesis assimilates to grain. 1.2.1.2 Drought Avoidance Drought avoidance is the ability of plants to maintain relatively high tissue water potential despite a shortage of soil moisture. Mechanisms for improving water uptake, storing in plant cell 6 N.K.S. Rao et al. and reducing water loss confer drought avoidance. Drought avoidance is performed by maintenance of turgor through increased rooting depth, efficient root system and increased hydraulic conductance and by reduction of water loss through reduced epidermal (stomatal and lenticular) conductance, reduced absorption of radiation by leaf rolling or folding and reduced evaporation surface (leaf area). Plants under drought condition survive by doing a balancing act between maintenance of turgor and reduction of water loss. 1.2.1.3 Drought Tolerance Drought tolerance is the ability to withstand water deficit with low tissue water potential. The responses of plants to tissue water deficit determine their level of drought tolerance. Drought tolerance is also defined as the ability of a plant to produce its economic product with minimum loss under water deficit environment in relation to the water-constraint-free management (Mitra 2001). The mechanisms of drought tolerance are maintenance of turgor through osmotic adjustment (a process which induces solute accumulation in cell), increase in elasticity in cell and decrease in cell size and desiccation tolerance by protoplasmic resistance. 1.2.2 Plant Responses to Abiotic Stresses 1.2.2.1 Drought A primary response of plants subjected to drought stress is the growth arrest. Shoot growth inhibition under drought reduces metabolic demands of the plant and mobilizes metabolites for the synthesis of protective compounds required for osmotic adjustment. Root growth arrest enables the root meristem to remain functional and gives rise to rapid root growth when the stress is relieved (Hsiao and Xu 2000). Lateral root inhibition has also been seen to be an adaptive response, which leads to growth promotion of the primary root, enabling extraction of water from the lower layers of soil (Xiong et al. 2006). Growth inhibition can arise due to the loss of cell turgor arising from the lack of water availability to the growing cells. Water availability to cells is low because of poor hydraulic conductance from roots to leaves caused by stomatal closure. Although a decrease in hydraulic conductance decreases the supply of nutrients to the shoot, it also prevents embolism in xylem and could constitute an adaptive response. Osmotic adjustment is another way by which plants cope with drought stress. Synthesis of compatible solutes like polyols and proline under stress prevents the water loss from cells and plays an important role in turgor maintenance (Blum 2005; DaCosta and Huang 2006). Modification of growth priorities as well as reduction in the performance of photosynthetic organs due to stress exposure leads to alterations in carbon partitioning between the source and sink tissues (Roitsch 1999). Hence, carbohydrates that contribute to growth under normal growth conditions are now available for selective growth of roots or for the synthesis of solutes for osmotic adjustment (Lei et al. 2006; Xue et al. 2008). 1.2.2.2 Flooding Flooding is a major environmental stress that severely limits crop productivity, and it has become a major problem worldwide. More than one third of the world’s irrigated area suffers due to flooding, frequently or otherwise. It may be due to heavy rainfall, faulty irrigation, unlevelled land, poor drainage or heavy soil texture. Various morpho-physiological, biochemical and anatomical changes are induced in root system during flooding, for example, reduction in the shoot-root relative growth; formation of thicker adventitious roots or air roots; aerenchyma formation and cuboidal packing of cells for enhancing the longitudinal transport of gases; the major shift in the carbohydrate metabolism of the plants, which move towards lactate; and ethanolic fermentation to provide the required ATP. Oxygen diffusion is 10,000 times slower in waterlogged soil as compared to aerated soil. Continued flooded conditions lead to lack of oxygen in the soil, restricting respiration of the growing roots and other living organisms. Soil