Sustainable Remedies for Abiotic Stress in Cereals

Contents
Editors and Contributors
Part I: Cereals and Abiotic Stress
1: Cereals Under Abiotic Stress: An Overview
1.1 Introduction
1.2 Major Abiotic Stresses in Cereals
1.2.1 Drought Stress
1.2.2 Salinity Stress
1.2.3 Heat Stress
1.2.4 Chilling, Cold, or Low Temperature Stress
1.2.5 Waterlogging
1.2.6 Light Flux
1.2.7 Heavy Metal Stress
1.3 Combined Stress Responses
1.4 Conclusion and Future Prospects
References
2: Organic Solutes in Cereals Under Abiotic Stress
2.1 Introduction
2.2 Cereals Under Abiotic Stress
2.3 Organic Solutes
2.3.1 Amino Acids and Their Derivatives
2.3.1.1 Proline
2.3.1.2 Polyamines
2.3.1.3 Proteins
2.3.2 Quaternary Ammonium Compounds: Glycine Betaine
2.3.3 Sugars
2.3.3.1 Disaccharides: Sucrose and Trehalose
2.3.3.2 Fructans
2.3.3.3 Raffinose Family Oligosaccharides (RFOs)
2.3.4 Sugar Alcohols
2.3.4.1 Mannitol
2.3.4.2 Inositols
2.4 Conclusion
References
3: Oxidative Stress and Antioxidant Enzymes in Cereals Under Abiotic Stress
3.1 Introduction
3.2 Oxidative Stress and ROS Generation in Cereal Crops
3.2.1 Superoxide Radical (O2-)
3.2.2 Hydrogen Peroxide (H2O2)
3.2.3 Hydroxyl Radical (OH)
3.2.4 Singlet Oxygen (1O2)
3.3 Sites of ROS Production in Plant Cells
3.3.1 Chloroplast
3.3.2 Mitochondria
3.3.3 Peroxisomes
3.3.4 Other Sources of ROS Generation in Plants
3.4 Effects of ROS
3.4.1 Lipid Peroxidation (LPO)
3.4.2 Oxidative Damage to DNA
3.4.3 Protein Oxidation
3.5 Antioxidant Defense
3.5.1 Enzymatic Antioxidants
3.5.1.1 Superoxide Dismutase (SOD)
3.5.1.2 Catalase (CAT)
3.5.1.3 Enzymes of Ascorbate-Glutathione Cycle
3.5.1.3.1 Ascorbate Peroxidase (APX)
3.5.1.3.2 Monodehydroascorbate Reductase (MDHAR)
3.5.1.3.3 Dehydroascorbate Reductase (DHAR)
3.5.1.4 Glutathione Reductase (GR)
3.5.1.5 Guaiacol Peroxidase (GPX)
3.5.2 Non-enzymatic Antioxidants
3.5.2.1 Ascorbic Acid (AsA)
3.5.2.2 Reduced Glutathione (GSH)
3.5.2.3 Tocopherols (Toc)
3.5.2.4 Carotenoids (Car)
3.5.2.5 Phenolic Compounds (Flavonoids)
3.6 Conclusion
References
Part II: Role and Responses Under Abiotic Stress
4: Maize: Role and Responses Under Abiotic Stress
4.1 Introduction
4.2 Drought Stress
4.3 Salinity Stress
4.4 Temperature Stress (High and Low)
4.5 Heavy Metal Stress
4.6 Management Strategies to Mitigate Stress in Maize
4.7 Molecular Approaches
4.8 Summary and Future Prospects
References
5: Sorghum: Role and Responses Under Abiotic Stress
5.1 Introduction
5.2 Nutritional Importance of Sorghum
5.3 Sorghum Response Towards Abiotic Stress
5.3.1 Drought Stress and Tolerance Mechanism of Sorghum
5.3.2 Effect of Developmental Stages in Drought Tolerance
5.3.3 Qualitative Trait Mapping (QTL) for Drought Resistance
5.3.4 Role of Epicuticular Wax in Drought Tolerance
5.3.5 Osmotic Adjustment and Antioxidant Capacity
5.4 Waterlogging and Tolerance Mechanism of Sorghum
5.5 Hot and Cold Stress
5.6 Salt Stress
5.7 Heavy Metal Stress
5.8 Conclusion and Future Perspective
References
6: Rice: Role and Responses Under Abiotic Stress
6.1 Introduction
6.2 Salt Stress in Rice
6.2.1 Overview
6.2.2 Causes of Salinity
6.2.3 Effects of Salinity Stress in Rice Crops
6.2.4 Morphophysiological Responses
6.2.5 Biochemical Responses
6.3 Ozone Stress in Rice
6.3.1 Overview
6.3.2 Causes of Ozone Stress
6.3.3 Effects of Ozone Stress in Rice
6.3.4 Physiological Effects on Rice Plants
6.3.5 Biochemical Effects on Rice Plants
6.4 Submergence Stress in Rice Plant
6.4.1 Overview
6.4.2 Causes of Submergence
6.4.3 Effects of Submergence in Rice
6.4.4 Physiological Responses
6.4.5 Biochemical Responses
6.5 Drought Stress in Rice
6.5.1 Overview
6.5.2 Causes of Drought
6.5.3 Effect of Drought Stress on Rice
6.5.4 Morphophysiological Responses
6.5.5 Biochemical Responses
6.6 Summary
References
7: Oats: Role and Responses Under Abiotic Stress
7.1 Introduction
7.2 Description of Different Abiotic Stresses
7.2.1 Heat Stress
7.2.2 Cold Tolerance
7.2.3 Drought
7.2.4 Waterlogging
7.2.5 Nutrient Use Efficiency
7.2.6 Water Use Efficiency
7.2.7 Other Stresses
7.2.8 Traditional Breeding Methods
7.3 Genetic Resources of Resistance/Tolerance Genes
7.4 Glimpses on Classical Genetics and Traditional Breeding
7.5 Brief on Diversity Analysis
7.5.1 Methods of Diversity Analysis
7.5.2 Phenotype-Based Diversity Analysis
7.5.3 Molecular Markers-Based Diversity Analysis
7.5.3.1 Molecular Markers
7.5.4 Estimation of Genetic Diversity Using Statistical Tools
7.5.4.1 D2 Statistics
7.5.4.2 Principal Component Analysis (PCA)
7.5.5 Software for Diversity Analysis
7.6 Association Mapping Studies
7.7 Brief Account of Molecular Mapping of Resistance/Tolerance Genes and QTLs
7.8 Marker-Assisted Breeding for Resistance/Tolerance Traits
7.9 Map-Based Cloning of Resistance/Tolerance Genes
7.10 Genomics-Aided Breeding for Resistance/Tolerance Traits
7.11 Recent Concepts and Strategies Developed
7.12 Brief on Genetic Engineering for Resistance/Tolerance Traits
7.13 Brief Account on Social, Political and Regulatory Issues
7.14 Future Perspectives
References
8: Millets: Role and Responses Under Abiotic Stresses
8.1 Introduction
8.2 Origin and Taxonomy
8.3 Domestication of Millets
8.4 Status of Millets in World and India
8.5 Millets and Various Abiotic Stresses
8.5.1 Agronomic Features
8.5.2 Millets and Drought Stress
8.5.3 Millets and Heat Stress
8.5.4 Millets and Salinity Stress
8.5.5 Millets and Water Logging Stress
8.5.6 Millets and Lodging
8.6 Morphological Responses of Millets to Various Abiotic Stresses
8.7 Biochemical and Physiological Responses to Various Abiotic Stresses in Millets
8.8 Molecular Responses of Millets to Various Abiotic Stresses
8.9 Crop Improvement
8.9.1 Genomic Tools Used for the Improvement of Millets
8.10 Nutritional and Health-Related Benefits of Millets
8.11 Future Prospects of Millets
References
9: Triticale (X Triticosecale Wittmack): Role and Responses Under Abiotic Stress
9.1 Introduction
9.2 History of Triticale
9.3 Production and the Area Under Cultivation
9.4 Factors Affecting Growth, Yield, and Yield Components
9.4.1 Drought Stress
9.4.2 Salinity Stress
9.4.3 Temperature
9.4.4 Nutrients (Deficiency or Toxicity)
9.5 Approaches to Improving Yield and Yield Components Under Stressful Conditions
9.5.1 The International Maize and Wheat Improvement Center (CIMMYT) Program
9.5.2 Plant Breeding and Resistant Cultivars
9.5.3 Intercropping
9.5.4 Bio-Fertilizers and Polyamines Under Abiotic Stress
9.6 Summary
References
10: Quinoa: Role and Responses Under Abiotic Stress
10.1 Introduction
10.2 Quinoa: A Promising Multipurpose Agricultural Crop
10.2.1 Historical Background
10.2.2 Plant Characteristic Attributes
10.2.3 An Overview of Genome
10.2.4 Genetic Diversity and Geographical Distribution
10.2.5 Nutritional Profile
10.2.6 Medicinal Importance
10.2.7 Economic Importance
10.3 Responses of Quinoa Under Diverse Abiotic Stresses
10.3.1 Salinity
10.3.1.1 Seed Germination and Growth
10.3.1.2 Photosynthesis
10.3.1.3 Nutritional Value of Seeds
10.3.1.4 Salt Tolerance in Quinoa
10.3.1.5 Mechanisms Implicated in Quinoa´s Salt Tolerance
10.3.1.5.1 Osmotic Regulation
10.3.1.5.2 Na+ Exclusion and Loading into Xylem
10.3.1.5.3 Potassium (K+) Retention
10.3.1.5.4 K+/Na+ Ratio
10.3.1.5.5 Salt Bladders
10.3.1.6 Genes Responsible for Salt Tolerance
10.3.2 Heat Stress
10.3.2.1 Seed Germination
10.3.2.2 Growth Parameters
10.3.2.3 Chlorophyll Fluorescence and Photosynthesis
10.3.2.4 Photoperiod, Flowering, and Seed Yield
10.3.2.5 Pollen Viability
10.3.2.6 Phenolics and Carotenoids in Seeds
10.3.2.7 Heat Shock Proteins
10.3.3 Drought
10.3.3.1 Abscisic Acid (ABA)
10.3.3.2 Photosynthetic Responses Using JIP Test
10.3.3.3 Gas Exchange, Root System, and Evapotranspiration
10.3.3.4 Seed Quality
10.3.3.5 Expression of Genes Under Water-Deficit Conditions
10.3.3.6 Field Studies Under Drought Conditions
10.3.3.7 Drought-Mediated Tolerance Mechanisms
10.3.4 Ultraviolet B (UV-B) Radiation, Frost, Waterlogging, and Heavy Metals
10.3.4.1 Ultraviolet B (UV-B)
10.3.4.2 Frost
10.3.4.3 Waterlogging
10.3.4.4 Nutritional Deficiency Stress
10.3.4.5 Heavy Metals
10.3.5 Responses of Quinoa Under Combination of Abiotic Stresses
10.4 Conclusions
References
Part III: Application of Organic Fertilizers and Phytohormones in Cereals Against Abiotic Stress
11: Cereals and Organic Fertilizers Under Abiotic Stress
11.1 Introduction
11.2 Plant Defense System Against Abiotic Stress
11.3 Main Categories of Organic Fertilizers
11.4 Organic Fertilizers as Agronomic Tools to Promote Plant Growth Under Abiotic Stresses
11.4.1 The Role of Organic Fertilizers in Enhancing Abiotic Stress in Cereals
11.4.2 Organic Fertilizers, Beneficial Effects, and Mode of Action
11.5 Concluding Remarks and Common Features
References
12: Cereals and Phytohormones Under Salt Stress
12.1 Introduction
12.2 Influence of Phytohormones on Plant Growth Exposed to Salt Stress
12.2.1 Effect of Auxin (IAA) on Plants Exposed to Salt Stress
12.2.2 Impact of Cytokinin (CK) on Plants Exposed to Salt Stress
12.2.3 Impact of Gibberellins (GAs) on Plants Exposed to Salt Stress
12.2.4 Effect of Ethylene on Plants Exposed to Salt Stress
12.2.5 Role of Brassinosteroids (BRs) Under Salt Stress Conditions
12.2.6 Impact of Jasmonates on Plants Exposed to Salt Stress
12.2.7 Impact of Salicylic Acid (SA) on Plants Exposed to Salt Stress
12.2.8 Role of Abscisic Acid (ABA) Under Salt Stress Conditions
12.3 Genetic Approaches Mediated Hormonal Homeostasis to Enhance the Salt Stress Tolerance in Crop Plants
12.4 Metabolic Approaches Mediated Hormonal Homeostasis to Enhance Salt Stress Tolerance in Crop Plants
12.5 Conclusions and Future Perspective
References
13: Cereals and Phytohormones Under Drought Stress
13.1 Phytohormones: Key Mediators of Cereal Responses to Drought Stress
13.2 Correlations Between Phytohormones and Drought Stress Tolerance in Cereals
13.3 Hormones´ Signalling for Drought Stress Response and Tolerance
13.3.1 Abscisic Acid (ABA)
13.3.2 Jasmonates (JAs)
13.3.3 Salicylic Acid (SA)
13.3.4 Ethylene (ET)
13.3.5 Auxins
13.3.6 Cytokinins (CKs)
13.3.7 Gibberellins (GA)
13.3.8 Brassinosteroids (BRs)
13.3.9 Strigolactones (SLs)
13.4 Hormone Signalling Crosstalk in Cereal Under Drought Stress Conditions
13.5 Factors Modifying the Phytohormonal Activity in Conferring Drought Tolerance in Cereal
13.5.1 Substances, Minerals, and Organic Amendments
13.5.2 Beneficial Microorganisms
13.5.2.1 Plant Growth-Promoting Fungi (PGPF)
13.5.2.2 Plant Growth-Promoting Rhizobacteria (PGPR)
13.6 Metabolic Engineering of Phytohormones: New Strategies in Cereal to Mitigate Drought Stress
13.6.1 Breeding
13.6.2 Genetic Engineering
13.7 Conclusion and Future Perspectives
References
14: Cereals and Phytohormones Under Temperature Stress
14.1 Introduction
14.2 Cereal Responses and Mechanisms of Tolerance to Cold Stress
14.3 Role of Phytohormones in Plant Development Under Low-Temperature Stress
14.4 Plant Responses and Mechanisms of Tolerance to Heat Stress
14.5 Role of Phytohormones in Plant Development Under High-Temperature Stress
14.6 Concluding Remarks
References
15: Cereals and Phytohormones Under Heavy Metal Stress
15.1 Introduction
15.1.1 Importance of Phytohormones in Cereals Under HM Stress
15.2 Classical Phytohormones Under Heavy Metal Stress
15.2.1 Auxins
15.2.2 Ethylene
15.2.3 Cytokinins
15.2.4 Gibberellins
15.2.5 Abscisic Acid (ABA)
15.2.6 Brassinosteroids
15.3 Molecular Phytohormones Under Heavy Metal Stress
15.3.1 Salicylic Acid
15.3.2 Nitric Oxide (NO)
15.3.3 Jasmonates
15.4 Newly Discovered Phytohormones Under Heavy Metal Stress
15.4.1 Strigolactones
15.5 Conclusion
References
16: Cereals and Phytohormones Under Mineral Deficiency Stress
16.1 Introduction
16.2 Mineral Nutrition in Plant Life
16.2.1 Non-Mineral Nutrients
16.2.2 Nutrients and Minerals
16.2.2.1 Macronutrients
16.2.2.2 Micronutrients
16.3 Nutrient Deficiency
16.4 Cross-Talk Between Macro- and Micronutrients Under Mineral Nutrient Deficiency
16.4.1 Interaction Between Pi, S, and Fe Nutrient Homeostasis in Cereals
16.4.2 Interaction Between N and Pi Nutrient Homeostasis in Cereals
16.4.3 Interaction Between N and Zn Nutrient Homeostasis in Cereals
16.4.4 Cross-Talk between Pi, Zn, and Fe Homeostasis in Cereals
16.4.5 Pi Availability Affects Zn Uptake in Cereals
16.4.6 Zn Availability Affects Pi Uptake in Cereals
16.4.7 Pi Availability Affects Fe Uptake and Homeostasis in Cereals
16.4.8 Fe Availability Affects Pi Uptake and Homeostasis in Cereals
16.4.9 Zn Availability Affects Fe Uptake and Homeostasis in Cereals
16.4.10 Fe Availability Affects Zn Uptake and Homeostasis in Cereals
16.5 Mineral Deficiency Affects Hormonal Homeostasis and the Role of Exogenous Phytohormones
References
17: Cereals and Phytohormones Under UV Stress
17.1 Introduction
17.2 Ultraviolet (UV) Stress
17.3 Responses of Phytohormones Under UV Stress
17.4 Phytohormones: Fundamental Entities of Plant Defense to Cope Abiotic Stresses
17.5 UV and Auxin
17.6 UV and Jasmonic Acid
17.7 UV and Brassinosteroid
17.8 Gibberellins
17.9 Abscisic Acid
17.10 Ethylene
17.11 Salicylic Acid
17.12 Cytokinin
17.13 Conclusion
References
18: Cereals and Phytohormones Under Ozone Stress
18.1 Introduction
18.1.1 Events After Ozone Exposure to Plant
18.2 Effect of Absorbed Ozone at the Cellular and Metabolic Level
18.3 Cereals Under Ozone Stress
18.3.1 Proposed Routes for Ozone-Induced Yield Reduction
18.4 Phytohormones and Ozone Stress
18.4.1 Abscisic Acid (ABA)
18.4.2 Auxins (IAA)
18.4.3 Ethylene (ET)
18.4.3.1 Mechanism of Action of Ozone-Induced ET on ABA-Induced Stomatal Closure
18.4.4 Gibberellins (GAs)
18.4.5 Salicylic Acid (SA)
18.4.6 Jasmonic Acid (JA)
18.5 Phytohormones in Alleviating of Ozone in Cereals
18.5.1 Rice
18.5.2 Wheat
18.5.3 Maize
18.6 Phytohormone Signaling and Their Interaction
18.7 Summary
References
Part IV: Improvement in Abiotic Stress Tolerance Through Biostimulants
19: Use of Biostimulants to Improve Salinity Tolerance in Cereals
19.1 Introduction
19.2 Salinity: Types, Causes, and Salt Uptake from the Soil
19.3 Cereals Responses to Salt Stress
19.3.1 Salt Adverse Effects and Physiological Responses in Cereals
19.3.1.1 Effect of Salinity on Seed Germination
19.3.1.2 Effect of Salinity on Vegetative and Reproductive Stages in Cereals
19.3.1.3 Salinity Effects on Mineral Nutrition and Plant Physiology
19.3.1.4 Effect on ROS and Carbohydrate Metabolisms
19.3.2 Strategies for Coping with Salinity in Cereal Cultures
19.3.2.1 Growth Regulation and Membrane Transport Systems Control
19.3.2.2 Na+ Exclusion
19.3.2.3 Osmoregulation
19.3.2.4 Antioxidant Defense System
19.3.2.5 Genetic Basis of Salt Stress Response in Cereals
19.4 Biostimulants as Alleviators of Salt Stress in Cereals
19.4.1 Microbial-Derived Biostimulants
19.4.2 Organic Amendment
19.4.3 Algae-Derived Biostimulants
19.4.4 Protein-Based Biostimulants
19.4.5 Chitosan
19.5 Role of Biostimulants in Improving Cereals Under Salt Stress
19.5.1 Improvement of Mineral Nutrition
19.5.2 Regulation of Ion Homeostasis and Mitigation of Deleterious Effects of Ion Toxicity
19.5.3 Production of Phytohormones
19.5.4 Accumulation of Osmoprotectants
19.5.5 Induction of Antioxidant System
19.5.6 Maintenance of Water Homeostasis
19.5.7 Restoration of Soil Structure and Quality
19.6 Conclusions and Perspectives
References
20: Use of Biostimulants to Improve Drought Tolerance in Cereals
20.1 Introduction
20.2 Cereal Production Under Drought Stress
20.2.1 Drought Stress Effects on Rhizosphere
20.2.2 Effects of Drought Stress on Physiological Traits in Cereals
20.2.3 Current Advances and Limitations for the Genetic Improvement of Drought-Tolerant Genotypes of Cereals
20.3 Biostimulants as a Tool for Improving Cereal Productivity Under Drought Risk
20.3.1 Microbial Biostimulants and Cereal Drought Tolerance
20.3.2 Nonmicrobial Biostimulants
20.3.3 Single, Dual, and Multi-combination of Biostimulants Against Drought
20.3.4 Biostimulants: Beneficial Effects and Mode of Action in Cereals Under Drought
20.4 Conclusions and Perspectives
References
21: Heat Stress in Cereals and Its Amelioration by Biostimulants
21.1 Introduction
21.2 Temperature Stress
21.3 Effect of Heat Stress on Major Cereals
21.3.1 Wheat
21.3.2 Rice
21.3.3 Maize
21.3.4 Barley
21.3.5 Oat
21.3.6 Pearl Millet
21.3.7 Sorghum
21.3.8 Rye
21.4 Pseudocereals
21.5 Biostimulants
21.5.1 Classification of Biostimulants
21.6 Biostimulants and Extreme Temperature Tolerance in Cereals
21.7 Conclusion
References
22: Use of Biostimulants to Increase Heavy Metal Tolerance in Cereals
22.1 Introduction
22.2 Classification
22.3 Production Technology of Biostimulants
22.4 Implication of Heavy Metal(s) in Cereals
22.4.1 Physiological Effects of Heavy Metals
22.4.2 Morphological Effects of Heavy Metals
22.4.3 Metabolic Effects of Heavy Metals
22.5 Implication of Biostimulants in Heavy Metal Tolerance
22.6 Commercialized Biostimulant Products for Cereals (Yakhin et al. 2017; Kumar and Aloke 2020; Hamid et al. 2021)
22.7 Conclusion
References
23: Use of Biostimulants to Improve UV Tolerance in Cereals
23.1 Introduction
23.2 Effect of UV-A, UV-B, and UV-C on Cereals
23.2.1 Effect of UV-A on Cereals
23.2.2 Effect of UV-B on Plants
23.2.3 Effect of UV-C on Cereals
23.3 Biostimulants Used for Improving the Plant Tolerance to UV Stress
23.3.1 Silicon
23.3.2 5-Aminolevulinic Acid (ALA)
23.3.3 Polyamines (PAs)
23.3.4 Cytokinesis
23.3.5 Nitric Oxide
23.3.6 Sitosterol
23.3.7 Magnetic Field
23.3.8 Salicylic Acid
23.3.9 Allantoin
References
24: Use of Biostimulants to Improve Ozone Tolerance in Cereals
24.1 Introduction
24.2 Ozone as an Important Abiotic Stress Factor Limiting the Production of Cereals
24.3 Improving Ozone Stress Tolerance Through Biostimulants in Cereals
24.3.1 Natural Compounds
24.3.2 Synthetic Compounds
24.4 Conclusions and Future Prospects
References
Part V: Application of Gene Editing Approaches and Nanotechnology for Induction of Abiotic Stress Tolerance
25: Genome Editingand miRNA-Based Approaches in Cereals under Abiotic Stress
25.1 Introduction
25.2 Genome Editing
25.2.1 Engineered Nucleases
25.2.1.1 Meganucleases
25.2.1.2 Zinc Finger Nuclease
25.2.1.3 Transcription Activator-Like Effector Nucleases (TALENs)
25.2.1.4 Clustered Regularly Interspaced ShortPalindromic Repeats (CRISPR/Cas9)
25.3 CRISPR for Crop Improvement
25.3.1 Improvement of Grain Yield
25.3.2 Improvement of Photosynthesis
25.3.3 Improvement for Biotic Stress Tolerance
25.3.4 Improvement for Abiotic Stress Tolerance
25.3.4.1 Drought Tolerance
25.3.4.2 Salinity Tolerance
25.3.4.3 Cold Tolerance
25.3.4.4 Heat Tolerance
25.3.4.5 Submergence Tolerance
25.4 miRNA-Based Approaches
25.4.1 Micro-RNA (miRNA) with Macro Functions
25.4.1.1 Salinity and Drought
25.4.1.2 Heat and Cold Stress
25.4.1.3 Hypoxia and Oxidative Stress
25.4.2 MiRNA-Micro-RNA with Macro Functions
25.4.3 Computational Identification of miRNA
25.4.4 miRNA Engineered Plants Against Abiotic Stresses
25.5 Bottlenecks in the Application of Genome Editing Tools for Crop Improvement
25.6 Conclusion and Future Prospects
References
26: Nanotechnology and Its Role in Cereal Crops under Abiotic Stress
26.1 Introduction
26.2 Applications of Nanotechnology in Agriculture
26.3 Entry of Nanoparticles (NPs) into the Plant System
26.4 Enhancement of Defensive System by Nanoparticles: Abiotic Stress Tolerance
26.4.1 Drought Stress
26.4.2 Salinity Stress
26.4.3 Heavy Metals
26.5 Conclusions and Future Strategies
References