EditorsDanny Geelen, Department of Plants and Crops, Faculty of Bioscience Engineering, Ghent University, Belgium.Lin Xu, Department of Plants and Crops, Faculty of Bioscience Engineering, Ghent University, Belgium.Series EditorChristian Stevens, Faculty of Bioscience Engineering, Ghent University, Belgium.

• List of Contributors xiiiSeries Preface xvPreface xviiPart I Introduction 11 Agricultural Functions and Action Mechanisms of Plant Biostimulants (PBs): an Introduction 3Patrick du Jardin, Lin Xu and Danny Geelen1.1 The Biostimulant Concept 31.2 The Chemistry of Bioactive Ingredients 91.2.1 Striving to Identify the Active Ingredient 91.2.2 Chemical Characterization of Traditional Biostimulants 101.2.3 Novelty by Targeted Modification of Known Bioactive Molecules 111.2.4 Approaches to Screen for New Molecules with Biostimulatory Activity 121.3 Defining Mode and Mechanism of Action 141.3.1 Journey to the Site of Action 141.3.2 Multiple Functions of Bioactive Ingredients 151.3.3 Tools for a Multilevel Analysis of PBs Action 161.4 Focusing on Key Traits Influenced by Biostimulants 171.4.1 Nutrient Use Efficiency (NUE) 171.4.2 Increasing Tolerance to Abiotic Stress 191.4.3 Crop Quality 221.5 Perspective 231.5.1 Biostimulants: A New Bandwagon to Move Agriculture Forward? 231.5.2 Integration of Biostimulants with Precision Agriculture (PA) 241.5.3 What Do We Need for the Future? 24Author Contributions 25Acknowledgement 25Further Reading 25References 25Part II Examples of Plant Biostimulants 312 Plant Biostimulants from Seaweed: An Overview 33Wendy A. Stirk, Kannan R.R. Rengasamy, Manoj G. Kulkarni and Johannes van Staden2.1 Introduction 332.2 Global Trends in Seaweed-Derived Plant Biostimulants 342.3 Production Technology 352.3.1 Methods of Production 352.3.2 Seaweed Biomass Variability 352.3.3 Shelf-Life 362.4 Beneficial Traits of Seaweed Biostimulants: Recent Developments 372.4.1 Improved Plant Growth 372.4.2 Increased Tolerance to Abiotic and Biotic Stresses 372.4.3 Biofortification 382.5 Major Biostimulants in Seaweed Extracts 382.5.1 Plant Hormones 382.5.2 Brassinosteroids 412.5.3 Betaines 422.5.4 Polyamines 422.5.5 Polymers 432.6 Concluding Remarks and Future Prospects 48Acknowledgement 48Abbreviations 48References 503 Seaweed Carbohydrates 57Oscar Goñi, Patrick Quille and Shane O’Connell3.1 Introduction 573.2 Fucoidan from Brown Algae 603.2.1 Detailed Description of Chemical Composition and Structure of Fucoidan 603.2.2 Experimental Methods for Chemical Characterization of Fucoidan 633.2.3 Fucoidan PB Activity and Potential Applications 643.3 Alginate from Brown Algae 643.3.1 Detailed Description of Chemical Composition and Structure of Alginate 643.3.2 Experimental Methods for Chemical Characterization of Alginate 663.3.3 Alginate PB Activity and Potential Applications 673.4 Carrageenan from Red Algae 693.4.1 Detailed Description of Chemical Composition and Structure of Carrageenan 693.4.2 Experimental Methods for Chemical Characterization of Carrageenan 713.4.3 Carrageenan PB Activities and Potential Applications 713.5 Ulvan from Green Algae 743.5.1 Detailed Description of Chemical Composition and Structure of Ulvan 743.5.2 Experimental Methods for Chemical Characterization of Ulvan 753.5.3 Ulvan PB Activities and Potential Applications 753.6 Laminarin from Brown Algae 773.6.1 Detailed Description of Chemical Composition and Structure of Laminarin 773.6.2 Experimental Methods for Chemical Characterization of Laminarin 783.6.3 Laminarin PB Activities and Potential Applications 783.7 Cellulose and Hemicellulose Derived Oligosaccharides 793.7.1 Detailed Description of Chemical Composition and Structure of Cellulose and Hemicellulose Oligosaccharides 793.7.2 Experimental Methods for Chemical Characterization of Cellulose and Hemicellulose Oligosaccharides 803.7.3 Cellulose and Hemicellulose Oligosaccharides PB Activity and Potential Applications 803.8 Conclusions 81Abbreviations 81References 824 Possible Role for Electron Shuttling Capacity in Elicitation of PB Activity of Humic Substances on Plant Growth Enhancement 97Richard T. Lamar4.1 Introduction 974.1.1 Chemical Nature of HS 974.1.2 Sources of Commercial HS 984.1.3 Formation of Coal-Derived HS and its Effect on HS Chemical Variation 984.2 Similar Responses of Plants to HS and Abiotic and Biotic Stresses 994.2.1 Stress Elicitation, Plant Stress Sensing and Commonality of Physiological Responses 994.2.2 Redox Activity of HS and Possible Role in Elicitation of Biostimulant Response 1004.2.3 Common Metabolic Events that are Shared by HS and Stress Elicitors 1004.3 Humic/Fulvic Elicitation Mechanism 111References 1125 Auxin: At the Crossroads Between Chemistry and Biology 123Sara Raggi, Siamsa M. Doyle and Stéphanie Robert5.1 Introduction: What is an Auxin? 1235.1.1 The Importance of Chemical Structure 1235.1.2 The History of Natural Auxins 1255.1.3 The Importance of Synthetic Auxins 1265.1.4 Auxin Gradients and the Regulation of Plant Growth 1265.2 Taking Advantage of Auxins: Industrial Applications 1285.2.1 Auxins as Rooting Agents for Plant Propagation 1285.2.2 Auxins as Herbicides 1295.3 Understanding Auxin: The Importance of Chemical Tools in Research 1305.3.1 Inhibitors of Auxin Metabolism as Research Tools 1305.3.2 Unravelling Auxin Transport with a Plethora of Chemical Tools 1345.3.3 Chemical Tools Reveal Complicated Auxin Perception and Signalling Pathways 1405.4 Conclusions 145Acknowledgement 146References 1466 Plant Biostimulants in Vermicomposts: Characteristics and Plausible Mechanisms 155Wei San Wong, Hong Tao Zhong, Adam Timothy Cross and Jean Wan Hong Yong6.1 Introduction 1556.2 Advantages of Vermicomposting 1576.3 General Characteristics of Vermicomposts 1596.3.1 Mineral Nutrient Composition of Vermicomposts 1596.3.2 Plant Growth Promoting Properties of Vermicompost 1616.4 Plant Growth Promoting Substances in Vermicomposts 1636.4.1 Phytohormones and Mass Spectrometric Evidence to Support Their Occurrence and Functions 1636.4.2 Vermicompost-Derived Phytohormones as Biostimulants for Plant Growth 1686.5 Benefits of Integrating Vermicomposts into the Current Plant Production Regime 1726.6 Conclusion 173References 173Part III Methods to Screen for New Biostimulants 1817 Exploring Natural Resources for Biostimulants 183Giovanni Povero7.1 Introduction 1837.2 Biological Screening Technologies 1877.2.1 Overview of Most Used Screening Protocols for Bioactives 1877.2.2 In vitro Bioassays 1877.2.3 ‘Acid Growth’ Tests 1907.2.4 Microphenotyping 1917.2.5 Genomic Investigation 1947.2.6 Phenomic Studies 1977.2.7 Other ‘-Omics’ 2007.3 Conclusions 201References 201Part IV Biostimulants’ Mode of Action 2058 Biostimulant Mode of Action: Impact of Biostimulant on Whole-Plant Level 207Elizabeth Wozniak, Adam Blaszczak, Pawel Wiatrak and Michael Canady8.1 Introduction 2078.2 Crop Growth and Development 2088.2.1 Crop Yield 2088.2.2 Crop Quality and Post-Harvest Stability 2108.2.3 Germination 2128.2.4 Shoot Growth 2138.2.5 Root Growth 2148.2.6 Bloom and Fruit Set 2158.3 Plant Physiology 2168.3.1 Nutrient Uptake and Distribution 2168.3.2 Abiotic and Biotic Stress 2188.4 Conclusion 220References 2219 Biostimulant Mode of Action: Impact of Biostimulant on Cellular Level 229Elizabeth Wozniak, Adam Blaszczak, Pawel Wiatrak and Michael Canady9.1 Reactive Oxygen Species (ROS) Control 2309.2 Membrane Stability and Function 2329.3 Enzyme Activity 2339.4 Production of Secondary Metabolites 2349.5 Production of Plant Growth Regulators (PGRs) 2359.6 Photosynthetic Pigments and Photosynthesis 2369.6.1 Photosynthetic Pigments 2369.6.2 Photosynthetic Processes 2379.7 Conclusions 237References 24010 Biostimulant Mode of Action: Impact of PBs on Molecular Level 245Lin Xu, Hoang Khai Trinh and Danny Geelen10.1 Molecular Tools to Unravel Small Molecules Mode of Action 24610.2 Biostimulant Impact on Plants on the Molecular Level 24610.2.1 Transcriptional Analysis 24710.2.2 Proteomic Studies 25010.2.3 Metabolomic Studies 25210.2.4 Multiple Approaches in Acquiring Omics Data: A Short Introduction 25410.3 Conclusions 254Acknowledgement 256References 256Part V Biostimulants – A Practical Guide 26111 Use of Plant Metabolites to Mitigate Stress Effects in Crops 263Nuria De Diego and Lukáš Spíchal11.1 Introduction 26311.2 Plant Metabolites Used for Stress Mitigation in Crops 26411.2.1 Amino Acids 26611.2.2 Polyamines 28511.2.3 Hormones 28611.2.4 Combined Application of Different Metabolites 28711.2.5 Common Mode of Action of the Plant Metabolites to Mitigate Stress 28711.3 Conclusion 289Acknowledgement 290References 290Index 301