Introduction to Plant Physiology (2008) PDF

Introduction to Plant Physiology This page intentionally left blank Introduction to Plant Physiology Fourth Edition William G. Hopkins and Norman P. A. Hüner The University of Western Ontario John Wiley & Sons, Inc. VICE PRESIDENT AND EXECUTIVE PUBLISHER Kaye Pace SENIOR ACQUISITIONS EDITOR Kevin Witt PRODUCTION SERVICES MANAGER Dorothy Sinclair PRODUCTION EDITOR Janet Foxman CREATIVE DIRECTOR Harry Nolan SENIOR DESIGNER Kevin Murphy EDITORIAL ASSISTANT Alissa Etrheim SENIOR MEDIA EDITOR Linda Muriello PRODUCTION SERVICES Katie Boilard/Pine Tree Composition COVER DESIGN David Levy COVER IMAGE ©Mark Baigent/Alamy This book was set in 10/12 Janson Text by Laserwords Private Limited, Chennai, India and printed and bound by Courier/Kendallville. The cover was printed by Courier/Kendallville. This book is printed on acid-free paper. Copyright © 2009 John Wiley & Sons, Inc. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording, scanning or otherwise, except as permitted under Sections 107 or 108 of the 1976 United States Copyright Act, without either the prior written permission of the Publisher, or authorization through payment of the appropriate per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, website www.copyright.com. Requests to the Publisher for permission should be addressed to the Permissions Department, John Wiley & Sons, Inc., 111 River Street, Hoboken, NJ 07030-5774, (201) 748-6011, fax (201) 748-6008, website www.wiley.com/go/permissions. To order books or for customer service, please call 1-800-CALL WILEY (225-5945). Library of Congress Cataloging-in-Publication Data: Hopkins, William G. Introduction to plant physiology / William G. Hopkins and Norman P. A. Hüner. –4th ed. p. cm. Includes index. ISBN 978-0-470-24766-2 (cloth) 1. Plant physiology. I. Hüner, Norman P. A. II. Title. QK711.2.H67 2008 571.2–dc22 2008023261 Printed in the United States of America 10 9 8 7 6 5 4 3 2 1 Preface When the ﬁrst edition of this text appeared thirteen the ﬁgure and the concepts described in the text. years ago, its writing was guided by several of objectives. At the same time, we are mindful of costs and hope that this has been done in a way that does not add The text should be suited for a semester course for signiﬁcant cost to the student. undergraduate students encountering the subject of plant physiology for the ﬁrst time. It was assumed The number of complex chemical structures in that the student would have completed a ﬁrst course many ﬁgures has been reduced and biosynthetic in botany or biology with a strong botanical compo- pathways have been simpliﬁed in order to provide nent. The book should provide a broad framework greater emphasis on fundamental principles. for those interested in pursuing advanced study in We have removed the traditional introductory plant physiology, but it should also provide the gen- chapter on Cells, Tissues, and Organs and eral understanding of plant function necessary for distributed some of this information in chapters to students of ecology or agriculture. which it pertains directly. In keeping with the above objective, the text should The list of references at the end of each chapter has focus on fundamental principles of how plants work been updated throughout the new edition. while attempting to balance the demands of bio- All life depends on energy and water. Unlike previ- chemistry and molecular biology on the one hand, ous editions, the fourth edition begins with four and traditional ‘‘whole-plant’’ physiology on the chapters that focus on the properties of water, other. osmosis, water potential, and plant–water relations, The text should be interesting and readable. It followed by a series of eight chapters dealing with should include some history so the student appreci- bioenergetics, primary plant metabolism, and plant ates how we arrived at our current understanding. It productivity. should also point to future directions and challenges A major change in this edition is the presence of in the ﬁeld. three new chapters (13, 14, and 15). Using the basic The sheer breadth of plant physiology and the information and concepts developed in chapters 1 rapidly expanding volume of literature in the ﬁeld to 12, these chapters focus on the inherent plastic- make it impossible to include all of the relevant ity of plants to respond to environmental change material in an entry-level text. Consequently, the on various time scales. This includes a discussion text must be selective and focused on those topics of abiotic and biotic stress, plant acclimation to that form the core of the discipline. At the same stress, and ﬁnally, long-term, heritable adaptations time, the student should be introduced to the sig- to environmental stress. niﬁcance of physiology in the role that plants play We have revised the treatment of hormones because in the larger world outside the laboratory. many instructors have told us that the separate While we have made every effort to retain the treatment of each hormone ﬁts their syllabus better. readability and overall approach of previous editions; The coverage of each hormone concludes with a we have also introduced a number of signiﬁcant changes general description of the current status of receptors in this fourth edition. Those changes include: and signal transduction pathways. A new chapter focuses on the molecular genetics of For this edition the illustration program has been ﬂower and fruit development. completely revised. Some ﬁgures have been deleted, others have been revised, and many new ﬁgures have A Glossary has been created for the new edition. been introduced. With the help of the publisher, we have also introduced color into the illustrations. The William G. Hopkins use of color improves the clarity of the ﬁgures, draws Norman P. A. Hüner attention to important elements in the ﬁgure, and London, Ontario helps students visualize the relationships between April 2008 v vi Preface To the Student This is a book about how plants work. It is about the in the literature almost daily. Many models and expla- questions that plant physiologists ask and how they go nations contained in this book may have been revised by about seeking answers to those questions. Most of all, the time the book appears on the market. If you ﬁnd a this book is about how plants do the things they do in particular topic interesting and wish to learn more about their everyday life. it, the listed publications at the end of each chapter are The well-known conservationist John Muir once your gateway into the relevant research literature. You wrote: When we try to pick out anything by itself, we ﬁnd can learn what has happened since this book was written it hitched to everything in the universe. Muir might well by seeking out reviews and opinions published in the have been referring to the writing of a plant physiology more recent editions of those same journals. textbook. The scope of plant physiology as a science In spite of its presumed objectivity, science is very broad, ranging from biophysics and molecular ultimately relies on the interpretation of experimental genetics to environmental physiology and agronomy. results by scientists— interpretations that are often Photosynthetic metabolism not only provides carbon found to be inadequate and ﬁlled with uncertainty. and energy for the growing plant, but also determines However, as results and observations accumulate, the capacity of the plant to withstand environmental interpretations are reﬁned and the degree of uncertainty stress. The growth and development of roots, stems, diminishes. This is the nature of scientiﬁc discovery leaves, and ﬂowers are regulated by a host of interacting and the source of the real excitement of doing science. factors such as light, temperature, hormones, nutrition, In this book, we have attempted to convey some sense and carbon metabolism. As a matter of practical necessity of this scientiﬁc process. more than scientiﬁc reality, we have treated many of We hope that, through this book, we are able to these topics in separate chapters. To get the most out of share with you some of our own fascination with the this book, we suggest you be aware of these limitations excitement, mystery, and challenge of learning about as you read and think about how various mechanisms plant physiology. are integrated to form a functional plant. Plant physiology is also a very active ﬁeld of study William G. Hopkins and new revelations about how plants work are reported Norman P. A. Hüner Contents Chapter 1 Plant Cells and Water 1 1.10 Aquaporins Facilitate the Cellular Movement of Water 13 1.1 Water has Unique Physical and Chemical 1.11 Two-Component Sensing/Signalling Systems are Properties 2 Involved in Osmoregulation 15 1.2 The Thermal Properties of Water are Biologically Summary 17 Important 3 Chapter Review 17 1.2.1 Water Exhibits a Unique Thermal Further Reading 17 Capacity 3 1.2.2 Water Exhibits a High Heat of Fusion and Heat of Vaporization 3 1.3 Water is the Universal Solvent 4 1.4 Polarity of Water Molecules Results in Cohesion Chapter 2 Whole Plant Water and Adhesion 4 Relations 19 1.5 Water Movement may be Governed by Diffusion 2.1 Transpiration is Driven by Differences in Vapor or by Bulk Flow 5 Pressure 20 1.5.1 Bulk Flow is Driven by Hydrostatic 2.2 The Driving Force of Transpiration is Differences Pressure 5 in Vapor Pressure 21 1.5.2 Fick’s First Law Describes the Process of 2.3 The Rate of Transpiration is Inﬂuenced by Diffusion 5 Environmental Factors 22 1.6 Osmosis is the Diffusion of Water Across a 2.3.1 What are the Effects of Humidity? 23 Selectively Permeable Membrane 6 2.3.2 What is the Effects of Temperature? 23 1.6.1 Plant Cells Contain an Array of Selectively 2.3.3 What is the Effect of Wind? 24 Permeable Membranes 7 2.4 Water Conduction Occurs via Tracheary 1.6.2 Osmosis in Plant Cells is Indirectly Energy Elements 24 Dependent 8 2.5 The Ascent of Xylem SAP is Explained by 1.6.3 The Chemical Potential of Water has an Osmotic Combining Transpiration with the Cohesive as Well as a Pressure Component 9 Forces of Water 27 1.7 Hydrostatic Pressure and Osmotic Pressure are 2.5.1 Root Pressure is Related to Root Two Components of Water Potential 11 Structure 28 1.8 Water Potential is the Sum of its Component 2.5.2 Water Rise by Capillarity is due to Adhesion and Potentials 11 Surface Tension 29 1.9 Dynamic Flux of H2 O is Associated with Changes 2.5.3 The Cohesion Theory Best Explains the Ascent of in Water Potential 12 Xylem Sap 30 vii viii Contents 2.6 Water Loss due to Transpiration must be 3.6 Cellular Ion Uptake Processes are Replenished 33 Interactive 52 2.6.1 Soil is a Complex Medium 33 3.7 Root Architecture is Important to Maximize Ion 2.7 Roots Absorb and Transport Water 34 Uptake 52 2.8 The Permeability of Roots to Water 3.7.1 A First Step in Mineral Uptake by Roots is Varies 35 Diffusion into the Apparent Free Space 53 2.9 Radial Movement of Water Through the Root 3.7.2 Apparent Free Space is Equivalent to the Apoplast Involves Two Possible Pathways 36 of the Root Epidermal and Cortical Summary 37 Cells 54 Chapter Review 37 3.8 The Radial Path of Ion Movement Through Further Reading 37 Roots 54 3.8.1 Ions Entering the Stele Must First be Transported BOX 2.1 Why Transpiration? 25 from the Apparent Free Space into the Symplast 54 Chapter 3 Roots, Soils, and Nutrient 3.8.2 Ions are Actively Secreted into the Xylem Uptake 39 Apoplast 55 3.8.3 Emerging Secondary Roots may Contribute to the 3.1 The Soil as a Nutrient Reservoir 40 Uptake of Some Solutes 55 3.1.1 Colloids are a Signiﬁcant Component of Most 3.9 Root-Microbe Interactions 56 Soils 40 3.9.1 Bacteria Other than Nitrogen Fixers Contribute to 3.1.2 Colloids Present a Large, Negatively Charged Nutrient Uptake by Roots 56 Surface Area 40 3.9.2 Mycorrhizae are Fungi that Increase the Volume of 3.1.3 Soil Colloids Reversibly Adsorb Cations from the the Nutrient Depletion Zone Around Soil Solution 41 Roots 57 3.1.4 The Anion Exchange Capacity of Soil Colloids is Summary 58 Relatively Low 41 Chapter Review 58 3.2 Nutrient Uptake 42 Further Reading 59 3.2.1 Nutrient Uptake by Plants Requires Transport of the Nutrient Across Root Cell BOX 3.1 Electrophysiology—Exploring Ion Membranes 42 Channels 44 3.2.2 Simple Diffusion is a Purely Physical Process 42 3.2.3 The Movement of Most Solutes Across Membranes Requires the Participation of Speciﬁc Transport Proteins 43 Chapter 4 Plants and Inorganic 3.2.4 Active Transport Requires the Expenditure of Nutrients 61 Metabolic Energy 43 4.1 Methods and Nutrient Solutions 62 3.3 Selective Accumulation of Ions by Roots 46 4.1.1 Interest in Plant Nutrition is Rooted in the Study of 3.4 Electrochemical Gradients and Ion Agriculture and Crop Productivity 62 Movement 46 4.1.2 The Use of Hydroponic Culture Helped to Deﬁne 3.4.1 Ions Move in Response to Electrochemical the Mineral Requirements of Plants 62 Gradients 46 4.1.3 Modern Techniques Overcome Inherent 3.4.2 The Nernst Equation Helps to Predict Whether an Disadvantages of Simple Solution Ion is Exchanged Actively or Passively 47 Culture 63 3.5 Electrogenic Pumps are Critical for Cellular 4.2 The Essential Nutrient Elements 65 Active Transport 49 4.2.1 Seventeen Elements are Deemed to be Essential for 3.5.1 Active Transport is Driven by ATPase-Proton Plant Growth and Development 65 Pumps 49 4.2.2 The Essential Nutrients are Generally Classed as 3.5.2 The ATPase-Proton Pumps of Plasma Membranes Either Macronutrients or and Vacuolar Membranes are Different 50 Micronutrients 65 3.5.3 K+ Exchange is Mediated by Two Classes of 4.2.3 Determining Essentiality of Micronutrients Transport Proteins 51 Presents Special Problems 65 Contents ix 4.3 Beneﬁcial Elements 66 Chapter 5 Bioenergetics and ATP 4.3.1 Sodium is an Essential Micronutrient for C4 Synthesis 77 Plants 66 4.3.2 Silicon May be Beneﬁcial for a Variety of 5.1 Bioenergetics and Energy Transformations in Species 67 Living Organisms 78 4.3.3 Cobalt is Required by Nitrogen-Fixing 5.1.1 The Sun is a Primary Source of Energy 78 Bacteria 67 5.1.2 What is Bioenergetics? 78 4.3.4 Some Plants Tolerate High Concentrations of 5.1.3 The First Law of Thermodynamics Refers to Selenium 67 Energy Conservation 79 4.4 Nutrient Functions and Deﬁciency 5.1.4 The Second Law of Thermodynamics Refers to Symptoms 67 Entropy and Disorder 79 4.4.1 A Plant’s Requirement for a Particular Element is 5.1.5 The Ability to do Work is Dependent on the Deﬁned in Terms of Critical Availability of Free Energy 80 Concentration 67 5.1.6 Free Energy is Related to Chemical 4.4.2 Nitrogen is a Constituent of Many Critical Equilibria 80 Macromolecules 68 5.2 Energy Transformations and Coupled 4.4.3 Phosphorous is Part of the Nucleic Acid Backbone Reactions 81 and has a Central Function in Intermediary 5.2.1 Free Energy of ATP is Associated with Coupled Metabolism 69 Phosphate Transfer Reactions 81 4.4.4 Potassium Activates Enzymes and Functions in 5.2.2 Free Energy Changes are Associated with Coupled Osmoregulation 69 Oxidation–Reduction Reactions 83 4.4.5 Sulfur is an Important Constituent of Proteins, 5.3 Energy Transduction and the Chemiosmotic Coenzymes, and Vitamins 70 Synthesis of ATP 85 4.4.6 Calcium is Important in Cell Division, Cell 5.3.1 Chloroplasts and Mitochondria Exhibit Speciﬁc Adhesion, and as a Second Messenger 70 Compartments 85 4.4.7 Magnesium is a Constituent of the Chlorophyll 5.3.2 Chloroplasts and Mitochondria Synthesize ATP by Molecule and an Important Regulator of Enzyme Chemiosmosis 90 Reaction 70 Summary 91 4.4.8 Iron is Required for Chlorophyll Synthesis Chapter Review 91 and Electron Transfer Further Reading 91 Reactions 71 BOX 5.1 Plastid Biogenesis 86 4.4.9 Boron Appears to have a Role in Cell Division and Elongation and Contributes to the Structural Integrity of the Cell Chapter 6 The Dual Role of Sunlight: Wall 73 Energy and 4.4.10 Copper is a Necessary Cofactor for Oxidative Information 93 Enzymes 73 6.1 The Physical Nature of Light 93 4.4.11 Zinc is an Activator of Numerous 6.1.1 Light is Electromagnetic Energy, Which Exists in Enzymes 73 Two Forms 93 4.4.12 Manganese is an Enzyme Cofactor as Well as 6.1.2 Light can be Characterized as a Wave Part of the Oxygen-Evolving Complex in the Phenomenon 94 Chloroplast 74 6.1.3 Light Can be Characterized as a Stream of Discrete 4.4.13 Molybdenum is a Key Component of Nitrogen Particles 94 Metabolism 74 6.1.4 Light Energy can Interact with Matter 95 4.4.14 Chlorine has a Role in Photosynthetic Oxygen 6.1.5 How Does One Illustrate the Efﬁciency of Light Evolution and Charge Balance Across Cellular Absorption and its Physiological Membranes 74 Effects? 97 4.4.15 The Role of Nickel is not Clear 74 6.1.6 Accurate Measurement of Light is Important in 4.5 Toxicity of Micronutrients 75 Photobiology 98 Summary 75 6.2 The Natural Radiation Environment 99 Chapter Review 76 6.3 Photoreceptors Absorb Light for use in a Further Reading 76 Physiological Process 100 x Contents 6.3.1 Chlorophylls are Primarily Responsible for Chapter 8 Energy Conservation in Harvesting Light Energy for Photosynthesis: CO2 Photosynthesis 100 Assimilation 129 6.3.2 Phycobilins Serve as Accessory Light-Harvesting Pigments in Red Algae and 8.1 Stomatal Complex Controls Leaf Gas Exchange Cyanobacteria 102 and Water Loss 130 6.3.3 Carotenoids Account for the Autumn 8.2 CO2 Enters the Leaf by Diffusion 132 Colors 103 8.3 How Do Stomata Open and Close? 133 6.3.4 Cryptochrome and Phototropin are Photoreceptors 8.4 Stomatal Movements are Also Controlled by Sensitive to Blue Light and UV-A External Environmental Factors 135 radiation 103 8.4.1 Light and Carbon Dioxide Regulate Stomatal 6.3.5 UV-B Radiation May Act as a Developmental Opening 135 Signal 105 8.4.2 Stomatal Movements Follow Endogenous 6.3.6 Flavonoids Provide the Myriad Flower Colors and Rhythms 136 Act as a Natural Sunscreen 105 8.5 The Photosynthetic Carbon Reduction (PCR) 6.3.7 Betacyanins and Beets 106 Cycle 136 Summary 107 8.5.1 The PCR Cycle Reduces CO2 to Produce a Chapter Review 107 Three-Carbon Sugar 137 Further Reading 107 8.5.2 The Carboxylation Reaction Fixes the CO2 137 Chapter 7 Energy Conservation in 8.5.3 ATP and NADPH are Consumed in the PCR Photosynthesis: Harvesting Cycle 138 Sunlight 109 8.5.4 What are the Energetics of the PCR Cycle? 139 7.1 Leaves are Photosynthetic Machines that 8.6 The PCR Cycle is Highly Regulated 139 Maximize the Absorption of Light 110 8.6.1 The Regeneration of RuBP is 7.2 Photosynthesis is an Oxidation–Reduction Autocatalytic 140 Process 112 8.6.2 Rubisco Activity is Regulated Indirectly by 7.3 Photosynthetic Electron Transport 114 Light 140 7.3.1 Photosystems are Major Components of the 8.6.3 Other PCR Enzymes are also Regulated by Photosynthetic Electron Transport Light 141 Chain 114 8.7 Chloroplasts of C3 Plants also Exhibit Competing 7.3.2 Photosystem II Oxidizes Water to Produce Carbon Oxidation Processes 142 Oxygen 117 8.7.1 Rubisco Catalyzes the Fixation of Both CO2 and 7.3.3 The Cytochrome Complex and Photosystem I O2 142 Oxidize Plastoquinol 119 8.7.2 Why Photorespiration? 143 7.4 Photophosphorylation is the Light-Dependent 8.7.3 In Addition to PCR, Chloroplasts Exhibit an Synthesis of ATP 120 Oxidative Pentose Phosphate Cycle 145 7.5 Lateral Heterogeneity is the Unequal Distribution Summary 149 of Thylakoid Complexes 122 Chapter Review 149 7.6 Cyanobacteria are Oxygenic 123 Further Reading 150 7.7 Inhibitors of Photosynthetic Electron Transport are Effective Herbicides 124 BOX 8.1 Enzymes 146 Summary 127 Chapter Review 127 Chapter 9 Allocation, Translocation, and Further Reading 128 Partitioning of Photoassimilates 151 BOX 7.1 Historical Perspective—The Discovery of Photosynthesis 113 9.1 Starch and Sucrose are Biosynthesized in Two Different Compartments 152 BOX 7.2 The Case for Two 9.1.1 Starch is Biosynthesized in the Stroma 152 Photosystems 125 9.1.2 Sucrose is Biosynthesized in the Cytosol 153 Contents xi 9.2 Starch and Sucrose Biosynthesis are Competitive 10.2.4 Limit Dextrinase is a Debranching Processes 154 Enzyme 176 9.3 Fructan Biosynthesis is An Alternative Pathway 10.2.5 α-Glucosidase Hydrolyzes Maltose 177 For Carbon Allocation 156 10.2.6 Starch Phosphorylase Catalyzes the 9.4 Photoassimilates are Translocated Over Long Phosphorolytic Degradation of Distances 156 Starch 177 9.4.1 What is the Composition of the Photoassimilate 10.3 Fructan Mobilization is Constitutive 178 Translocated by the Phloem? 158 10.4 Glycolysis Converts Sugars to Pyruvic 9.5 Sieve Elements are the Principal Cellular Acid 178 Constituents of the Phloem 159 10.4.1 Hexoses Must be Phosphorylated to Enter 9.5.1 Phloem Exudate Contains a Signiﬁcant Amount of Glycolysis 178 Protein 160 10.4.2 Triose Phosphates are Oxidized to 9.6 Direction of Translocation is Determined by Pyruvate 180 Source-Sink Relationships 161 10.5 The Oxidative Pentose Phosphate Pathway is an 9.7 Phloem Translocation Occurs by Mass Alternative Route for Glucose Transfer 161 Metabolism 180 9.8 Phloem Loading and Unloading Regulate 10.6 The Fate of Pyruvate Depends on the Availability Translocation and Partitioning 163 of Molecular Oxygen 181 9.8.1 Phloem Loading can Occur Symplastically or 10.7 Oxidative Respiration is Carried out by the Apoplastically 164 Mitochondrion 182 9.8.2 Phloem Unloading May Occur Symplastically or 10.7.1 In The Presence of Molecular Oxygen, Pyruvate Apoplastically 166 is Completely Oxidized to CO2 and Water by 9.9 Photoassimilate is Distributed Between the Citric Acid Cycle 182 Different Metabolic Pathways and Plant 10.7.2 Electrons Removed from Substrate in the Citric Organs 166 Acid Cycle are Passed to Molecular Oxygen 9.9.1 Photoassimilates May be Allocated to a Variety of Through the Mitochondrial Electron Transport Metabolic Functions in the Source or The Chain 183 Sink 167 10.8 Energy is Conserved in the Form of ATP in 9.9.2 Distribution of Photoassimilates Between Accordance with Chemiosmosis 185 Competing Sinks is Determined by Sink 10.9 Plants Contain Several Alternative Electron Strength 168 Transport Pathways 186 9.10 Xenobiotic Agrochemicals are Translocated in the 10.9.1 Plant Mitochondria Contain External Phloem 170 Dehydrogenases 186 Summary 170 10.9.2 Plants have a Rotenone-Insensitive NADH Chapter Review 171 Dehydrogenase 186 Further Reading 171 10.9.3 Plants Exhibit Cyanide-Resistant Respiration 187 10.10 Many Seeds Store Carbon as Oils that are Converted to Sugar 188 Chapter 10 Cellular Respiration: 10.11 Respiration Provides Carbon Skeletons for Unlocking the Energy Stored Biosynthesis 189 in Photoassimilates 173 10.12 Respiratory Rate Varies with Development and 10.1 Cellular Respiration Consists of a Series of Metabolic State 191 Pathways by Which Photoassimilates are 10.13 Respiration Rates Respond to Environmental Oxidized 174 Conditions 192 10.2 Starch Mobilization 175 10.13.1 Light 192 10.2.1 The Hydrolytic Degradation of Starch Produces 10.13.2 Temperature 192 Glucose 175 10.13.3 Oxygen Availability 193 10.2.2 α-Amylase Produces Maltose and Limit Summary 193 Dextrins 176 Chapter Review 194 10.2.3 β-Amylase Produces Maltose 176 Further Reading 194 xii Contents Chapter 11 Nitrogen Assimilation 195 12.2 Carbon Economy is Dependent on the Balance Between Photosynthesis and 11.1 The Nitrogen Cycle: A Complex Pattern of Respiration 214 Exchange 195 12.3 Productivity is Inﬂuenced by a Variety of 11.1.1 Ammoniﬁcation, Nitriﬁcation, and Environmental Factors 215 Denitriﬁcation are Essential Processes in the 12.3.1 Fluence Rate 215 Nitrogen Cycle 196 12.3.2 Available CO2 216 11.2 Biological Nitrogen Fixation is Exclusively 12.3.3 Temperature 218 Prokaryotic 196 12.3.4 Soil Water Potential 219 11.2.1 Some Nitrogen-Fixing Bacteria are Free-Living 12.3.5 Nitrogen Supply Limits Productivity 219 Organisms 196 12.3.6 Leaf Factors 220 11.2.2 Symbiotic Nitrogen Fixation Involves Speciﬁc Summary 221 Associations Between Bacteria and Chapter Review 222 Plants 197 Further Reading 222 11.3 Legumes Exhibit Symbiotic Nitrogen Fixation 197 11.3.1 Rhizobia Infect the Host Roots, Which Induces Nodule Development 198 11.4 The Biochemistry of Nitrogen Fixation 200 Chapter 13 Responses of Plants to 11.4.1 Nitrogen Fixation is Catalyzed by the Enzyme Environmental Stress 223 Dinitrogenase 200 13.1 What is Plant Stress? 223 11.4.2 Nitrogen Fixation is Energetically 13.2 Plants Respond to Stress in Several Different Costly 201 Ways 224 11.4.3 Dinitrogenase is Sensitive to Oxygen 202 13.3 Too Much Light Inhibits 11.4.4 Dinitrogenase Results in the Production of Photosynthesis 225 Hydrogen Gas 202 13.3.1 The D1 Repair Cycle Overcomes Photodamage 11.5 The Genetics of Nitrogen Fixation 203 to PSII 227 11.5.1 NIF Genes Code for Dinitrogenase 203 13.4 Water Stress is a Persistent Threat to Plant 11.5.2 NOD Genes and NIF Genes Regulate Survival 229 Nodulation 203 13.4.1 Water Stress Leads to Membrane 11.5.3 What is the Source of Heme For Damage 230 Leghemoglobin? 204 13.4.2 Photosynthesis is Particularly Sensitive to Water 11.6 NH3 Produced by Nitrogen Fixation is Stress 230 Converted to Organic Nitrogen 204 13.4.3 Stomata Respond to Water Deﬁcit 230 11.6.1 Ammonium is Assimilated by 13.5 Plants are Sensitive to Fluctuations in GS/GOGAT 204 Temperature 233 11.6.2 PII Proteins Regulate GS/GOGAT 205 13.5.1 Many Plants are Chilling Sensitive 233 11.6.3 Fixed Nitrogen is Exported as Asparagine and 13.5.2 High-Temperature Stress Causes Protein Ureides 206 Denaturation 234 11.7 Plants Generally Take up Nitrogen in the Form 13.6 Insect Pests and Disease Represent Potential of Nitrate 207 Biotic Stresses 235 11.8 Nitrogen Cycling: Simultaneous Import and 13.6.1 Systemic Acquired Resistance Export 208 Represents a Plant Immune 11.9 Agricultural and Ecosystem Productivity is Response 236 Dependent on Nitrogen Supply 209 13.6.2 Jasmonates Mediate Insect and Disease Summary 211 Resistance 237 Chapter Review 211 13.7 There are Features Common to all Further Reading 211 Stresses 237 Summary 238 Chapter 12 Carbon and Nitrogen Assimilation and Plant Chapter Review 238 Productivity 213 Further Reading 238 12.1 Productivity Refers to an Increase in BOX 13.1 Monitoring Plant Stress by Biomass 213 Chlorophyll Fluorescence 228 Contents xiii Chapter 14 Acclimation to Environmental 15.2.2 The C4 Syndrome is Usually Associated with Stress 241 Kranz Leaf Anatomy 265 15.2.3 The C4 Syndrome has Ecological 14.1 Plant Acclimation is a Time-Dependent Signiﬁcance 265 Phenomenon 242 15.2.4 The C4 Syndrome is Differentially Sensitive to 14.2 Acclimation is Initiated by Rapid, Short-Term Temperature 265 Responses 242 15.2.5 The C4 Syndrome is Associated with Water 14.2.1 State Transitions Regulate Energy Distribution Stress 266 in Response to Changes in Spectral 15.3 Crassulacean Acid Metabolism is an Adaptation Distribution 242 to Desert Life 267 14.2.2 Carotenoids Serve a Dual Function: Light 15.3.1 Is CAM a Variation of the C4 Harvesting and Photoprotection 244 Syndrome? 268 14.2.3 Osmotic Adjustment is a Response to Water 15.3.2 CAM Plants are Particularly Suited to Dry Stress 247 Habitats 269 14.2.4 Low Temperatures Induce Lipid Unsaturation 15.4 C4 and CAM Photosynthesis Require Precise and Cold Regulated Genes in Cold Tolerant Regulation and Temporal Integration 269 Plants 248 15.5 Plant Biomes Reﬂect Myriad Physiological 14.2.5 Q10 for Plant Respiration Varies as a Function of Adaptations 270 Temperature 248 15.5.1 Tropical Rain Forest Biomes Exhibit the 14.3 Long-Term Acclimation Alters Greatest Plant Biodiversity 270 Phenotype 249 15.5.2 Evapotranspiration is a Major Contributor to 14.3.1 Light Regulates Nuclear Gene Expression and Weather 271 Photoacclimation 249 15.5.3 Desert Perennials are Adapted to Reduce 14.3.2 Does the Photosynthetic Apparatus Respond to Transpiration and Heat Load 272 Changes in Light Quality? 252 15.5.4 Desert Annuals are Ephemeral 273 14.3.3 Acclimation to Drought Affects Shoot–Root Summary 273 Ratio and Leaf Area 253 Chapter Review 274 14.3.4 Cold Acclimation Mimics Further Reading 274 Photoacclimation 254 14.4 Freezing Tolerance in Herbaceous Species is a Complex Interaction Between Light and Low Chapter 16 Development: An Temperature 255 Overview 275 14.4.1 Cold Acclimated Plants Secrete Antifreeze Proteins 256 16.1 Growth, Differentiation, and 14.4.2 North Temperate Woody Plants Survive Development 275 Freezing Stress 256 16.1.1 Development is the Sum of Growth and Differentiation 275 14.5 Plants Adjust Photosynthetic Capacity in Response to High Temperature 257 16.1.2 Growth is an Irreversible Increase in Size 276 14.6 Oxygen may Protect During Accimation to 16.1.3 Differentiation Refers To Qualitative Changes Various Stresses 258 That Normally Accompany Growth 276 Summary 259 16.2 Meristems are Centers of Plant Chapter Review 259 Growth 277 Further Reading 260 16.3 Seed Development and Germination 279 Chapter 15 Adaptations to the 16.3.1 Seeds are Formed in the Flower 279 Environment 261 16.3.2 Seed Development and Maturation 280 16.3.3 Seed Germination 281 15.1 Sun and Shade Adapted Plants Respond 16.3.4 The Level and Activities of Various Hormones Differentially to Irradiance 262 Change Dramatically During Seed 15.2 C4 Plants are Adapted to High Temperature and Development 283 Drought 263 16.3.5 Many Seeds Have Additional Requirements for 15.2.1 The C4 Syndrome is Another Biochemical Germination 284 Mechanism to Assimilate CO2 263 16.4 From Embryo to Adult 285 xiv Contents 16.5 Senescence and Programmed Cell Death are the 17.5.3 Calcium-Based Signaling 301 Final Stages of Development 286 17.5.4 Transcriptional-Based Signaling 303 Summary 287 17.6 There is Extensive Crosstalk Among Signal Chapter Review 287 Pathways 303 Further Reading 288 Summary 304 Chapter Review 304 BOX 16.1 Development in a Mutant Further Reading 304 Weed 282 BOX 17.1 Cytoskeleton 295 BOX 17.2 Ubiquitin and Chapter 17 Growth and Development of Proteasomes—Cleaning up Unwanted Cells 289 Proteins 302 17.1 Growth of Plant Cells is Complicated by the Presence of a Cell Wall 289 Chapter 18 Hormones I: Auxins 305 17.1.1 The Primary Cell Wall is a Network of Cellulose Microﬁbrils and Cross-Linking 18.1 The Hormone Concept in Plants 305 Glycans 289 18.2 Auxin is Distributed Throughout the 17.1.2 The Cellulose–Glycan Lattice is Embedded in a Plant 306 Matrix of Pectin and Protein 290 18.3 The Principal Auxin in Plants is Indole-3-Acetic 17.1.3 Cellulose Microﬁbrils are Assembled at the Acid (IAA) 307 Plasma Membrane as they are Extruded into the 18.4 IAA is Synthesized from the Amino Acid Cell Wall 292 l-Tryptophan 309 17.2 Cell Division 292 18.5 Some Plants do not Require Tryptophan for IAA 17.2.1 The Cell Cycle 292 Biosynthesis 310 17.2.2 Cytokinesis 293 18.6 IAA may be Stored as Inactive 17.2.3 Plasmodesmata are Cytoplasmic Channels that Conjugates 310 Extend Through the Wall to Connect the 18.7 IAA is Deactivated by Oxidation and Conjugation Protoplasts of Adjacent Cells 294 with Amino Acids 311 17.3 Cell Walls and Cell Growth 294 18.8 Auxin is Involved in Virtually Every Stage of 17.3.1 Cell Growth is Driven by Water Uptake and Plant Development 311 Limited by the Strength and Rigidity of the Cell 18.8.1 The Principal Test for Auxins is the Stimulation Wall 296 of Cell Enlargement in Excised 17.3.2 Extension of the Cell Wall Requires Tissues 311 Wall-Loosening Events that Enable 18.8.2 Auxin Regulates Vascular Load-Bearing Elements in the Wall to Yield to Differentiation 311 Turgor Pressure 296 18.8.3 Auxin Controls the Growth of Axillary 17.3.3 Wall Loosening and Cell Expansion is Buds 313 Stimulated by Low Ph and Expansins 297 18.9 The Acid-Growth Hypothesis Explains Auxin 17.3.4 In Maturing Cells, a Secondary Cell Wall is Control of Cell Enlargement 314 Deposited on the Inside of the Primary 18.10 Maintenance of Auxin-Induced Growth and Wall 298 Other Auxin Effects Requires Gene 17.4 A Continuous Stream of Signals Provides Activation 316 Information that Plant Cells Use to Modify 18.11 Many Aspects of Plant Development are Linked Development 298 to the Polar Transport of Auxin 317 17.4.1 Signal Perception and Transduction 299 Summary 320 17.4.2 The G-Protein System is a Ubiquitous Receptor Chapter Review 321 System 299 Further Reading 321 17.5 Signal Transduction Includes a Diverse Array of Second Messengers 300 BOX 18.1 Discovering Auxin 307 17.5.1 Protein Kinase-Based Signaling 300 17.5.2 Phospholipid-Based Signaling 300 BOX 18.2 Commercial Applications of Auxins 314 Contents xv Chapter 19 Hormones II: 20.2 Cytokinins are Synthesized Primarily in the Root Gibberellins 323 and Translocated in the Xylem 341 20.3 Cytokinins are Required for Cell 19.1 There are a Large Number of Proliferation 343 Gibberellins 323 20.3.1 Cytokinins Regulate Progression through the 19.2 There are Three Principal Sites for Gibberellin Cell Cycle 343 Biosynthesis 324 20.3.2 The Ratio of Cytokinin to Auxin 19.3 Gibberellins are Terpenes, Sharing a Core Controls Root and Shoot Initiation in Callus Pathway with Several Other Hormones and a Tissues and the Growth of Axillary Wide Range of Secondary Products 325 Buds 344 19.4 Gibberellins are Synthesized from 20.3.3 Crown Gall Tumors are Genetically Engineered Geranylgeranyl Pyrophosphate to Overproduce Cytokinin and Auxin 345 (GGPP) 327 20.3.4 Cytokinins Delay Senescence 346 19.5 Gibberellins are Deactivated by 20.3.5 Cytokinins Have an Important Role in 2β-Hydroxylation 329 Maintaining the Shoot Meristem 347 19.6 Growth Retardants Block the Synthesis of 20.3.6 Cytokinin Levels in the Shoot Apical Meristem Gibberellins 329 Are Regulated by Master Control 19.7 Gibberellin Transport is Poorly Genes 348 Understood 330 20.4 Cytokinin Receptor and Signaling 350 19.8 Gibberellins Affect Many Aspects of Plant 20.4.1 The Cytokinin Receptor is a Membrane-Based Growth and Development 330 Histidine Kinase 350 19.8.1 Gibberellins Stimulate Hyper-elongation of 20.4.2 The Cytokinin Signaling Chain Involves a Intact Stems, Especially in Dwarf and Rosette Multistep Transfer of Phosphoryl Groups to Plants 330 Response Regulators 351 19.8.2 Gibberellins Stimulate Mobilization of Nutrient Summary 353 Reserves During Germination of Cereal Chapter Review 353 Grains 332 Further Reading 354 19.9 Gibberellins Act by Regulating Gene Expression 333 BOX 20.1 The Discovery of Cytokinins 341 Summary 336 Chapter Review 336 BOX 20.2 Tissue Culture has Made Possible Large-Scale Cloning of Plants by Further Reading 337 Micropropagation 345 BOX 19.1 Discovery of Gibberellins 325 Chapter 21 Hormones IV: Abscisic Acid, BOX 19.2 Commercial Applications of Ethylene, and Gibberellins 330 Brassinosteroids 355 21.1 Abscisic Acid 355 BOX 19.3 Della Proteins and the Green 21.1.1 Abscisic Acid is Synthesized from a Carotenoid Revolution 335 Precursor 355 21.1.2 Abscisic Acid is Degraded to Phaseic Acid by Oxidation 357 Chapter 20 Hormones III: 21.1.3 Abscisic Acid is Synthesized in Mesophyll Cells, Cytokinins 339 Guard Cells, and Vascular Tissue 357 20.1 Cytokinins are Adenine Derivatives 339 21.1.4 Abscisic Acid Regulates Embryo Maturation and 20.1.1 Cytokinin Biosynthesis Begins with the Seed Germination 358 Condensation of an Isopentenyl Group with the 21.1.5 Abscisic Acid Mediates Response to Water Amino Group of Adenosine Stress 358 Monophosphate 339 21.1.6 Other Abscisic Acid Responses 359 20.1.2 Cytokinins may be Deactivated by Conjugation 21.1.7 ABA Perception and Signal or Oxidation 340 Transduction 359 xvi Contents 21.2 Ethylene 362 22.4 Phytochrome and Cryptochrome Mediate 21.2.1 Ethylene is Synthesized from the Amino Acid Numerous Developmental Responses 379 Methionine 362 22.4.1 Seed Germination 379 21.2.2 Excess Ethylene is Subject to 22.4.2 De-Etiolation 380 Oxidation 364 22.4.3 Shade Avoidance 381 21.2.3 The Study of Ethylene Presents a Unique Set of 22.4.4 Detecting End-of-day Signals 381 Problems 364 22.4.5 Control of Anthocyanin Biosynthesis 382 21.2.4 Ethylene Affects Many Aspects of Vegetative 22.4.6 Rapid Phytochrome Responses 382 Development 364 22.4.7 PhyA may Function to Detect the Presence of 21.2.5 Ethylene Receptors and Signaling 365 Light 383 21.3 Brassinosteroids 367 22.5 Chemistry and Mode of Action of Phytochrome 21.3.1 Brassinosteroids are Polyhydroxylated Sterols and Cryptochrome 383 Derived from the Triterpene 22.5.1 Phytochrome is a Phycobiliprotein 383 Squalene 367 22.5.2 Phytochrome Signal Transduction 384 21.3.2 Several Routes for Deactivation of 22.5.3 Cryptochrome Structure is Similar to DNA Brassinosteroids have been Identiﬁed 369 Repair Enzymes 386 21.3.3 Brassinolide receptors and Signaling 369 22.5.4 Cryptochrome Signal Transduction 386 Summary 369 22.6 Some Plant Responses are Regulated by UV-B Chapter Review 370 Light 387 Further Reading 370 22.7 De-Etiolation in Arabidopsis: A Case Study in Photoreceptor Interactions 387 BOX 21.1 The Discovery of Abscisic Summary 388 Acid 356 Chapter Review 389 Further Reading 389 BOX 21.2 The Discovery of Ethylene 363 BOX 22.1 Historical Perspectives—The BOX 21.3 Mitogenactivated Protein Kinase: A Discovery of Phytochrome 375 Widespread Mechanism for Signal Transduction 366 Chapter 23 Tropisms and Nastic Movements: Orienting Plants Chapter 22 Photomorphogenesis: in Space 391 Responding to Light 373 23.1 Phototropism: Reaching for the Sun 392 22.1 Photomorphogenesis is Initiated by 23.1.1 Phototropism is a Response to a Light Photoreceptors 373 Gradient 392 22.2 Phytochromes: Responding to Red and Far-Red 23.1.2 Phototropism is a Blue-Light Light 374 Response 393 22.2.1 Photoreversibility is the Hallmark of 23.1.3 Phototropism Orients a Plant for Optimal Phytochrome Action 376 Photosynthesis 393 22.2.2 Conversion of Pr to Pfr in Etiolated Seedlings 23.1.4 Fluence Response Curves Illustrate the Leads to a Loss of Both Pfr and Total Complexity of Phototropic Phytochrome 377 Responses 394 22.2.3 Light Establishes a State of Dynamic 23.1.5 The Phototropic Response is Attributed to a Photoequilibrium Between Pr and Lateral Redistribution of Diffusible Pfr 378 Auxin 395 22.2.4 Phytochrome Responses can be Grouped 23.1.6 Phototropism and Related Responses are According to their Fluence Regulated by a Family of Blue-Sensitive Requirements 378 Flavoproteins 396 22.3 Cryptochrome: Responding to Blue and UV-A 23.1.7 A Hybrid Red/Blue Light Photoreceptor has Light 379 been Isolated from a Fern 397 Contents xvii 23.1.8 Phototropin Activity and Signal 24.2.2 Light Resets the Biological Clock on a Daily Chain 397 Basis 425 23.1.9 Phototropism in Green Plants is Not Well 24.2.3 The Circadian Clock is Understood 398 Temperature-Compensated 426 23.2 Gravitropism 398 24.2.4 The Circadian Clock is a Signiﬁcant Component 23.2.1 Gravitropism is More than Simply Up and in Photoperiodic Time Down 399 Measurement 427 23.2.2 The Gravitational Stimulus is the Product of 24.2.5 Daylength Measurement Involves an Interaction Intensity and Time 399 Between an External Light Signal and a 23.2.3 Root Gravitropism Occurs in Four Circadian Rhythm 428 Phases 401 24.2.6 The Circadian Clock is a Negative Feedback 23.3 Nastic Movements 405 Loop 429 23.3.1 Nyctinastic Movements are Rhythmic 24.3 Photoperiodism in Nature 430 Movements Involving Reversible Turgor Summary 431 Changes 406 Chapter Review 432 23.3.2 Nyctinastic Movements are due to Ion Fluxes Further Reading 432 and Resulting Osmotic Responses in Specialized Motor Cells 407 BOX 24.1 Historical Perspectives: The 23.3.3 Seismonasty is a Response to Mechanical Discovery of Photoperiodism 414 Stimulation 409 Summary 410 BOX 24.2 Historical Perspectives: The Chapter Review 411 Biological Clock 422 Further Reading 411 BOX 23.1 Methods in the Study of Gravitropism 400 Chapter 25 Flowering and Fruit Development 433 25.1 Flower Initiation and Development Involves the Chapter 24 Measuring Time: Controlling Sequential Action of Three Sets of Development by Photoperiod Genes 433 and Endogenous 25.1.1 Flowering-Time Genes Inﬂuence the Duration Clocks 413 of Vegetative Growth 434 24.1 Photoperiodism 414 25.1.2 Floral-Identity Genes and Organ-Identity Genes 24.1.1 Photoperiodic Responses may be Characterized Overlap in Time and Function 436 by a Variety of Response Types 415 25.2 Temperature can Alter the Flowering Response 24.1.2 Critical Daylength Deﬁnes Short-Day and to Photoperiod 437 Long-Day Responses 415 25.2.1 Vernalization Occurs most Commonly in 24.1.3 Plants Actually Measure the Length of the Dark Winter Annuals and Biennials 438 Period 417 25.2.2 The Effective Temperature for Vernalization is 24.1.4 Phytochrome and Cryptochrome are the Variable 439 Photoreceptors for Photoperiodism 418 25.2.3 The Vernalization Treatment is Perceived by the 24.1.5 The Photoperiodic Signal is Perceived by the Shoot Apex 440 Leaves 419 25.2.4 The Vernalized State is 24.1.6 Control of Flowering by Photo- Transmissible 440 period Requires a Transmissible 25.2.5 Gibberellin and Vernalization Operate through Signal 420 Independent Genetic Pathways 440 24.1.7 Photoperiodism Normally Requires a Period of 25.2.6 Threee Genes Determine the Vernalization High Fluence Light Before or After the Dark Requirement in Cereals 441 Period 421 25.3 Fruit Set and Development is Regulated by 24.2 The Biological Clock 423 Hormones 442 24.2.1 Clock-Driven Rhythms Persist Under Constant 25.3.1 The Development of Fleshy Fruits can be Conditions 423 Divided into Five Phases 442 xviii Contents 25.3.2 Fruit Set is Triggered by Auxin 442 27.3.3 Cyanogenic Glycosides are A Natural Source of 25.3.3 Ripening is Triggered by Ethylene in Hydrogen Cyanide 466 Climacteric Fruits 444 27.3.4 Glucosinolates are Sulfur-Containing Precursors Summary 445 to Mustard Oils 466 Chapter Review 446 27.4 Phenylpropanoids 467 Further Reading 446 27.4.1 Shikimic Acid is a Key Intermediate in the Synthesis of Both Aromatic Amino Acids and BOX 25.1 Ethylene: It’s a Gas! 445 Phenylpropanoids 468 27.4.2 The Simplest Phenolic Molecules are Essentially Chapter 26 Temperature: Plant Deaminated Versions of the Corresponding Development and Amino Acids 468 Distribution 447 27.4.3 Coumarins and Coumarin Derivatives Function as Anticoagulants 468 26.1 Temperature in the Plant 27.4.4 Lignin is a Major Structural Component of Environment 447 Secondary Cell Walls 470 26.2 Bud Dormancy 449 27.4.5 Flavonoids and Stilbenes have Parallel 26.2.1 Bud Dormancy is Induced by Biosynthetic Pathways 471 Photoperiod 450 27.4.6 Tannins Denature Proteins and Add an 26.2.2 A Period of Low Temperature is Required to Astringent Taste to Foods 472 Break Bud Dormancy 451 27.5 Secondary Metabolites are Active Against Insects 26.3 Seed Dormancy 451 and Disease 474 26.3.1 Numerous Factors Inﬂuence Seed 27.5.1 Some Terpenes and Isoﬂavones have Insecticidal Dormancy 451 and Anti-Microbial Activity 474 26.3.2 Temperature has a Signiﬁcant Impactl on Seed 27.5.2 Recognizing Potential Pathogens 475 Dormancy 453 27.5.3 Salicylic Acid, a Shikimic Acid Derivative, 26.4 Thermoperiodism is a Response to Alternating Triggers Systemic Acquired Temperature 454 Resistance 475 26.5 Temperature Inﬂuences Plant 27.6 Jasmonates are Linked to Ubiquitin-Related Distribution 454 Protein Degradation 476 Summary 457 27.7 Alkaloids 476 Chapter Review 457 27.7.1 Alkaloids are a Large Family of Chemically Further Reading 457 Unrelated Molecules 476 27.7.2 Alkaloids are Noted Primarily for their BOX 26.1 Bulbs and Corms 450 Pharmacological Properties and Medical Applications 476 Chapter 27 Secondary Metabolites 459 27.7.3 Like Many Other Secondary Metabolites, Alkaloids Serve as Preformed Chemical Defense 27.1 Secondary Metabolites: A.K.A Natural Molecules 479 Products 459 Summary 479 27.2 Terpenes 460 Chapter Review 480 27.2.1 The Terpenes are a Chemically and Functionally Diverse Group of Molecules 460 Further Reading 480 27.2.2 Terpenes are Constituents of Essential Oils 460 Appendix Building Blocks: Lipids, Proteins, 27.2.3 Steroids and Sterols are Tetracyclic and Carbohydrates 481 Triterpenoids 462 I.1 Lipids 481 27.2.4 Polyterpenes Include the Carotenoid Pigments I.2 Proteins 483 and Natural Rubber 462 I.3 Carbohydrates 485 27.3 Glycosides 463 I.3.1 Monosaccharides 485 27.3.1 Saponins are Terpene Glycosides with Detergent Properties 464 I.3.2 Polysaccharides 486 27.3.2 Cardiac Glycosides are Highly Toxic Steroid Glycosides 465 Index/Glossary 489 Cytosol PIP H2O H2O H2O TIP H2O Vacuole 1 Plant Cells and Water Without water, life as we know it could not exist. Water able medium for the uptake and distribution of mineral is the most abundant constituent of most organisms. nutrients and other solutes required for growth. Many The actual water content will vary according to tissue of the biochemical reactions that characterize life, and cell type and it is dependent to some extent such as oxidation, reduction, condensation, and hydrol- on environmental and physiological conditions, but ysis, occur in water and water is itself either a reactant water typically accounts for more than 70 percent by or a product in a large number of those reactions. The weight of non-woody plant parts. The water content transparency of water to visible light enables sunlight of plants is in a continual state of ﬂux, depending to penetrate the aqueous medium of cells where it can be on the level of metabolic activity, the water status used to power photosynthesis or control development. of the surrounding air and soil, and a host of other Water in land plants is part of a very dynamic sys- factors. Although certain desiccation-tolerant plants tem. Plants that are actively carrying out photosynthesis may experience water contents of only 20 percent and experience substantial water loss, largely through evap- dry seeds may contain as little as 5 percent water, oration from the leaf surfaces. Equally large quantities both are metabolically inactive, and resumption of of water must therefore be taken up from the soil and signiﬁcant metabolic activity is possible only after the moved through the plant in order to satisfy deﬁciencies water content has been restored to normal levels. that develop in the leaves. For example, it is estimated Water ﬁlls a number of important roles in the that the turnover of water in plants due to photosynthesis physiology of plants; roles for which it is uniquely suited and transpiration is about 1011 tonnes per year. because of its physical and chemical properties. The This constant ﬂow of water through plants is a thermal properties of water ensure that it is in the liq- matter of considerable signiﬁcance to their growth and uid state over the range of temperatures at which most survival. The uptake of water by cells generates a pres- biological reactions occur. This is important because sure known as turgor; in the absence of any skeletal most of these reactions can occur only in an aqueous system, plants must maintain cell turgor in order to medium. The thermal properties of water also con- remain erect. As will be shown in later chapters, the tribute to temperature regulation, helping to ensure that uptake of water by cells is also the driving force for plants do not cool down or heat up too rapidly. Water cell enlargement. Few plants can survive desiccation. also has excellent solvent properties, making it a suit- There is no doubt that the water relations of plants and 1 2 Chapter 1 / Plant Cells and Water plant cells are fundamental to an understanding of their physiology. − − This chapter is concerned with the water relations of cells. Topics to be addressed include the following: O a review of the unique physical and chemical prop- H H erties of water that make it particularly suitable as a + + medium for life, physical processes that underlie water movement in A. plants, including diffusion, osmosis, and bulk ﬂow as mechanisms for water movement, and H O the chemical potential of water and the concept of water potential. O These concepts provide the basis for understanding H water movement within the plant and between the plant H and its environment, to be discussed in Chapter 2. O H 1.1 WATER HAS UNIQUE O PHYSICAL AND CHEMICAL H PROPERTIES The key to understanding many of the unique properties H H of water is found in the structure of the water molecule H O and the strong intermolecular attractions that result from that structure. Water consists of an oxygen atom covalently bonded to two hydrogen atoms (Figure 1.1). B. The oxygen atom is strongly electronegative, which FIGURE 1.1 (A) Schematic structure of a water molecule. means that it has a tendency to attract electrons. One (B) The hydrogen bond (dashed line) results from the consequence of this strong electronegativity is that, in electrostatic attraction between the partial positive the water molecule, the oxygen tends to draw electrons charge on one molecule and the partial negative charge away from the hydrogen. The shared electrons that on the next. make up the O—H bond are, on the average, closer to the oxygen nucleus than to hydrogen. As a consequence, distribution makes water a polar molecule. Overall, the oxygen atom carries a partial negative charge, and a water remains a neutral molecule, but the separation of corresponding partial positive charge is shared between partial negative and positive charges generates a strong the two hydrogen atoms. This asymmetric electron mutual (electrical) attraction between adjacent water TABLE 1.1 Some physical properties of water compared with other molecules of similar molecular size. Because thermal properties are deﬁned on an energy-per-unit mass basis, values are given in units of joules per gram Speciﬁc Melting Heat of Boiling Heat of Molecular heat point fusion point vaporization mass (Da) (J/g/˚C) (˚C) (J/g) (˚C) (J/g) Water 18 4.2 0 335 100 2452 Hydrogen sulphide 34 — −86 70 −61 — Ammonia 17 5.0 −77 452 −33 1234 Carbon dioxide 44 — −57 180 −78 301 Methane 16 — −182 58 −164 556 Ethane 30 — −183 96 −88 523 Methanol 32 2.6 −94 100 65 1226 Ethanol 46 2.4 −117 109 78 878 1.2 The Thermal Properties of Water are Biologically Important 3 molecules or between water and other polar molecules. and ethanol (CH3 CH2 OH) to temperatures much closer This attraction is called hydrogen bonding (Figure 1.1). to that of water. This is because the presence of oxy- The energy of the hydrogen bond is about 20 kJ mol−1. gen introduces polarity and the opportunity to form The hydrogen bond is thus weaker than either covalent hydrogen bonds. or ionic bonds, which typically measure several hundred kJ mol−1 , but stronger than the short-range, transient attractions known as Van der Waals forces (about 4 kJ 1.2.1 WATER EXHIBITS A UNIQUE mol−1 ). Hydrogen bonding is largely responsible for the THERMAL CAPACITY many unique properties of water, compared with other The term speciﬁc heat1 is used to describe the thermal molecules of similar molecular size (Table 1.1). capacity of a substance or the amount of energy that can In addition to interactions between water molecules, be absorbed for a given temperature rise. The speciﬁc hydrogen bonding also accounts for attractions between heat of water is 4.184 J g−1 ◦ C−1 , higher than that of water and other molecules or surfaces. Hydrogen bond- any other substance except liquid ammonia (Table 1.1). ing, for example, is the basis for hydration shells that Because of its highly ordered structure, liquid water also form around biologically important macromolecules has a high thermal conductivity. This means that it such as proteins, nucleic acids, and carbohydrates. rapidly conducts heat away from the point of application. These layers of tightly bound and highly oriented water The combination of high speciﬁc heat and thermal con- molecules are often referred to as bound water. It ductivity enables water to absorb and redistribute large has been estimated that bound water may account for amounts of heat energy without correspondingly large as much as 30 percent by weight of hydrated protein increases in temperature. For plant tissues that consist molecules. Bound water is important to the stability of largely of water, this property provides for an excep- protein molecules. Bound water ‘‘cushions’’ protein, tionally high degree of temperature stability. Localized preventing the molecules from approaching close overheating in a cell due to the heat of biochemical enough to form aggregates large enough to precipitate. reactions is largely prevented because the heat may Hydrogen bonding, although characteristic of be quickly dissipated throughout the cell. In addition, water, is not limited to water. It arises wherever large amounts of heat can be exchanged between cells hydrogen is found between electronegative centers. and their environment without extreme variation in the This includes alcohols, which can form hydrogen bonds internal temperature of the cell. because of the—OH group, and macromolecules such as proteins and nucleic acids where hydrogen bonds | between amino (−NH2 ) and carbonyl (C = O) groups 1.2.2 WATER EXHIBITS A HIGH | HEAT OF FUSION AND HEAT help to stabilize structure. OF VAPORIZATION Energy is required to cause changes in the state of any substance, such as from solid to liquid or liquid to gas, 1.2 THE THERMAL PROPERTIES without a change in temperature. The energy required OF WATER ARE to convert a substance from the solid to the liquid state BIOLOGICALLY IMPORTANT is known as the heat of fusion. The heat of fusion for water is 335 J g−1 , which means that 335 J of energy are Perhaps the single most important property of water is required to convert 1 gram of ice to 1 gram of liquid that it is a liquid over the range of temperatures most water at 0◦ C (Table 1.1). Expressed on a molar basis, compatible with life. Boiling and melting points are the heat of fusion of water is 6.0 kJ mol−1 (18 g of water generally related to molecular size, such that changes per mole × 335 J g−1 ). The heat of fusion of water is of state for smaller molecules occur at lower temper- one of the highest known, second only to ammonia. atures than for larger molecules. On the basis of size The high heat of fusion of water is attributable to alone, water might be expected to exist primarily in the large amount of energy necessary to overcome the the vapor state at temperatures encountered over most of the earth. However, both the melting and boiling points of water are higher than expected when compared 1 Speciﬁc heat is deﬁned as the amount of energy required to with other molecules of similar size, especially ammonia raise the temperature of one gram of substance by 1◦ C (NH3 ) and methane (CH4 ) (Table 1.1). Molecules such (usually at 20◦ C). The speciﬁc heat of water is the basis for as ammonia and the hydrocarbons (methane and ethane) the deﬁnition of a quantity of energy called the calorie. The are associated only through weak Van der Waals forces speciﬁc heat of water was therefore assigned the value of 1.0 and relatively little energy is required to change their calorie. In accordance with the International System of Units state. Note, however, that the introduction of oxygen (Système Internationale d’Unites, or SI), the preferred unit raises the boiling points of both methanol (CH3 —OH) for energy is the joule (J). 1 calorie = 4.184 joules. 4 Chapter 1 / Plant Cells and Water strong intermolecular forces associated with hydrogen the ability to partially neutralize electrical attractions bonding. between charged solute molecules or ions by surround- The density of ice is another important property. ing the ion or molecule with one or more layers of At 0◦ C, the density of ice is less than that of liquid oriented water molecules, called a hydration shell. water. Thus water, unlike other substances, reaches Hydration shells encourage solvation by reducing the its maximum density in the liquid state (near 4◦ C), probability that ions can recombine and form crystal rather than as a solid. This occurs because molecules structures (Figure 1.2). in the liquid state are able to pack more tightly than in The polarity of molecules can be measured by the highly ordered crystalline state of ice. Consequently, a quantity known as the dielectric constant. Water ice ﬂoats on the surface of lakes and ponds rather has one of the highest known dielectric constants than sinking to the bottom where it might remain (Table 1.2). The dielectric constants of alcohols are year-round. This is extremely important to the survival somewhat lower, and those of nonpolar organic liquids of aquatic organisms of all kinds. such as benzene and hexane are very low. Water is Just as hydrogen bonding increases the amount of thus an excellent solvent for charged ions or molecules, energy required to melt ice, it also increases the energy which dissolve very poorly in nonpolar organic liquids. required to evaporate water. The heat of vaporization Many of the solutes of importance to plants are charged. of water, or the energy required to convert one mole of On the other hand, the low dielectric constants of liquid water to one mole of water vapor, is about 44 kJ nonpolar molecules helps to explain why charged mol−1 at 25◦ C. Because this energy must be absorbed solutes do not readily cross the predominantly nonpolar, from its surroundings, the heat of vaporization accounts hydrophobic lipid regions of cellular membranes. for the pronounced cooling effect associated with evap- oration. Evaporation from the moist surface cools the surface because the most energetic molecules escape the 1.4 POLARITY OF WATER surface, leaving behind the lower-energy (hence, cooler) MOLECULES RESULTS IN molecules. As a result, plants may undergo substantial heat loss as water evaporates from the surfaces of leaf COHESION AND ADHESION cells. Such heat loss is an important mechanism for tem- The strong mutual attraction between water molecules perature regulation in the leaves of terrestrial plants that resulting from hydrogen bonding is also known as cohe- are often exposed to intense sunlight. sion. One consequence of cohesion is that water has an exceptionally high surface tension, which is most evident at interfaces between water and air. Surface ten- sion arises because the cohesive force between water 1.3 WATER IS THE UNIVERSAL molecules is much stronger than interactions between SOLVENT water and air. The result is that water molecules at the surface are constantly being pulled into the bulk water The excellent solvent properties of water are due to the (Figure 1.3). The surface thus tends to contract and highly polar character of the water molecule. Water has behaves much in the manner of an elastic membrane. A O H H H H H O O H O H H O H H H H O O H H H

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