Herbicides and Plant Physiology Second Edition Professor Andrew H. Cobb Dr John P.H. Reade Crop and Environment Research Centre Harper Adams University College Newport Shropshire UK A John Wiley & Sons, Ltd., Publication Herbicides and Plant Physiology, Second Edition Andrew H. Cobb and John P.H. Reade © 2010 A.H. Cobb and J.P.H. Reade. ISBN: 978-1-405-12935-0 This edition fi rst published 2010 © 2010 A.H. Cobb and J.P.H. Reade Blackwell Publishing was acquired by John Wiley & Sons in February 2007. Blackwell’s publishing programme has been merged with Wiley’s global Scientifi c, Technical, and Medical business to form Wiley-Blackwell. 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Includes bibliographical references and index. ISBN 978-1-4051-2935-0 (pbk. : alk. paper) – ISBN 978-1-4443-2780-9 (e-book) 1. Plants–Effect of herbicides on. 2. Plant physiology. 3. Herbicides–Physiological effect. 4. Weeds–Control. I. Reade, John P. H. II. Title. SB951.4.C63 2010 632′.954–dc22 2010016815 A catalogue record for this book is available from the British Library. Set in 10/12 pt Times by Toppan Best-set Premedia Limited Printed in Singapore 1 2010 Contents Preface vii 1 An Introduction to Weed Biology 1 1.1 Introduction 1 1.2 Distribution 1 1.3 The importance of weeds 2 1.4 Problems caused by weeds 3 1.5 Biology of weeds 7 1.6 A few examples of problem weeds 17 1.7 Positive attributes of weeds 19 1.8 The ever-changing weed spectrum 20 1.9 Weed control 23 References 24 2 Herbicide Discovery and Development 27 2.1 Introduction 27 2.2 Markets 28 2.3 Prospects 30 2.4 Environmental impact and relative toxicology 32 2.5 The search for novel active ingredients 35 2.6 The search for novel target sites 40 2.7 Mode of action studies 41 2.8 A lower limit for rates of herbicide application? 47 References 48 3 Herbicide Uptake and Movement 50 3.1 Introduction 50 3.2 The cuticle as a barrier to foliar uptake 50 3.3 Physicochemical aspects of foliar uptake 55 3.4 Herbicide formulation 57 3.5 Uptake by roots from soil 63 3.6 Herbicide translocation from roots to shoots 64 3.7 A case study: the formulation of acids 65 iii iv Contents 3.8 Recent developments 67 References 68 4 Herbicide Selectivity and Metabolism 70 4.1 Introduction 70 4.2 General principles 71 4.3 Herbicide safeners and synergists 81 References 86 5 Herbicides That Inhibit Photosynthesis 87 5.1 Introduction 87 5.2 Photosystems 87 5.3 Inhibition at Photosystem II 89 5.4 Photodamage and repair of Photosystem II 98 5.5 Structures and uses of Photosystem II inhibitors 100 5.6 Interference with electron fl ow at Photosystem I 100 5.7 RuBisCo activase 106 5.8 How treated plants die 107 References 114 6 Inhibitors of Pigment Biosynthesis 116 6.1 Introduction: structures and functions of photosynthetic pigments 116 6.2 Inhibition of chlorophyll biosynthesis 116 6.3 Inhibition of carotenoid biosynthesis 122 6.4 Inhibition of plastoquinone biosynthesis 127 6.5 How treated plants die 130 6.6 Selectivity and metabolism 130 References 131 7 Auxin-Type Herbicides 133 7.1 Introduction 133 7.2 Structures and uses of auxin-type herbicides 134 7.3 Auxin, a natural plant growth regulator 136 7.4 Auxin receptors, gene expression and herbicides 140 7.5 Signal transduction 143 7.6 Auxin transport 147 7.7 An ‘auxin’ overdose 149 7.8 How treated plants die 149 7.9 Selectivity and metabolism 151 References 155 8 Inhibitors of Lipid Biosynthesis 157 8.1 Introduction 157 8.2 Structures and uses of graminicides 157 Contents v 8.3 Inhibition of lipid biosynthesis 162 8.4 Anti-auxin activity of graminicides 168 8.5 How treated plants die 170 8.6 Selectivity 171 References 174 9 The Inhibition of Amino Acid Biosynthesis 176 9.1 Introduction 176 9.2 Overview of amino acid biosynthesis in plants 176 9.3 Inhibition of glutamine synthase 177 9.4 Inhibition of EPSP synthase 181 9.5 Inhibition of acetolactate synthase 185 9.6 Inhibition of histidine biosynthesis 195 References 197 10 The Disruption of Cell Division 200 10.1 Introduction 200 10.2 The cell cycle 200 10.3 Control of the cell cycle 201 10.4 Microtubule structure and function 204 10.5 Herbicidal interference with microtubules 205 10.6 Selectivity 208 References 209 11 The Inhibition of Cellulose Biosynthesis 210 11.1 Introduction 210 11.2 Cellulose biosynthesis inhibitors 211 11.3 Selectivity 214 References 214 12 Herbicide Resistance 216 12.1 Introduction 216 12.2 Mechanisms of herbicide resistance 217 12.3 How resistance occurs 227 12.4 Chronology of herbicide resistance 228 12.5 Herbicide resistance case study – black-grass (Alopecurus myosuroides Huds) 229 12.6 The future development of herbicide resistance 233 References 233 13 Herbicide-Tolerant Crops 238 13.1 Introduction 238 13.2 History of genetically modifi ed, herbicide-tolerant crops 238 vi Contents 13.3 How genetically modifi ed crops are produced 239 13.4 Genetically engineered herbicide tolerance to glyphosate 243 13.5 Genetically modifi ed herbicide tolerance to glufosinate 246 13.6 Genetically modifi ed herbicide tolerance to bromoxynil 247 13.7 Genetically modifi ed herbicide tolerance to sulfonylureas 248 13.8 Genetically modifi ed herbicide tolerance to 2,4-D 248 13.9 Genetically modifi ed herbicide tolerance to fops and dims 248 13.10 Genetically modifi ed herbicide tolerance to phytoene desaturase 249 13.11 Herbicide tolerance due to engineering of enhanced metabolism 249 13.12 Herbicide tolerance through means other than genetic modifi cation 250 13.13 Genetically modifi ed high-tolerance crops in practice: the UK Farm-Scale Evaluations, 2000–2 251 13.14 Future developments 256 References 256 14 Further Targets For Herbicide Development 259 14.1 Introduction 259 14.2 Protein turnover 260 14.3 Biological control of weeds 263 14.4 Natural products as leads for new herbicides 265 References 269 Glossary 270 Index 277 Preface Whilst recently acting in the capacity of External Examiner at a UK Higher Education Institution, AHC encountered the following question on an examination paper: “ Discuss why, after 50 years of using herbicides, weeds are still a major problem in agroecosystems.” The short answer, of course, is that the “ weed problem ” will never go away as long as crops are grown and harvested. The grower needs to be forever vigilant of the changing weed spectrum and of the need to use the appropriate methods available to ensure adequate control, including both crop and chemical rotation. The theory then is relatively simple, but in practice the grower is put under increasing pressure by variable returns, to repeat- edly grow monocultures and rely on the cheapest methods of weed control and chemical inputs available. Too often a farmer will adopt a wait and see approach and then resort to a fi re - fi ghting, last - minute strategy for weed control. Variable returns and environmental considerations are indeed forcing the farmer to re - examine the need for all chemical inputs, including fertilisers and crop protection agents. This will reduce costs but with a consequence of lower yields and reduced crop quality. On the other hand, a more judicious, measured use of agrochemicals at an appropriate dose may provide the most cost effective management of a weed problem. To do this, the farmer must know the land, understand its previous history of cropping and have good records of the previous usage of crop protection chemistry. Even then, as we develop new cropping practices, such as minimum cultivation or even organic farming, we can expect the weed population and spectrum to change, population density to alter and different species to gain prominence. These all create new and demanding challenges to weed management and control strategies. Our starting point then, in a study of herbicides, must be to understand weed biology and the competing demands of weeds and crops. Only then is refl ection possible to choose the most appropriate herbicide for the weed issue in question. Subsequent chapters include considerations of how herbicides have been discovered and developed (Chapter 2 ), how they gain entry into plants and move to their site of action (Chapter 3 ), and the basis for herbicide selectivity (Chapter 4 ). Detailed accounts are then given of how herbicides interact with the major physiological processes in plants leading to plant death. This begins with the inhibition of photosynthesis (Chapter 5 ), followed by pigment biosynthesis (Chapter 6 ), interactions with the plant growth regula- tor, auxin (Chapter 7 ), lipid biosynthesis (Chapter 8 ), amino acid biosynthesis (Chapter vii viii Preface 9 ), cell division (Chapter 10 ) and cellulose biosynthesis (Chapter 11 ). Chapter 12 gives a detailed and up to date presentation on herbicide resistance, leading to an account of the development and underlying science of herbicide tolerant crops (Chapter 13 ). Finally, Chapter 14 highlights further targets for herbicide development that may yield new products in the following decade(s). In reviewing literature for this second edition, it may be tempting at a superfi cial level to believe that there is little new to report in this discipline since the fi rst edition was published in 1992. Indeed, superfi cially, there are few novel modes of action to report with little exploitation of new target sites. A more detailed scrutiny, however, reveals the development of new triazine inhibitors of PSII (p 98), of the HPPD inhibitors (p 128) , more clarity on the mode of action of the chloroacetamides (p 167) and the discovery of a new class of microtubule inhibitors (p 205) and inhibitors of cellulose biosynthesis (p 213) to name but fi ve. Why have there been fewer herbicide discoveries in the recent years? Two main contributory factors come to mind. The fi rst is the increasing costs to develop a new product that satisfi es all the regulatory requirements. The second lies in the use of molecular approaches to induce herbicide tolerance into crops, thereby extend- ing the usage of existing products. These areas are considered further in Chapters 2 and 13 , respectively. Our understanding of the metabolism, environmental impact, genetics and molecular biology of herbicide action, particularly in relation to resistance, has also greatly improved. Indeed, this understanding has led to lower doses of new agrochemicals that are more effective than previous versions and are more environmentally benign. Furthermore, new methods have been established for high - throughput screening (p 38) to discover new agrochemicals. Since the fi rst edition of this book our understanding of plant physiology and metabo- lism in general has improved, especially utilising our knowledge of the A rabidopsis genome. This has generated potential for new target sites that may yield the new herbicides of the coming decades. The need for crop protection agents remains essential, as eloquently summarised by Len Copping (2001) . “ In the time it takes to read this sentence, another 20 people will have been added to the world ’ s population. And by this time next week, enough people will have been born to establish a new city about the size of Birmingham in the UK or Detroit in the US. This rapid expansion is forecast to stabilise by 2050 at 11 billion, a 100% increase from 1998. Enormous sociological and economic progress must occur to allow such increase without apocalyptic penalty, but of primary importance will be our ability to produce food in suffi cient quantity and of appropriate quality to sustain an acceptable standard of living. ” It is our view that only with the intelligent use of agrochemicals and especially crop protection agents will the food security of future generations be secured. The Farm Ministers of the European Union have agreed to reduce the agrochemicals used in European agriculture, ostensibly to enhance the protection of human health and the environment. This could lead to the exclusion of any pesticide containing active ingredients considered hazardous, with the result that 15% of existing crop protection agents will be deregistered. As for herbicides, this could lead to the loss of active ingre- dients with an auxin - type mode of action, in addition to pendimethalin, linuron and ioxynil. All valuable components of our crop protection armoury. Ironically, even though Preface ix these active ingredients could be banned for use by European farmers, we as consumers will still be eating perfectly safe food imported from non - European countries using the very same chemicals! This directive comes at a time when European politicians are, quite rightly, expressing concerns about food security and rising food prices. Yet, this decision could result in much lower crop yields, probably leading to further increases in food costs. It is hoped that the European Parliament will eventually take a more balanced view and that common sense and a pragmatic agricultural solution will prevail. It is our view that more work is needed to communicate to the general public and EU legislators the distinction between hazard and risk. Thus, the caffeine in our morning coffee or the addition of salt to our food can kill (hazard), but the dose ingested will not (risk). Similar arguments can be presented for the natural but potentially toxic secondary metabolites that feature in our everyday diet, such as in potatoes or nuts. Or, applying the same logic to synthetic chemistry, to shampoos, aspirin and paracetamol. The latter is a most interesting and relevant example of a synthetic molecule that is widely used, since it is well established that paracetamol is hazardous, as it can cause fatal liver damage at high concentrations, but the actual risk of the dose needed to cure a headache is very low. We hope that this volume will become a useful resource for those working in the plant protection industries and for advanced level undergraduates and postgraduate students of agricultural chemistry, plant physiology and biochemistry, and applied biology. Andrew H. Cobb John P.H. Reade Reference Copping , L. ( 2001 ) The Crop - Protection Industry . C hemistry and Industry , Issue 16 , 500 – 501 , 20 August 2001. Chapter 1 An Introduction to Weed Biology 1.1 Introduction The human race has been farming for over 10,000 years. Weeds would have been an unwelcome presence alongside crops ever since the fi rst farmers saved and planted seeds in the region that is now present - day Turkey and the Middle East. Indeed, when these early farmers noticed a different plant growing, decided they did not want it and pulled it up, they were carrying out a form a weed control that is still used today: hand roguing. But what are weeds? Weeds are all things to all people, depending on the viewpoint of the individual. To some they are plants growing where they are not wanted; to others they are plants growing in the wrong place, in the wrong quantity, at the wrong time; and to some they are regarded as plants whose virtues have yet to be fully discovered! The need to control weeds only arises when they interfere with the use of the land, and this is usually in the presence of a crop, such as in agriculture and horticulture. Weed control may also be necessary in other situations including amenity areas, such as parks and lawns, in water courses, or on paths and drives where the presence of plants may be regarded as unsightly. It should not be overlooked, however, that weeds contribute to the biodiversity of ecosys- tems and should only be removed when fi nancial or practical implications make their presence unacceptable. With this in mind an appropriate defi nition of a weed is: Any plant adapted to man - made habitats and causing interference of the use of those habitats (Lampkin, 1990 ) 1.2 Distribution On a global basis only about 250 species are suffi ciently troublesome to be termed weeds, representing approximately 0.1% of the world ’ s fl ora. Of these, 70% are found in 12 families, 40% alone being members of the Gramineae and Compositae. Interestingly, 12 crops from 5 families provide 75% of the world’ s food and the same 5 families provide many of the worst weeds (Table 1 .1) . This implies that our major crops and weeds share certain characteristics and perhaps common origins. Herbicides and Plant Physiology, Second Edition Andrew H. Cobb and John P.H. Reade © 2010 A.H. Cobb and J.P.H. Reade. ISBN: 978-1-405-12935-0 1