Principles of Weed Control: 4th edition

By California Weed Science Society

The is a weed management book with a focus on California's unique mix of crops, but with relevance to other areas as well. The book provides the basics of weed management in agronomic crops as well as tree and vine crops, vegetable crops and turf and landscape. Featured also are aquatic weed management, forestry and range management as well as ind

• Language: English
• Publisher: California Weed Science Society
• Release date: Dec 5, 2014
• ISBN: 9780692354902

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Copyright © 1985, 1989, 2002, 2014

By the California Weed Science Society

All Rights Reserved.

ISBN: 978-0-692-30482-2

No part of this book may be reproduced, stored, introduced into a retrieval system, or otherwise copied in any form electronic or printed, without the prior written approval of the publisher, except in brief quotations in reviews or citations.

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The fourth edition of Principles of Weed Control is dedicated to all of our members that are no longer with us. We miss them as friends and colleagues that brought so much to our lives and to the Society. They are not forgotten. The list below is just the few that were remembered and celebrated during the business banquet at one of the annual conferences. We know there are many others and we apologize for not having their names to add to this list.

Sam Armentrout

Warren E. Bendixon

James Dewlen

Kenneth W. Dunster

Bill B. Fisher

Don Grivna

Elaine Hale

John Inman

W.B. (Jim) McHenry

Larry Mitich

Neil Phillips Sr

Ed Stillwell

Alvin (Jack) Warren

Tom Yutani

Contents

COVER

TITLE PAGE

COPYRIGHT

DEDICATION

Preface to the Fourth Edition

List of Authors

Introduction

CHAPTER 1. Plants

PLANT STRUCTURE

PLANT FUNCTION

PLANT GROWTH AND DEVELOPMENT

ENVIRONMENT AND PLANT GROWTH

CHAPTER 2. Weed Biology and Ecology

WHAT MAKES A PLANT A WEED?

WEED IDENTIFICATION

Monocot (grass) weed identification

Eudicot (broadleaf) weed identification

WEED CLASSIFICATION

WEED BIOLOGY

THE ECOLOGICAL IMPACT OF WEEDS

CHAPTER 3. Cultural, Mechanical, and Physical Methods of Weed Management

CULTURAL CONTROL

MECHANICAL CONTROL

PHYSICAL CONTROL

CHAPTER 4. Biological Control Methods

TYPES OF BIOLOGICAL CONTROL

TYPES OF BIOLOGICAL CONTROL ORGANISMS

TYPES OF DAMAGE FROM BIOLOGICAL CONTROL AGENTS

STEPS IN A CLASSICAL BIOLOGICAL CONTROL PROGRAM

ADVANTAGES OF BIOLOGICAL CONTROL

LIMITATIONS OF BIOLOGICAL CONTROL

THE FUTURE OF BIOLOGICAL CONTROL OF WEEDS

CHAPTER 5. Chemical Control Methods

INTRODUCTION TO CHEMICAL CONTROL

SOIL APPLICATIONS

FOLIAR APPLICATIONS

ADJUVANTS

APPLICATION METHODS AND EQUIPMENT

SOIL INCORPORATION

IRRIGATION

HERBICIDE SYMPTOMS AND DIAGNOSIS OF FIELD INJURY

SOIL FUMIGATION

CHAPTER 6. Herbicides

HERBICIDE CLASSIFICATION SYSTEMS

MODE OF ACTION

CHAPTER 7. Herbicide Resistant Weeds and Crops

HERBICIDE TOLERANT AND RESISTANT WEEDS

MANAGEMENT OF HERBICIDE-RESISTANT WEEDS

HERBICIDE-RESISTANT CROPS

CHAPTER 8. Registration, Regulation, and Safe Use of Pesticides

REGISTRATION AND REGULATION OF PESTICIDES

REGISTRATION OF PESTICIDES IN CALIFORNIA

ENFORCEMENT OF PESTICIDE LAWS AND REGULATIONS

WORKER HEALTH AND SAFETY

PESTICIDE STORAGE, TRANSPORTATION, AND DISPOSAL

ENVIRONMENTAL MONITORING AND PEST MANAGEMENT

ENDANGERED SPECIES PROTECTION

LEGISLATIVE EFFECTS ON PESTICIDE REGISTRATION AND USE

DIVISION OF OCCUPATIONAL SAFETY AND HEALTH

SAFETY AROUND AGRICULTURAL AIRCRAFT OPERATIONS

CHAPTER 9. Environmental Fate of Herbicides

INTRODUCTION

DISSIPATION AND MOVEMENT OF APPLIED HERBICIDES

OFF-SITE MOVEMENT OF HERBICIDES: PATHWAYS AND MITIGATION

HERBICIDE METABOLISM IN PLANTS

CHAPTER 10. Agronomic Crops

INTRODUCTION

ALFALFA (Medicago sativa)

SMALL GRAINS (Wheat [Triticum aestivum and T. durum], Triticale (× Triticosecale) Barley [Hordeum vulgare], Rye [Secale cereal] and Oats [Avena sativa])

CORN (Zea mays)

COTTON (Gossypium spp.)

DRY BEANS (Phaseolus spp.)

IRRIGATED PASTURES

RICE (Oryza sativa)

SAFFLOWER (Carthamus tinctorius)

SORGHUMS (Sorghum bicolor)

SUGAR BEETS (Beta vulgaris L.)

CHAPTER 11. Vegetable Crops

INTRODUCTION

ASPARAGUS FAMILY—ASPARAGACEAE

CARROT FAMILY—APIACEAE

COMPOSITE (SUNFLOWER) FAMILY—ASTERACEAE

CUCURBIT FAMILY—CUCURBITACEAE

GOOSEFOOT FAMILY—CHENOPODIACEAE

GRASS FAMILY—POACEAE

LILY FAMILY—LILIACEAE

MORNING GLORY FAMILY—CONVOLVULACEAE

MUSTARD FAMILY—BRASSICACEAE

NIGHTSHADE FAMILY—SOLANACEAE

CHAPTER 12. Tree, Vine, and Soft-Fruit Crops

THE TREE, VINE, AND SOFT-FRUIT INDUSTRY

PLANTING AND MANAGEMENT SYSTEMS

IMPACT OF WEEDS ON CULTURAL PRACTICES

UNIQUE WEED PROBLEMS

METHODS USED IN WEED MANAGEMENT

MECHANICAL WEED CONTROL

CHEMICAL WEED CONTROL

CULTURAL WEED CONTROL

CHAPTER 13. Weed Control in Ornamental Production, Landscape Plantings and Turfgrass

WEED MANAGEMENT METHODS

ORNAMENTAL PRODUCTION

LANDSCAPE PLANTINGS

TURFGRASS

CHAPTER 14. Forest, Rangeland, and Habitat Conservation Lands

FOREST VEGETATION MANAGEMENT:

WEED MANAGEMENT ON RANGELAND

HABITAT CONSERVATION LANDS:

CHAPTER 15. Terrestrial Non-Crop Areas and Aquatic Weed Control

TERRESTRIAL NON-CROP AREAS

AQUATIC VEGETATION MANAGEMENT

Glossary

Useful Conversion Factors

Preface to the Fourth Edition

The following is from the Preface to the First Edition:

"Early in 1982, a conclave of unsuspecting souls decided that the sum total of knowledge available within the membership of the California Weed Conference (the original name of CWSS—editor’s note) could form the basis for the publication of a textbook dealing primarily with the applied aspects of weed control in California. They speculated that the end result would benefit anyone interested in pursuing or continuing a career in weed science and, further, that such an undertaking could be accomplished in a relatively short period (i.e less than two years). Although the goal never changed, a number of editorial and publication schedules did, and the following comments are deemed appropriate to indicate to the reader the basic thinking that accompanied the many months of dedicated effort represented by this book.

The purpose of this textbook is to provide access to the fundamental principles and concepts of weed control in California as understood by individual experts and based on their research, experimentation, observations, and practical experience. Some 70 authors contributed to the success of this book. Their individual areas of expertise and writing styles are reflected within their respective chapters and sections."

As much as things have changed enormously in the past 32 years, some things are the same. This fourth edition has 61 authors, several of whom were contributors to previous editions, some to all editions. We started this revision in 2009 and it has taken us more than five years to put this together. We believe that the result has been equal to all three previous editions, e.g. the most accurate, field-tested information on managing weeds and invasive plants in all of the varied farming, domesticated and natural ecosystems in California.

Most importantly, Principles of Weed Control is now published solely in electronic format. We have struggled for years with printing, warehousing, and selling this book. So now we have entered the 21st Century. An E-book will allow us to make it available at low cost, eliminate burdensome handling issues and make revisions far easier for subsequent CWSS Boards of Directors and members. Many people have contributed to this book beyond the authors listed in the publication. The Co-Chairs of the Editorial Committee extend our heartfelt gratitude to Dr. Thomas Miller, the textbook committee members, Chapter lead authors, the Board of Directors of CWSS, our business office staff (the PAPA ladies), and all of the readers of this book that have made continuing this marvelous ordeal worthwhile. We also recognize that we stand on the shoulders of giants; this book, indeed this field of Weed Science would not exist without the dedication and hard work of people such as E.W. Hilgard, F.J. Smiley, Walter Ball. W.W. (Doc) Robbins, Alden Crafts, William A Harvey, and many others who created the field now known as Weed Science starting over a century ago.

Steven A. Fennimore and Carl Bell,

Co-Chairs of the Textbook Committee

List of Authors

Principles of Weed Control, 4th Edition

Brenna Aegerter

University of California, Stockton, CA

Harry S. Agamalian

Emeritus, University of California, Salinas, CA

Norman B. Akesson

Emeritus, University of California, Davis, CA

Kassim Al- Khatib

University of California, Davis, CA

Lars Anderson

Retired, United States Department of Agriculture, Agricultural Research Service, Davis, CA

Mohammad Bari

Artichoke Research Advisory Board, Salinas, CA

Carl Bell

Emeritus, University of California, San Diego, CA

Andre Biscaro

University of California, Lancaster, CA

Ray Brinkmeyer

Dow AgroSciences, Indianapolis, IN

Mick Canevari

Emeritus, University of California, Stockton, CA

David Cheetham

Helena Chemical Co., Chico, CA

Lawrence Clement

University of California, Fairfield, CA

Surendra Dara

University of California, San Luis Obisbo, CA

J.S. Davy

University of California, Red Bluff, CA

Oleg Daugovish

University of California, Ventura, CA

Joseph M. DiTomaso

University of California, Davis, CA

Matt Ehlhardt

The Lyman Group, Woodland, CA

Robert Ehn

California Garlic and Onion Research Advisory Board, Clovis, CA

Clyde L. Elmore

Emeritus, University of California, Davis, CA

Steven A. Fennimore

University of California, Salinas, CA

Julie Finzell

University of California, Bakersfield, CA

Albert J. Fischer

University of California, Davis, CA

Bradley D. Hanson

University of California, Davis, CA

Will Harrison

Target Specialty Products, Santa Fe, CA

Kurt Hembree

University of California, Fresno, CA

James E. Hill

University of California, Davis, CA

Jodie S. Holt

University of California, Riverside, CA

Bob Hutmacher

University of California, Five Points, CA

John Jachetta

Dow AgroSciences, Indianapolis, IN

Marie Jasieniuk

University of California, Davis, CA

Amit J. Jhala

University of Nebraska, Lincoln, NE

Scott Johnson

Wilbur-Ellis Co., Stockton, CA

Thomas Kearney

Emeritus, University of California, Woodland, CA

Don E. Koehler

Retired California Department of Pesticide Regulation, Sacramento, CA

Guy B. Kyser

University of California, Davis, CA

Michelle LeStrange

University of California, Visalia, CA

W. Thomas Lanini

Emeritus, University of California, Davis, CA

Benn W. Laverty III

California Safety Training Corporation, Bakersfield, CA

Milton E. McGiffen, Jr.

University of California, Riverside, CA

Alan McHughen

University of California, Riverside, CA

Gene Miyao

University of California, Woodland, CA

Richard Molinar

Emeritus, University of California, Fresno, CA

Robert Mullen

Emeritus, University of California, Stockton, CA

Doug Munier

Emeritus, University of California, Orland, CA

Robert F. Norris

Emeritus, University of California, Davis, CA

Joe Nunez

University of California, Bakersfield, CA

Patrick O’Connor-Marer

Retired, University of California, Davis, CA

Steve Orloff

University of California, Yreka, CA

Jutta Pils

DuPont Crop Protection, Newark, DE

Michael J. Pitcairn

California Department of Food and Agriculture, Sacramento, CA

John Roncoroni

University of California, Napa, CA

Anil Shrestha

California State University, Fresno, CA

Lincoln Smith

USDA Agricultural Research Service, Albany, CA

Richard Smith

University of California, Salinas, CA

Scott J. Steinmaus

California Polytechnic State University, San Luis Obispo, CA

C. Scott Stoddard

University of California, Merced, CA

John Troiano

California Department of Pesticide Regulation, Sacramento, CA

Cheryl Wilen

University of California, San Diego, CA

Rob Wilson

University of California, Tulelake, CA

Dale M. Woods

California Department of Food and Agriculture, Sacramento, CA

Steven D. Wright

University of California, Visalia, CA

Introduction

Scott J. Steinmaus¹ and Carl E. Bell²

1. California Polytechnic State University, San Luis Obispo, CA

2. Emeritus, University of California, San Diego, CA

The fourth edition of Principles of Weed Control continues the efforts of the members of the California Weed Science Society (CWSS) to provide the most current, accurate and high quality information on weeds and their management in California and beyond. The book relies on the individual and collective knowledge of 61 authors, each an expert in their particular corner of the weed science universe in the most diverse state in the USA. This is more than a small challenge:

• California has the highest value agriculture in the US ($45 billion in farm gate sales in 2013)

• California has the largest population of any state (35 million in the 2010 census)

° Which means the most number of roads, housing developments, businesses, schools, shopping centers, etc

• California has the greatest habitat, climatic and biological diversity of any state

All of these factors in California create weed problems requiring unique management strategies and methods.

As with previous editions, we have made some changes. Chapter 3, Vegetation Management Systems, has been dropped because it duplicated information and concepts found in several other chapters. Chapters 4 and 5 have been combined to have one chapter (Chapter 3 in this edition) on Cultural, Physical, and Mechanical Control. Chapter 7, Herbicide Tolerant Crops and Weeds (Chapter 9 in the third edition) now includes information on herbicide resistant weeds, which was not a significant problem when the third edition was written. Chapter 9 is new and covers the important subject of Environmental Fate of Herbicides. The Ornamentals and Turfgrass Chapters have been combined (Chapter 13 in this edition).

HISTORY OF WEEDS

Weeds have been with us since humans first began domesticating wild plants and creating food crops many millennia ago. As this domestication progressed, it must have soon become apparent that any other plant species present among the food plants was reducing the amount of useful food produced. A simple and common definition in use today is that a weed is a plant out of place, a logical conclusion of anyone that has tried to grow something useful or edible.

Efforts to minimize the growth and numbers of unwanted plants and to maximize the production of useful plants surely began in early historical times. But there does not seem to be much evidence that weed control practices changed much for millennia; people relied peincipally on hand weeding and plowing. The single most important weed management improvement was drill seeding of crops invented by Jethro Tull in England in 1701. Having crops in rows let farmers recognize the weeds, e.g. the ones not in rows, which made hand weeding much easier and far less damaging to the crop.

A detailed history of the worldwide development of weed control is not the aim of this publication. Excellent general histories are available elsewhere. There are, however, milestones in the development of weed science, particularly in California, which should be mentioned.

HISTORY OF WEED SCIENCE

Although for many people weed control began with the development of 2,4-D in 1945, there were important beginnings long before. The recognition by botanists of weeds as a category of the plant world began at least a century ago. Publications on identification of weeds began to appear in the United States before 1900. Some included limited discussions of control methods. These bulletins were often published to meet farmers’ needs and the control information was based on field experiments rather than research programs. Two of the earliest publications on weed control in the state were The California Agricultural Experiment Station Report for 1890, which included a 15-page section on The Weeds of California by E. W. Hilgard, and, in 1911, Frederic T. Bioletti’s California Agricultural Experiment Station Circular 69 titled The Extermination of Morningglory (e.g. field bindweed—authors note).

By the 1920s most states had weed manuals of some sort. In 1922, the California Department of Agriculture published Weeds of California and Methods of Control by F. J. Smiley, with contributions by members of the department. This was succeeded by the 1941 publication of Weeds of California by W. W. Robbins, Margaret K. Bellue, and Walter S. Ball. In the forward to that publication, W. C. Jacobsen stated, The year 1922 marked the beginning of a real and active interest in weeds and weed control on the part of official agencies in California.

It was in 1922 that W. W. (Doc) Robbins came from Colorado to teach botany at the University of California at Davis. His background had included work on weed control, and in 1929 he persuaded the California Department of Agriculture to bring Walter S. Ball from Colorado to act as the department’s specialist in that subject. This early core group was further strengthened in 1931 when Alden S. Crafts was hired in the botany department as a full-time research botanist to work on weed control. Thus, by 1931 California had a nucleus of trained people to develop a weed-control program that included teaching, research and regulation.

Non-selective Weed Control

Let us see where weed control stood in the early 1930s—some 80 years ago. The conversion of agricultural systems to tractor power was proceeding rapidly, making possible new improvements in cultivation equipment, much of which was developed by the farmers themselves to meet their needs. Different kinds of blades and the rod weeder developed earlier could now be used for numerous weeding chores on a large scale. The potential for using tractor-mounted sprayers to treat large areas was being realized.

Herbicides were just then beginning to become part of farming practice. Selective weed control in crops with herbicides was not yet an accepted practice, although copper sulfate, iron sulfate, and sulfuric acid had been tested. Limited used was made of sulfuric acid on cereals and onions but the problems associated with handling a strong acid prevented wide-scale use. Paul Sharp, Director of the UC Agricultural Experiment Station presented a detailed chronological (1931–1950) report on research work by UC Davis scientists on these subjects to the attendees of the 3rd California Weed Conference in 1951. Fortunately this presentation was captured in the Proceedings of the conference. It was clear that diligent efforts were being made to solve weed problems for farmers, public agencies, and private industry from the beginning of weed science in California, an effort we try to maintain today.

Nonselective herbicides, such as arsenic, orchard heating oil, salt (sodium chloride), sodium chlorate, and carbon bisulfide as a fumigant, were the available remedies, primarily for perennial weeds. The application rates of these compounds seem tremendous as compared to today’s herbicide use rates. Salt was used at 20 tons/A or more, sodium chlorate at 600–1000 lb/A, carbon bisulfide at 320 gal (3200 lb)/A, heating oil at 100–300 gal/A, and arsenic in various forms at from several pounds as sodium arsenate to hundreds of pounds as dry white arsenic.

Biological control was under investigation; the first release of insects for weed control in the United States occurred in 1946 (Chysolina beetles for control of Klamath weed (Hypericum perforatum)). Work in California was motivated by successes in the control of prickly pear cactus with an introduced insect in Australia.

Seed and weed laws were enacted in many states by the early 1930s, but ones pertaining to weeds were not always enforced, due in part to lack of suitable herbicides and of adequate funding. However, weed supervisors became important officials in a number of states, particularly in the Midwest, with the authority to enforce control of certain weeds. Specific weeds were often targeted for eradication, and sodium chlorate and carbon bisulfide were used in large quantities in the Midwest and the West for field bindweed control. The government programs were often used to provide work for the unemployed, and they had the side effect of stimulating research into both weeds and herbicides.

Farmer meetings on the subject of weeds led by UC Agricultural Experiment Station and Extension Service personnel were common in almost every state as farmers became more conscious of losses caused by weeds. Basic farm practices, such as sowing clean seeds, preventing seed formation of existing weeds, cultivation, mowing, and hand hoeing of individual weeds, were emphasized in these meetings. Doc Robbins became famous throughout California for his rallying cry of Eternal Vigilance uttered while brandishing a hoe.

Several state weed conferences were organized in the 1930s, including the Western Weed Control Conference in 1938 (now the Western Society of Weed Science), which was a spin-off from the Western Plant Quarantine Board. This conference exhibited recognition of the importance of weeds and the value of exchanging information among weed workers. Most of the 11 western US Land Grant Universities with experiment station programs had at least part-time weed research workers in their botany or agronomy departments. In 1935, in response to congressional pressure and funding, the United States Department of Agriculture (USDA) set up a regional weed-research project with field research stations in five states.

Selective Herbicides

Thus, by the start of the 1940s, there had begun a steady development of weed science in all of its aspects. This advance was slowed with the advent of World War II. The war years, nevertheless, brought the beginning of a revolution in weed science that stemmed from plant hormone research begun in the 1930s. The potential of vegetation control with chemicals was recognized and investigated under wartime secrecy. In late 1944, the secret was out: (2,4-dichlorophenoxy) acetic acid (now known as 2,4-D) had been found capable of destroying annual weeds at rates of a few ounces per acre, and certain perennial weeds at slightly higher rates. Moreover, it could be used selectively to control broadleaved weeds in corn, small grains, grass pastures, and lawns. The phenoxy compounds were translocated in the plant and could cause destruction of deep roots of perennial weeds; they could be applied at low volume by ground or air. Indeed, they appeared as miracles to those weed workers who had labored with pounds and tons per acre of nonselective, soil-acting herbicides. Weed science was never to be the same again. All that had gone before became ancient history.

Although the development of synthetic organic herbicides had begun in the late 1930s and early 1940s with dinitrophenol salts used as selective herbicides, such chemicals had the disadvantages of relatively high human toxicity, staining of skin and clothing, and the requirement of high volumes of solution per acre to give adequate wetting of the foliage. Nevertheless, prior to the introduction of 2,4-D, they were used extensively in California in small grains, often with spray rigs carrying 1000 gallons of solution and applying 700–1000 gal/A. Despite the availability of 2,4-D, their use on onions continued, since their selectivity was based on selective plant wetting.

The selectivity of the phenoxy compounds was based on physiological factors within the plant, a phenomenon that excited plant physiologists throughout the world and generated large quantities of research. This was a breakthrough for the acceptance of weed research as a valid field of scientific inquiry. There were important facts to be learned from studying the action of herbicides on plants. Much of the work was aimed not necessarily at control of weeds but at understanding the physiology of plants, including both crops and weeds. Thus, weed control became weed science with the infusion of scientific studies of the physiology, biochemistry, and anatomy of weedy plants.

The increased interest in weed control that accompanied the wide acceptance of 2,4-D was not lost on chemical companies looking for postwar products. Screening programs were developed by major chemical and drug companies that previously had never thought to look for herbicidal activity in the multitude of organic compounds they produced or were capable of producing. Early screening and product development of herbicides became the province of the chemicals industry while the academic community moved toward field screening and adaptation to local crop and weed problems and into basic research studies (e.g., mode-of-action research).

New herbicides came on the market in ever-increasing numbers. Use recommendations became a major task for extension services, experiment stations, and the USDA. As farmers were eager to use the new herbicides and industry was interested in getting new products on the market and into use, a major issue regarding herbicide recommendation arose: how much local testing was adequate for such factors as soil type, organic matter, rainfall, irrigation practice, soil and air temperature, crop variety, etc.? Specialists were brought in to help provide such information, with the serendipitous result of broadening the scientific base of weed science. The need for pooling information led to the formation of many state and regional weed-control conferences.

The North Central Weed Control Conference was organized in 1944, the Northeastern in 1947, and the Southern in 1948. These regional conferences came together in 1949 as the Association of Regional Weed Control Conferences and in 1951 initiated the publication Weeds. A joint meeting was held in 1953, and the Weed Science Society of America organized in 1954, with the first meeting being held in 1956.

The California Weed Conference was organized in 1949, largely through the efforts of Doc Robbins and Walter Ball of the California Department of Agriculture. Participation by all interest groups and agencies was encouraged. The meeting was an instant success. Renamed the California Weed Science Society in 1985, the conference has met continuously ever since and will do so as long as there are weeds, weed scientists and weed practitioners in California (e.g forever).

CURRENT AND FUTURE WEED SCIENCE

As the number of herbicides increased and field use expanded, the issues of weed science began to change. Early interest centered on finding herbicides that were effective in controlling weeds, and rates into the tons-per-acre range were accepted to accomplish this purpose. As herbicides that were selective on certain crops were discovered, concerns about crop injury spread. As a result, application rates were refined to provide weed control with crop safety. Wide-scale use of 2,4-D produced concerns about drift to nearby sensitive crops. It should be understood that there had never before been a herbicide the chemical drift of which could damage such crops as grapes, cotton, melons, tomatoes, etc., miles away from the original application point. This hazard resulted in new government regulations and research into application techniques and air-mass movement. Crop-damage evaluations and estimates of crop losses resulting from non-target chemical applications produced a new breed of consultants skilled in legal testimony.

Environmental and food safety concerns

With urbanization of farming communities and areas, agricultural sources of pollutants, which affect air, water, and food, became a concern to the general public. At the same time, public concerns regarding pesticide residues in food were growing. The 1958 Food Additives Amendment to the Food, Drugs and Cosmetic Act of 1938 included the Delaney Clause, which stipulated zero tolerance for potential carcinogens in food. The zero tolerance requirement was difficult to meet because raw foods were not held to the same standard as processed foods and pesticides can now be detected at very low levels. The publication of Rachel Carson’s Silent Spring in 1962 heightened worries about pesticides in food and the environment. Further, a 1993 National Academy of Science (NAS) study found that children were more sensitive to pesticides in their diet than adults were. The unanimous passage of the Food Quality Protection Act (FQPA) by the US Congress in 1996 appeared to be a consequence of the NAS study, along with increasing concerns about pesticides and food safety. FQPA changed how food tolerances for pesticides were calculated, but also targeted certain classes of pesticides. The pesticide groups of greatest concern were the organophosphates (OP’s) and the carbamates, both of which had caused acute toxic problems, including death, for farmers and farmworkers. While most of the OP’s and carbamates, especially the most toxic chemicals, were insecticides, a few are herbicides. Weed scientists with public agencies, universities, private industry and agricultural organizations have devoted many, many hours to develop or retain herbicide registrations for the hundreds of minor crops grown in California.

Seven pesticides (all herbicides) have been detected in groundwater and, consequently, are on the California Code of Regulations Groundwater Protection List. Three of the six are triazines. Uses of these herbicides are strictly regulated to prevent further contamination of groundwater.

Sources of air pollution related to weed management are herbicide drift, dust from tillage operations, and smoke from agricultural burning. Particulate Matter that is 10 microns or less (PM10) is a major constituent of air pollution throughout California. Agricultural sources of PM10 are sufficiently high that it will likely attract more attention from air quality boards across the state as attempts are made to reduce this pollutant.

Socio-economically acceptable weed management tools.

It currently costs $150–250 million and 9 years to develop and register a new herbicide. These figures represent a large investment even for a major corporation. Many of the modes of action discovered in the middle 20th century have been pursued and developed. The next generation of herbicides will require unusual ingenuity on the part of research and development. Add to this the fact that chemical manufacturers focus on large markets, and thus, are not registering minor-use chemicals for specialty crops. This has a significant impact in California because much of agriculture in this state is comprised of specialty crops. Registration of minor-use pesticides has been hastened by the passage of FQPA, which ultimately sets guidelines for EPA registrations. However, the net effect of FQPA is to push tolerances for pesticide residues in food even lower than present levels. All these forces are likely to reduce herbicide availability for future California growers.

Developing new technologies, refining old technologies, or utilizing all technologies in new ways are likely approaches for the future. Soil steaming, flaming, polyculture, mulching, solarization, are but a few of the older methods that might be refined, especially for the ever-expanding organic crop production systems. Chapter 3 provides a comprehensive review of these methods that are still a mainstay of production agriculture. Computer controlled robotic cultivation for weed control is on the cusp of becoming reality in California as an alternative to herbicides and hand labor. Genetically engineered herbicide resistant crops (HRC) have shown promise, but not without controversy or serious unexpected problems (see Chapter 7 for a thorough review of this topic). Several economically viable examples of classical biological control have been developed for weeds on non-cultivated land (see Chapter 4). However, classical biological control has traditionally been thought to act too slowly and provide insufficient control for the demands of crop production.

A worldwide market is a reality, placing an even greater emphasis on efficient crop production and pest management in order that California and the U.S. remain competitive. As more food comes into the state from different countries, there will likely be an increased demand by the public to verify that food was produced according to California or U.S. food safety regulations.

The portion of the labor force actually working on the farm in California is currently below 2%, and continues to decrease. These shifting demographics will further detach society from agriculture. Consequently, society may become less sympathetic to the constraints under which a grower must operate. Fewer herbicides, the demand for greater efficiency, and fewer people to carry out the actual farm operations will be a challenge to California’s future agriculturists.

A biological and ecological understanding of weeds

A complete biological and ecological understanding of weeds will identify their weaknesses and strengths (see Chapters 1 and 2). With this information we can focus control efforts on weed weaknesses and not waste efforts on their strengths. Work in this area will likely focus on mechanisms of weed seed dormancy, weed-crop competition for resources, seedbank and population dynamics, population genetics of weeds, and allelopathy in weeds and crops. An objective of the ecological approach might seek to more efficiently close all non-crop niches in time and space that would otherwise be occupied by weeds. The development of predictive models with this information will help weed control practitioners to anticipate weed problems rather than react to them.

In non-crop and natural areas, weed control frequently fails because of poor understanding of weed biology and ecology. For example, control measures are applied to annual weeds that have already set seed or to perennial weeds in such a way as to increase rather than diminish plant populations. A particular problem is applying the wrong herbicide or the right herbicide in the wrong amount or at the wrong time. These problems are mostly a matter of education and developing weed management professionals in these areas. Education and professional development are principle components of the CWSS mission.

Precision timing and placement of management measures

Future weed control will apply control measures only when and where weeds are troublesome. In crops, control measures need only be applied for some critical period that is much less than the entire season. A model, called the critical period for weed control identifies the time when weed control should begin and end for a given crop. Even though critical periods have been established for several cropping systems, they have not been broadly adopted. The reasons for this lack of adoption probably include the lack of precise fit of a model to a particular site and also the desire of many farmers to try to achieve 100% weed control in most situations. In non-crop areas, such as ornamental landscapes, industrial areas, roadsides and natural areas the goal is to balance weed control with environmental concerns. This will require a continued emphasis on precision in weed control practice and improved decision making by practitioners.

Global Positioning Systems (GPS) and Global Information Systems (GIS) technologies are being utilized for precision placement of herbicides. Technologies will probably be further refined which take into account seed dispersal due to tillage or harvesting so that precision preemergent treatments can be applied. Current technologies utilize chlorophyll-sensing electric eyes to distinguish weed foliage from bare ground in vineyards and orchards enabling a rapid-fire valve cartridge to spray only the weed foliage and not bare ground. Future technologies will likely use leaf color, leaf shape or leaf temperature to distinguish between weed and crop foliage. Nozzle manufacturers will continue to work on technologies that reduce the potential for off target applications (i.e. drift). The goal of such technologies is to ensure that herbicide applications go only where they are intended. Other future approaches to weed management might be further improvements in precision placement of water and nutrients so that only the crop has access to these resources.

CONCLUSION

Regardless of the actual approach taken in the future, it will have to operate within the constraints of the current socio-political atmosphere and available technology. The most efficient solutions to future problems in weed science are likely to come from those who have a complete understanding of the entire ecosystem, whether it is agricultural, natural or domesticated. Many of our weed problems are the direct and indirect result of our ancestors’ attempts to manage weeds. Similarly, our efforts to manage weeds will almost certainly create unique problems with which succeeding generations will have to contend.

Thus, weed science has come of age, a discipline in its own right. Much of its strength comes from its foundations in basic sciences. Its vitality comes from the cooperation of the many sectors involved—university, regulatory, practitioners, and industry. We believe that the readers of this book will gain a better understanding of weeds, weed management and weed science. Only the well informed are going to participant in new solutions to weed problems.

CHAPTER 1

Plants

Jodie S. Holt¹

1. University of California, Riverside, CA

Botany is the field of basic science dealing with the study and inquiry into the form, function, development, diversity, reproduction, evolution, and uses of plants and their interactions within the biosphere (Botanical Society of America 2010). Applied botanists, such as weed scientists, use basic information about plants to solve vegetation-management problems. This introductory chapter presents a brief overview of plant characteristics, which must be understood in order to develop effective weed management strategies.

PLANT STRUCTURE

The classification of living organisms has undergone major revision in the past decade due to new molecular tools and DNA-based evidence. The highest level of organization, above the Kingdom, is now the Domain. Bacteria and Archaea are the two prokaryotic Domains, which contain only single-celled organisms that lack a nuclear membrane and organelles. Eukarya is the eukaryotic Domain, in which four kingdoms are recognized: Fungi, Animalia, Protista, and Plantae (although some systematists no longer recognize Protista). All terrestrial plants, including most weeds, are in kingdom Plantae. Algae are simple photosynthetic organisms classified either as Bacteria (single-celled blue-green algae) or Protista (multicellular seaweeds); some algae are considered weeds, as well (see Chapter 15). Embryophytes, or higher plants, form embryos, and most have vascular or conducting tissues. While some non-vascular plants can be weedy (e.g., mosses and liverworts), most weeds are in the category known as higher vascular plants, or seed plants, which includes both gymnosperms and angiosperms. These plants are highly complex and composed of different kinds of cells, tissues, tissue systems, and organs.

Plant cells

The fundamental structural unit of a plant is the cell (Figure 1). Cells of higher plants are compartmentalized into discrete, membrane-bound particles called organelles, each of which has a unique structure and function. The organelles of a typical cell are the nucleus, containing the genetic material (DNA) of the plant and acting as the control center of the cell; mitochondria, the site of respiration; chloroplasts, the site of photosynthesis; vacuoles, fluid-filled organelles that sequester and store waste materials; and endoplasmic reticulum, which functions in cell wall synthesis. A nonliving cell wall surrounds plant cells and provides structure, while a plasma membrane (plasmalemma) occurs inside the wall and completely surrounds the cell contents, or cytoplasm. The plasmalemma is differentially permeable and is the major barrier to movement of solutes into the cell. Other parts of a typical cell are plasmodesmata, ribosomes, Golgi bodies, microbodies, and other types of plastids (Figure 1).

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Figure 1. A typical plant cell and its parts.

There are many different types of plant cells that perform a variety of different functions. At maturity, some cells are still living (they contain cytoplasm), while others are dead. Collectively, all nonliving parts of plants, including cell walls, intercellular spaces, and nonliving cells, are called the apoplast, while all living portions of the plant, including the plasmalemma and cytoplasm of living cells, are called the symplast. In general, water and solutes can flow freely through apoplastic regions of a plant, while flow into symplastic regions is regulated and ultimately requires passing through the plasmalemma. Herbicide uptake into cells is also subject to regulation by the plasmalemma. Herbicides exert their effect on plants by disrupting specific cellular and molecular sites and processes.

Plant tissues

Groups of cells with a common origin that perform a particular collective function are called tissues. The major plant tissues are meristematic tissues, the sites of growth and differentiation; epidermis, which covers the entire plant body and protects it from desiccation; vascular tissues (xylem, phloem), which transport water, minerals, and food materials throughout the plant; and ground tissues, which form the bulk of a plant and serve a variety of functions. Many herbicides act on a specific tissue; for example, the auxin-type growth regulators are especially toxic to meristematic tissues. Herbicides contact plants at the epidermis; for a herbicide to injure a plant, it must pass through the epidermis and enter the symplast or be transported through the vascular tissue to another site of action. Thus, basic information about plant structure and function is necessary in order to select appropriate chemicals for weed control.

Plant organs

Organs are the largest structural units of a plant and are composed of groups of tissues that perform a specific function. The aboveground plant organs (stem, leaves, buds, and flowers) are referred to as the shoot system, while the root system is the primary underground plant organ. Angiosperms, the phylum in the plant kingdom containing most terrestrial weeds and crops, are separated into two classes based on the number of cotyledons in the seed. Dicotyledons (dicots), such as wild mustard (Brassica kaber) and bean (Phaseolus spp.), have two cotyledons, while monocotyledons (monocots), such as johnsongrass (Sorghum halepense) and corn (Zea mays), have one. These two large groups of plants typically have different types, arrangements, and locations of organs. Dicots have broad leaves with veins radiating from a midvein, a taproot and/or fibrous root system, and flower parts in multiples of four or five. In contrast, monocots generally have long, narrow leaves with parallel veins, a fibrous root system, and flower parts in multiples of three. The structure of typical dicot and monocot plants is shown in Figure 2. Because of pronounced structural differences between dicots and monocots, weed control methods can often be targeted specifically at one of these groups.

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Figure 2. Structure of (a) a mature dicot and (b) a mature monocot plant.

Roots

The roots of plants provide anchorage to the soil, absorb water and solutes, and conduct this material upward to the shoot via the xylem. Older roots may also act as storage organs for starch, which is synthesized from sugars that are produced in photosynthesis in leaves and transported via the phloem to the root. Roots possess an apical meristem near the root tip from which primary root growth continues to occur during the life of the plant. In addition, lateral roots and sometimes underground shoots originate from a specialized meristematic cell layer called the pericycle (Figure 3).

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Figure 3. (a) Section of a plant showing transport pathways. Symplastic transport from leaves to roots is shown by solid line, apoplastic transport from roots to leaves is shown by broken line. (b) Leaf cross-section showing tissues and cell layers. (c) Stomata, closed and open, showing configuration of guard cells. (d) Vascular tissues in the stem, showing xylem (left) and phloem (right). (e) Root cross-section showing Casparian strip of the endodermis and other tissues and cell layers.

Epidermal tissue comprises the outer cell layer of all roots. Epidermal cells of the root near the tip produce root hairs, which are cellular extensions that increase the surface area, and thus, the absorbing capacity of the root. The vascular tissue in the root, as in all plant organs, is composed of xylem and phloem. Cells of the xylem, tracheids, and vessel elements, are elongated cells that form thick secondary walls (Figure 3). These cells die after the walls are formed, producing the xylem tissue in which water and nutrients are transported throughout the plant. The phloem tissue, generally located to the outside of the xylem, is composed of sieve-tube elements and companion cells (Figure 3). This tissue functions in the long-range transport of food materials, mostly sugars, which are produced in the upper portions of the plant. A major distinction between these two plant tissues is that at maturity, phloem cells are alive (symplastic) while xylem cells are dead (apoplastic); together they form the vascular cylinder of the plant.

Outside the vascular cylinder is a single cell layer, the endodermis, where plants regulate the flow of materials from the soil solution into the vascular tissue. This regulation is critically important to plant survival and occurs by means of specially modified walls of the endodermal cells. These cell walls contain a continuous, impermeable, waxy band, called the Casparian strip (Figure 3), which forces entrance of absorbed solutes through only the living portion, or symplast, of the endodermal cells via the plasmalemma. The cortex, a type of ground tissue, comprises the remainder of the root and functions in food storage.

The two types of root systems in plants are the taproot system, which is typical of dicots, and the fibrous root system, which is typical of monocots (Figure 2). Taproots develop from the seedling primary root and have lateral roots extending from them. Fibrous roots develop as adventitious roots from underground parts of stems after the seedling primary root dies. Many plants with taproots also form fibrous roots that are close to the soil surface. Roots can penetrate deep into the soil; the extent of the root system of a plant usually exceeds that of its shoot. However, fibrous root systems are generally more shallowly rooted than are taproot systems.

Stems

The main axis of the plant shoot is the stem (Figure 2). Stems are divided into nodes, where leaves are attached, and internodes, the areas between nodes. Stems function to support the aboveground plant parts, store and conduct water and solutes from the roots to the shoot, and store and conduct products of photosynthesis from the leaves to other aboveground plant parts and to the roots. In addition, green stems have the capacity to perform photosynthesis. Just as in roots, stems possess an apical meristem at the shoot tip, from which primary growth continues to occur. Axillary buds located in the axils of leaves are also meristematic and grow into lateral branches on the plant (Figure 2). Buds on the plant shoot may grow into vegetative parts (stems and/or leaves) or may undergo transition to the floral state and produce flowers. Buds characteristically possess seasonal periods of inactivity, or dormancy, which are regulated by environmental conditions and internal hormones of the plant.

The internal anatomy of the plant stem is similar to that of the root but is more complex due to the presence of leaves and buds. Plant shoots are covered on their outer surfaces by a waxy cuticle that is secreted by the epidermal tissue. The cuticle is the primary barrier to gas and water movement into and out of the plant and also represents the major barrier to entry of foliar-applied herbicides into plants. Inside the epidermis is a cortex similar to that of the root, except that these cells in young stems may contain chloroplasts and engage in photosynthesis. The vascular bundles of the stem are dispersed throughout the cortex. As the cells of the stem and leaves develop, some differentiate to form cells of the phloem and others to form cells of the xylem. This results in an interconnecting and continuous system for transport between roots and shoots (Figure 3). Stems do not have an endodermis such as is found in roots. The innermost part of dicot stems is the ground tissue known as pith.

Many plants also possess stems that are modified for specialized functions. For example, horizontal stems, such as the underground rhizomes of quackgrass (Elytrigia repens) and the aboveground stolons of strawberry (Fragaria spp.), can reproduce vegetatively by growing from nonflowering buds. Enlarged underground stems, such as the tubers of yellow nutsedge (Cyperus esculentus), the bulbs of onion (Allium spp.), and the corms of wild hyacinth (Dichelostemma pulchella), can store food materials. Other types of modified stems include thorns and vines.

Belowground reproductive structures

Plants can be classified according to whether they complete the life cycle from seed to seed in one year or less (annual plants), live longer than one year but less than two years (biennial plants), or live longer than two years (perennial plants). Perennial plants may reproduce several times before dying and often produce belowground vegetative (nonflowering) structures that store carbohydrates over the winter or dormant season (see discussion of stems and roots, above). In perennial plants where the shoot system dies back each year, belowground structures also possess buds that form new shoots when the growing season resumes. Modified stems, such as rhizomes, tubers, bulbs, corms, as well as long-lived taproots can contribute to vegetative reproduction of perennial plants in this way.

Leaves

The leaves of a plant are attached to the stems at the nodes and consist of a flattened blade, a petiole that supports the blade, and in some plants, stipules at the base of the petiole (Figure 2). In grass plants (monocots), the petiole forms a sheath around the stem for a short distance above the node, while in some dicots, the petiole is lacking entirely and the blade is sessile, that is, attached directly at the node. Leaf blades may be simple, all in one unit, or compound, composed of a number of separate parts called leaflets. There are many types of compound leaves. Although leaf size and to some extent shape may vary considerably with environment, the type of leaf and its arrangement on a plant are often good diagnostic clues to use in plant identification.

The primary function of leaves is photosynthesis, in which light, carbon dioxide (CO2), and water (H2O) are utilized to produce energy-rich carbohydrates. Sandwiched between the upper and lower epidermis of the leaf are two types of ground tissue, both of which function in photosynthesis. The palisade parenchyma, near the upper surface, consists of elongated, closely packed cells, while the spongy parenchyma, near the lower surface, is made up of irregularly shaped, loosely packed cells surrounded by air spaces (Figure 3). Cells of both tissues contain an abundance of chloroplasts. The cuticle and epidermis cover both outer surfaces of the leaf. Movement of gases into and out of the leaf occurs through microscopic pores that are usually most abundant on the lower leaf surface. Two crescent-shaped, osmotically sensitive guard cells surround each pore. These pores and their surrounding guard cells are called stomata and regulate the flow of CO2 into the leaf, and oxygen (O2) and H2O out of the leaf. Thus, CO2 for photosynthesis can enter the leaf directly, without having to pass through the epidermal cells and the cuticle. Vascular tissue is present in leaves as shown in Figure 3.

Flowers

Flowers are organs on the shoots of plants that function to produce a new generation through sexual reproduction, which results in the production of seeds. The parts of a flower are attached to the receptacle, the swollen region at the end of the flower stalk, or peduncle (Figure 4). These parts are attached to the receptacle in whorls, that is, parts of the same type are attached at the same level. The outermost or lowest whorl is the calyx, composed of sepals, which serves to protect the flower when it is in the bud stage. The corolla, composed of petals, is the next whorl, which functions in attracting pollinators to the flower. Inside the calyx and corolla (collectively called the perianth) are found the fertile parts of the flower, the stamens and the pistil(s). Stamens consist of a stalk, or filament, and anther, which produces pollen and ultimately, sperm cells. Pistils are composes of a stigma, the receptive surface where pollen is deposited, stalk, called the style, and ovary, which contains ovules and eggs (Figure 4).

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Figure 4. Generalized picture of a flower, which contains four whorls (rows) of parts: sepals, petals, stamens, and pistils.

Pollination is the transfer of pollen grains from an anther to the stigma of a pistil. When a pollen grain reaches the stigma, it germinates and grows down the style into the ovary, where the sperm cells are released. Fertilization occurs when a sperm and egg cell fuse. A special feature of flowering plants is double fertilization, which occurs when a second sperm cell fuses with two other nuclei in the ovule, forming the endosperm, a nutritive tissue that supports the developing embryo. After fusion of sperm and egg cells, an embryo forms inside the ovule and the ovule develops into a seed. The ovary and sometimes associated structures develop into a fruit that protects and disperses the seeds.

PLANT FUNCTION

The many biochemical reactions occurring in an organism are known as metabolism. These processes occur within organelles in plant cells. Metabolic reactions are divided into anabolic, or synthesis, reactions that require energy, and catabolic, or degradation, reactions that release energy. The major anabolic reactions in plants include photosynthesis and synthesis of nitrogen-containing compounds, such as proteins and nucleotides. Catabolic reactions include respiration and breakdown of other energetic compounds accompanied by the release of available energy. Herbicides generally have a phytotoxic effect on plants by blocking or disrupting one or more metabolic reactions.

Photosynthesis—food production in plants

Through the anabolic process of photosynthesis, green plants convert the light energy of the sun to biochemical energy in the form of carbohydrates. The presence of chlorophyll in chloroplasts of plant cells, as well as light, CO2, and H2O are required for photosynthesis to occur. The general equation for photosynthesis is:

light

6CO2 + 6H2O → C6H12O6 + 6O2

Photosynthesis in higher plants occurs in chloroplasts as a two-fold process involving light-dependent photochemical reactions, the light reactions, and light-independent enzymatic reactions, the dark reactions or Calvin cycle. In the light reactions, energy-rich compounds (ATP and NADPH) are produced that are subsequently used in the dark reactions to produce sugars from CO2. These six-carbon sugars can then be resynthesized into a variety of carbohydrates such as starch, sucrose, or cellulose. The food substances that plants produce become directly or indirectly the food supply of all animals on earth.

Respiration

In order to grow, plants also must catabolize carbohydrates to derive energy for all cellular needs. The process of breaking down a carbohydrate into its simpler components of CO2 and H2O with the release of biologically utilizable energy (ATP) is known as respiration and occurs in the mitochondria of cells. The general equation for respiration is:

C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

Respiration is a complex series of chemical reactions involving many different enzymes and electron carrying molecules. All living cells depend on this process to provide energy for their life processes. Respiration in most organisms is aerobic, utilizing free oxygen from the air to completely oxidize carbohydrates and other fuel molecules. Aerobic respiration is essentially the chemical reverse of photosynthesis.

Transport of carbohydrates

After carbohydrates have been produced, they must be transported to areas where they are either used or stored. If use occurs within the leaf, transport occurs via the cytoplasm and is of relatively short distance. However, if use or storage occurs in other parts than the leaf of production, long-distance transport, called translocation, is necessary. Translocation occurs in the symplastic portion of the plant, which is a continuous system of living plant cells connected by phloem tissue (Figure 3).

The translocation of carbohydrates within plants is explained by the pressure-flow hypothesis, in which the driving force for phloem transport is the mass flow of sugars dissolved in water from regions of high concentration to regions of low concentration. Flow occurs from sources, or areas of production or storage of carbohydrates (photosynthetically active leaves, underground storage structures), to sinks, or areas of use or storage (actively growing shoot tips, actively respiring roots, underground storage structures). Foliar-applied herbicides, such as glyphosate and 2,4-D, penetrate the plant cuticle, enter the symplast by passing into the cytoplasm of epidermal cells, and are translocated throughout the plant by way of the phloem. Information on the primary transport pathways and relative mobility for a number of herbicides is provided in Table 1 of Chapter 6.

Storage organs present an interesting case in transport because they may act as either sources or sinks, depending on the physiological state of the plant (Figure 5). In early spring, when new shoots are being produced from perennial roots systems, underground organs may act as a source, translocating carbohydrates to actively growing parts of the plant. During flowering, which requires large amounts of energy, stored underground carbohydrate reserves are generally at their lowest point. Later in the year, when leaves are producing carbohydrates for storage, the same underground storage organs may act as translocation sinks. Winter dormant periods are when stored reserves are highest (Figure 5). This seasonal cycling of translocation and stored carbohydrate reserves is important to consider in weed control, for it regulates both movement of herbicides in phloem throughout a perennial plant and regeneration of shoots following damage to roots.

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Figure 5. Seasonal progression of stored carbohydrates in roots of perennial plants.

Transport and evaporation of water

Stomata are the openings in plants through which the majority of gas exchange occurs. Guard cells are stimulated to open by sunlight as well as by low internal CO2 concentrations in leaf tissue. Open stomata take in CO2 for photosynthesis and lose water vapor by evaporation from cells inside the stomatal opening. This evaporation from leaves, called transpiration, regulates the temperature of the transpiring organ, which is cooled as water evaporates. However, if the water lost through open stomata is not replaced, guard cells may become less turgid, resulting in stomatal closure (Figure 3). Although plants can wilt if transpired water is not replaced, the regulation of guard cell opening and closing by turgor pressure reduces the chance of plant death from water loss. The amount of water used by some species in transpiration can be very large. For example, many herbaceous crops and weeds can transpire 1 to 2 gallons of water per day. Thus, plants are obviously in a position of compromise for survival. The need for CO2 for photosynthesis must be balanced with the loss of H2O through transpiration. As a result, relatively large amounts of water must be supplied to plants for maximum productivity.

Long distance transport of water and mineral nutrients in plants occurs in response to water-potential gradients produced by transpiration. As water evaporates from a plants’ leaf surfaces, more water moves to those surfaces to fill the deficit, effectively pulling water and dissolved substances through the plant from the soil to the leaves. The movement of water in plants occurs in the apoplast, a continuous system of cell walls, intercellular spaces, and xylem tissue (Figure 3).

Water and nutrients enter roots through root hairs and move towards the vascular cylinder largely through apoplastic walls and spaces. At the endodermis, however, movement of any water solution through cell walls is restricted by the Casparian strip (Figure 3), and therefore, must move into the symplast at the cytoplasm of the endodermal cells in order to reach the xylem. Thus, everything entering the vascular tissue, including soil-applied herbicides such as the triazines and ureas, must first pass through the filtering process of the plasmalemma of the endodermis before it can be transported throughout the plant. Once past the endodermal cells, water and nutrients move into the xylem tissue and are transported apoplastically upward throughout the plant. Although water is used by plants to conduct metabolic processes and for growth, most of the water that moves through plants is lost through transpiration.

PLANT GROWTH AND DEVELOPMENT

Plant growth consists of cell division, enlargement, and differentiation, and occurs in the meristematic regions of the plant. As described above, plant meristems are located at the shoot and root tips, in buds, and in various internal tissues (e.g., pericycle). Thus, any method of weed control that affects meristems may prevent normal growth of plant cells. Plant development is the pattern in which cells become organized into tissues and organs to result in the mature form of the plant. Basic differences in seed and seedling growth and development occur between dicots and monocots, which are important both in weed identification and in selecting weed control methods.

Seeds and germination

Even during storage, a seed is a living unit composed of three basic parts: an embryo, a source of nutrition, and an external covering or seed coat (Figure 6). Carbohydrates are usually stored in cotyledons of seeds of dicots and in the endosperm of seeds of monocots. Although seeds are living, when they are relatively dehydrated they remain dormant, a condition in which all metabolic processes occur at very slow rates. To activate the plant embryo, seeds of most species need only to be saturated with water and placed in a suitable environment, which results in germination. Most weed seeds, in contrast to crop seeds, also require light for germination, which therefore occurs at or near the soil surface. Once the microenvironmental conditions that allow germination to proceed are met, further events occur within the seed that allow the seedling to develop.

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Figure 6. Seed, germination, and emergence of bean, a typical dicot.

Since the plant embryo relies on its own carbohydrate reserves for nutrition, a mechanism is required to mobilize those reserves for use during germination. Very soon after the seed begins to absorb water (imbibition), the activated embryo produces a plant hormone called gibberellin (GA). Gibberellin diffuses from the embryo to the aleurone layer just underneath the seed coat. The presence of GA signals the aleurone layer to produce and secrete an enzyme, a-amylase, which breaks down the starchy cotyledons or endosperm to sugars. Usually by the third or fourth day after imbibition, the embryo has grown in size since it is now able to utilize energy from the catabolized sugars. After about a week, the stored carbohydrates are completely degraded and used, but by that time the embryo usually is no longer dependent on its reserves since the seedling plant has emerged from the soil and has become photosynthetic. The first true leaves derive energy from the environment by photosynthesis and the roots absorb water and minerals from the soil.

While dry seeds in the soil are difficult to kill, there are many steps in the process of germination that may be disrupted (see Chapter 3). Furthermore, the seedling is considered the most vulnerable stage of a plant’s life cycle and is generally the easiest stage to control by physical or chemical means.

Growth

As the seed germinates, the emergence of the primary root, or radicle, from the seed usually occurs first in both dicots and monocots, followed by the expansion of the seedling shoot. Both organs grow rapidly. As described above, both the root tip and the