Cardinal temperatures for plants vary with species (e.g., temperate versus tropical, and cool season...

Cardinal temperatures for plants vary with species (e.g., temperate versus tropical, and cool season versus warm season), stage of development (young tissue is usually more temperature s[r]

Principles of Plant Science Environmental Factors and Technology in

Growing Plants

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Principles of Plant Science Environmental Factors and Technology in

Growing Plants

Dennis R Decoteau

The Pennsylvania State University

Upper Saddle River, New Jersey 07458

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10 9 8 7 6 5 4 3 2 1 ISBN: 0-13-016301-5

To my Pennsylvania family (Chris and Megan) for their patience, understanding, and support,

and

To my New Hampshire family (Mom, Bobby, Hank, Priscilla, Donald, and Carol) for their constant support and encouragement

in bringing up the youngest.

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PART 1 AN OVERVIEW OF THEPLANT SCIENCES 1

CHAPTER 1 Introduction to the Plant Sciences 3

Plants in Society 3Plants in Industries 3Plants in Sciences 4

CHAPTER 2 Plants and Society 7

History of Agriculture 7The First Cultivated Plants 8Development of Agricultural Crops 10Timeline for Domestication of Important Crops 11Contemporary Crop Improvement Programs 11

CHAPTER 3 Plants as Industries 15

Historical Periods of the United States and the Development ofPlant-Related Agricultural Industries 15

Colonists and Early Settlers 15Post-Civil War 16

Pre-World War I 16Post-World War II 16Relatively Recent Time 17U.S Agriculture and Crop Production 17Exports 17

Imports 20Agronomic Segments of Crop Production Industries 20Cereal or Grain Crops 20

Forage Crops 22Fiber Crops 22Tobacco 22Oilseed Crops 22Horticultural Segments of Crop Production Industries 22Vegetable Crops 22

Fruit and Nut Crops 23

vii

Nursery and Greenhouse Crops 24Niche Crops 25

Home Gardens 25The Sciences of Plants 29

CHAPTER 4 The Sciences of Plants 29

Botany 29The “Basic” Botanical Sciences 30Examples of Form and Structure Disciplines 30Examples of Growth and Development Disciplines 30The “Applied” Botanical Sciences or Plant Sciences 31Examples of Commodity-Based Disciplines of Plant Science 32Examples of Cross-Commodity Disciplines of Plant Science 32Plant Classification Schemes 33

Botanical 33Use 34Life Cycle 34Other Classifications 34

PART 2 BASICS OF PLANT GROWTH ANDDEVELOPMENT 37

CHAPTER 5 Introduction to Plant Growth and Development 39

Plant Organs 39Stems 39Leaves 40Flowers 40Roots 40Plant Growth 40Stem Growth 40Leaf Growth 41Flower Growth 41Root Growth 43Determinate Versus Indeterminate Growth 43Measurement of Growth 44

Development 44Seedling Phase 45Juvenile Phase 45Reproductive Phase 45Senescence Phase 46

CHAPTER 6 An Overview of Photosynthesis and Respiration 49

Photosynthesis 49Overview 49

Major Stages of Photosynthesis 54Respiration 60

Overview of Respiration 60Major Phases of Respiration 61Fermentation Metabolism 64

CHAPTER 7 Plant Hormones 67

Definitions 67General Mechanisms of Action 68Classes of Plant Hormones 69Auxins 69

Gibberellins 69Cytokinins 71Abscisic Acid 71Ethylene 72Other Compounds Exhibiting Plant HormoneCharacteristics 72

Practical Uses of Growth Regulators in the Plant Sciences 74Rooting 74

Height Control 75Herbicides 75Branching and Shoot Growth 76Flowering 76

Fruiting 77Ripening 77Fruit Abscission 77Enhance Postharvest Life 78

CHAPTER 8 Some Ecological Principles in Plant Growth

and Production 81

Important Ecological Concepts 81Organism Groupings 81Significant Interactions Among Organisms 83Farm as Ecosystems 84

PART 3 ENVIRONMENTAL FACTORS THAT INFLUENCE

CHAPTER 9 Introduction to the Role of the Environment

in Plant Growth and Development 89

Environment 89Weather and Climate 90Plant Stress 90

Reasons to Study How the Environment Affects Plant Growth 91

Methods for Studying How the Environment Affects Plant Growth and Development 92

CHAPTER 10 Overview of the Aerial Environment 97

The Ecosphere and the Aerial Environment 97Irradiance and Temperature 99

The Earth’s Atmosphere 99Above-Ground Living Organisms 100

CHAPTER 11 Irradiance 103

Background 103Characteristics of Sunlight 103Photochemical Reactions 107Variations in the Light Environment 110Radiation Measurements and Instrumentation 114Effects on Plants 115

Seed Germination 115Uptake of Radiation by Plants 117Ultraviolet Radiation 117

Plant Uses of Radiant Energy 118Distribution of Light in Plant Canopies 130Agricultural Technologies That Affect Light 132Plant Density 132

Weeds 132Row Orientation 133Supplemental Lighting 133Filtering Light 134

Practical Uses of Shade 134Planting Considerations in the Shade Environment 135Pruning and Training 137

Reflective Plastic Mulches 137

CHAPTER 12 Temperature 143

Background Information 143Radiation and Heat 143Temperature Changes with Altitude, Seasons and Latitude,and Time of Day 144

Effects of Temperatures on Physical and ChemicalProcesses 147

Frost Types 147Weather Conditions Causing Frost 148Frost Versus Freeze 148

The Freezing Process 148Length of Growing Season 148Topographic Factors That Affect Frosts and Freezes 149Wind Chill 151

Effects on Plants 151Effects of Temperatures on Biochemical Processes 151Cardinal Temperatures 152

Potential Differences Between Air and Plant SurfaceTemperatures 154

Temperature Effects on Photosynthesis and Respiration 154

Membranes 156Chilling Injury 157Freezing Injury 157Vernalization 159Crop-Specific Responses to Cold Conditions 159High Temperature 162

Temperature Acclimation or Hardening 165Agricultural Technologies That Affect Temperature 165Frost Protection 165

Methods of Alleviating Excessive Heat 179Influence of Vegetation Cover on Surface Temperatures 180

Use of Plants for Energy Conservation Around Homesand Buildings 180

Harvesting According to Accumulated Heat Units 182Commodity Cooling to Extend Postharvest Life 182Harvest Procedures to Reduce Temperature Stress 183Cooling Procedures After Harvest 184

CHAPTER 13 Atmospheric Gases 189

Background Information 189Evolution of the Earth’s Atmosphere 189Some Basic Properties of Gases 190Common Atmospheric Gases 191The Vertical Structure of the Atmosphere 194The Greenhouse Effect 195

Effects on Plants 197Stomata 197Photosynthesis 198Transpiration 200Wilting 200Diseases and Insects 201

Agricultural Technologies That Affect Gases 201Spacing and Crowding 201

Antitranspirants 201Modified Atmosphere Storage of Fruits and Vegetables 202

Air Pollution 202

CHAPTER 14 Air Pollutants 205

Background Information 205Sources of Air Pollutants 205Common Air Pollutants 207Trace Gases 211

Variables That Can Affect the Various Air Pollutants 211Sources of Air Pollution 212

Effects of Topography on Dispersion of Air Pollutants 212Effects of Plants 213

Symptoms 213Factors Influencing Air Pollution Injury to Plants 217Types of Crop Losses Due to Air Pollutants 218Photosynthesis and Dry Matter Production 218Agricultural Technologies That Affect Air Pollution 218Bioindicator Plants for Air Pollutants 218

CHAPTER 15 Mechanical Disturbances 225

Background Information 225Physical Mechanical Disturbance 225Biological Mechanical Disturbance 227Effects on Plants 227

Release of Stress Ethylene by Plants 227Physical Mechanical Disturbances 228Biological Mechanical Disturbances 238Agricultural Technologies and Mechanical Disturbances 244Physical Mechanical Disturbances 244

Biological Mechanical Disturbances 256Special Considerations of Mechanical Disturbances of theRhizosphere 260

CHAPTER 16 Overview of the Rhizosphere 271

Soil 271Soil Types 271

Soil Fertility 273Organic Matter 273Soil Horizons 273Soil-Forming Factors 274The Water Cycle and the Groundwater System 276Plant Roots 278

Soil Organisms and Allelochemicals 279

CHAPTER 17 Water 283

Background Information 284Unique Properties of Water 284Water Potential 286

Forms of Atmospheric Moisture 287The Supply of Water by the Soil 287Effects on Plants 288

Classification of Plants Based on Water Use 288Water Acquisition via Roots 289

Water Movement Through Plants 289Transpiration 290

Measuring Soil Moisture 291Efficient Water Use by Plants 295Drought Stress 295

Flooding and Anaerobiosis 296Acid Deposition 297

Agricultural Technologies That Affect Water 300Methods of Evaporative Control 300Types of Farming Systems Based on Water Use 301Irrigation 303

Types of Irrigation Systems 305Drainage 308

Wind Screens 311Hydroponics 311Hydrophilic Gels 312

CHAPTER 18 Nutrients 317

Background Information 317Methods of Nutrient Acquisition by Plants 317Essential Plant Nutrients 318

Soil pH 321Biogeochemistry 322Effects on Plants 327Critical Nutrient Concentrations 327Crop Removal Values 328

Agricultural Technologies That Affect Nutrients 329Adjusting Soil pH 329

Nutrient Replacement 330Soil and Tissue Analysis 334Rotations 335

Salt Injury 335Phytoremediation 337

CHAPTER 19 Soil Organisms 341

Background Information 341Microorganisms 341Macroorganisms 348Comparative Organism Activity 351Effects on Plants 352

Microorganisms 352Macroorganisms 354Competition for Nutrients 355Agricultural Technologies That Affect Soil Organisms 356Some Generalizations on the Effect of Agricultural Systems

on Soil Organisms 356Effect of Planting New Ground 357Disease Control by Soil Management 357

CHAPTER 20 Allelochemicals 361

Background Information 362Examples 362

Effects on Plants 364Classification and Chemical Nature of Allelochemicals364

Physiological Action of Allelochemicals 365Sources and Names of Allelochemicals 366Agricultural Technologies That Affect Allelochemicals 368Weed Control 368

Cover Crops 368Plant Autotoxicity 368

APPENDIX A Conversion Factors for International System of

Units (SI Units) to and from Non-SI Units 371 APPENDIX B Conversion Factors for Some Commonly Used

Non-International System of Units (Non-SI Units) 375

APPENDIX C Some Physical Constants or Values 377 APPENDIX D Some Prefixes Used to Define Multiples of

International System of Units (SI Units) Conversions 379

APPENDIX E Compilation of Internet Resources 381

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The environmental factors that affect plant growth and developmentare discussed in one of two broad categories: the aerial environment orthe rhizosphere (Fig P.1) The factors covered in the aerial environmentsection include irradiance (light), temperature, gases, air pollutants, andmechanical disturbances The factors covered in the rhizosphere sectioninclude water, mineral nutrients, soil organisms, and allelochemicals Al-though some of the environmental factors (e.g., temperature, water, at-mospheric gases) may affect both the aerial environment and therhizosphere, for discussion they were placed in the section that their ef-fect was considered greater.

Material on each environmental factor covered is presented in threesections The first section provides background information on the spe-cific environmental factor Appropriate physical definitions and expla-nations are presented The second section discusses how theenvironmental factor affects plant growth Direct effects on biologicalprocesses and specific effects on plants are discussed The third sectionillustrates how agricultural technologies affect the environmental factor,especially for the benefit of plant growth and production

xvii

This book is designed for use in plant science (or horticulture) courses that would

be taken before a student enrolls in the various advanced plant production coursessuch as agronomy, crop science, vegetable crops, small fruits, pomology, and flori-culture The material may also be utilized for plant growth and development or ap-plied introductory plant physiology courses taught at universities, junior colleges, orcommunity colleges

The primary emphasis of this book is on the environmental factors and theirrole in plant growth and production Its intent is to provide sufficient introductorymaterial on the various environmental factors and examples of effects of these en-vironmental factors on plant growth and development to facilitate further discus-sions and study

Sources of Additional Information

Complete and exhaustive discussions on the fields of plant science and the influence

of the various environmental factors on plant growth are beyond the scope of anysingle textbook Further information on the plant sciences can be gleaned fromother introductory textbooks on plant science, crop science, and horticulture In-formation from local extension services should serve as important sources of currentlocation-specific marketing and crop production information More in-depth infor-mation on the environment and plant growth can be obtained from plant physiol-ogy, plant ecology, and plant ecophysiology texts and/or articles on specific topics

in scientific journals

Target Audience and Uses of the Book

Aerial Environment

Soil level Rhizosphere

Air pollutants Gases Irradiance Temperature Mechanical disturbances Water

Allelochemicals Soil organisms Gases Water Mineral nutrients Temperature

FIGURE P.1 Examples of environmental factors that can affect plant growth and development.

PREFACE xix

Acknowledgments

Several colleagues provided important feedback I am grateful to Craig Anderson,University of Arkansas; Mark Bennett, Ohio State University; Rebecca Darnell, Uni-versity of Florida; Robert Gough, Montana State University; Wallace G Pill, Univer-sity of Delaware; Robert P Rice, Jr., California Polytechnic State University—SanLuis Obispo; Mark Rieger, University of Georgia; and Sudeep Vyapari, Sam HoustonState University, for their invaluable assistance

My colleagues at the Pennsylvania State University are thanked for their port Special thanks go out to the many students that I have taught for allowing me

sup-to learn along with them Dr E Jay Holcomb, Professor of Floriculture, is gratefullyacknowledged for co-teaching with me the “Environmental Effects on HorticulturalCrop Growth” course that is taught in the Department of Horticulture at Penn StateUniversity Finally, I would like to thank all the editors and professionals at PrenticeHall for helping me pull this together and keeping me on track

About the Author

Dr Dennis R Decoteau is a Professor and the Head of the Department of ture at The Pennsylvania State University His previous academic appointments in-clude serving as Professor and Chair of the Department of Horticulture at ClemsonUniversity Dr Decoteau received his B.S degree in Environmental Studies from theUniversity of Maine at Fort Kent and his M.S and Ph.D degrees in Plant Science with

Horticul-an emphasis in Environmental PlHorticul-ant Physiology from the University of setts He has won numerous teaching and research awards, including the Outstand-ing Teacher Award in Horticulture from Clemson University, the OutstandingResearch Paper on Teaching from the American Society for Horticultural Sciences,the L M Ware Distinguished Research Award from the Southern Region of theAmerican Society for Horticultural Science, and the IV Congreso International DeNeuvas Technologias Agricolas Award Dr Decoteau is the current Vice President,Education Division, and member of the Board of Directors for the American Soci-ety for Horticultural Science and has taught several undergraduate and graduatecourses, including “Environmental Effects on Horticultural Crop Growth.” He haswritten extensively in the scientific press and for the general public and is the author

Massachu-of Vegetable Crops, also published by Prentice Hall.

The New York Times eThemes archive, and other agriculture-related links.

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P A R T

1

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Introduction to the Plant Sciences

1

Plant science is a specialized area of study of botany that emphasizes theuse of plants in agricultural applications This study area of botany is alsoreferred to as the applied phases of plant study, the plant sciences, oreconomic botany The plant sciences can be divided into more special-ized fields that can be broadly categorized by commodity (such as agron-omy, horticulture, and forestry) or cross commodity (such as plantpathology, entomology, integrative pest management) orientation Thisintroductory chapter provides an overview of the historical importance

of plants in the development of societies, industries, and science Thesesubjects will be covered more in-depth in Chapters 2 through 4

Plants in Society

Plants are intimately connected with the development of societies andmany of the fields of science Early humans undoubtedly observed thatplants (a major source of food and other raw products) were greatly af-fected by the environment Seasonal cropping cycles, serendipitous suc-cessful plantings, and beneficial uses of certain plants were noted (aspaintings, drawings, and written and oral stories) by these early ancestorsand passed on to future generations As a result, humans became lessnomadic, which allowed communities and societies to evolve

Plants in Industries

The success of most of today’s agricultural plant production businessesdepends on growing crops to acceptable quality in economical techno-logical ways in the available environment so that the plants can be soldfor a profit (Fig 1.1) Agriculturalists have long known the importance

4 PART 1 An Overview of the Plant Sciences

FIGURE 1.1 A roadside market can be successful if appropriate crops are selected for the

growing environment and if these crops can be sold at a profit.

Courtesy of Dr Mike Orzolek, Penn State University.

of the environment on plant growth, and early farmers were some of the first to nipulate or modify the growing environment to provide a more conducive environ-ment for plant growth Protective shelters were built, water needs of the plant wereprovided for, and sunlight was modified (by shading or by training and pruningplant material) to provide more optimal growing environments to enhance plantgrowth and production

ma-Plants in Science

As growing plants (and animals) became more specialized and agriculture ued to evolve, the understanding of the role of the environment in successful foodand fiber production became more of a science The plant-growing environmentwas determined to be a myriad of environmental factors that directly influenced thephysiology of the plant and how well it grew Botany, or the study of plants, devel-oped as science and investigations on the taxonomy, anatomy, morphology, physiol-ogy, and ecology of plants were institutionalized as fields of advanced study atinstitutions of higher learning

contin-The areas of plant study that placed major emphasis on the use of plants by mans or as agricultural products (or crops) became known as the plant sciences Theplant sciences also became institutionalized with the establishment of the land grantuniversities (with their emphasis on the agricultural and mechanical sciences), andfederal and state funding for the establishment of the various state experiment stationsand extension services The plant sciences evolved to specialized crop commodityfields such as agronomy, horticulture, and forestry, and specialized cross commodityfields such as plant pathology, entomology, and integrated pest management

hu-CHAPTER 1 Introduction to the Plant Sciences 5

Summary

Plant science is a field of botany that emphasizes the use of plants by humans, often

in the context of agricultural production Domesticating plants often coincidedwith or was the precursor for the evolution of societies and communities As plantgrowing became more successful, specialty disciplines such as agronomy, horticul-ture, forestry, pathology, and entomology were developed and institutionalized

Review Questions

1 What are some of the ways our early ancestors passed on information onplants and plant culture?

2 How can the plant-growing environment be manipulated?

3 Define plant sciences

Selected References

Janick, J 1979 Horticultural science 3rd ed San Francisco: W H Freeman.

Norstog, K., and R W Long 1976 Plant biology Philadelphia: W B Saunders Stern, K R 1991 Introductory plant biology 5th ed Dubuque, IA: Wm C Brown.

Selected Internet Sites

www.aces.uiuc.edu/~sare/history.htm/ A Brief History of Agriculture, University ofIllinois College of A.C.E.S

www.agr.ca/backe.htm/ Agriculture and Agri-Food Canada: History, Agriculture andAgri-Food Canada

USDA Economic Research Service

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Plants and Society

2

The human race came into existence about two million years ago, and it

is believed that our earliest ancestors were food gatherers that spent most

of their time hunting animals, catching fish, and collecting edible plants

Ancient cave art often depicted important plants that were used for food

Plants also appear to have been used by primitive humans for uses otherthan food, as flowers for ornamental purposes and plant fiber in clothwere included in some of the burials of 60,000 years ago

It is suspected that humans were in the Americas as early as 15,000years ago The first Americans probably crossed the Bering Strait fromAsia during the most recent Ice Age, following large game during thegame’s migration Eventually the humans settled in the Americas wherelarge numbers of game were present

This chapter provides a summary of the history of agriculture with adiscussion of important events that paralleled the evolution of societies

One of the most important events in the establishment of agriculture wasthe selection and domestication of important plants and the develop-ment of agricultural crops Events that led to the development of agri-cultural crops are presented, including a discussion of contemporarycrop improvement programs

History of Agriculture

Evidence indicates that agriculture and the plant sciences originated ing the Neolithic Age (around 8000 BC) in the semiarid mountainous re-gions near the river valleys of Mesopotamia, in present-day Iraq Aboutthat time, men and women began collecting grain plants and keeping do-mesticated animals such as sheep and dogs

dur-Eventually, through cultivation of plants and domestication of animals,humans were able to produce sufficient food to free them from the con-stant search for their next meal With the development of a dependablefood source, humans began to live together and villages and towns came

8 PART 1 An Overview of the Plant Sciences

into existence Wheat and barley seeds dated at about 6750 BC have been found inevacuations in Iraq Early centers of crop development include the Near East, South-east Asia, Meso-America, and possibly Western Sudan The further domestication ofplants and animals paralleled the development of villages, towns, and cities along theNile around 3500 BC in present-day Egypt The Egyptians are credited with develop-ing technologies for food storage, such as pickling and drying, and the agriculturaltechnologies of drainage, irrigation, and land preparation

The Romans, around 500 BC to 500 AD, developed efficient agricultural duction systems that became the cornerstone of their strength During the collapse

pro-of the Roman Empire (around 500 AD) and before the Renaissance, monasteriesbecame important reservoirs for agricultural skills perfected by the Romans Monas-teries often had cultivated fields of grain and vegetables as well as orchards Themonks in the monasteries maintained important collections of herbal and medici-nal plants Many of the plants kept in cultivation in the monasteries would becomeimportant later during the Renaissance

Because the earliest uses of many agricultural plants were medicinal, herbals aresome of the earliest and most important plant science manuals Their origin traces tothe Greek interest in cataloging and describing plants As originators of the study ofbotany, the Greeks produced writings that listed common plants and often suppliedtheir medicinal usage For much of the Middle Ages, there was little distinction be-tween medicine and botany, as plants were used to cure ills

By the second half of the 1500s botany had established itself as a science Thedrawings of the plants became more accurate, and, with the exploration of newworlds, many new plants were discovered and cataloged It was fashionable duringthat period of history for the aristocracy throughout Europe to maintain largegardens full of exotic plants and to have extensive collections of herbals in their li-braries

The First Cultivated Plants

The tribes in the Old World that engaged in hunting and in gathering wild edibleplants made attempts to domesticate dogs, goats, and possibly sheep as early as 8000

BC Centers of origin of major cultivated crops are found in both the Old and Newworlds (Table 2.1) A plant’s center of origin is the geographical area where a species

is believed to have evolved through natural selection from its ancestors This is alsothe plant’s center of diversity where a pool of genes exists for use by plant breeders

in crop improvement programs

Earliest evidence of plant cultivation in the Americas appears to be about 10,000years ago in South America, where potatoes and a variety of other crops were do-mesticated Agriculture probably began in Mexico about 5000 BC Early cultivatedplants included maize, avocado, squash, and chili peppers Research also indicatesthat agriculture arose very early in the Andean valleys of Peru It seems probable thatagriculture originated independently in at least two regions in the New World—Mexico and Peru—and the earliest site may have been Peru

CHAPTER 2 Plants and Society 9

TABLE 2.1 Centers of Origin of Important Cultivated Plants and Commercially

Important Crops that Originated from These Centers

India (India and Burma)—Important crops that originated from this center include orange, other citrus, mango, black pepper, many legumes, and gourds.

Indo-Malaysia (Indochina, Malaysia, Java, Borneo, Sumatra, and Philippines)—Important crops that originated from this center include giant bamboo, ginger, banana, coconut, and nutmeg.

Central Asia—Important crops that originated from this center include common wheat, pistachio, apricot, pears, apple, garden pea, broad bean, carrot, radish, garlic, spinach, and mustard.

Near East (Asia Minor, Iran, Transcaucasia, and Turkmenistan Highlands)—Important vegetable crops that originated from this center include poppy, fig, pomegranate, cherry, hazelnut, cantaloupe, cabbage, lettuce, and muskmelons.

Mediterranean—Important crops that originated from this center include olive, peppermint, lavender, thyme, sage, rosemary, beet, parsley, leek, chive, celery, parsnip, and rhubarb.

Abyssinia (Ethiopia and Somaliland)—Important crops that originated from this center include wheats, barley, sesame, coffee, okra, and garden cress.

New World

Southern Mexico and Central America (South Mexico, Guatemala, El Salvador, Honduras, Nicaragua, and Costa Rica)—Important crops that originated from this center include chayote, upland cotton, papaya, agave, cacao, corn, common bean, lima bean, sweet potato, pepper, and cherry tomato.

South America (Peru, Ecuador, and Bolivia)—Important crops that originated from this center include lima bean, pepper, coca, tobacco, potato, tomato, and pumpkin.

Chiloé (South Chile)—An important crop that originated from this center is the white potato Brazil–Paraguay—Important crops that originated from the semiarid region include peanut and pineapple; important crops from the tropical Amazon region include manioc and the rubber tree.

Sources: Adapted from G Acquaah, Horticulture: Principles and Practices (Upper Saddle River, NJ: Prentice

Hall, 1999); J Janick, Horticultural Science, 3rd ed (San Francisco: W H Freeman, 1979).

Maize (corn) appears to have been introduced into North America from SouthAmerica about 1,700 years ago and was a relatively minor crop until about 800 yearsago Different varieties of maize with different cob shapes and sizes appeared in dif-ferent regions Europeans recorded the existence of short- and long-season corn va-rieties, but it is not known when or how Native Americans developed these varieties.Also about this time, the common bean was introduced

10 PART 1 An Overview of the Plant Sciences

Development of Agricultural Crops

All of the modern crops (plants used in agriculture) that we produce today had theirearliest beginnings as wild plants Early plant gatherers applied self-serving criteria

to decide which plants to gather Of utmost importance was size As a result of tinually choosing the largest fruit of the wild plants through the years, many of thecrops that underwent domestication have bigger fruits than their wild ancestors had.Another criterion used by plant gatherers was taste Many wild seeds are bittertasting, yet their fruits are sweet and tasty This was important so seed would not bechewed up and eaten and instead would be expelled Other criteria for gatheringwere fleshy or seedless fruit, oily fruits and seeds, and plants with fiber for clothes.Other plants produced fruit that are adapted to being eaten by a particular animal.Strawberries are often eaten by birds, acorns by squirrels, and mangoes by bats As aresult of eating these plants, seeds and other plant parts were dispersed This was thebeginning step in the process of domestication

con-Human latrines may have been testing grounds for the first crop breeders, asthese may have been primary spots where seeds of ingested fruits would have beendeposited These were also areas where the soils had greater nutrient concentrationsthan in nonlatrine areas

Garbage dumps where food scraps were scattered may have also played a role inearly plant domestication and growing Spoiled or rotten fruit would have beenplaced in these areas and the seeds from these fruits could have germinated Alsoseeds of consumed fruits may have been deposited into these areas and provided an-other source for future plant generations

Other methods of seed dispersal are unintentional Dispersal may occur by theplants or their seeds being blown about in the wind or floating in the water Manywild plants have specialized mechanisms that scatter seeds and generally make themunavailable to humans Most wild peas have pods that explode (split open) when theseeds within the pod are mature Because humans collected only the pods that didn’texplode, this nonexploding trait was passed on during domestication

For the survival of plant species, seeds produced by the plant had a scattered mination rate (i.e., they didn’t germinate at one time) This resulted in plants nothaving all their seeds germinating at a time when possibly the climate or some otherfactor may not have been conducive for early plant growth and development Thickseed coats contributed to seeds undergoing a scattered germination rate Many eco-logically advantageous traits such as thick seed coats have been reduced or removedduring domestication to improve the efficiency of seedling establishment and plantgrowth in large-scale field plantings

ger-During the 10,000 or 11,000 years that have passed since the beginning of plantdomestication, the plants that humans selected as useful to them have undergoneprofound changes Artificial selection of plants during the ages differs considerablyfrom natural selection During natural selection the plants have properties thatpreadapt them to a wide variety of environmental conditions, hopefully ensuringcontinuation of the species Artificial selection breaks down these stabilized systems,creating gene combinations that possibly could not survive in the wild

CHAPTER 2 Plants and Society 11

Timeline for Domestication of Important Crops

During the development of agriculture and plant domestication some plants weredomesticated earlier or easier than others were The earliest domesticated plants ap-pear to have been the Near Eastern crops of cereals and legumes (such as wheat, bar-ley, and peas) around 10,000 years ago These crops may have been domesticatedearlier because they came from wild ancestors that had many characteristics thatwere advantageous for the process of domestication Some of these advantages in-clude seeds that were edible in the wild and could be readily stored and plants thatwere easily grown from sowing, grew quickly, were self-pollinated Overall few ge-netic changes were required of these crops to go from wild plants to domesticatedplants

Fruit and nut domestication probably began around 4000 BC The fruit andnut crops typically are not harvested until 3 to 5 years after planting To growthese types of crops, people needed to be committed to settled life and could nolonger be seminomadic Fruit and nut crops are often grown by cuttings, whichhas the advantage that the progeny or descendants of the original plants are iden-tical to the original plant Some fruit trees cannot be grown from cuttings, and itwas found to be a waste of effort to grow them from seed Instead, grafting tech-niques had to be discovered and perfected before suitable domestication couldoccur

Contemporary Crop Improvement Programs

Improvement to crops has been a continuous process since humans began ing and then growing plants for their use In the process of selecting and using cer-tain plants, primitive humans passed on these chosen plants and the characteristicsthat made them be selected to the next set of plants that grew The criteria for se-lection may have been fruit size, color, taste, fast rate of growth, resistance to diseases

collect-or insects, collect-or simply the lack of toxins to humans This process of selection was fective, as many of our contemporary cultivated plants no longer resemble theirprimitive ancestors

ef-Plant breeding as a science is the systematic improvement of plants and has onlybeen in existence in the last couple of decades Plant genetics is the study of themechanisms of heredity of plant traits and is the underlying science of plant breed-ing Gregor Mendel did the pioneering work on genetic inheritance in 1865 usinggarden peas Today’s plant breeding and genetic programs continue to utilize thebasic concepts of genetics and controlled plant crosses Crossing is the transfer ofgenes (in pollen) from one plant to another

Some plants are self-pollinated, which means that the flowers of the plant use itsown pollen for pollination Seeds from these plants produce uniform plants verymuch like the parents Mass selection and pedigree selection are breeding methodsfor self-pollinated crops Mass selection involves selecting the best plants based on

12 PART 1 An Overview of the Plant Sciences

appearance (phenotype) Pedigree selection is conducted after creating variability

by controlled crossing of two parents

Cross-pollination is the process of transferring pollen grains from one plant anddepositing them on the stigma of the flower of a different one Seeds from plantsthat cross-pollinate produce hybrid (nonuniform) plants Hybrid vigor or heterosisrefers to the increase in vigor shown by certain crosses as compared to that of eitherparent Many cross-pollinated agricultural crops (such as apple) are also vegetativelypropagated Mass selection, recurrent selection, and other methods of breedingmay be applied to cross-pollinated species Many of today’s crop improvement pro-grams also utilize the modern concepts of molecular genetics, tissue culture, and ge-netic engineering

Plant domestication began when early agriculturalists began selecting and tivating wild plants that had desirable characteristics such as large fruit size, sweettaste, fast growth, and disease and insect resistance The earliest domesticated plantsappear to have been cereals and legumes (such as wheat, barley, and peas) Crop do-mestication is a continual process, carried on in today’s plant breeding and crop im-provement programs

3 What are some of the methods of seed dispersal that are used by plants?

4 What were some of the criteria used by early plant gatherers in selectingplants?

5 Why did cereal and legume crops undergo domestication easier and earlierthan fruit and nut crops?

6 What is plant breeding?

CHAPTER 2 Plants and Society 13

Selected References

Acquaah, G 1999 Horticulture: Principles and practices Upper Saddle River, NJ:

Prentice Hall

Janick, J 1979 History’s ancient roots HortScience 14:299–313.

Janick, J 1979 Horticultural science 3rd ed San Francisco: W H Freeman.

Norstog, K., and R W Long 1976 Plant biology Philadelphia: W B Saunders.

Selected Internet Sites

www.aces.uiuc.edu/~sare/history.htm/ A Brief History of Agriculture, University ofIllinois College of A.C.E.S

www.agr.ca/backe.htm/ Agriculture and Agri-Food Canada: History, Agriculture andAgri-Food Canada

USDA Economic Research Service

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This introductory chapter provides an overview of the historical portance of plants in the development of plant-related agricultural in-dustries and the current status of U.S agriculture and crop production.

im-To provide a better understanding of contemporary plant-related cultural industries, important agronomic and horticultural industries arepresented

agri-Historical Periods of the United States and the

Development of Plant-Related Agricultural Industries

Colonists and Early Settlers

American colonists were largely self-supporting, growing plants for theirown use Early settlers endured a rough pioneer life while adapting to newenvironments Small family farms predominated, except for some rela-tively large plantations that were developed in the southern coastal areas

The settlers used relatively simple tools such as wagons, plows, rows, axes, rakes, scythes, forks, and shovels All seed sowing was done byhand Most of the early settlers tended to live near forested areas, whichprovided wood for housing, fencing, and fuel

har-In 1819, Jethro Tull helped revolutionize farming by developing andpatenting an iron plow with interchangeable parts By the 1820s and 1830sfarmers, blacksmiths, and other innovators introduced various modifica-tions to the plow that provided a sharper and stronger cutting edge with

16 PART 1 An Overview of the Plant Sciences

smoother surfaces so that the soil did not stick to either the plowshare or the board John Deere and Leonard Andrus began manufacturing steel plows in 1837.Most early settlers planted a mixture of crops both for home consumption andfor market Field corn was often a mainstay because it gave high, reliable yields Di-versification was also needed to supply the various food components for the hometable, and vegetables were mainstays of these plantings

mold-Manufacturing was making its way into the cities and into agriculture by the1850s The two-horse straddle row cultivator was patented in 1856 The change fromhand power to horses characterized the first American agricultural revolution

Post–Civil War

Many products that were developed prior to or during the Civil War increased theproductivity of each laborer in harvesting, planting, and cultivating fields In 1800,approximately 75% of the population was directly engaged in agricultural produc-tion By 1850, it was less than 60%, and by 1900 less than 40% of the population wasengaged in agricultural production

During these times the number of farms began to decline and farm size increased

As both farming and manufacturing became more productive, people could choose

a broader range of career options in such fields as medicine, law, science, government,and entertainment The increased productivity of farmworkers led to surpluses of agri-cultural products and thus lower prices This affected the livelihood of many farm-workers, and the supply of available labor at times exceeded the demands, resulting

in unemployment By the 1890s agriculture became increasingly more mechanized.Industrial expansion, which began about 1895, established concentrated popu-lation centers or cities Agricultural improvements made more food available to sup-port a larger nonagricultural population These large population centers becamelargely dependent on special producers for their food supply, and, as a result, com-mercial production of many agricultural crops developed near population centers.During the 1910s, big open-geared tractors came into use in areas of extensivefarming In 1926, a successful light tractor was developed, and during the 1930s theall-purpose, rubber-tired tractor came into wide use By the 1930s, 58% of all farmshad cars, 34% had telephones, and 13% had electricity

Pre–World War II

For much of the early 1900s, American agricultural policy was guided by the ophy that society would best be served by traditional family-size, owner-operatedfarms These family farms relied heavily on local labor, supplies, and consumers tosustain their business

philos-Post–World War II

After 1945, the rapid transition to volume marketing systems began The ment in technology in food and fiber handling systems (including refrigeration) de-veloped during wartime, the changing economic structure of American agriculture,

improve-CHAPTER 3 Plants as Industries 17

and the highway expansion of the 1950s all favored those growers who could supplythe market with a large volume over a prolonged period Small farms were ineffi-cient and many either failed or enlarged to meet new challenges Growers enlargedthrough purchases of additional land or through production/marketing coopera-tives They maintained competitiveness by adopting new technology or by stressinghigh quality in crop production and handling During these times, much less varietywas available than today in terms of number, form, and quality of crop products.The change from horses to tractors after 1945 and the adoption of a group oftechnological practices characterized the second American agricultural revolution

By 1954, the number of tractors on farms exceeded the number of horses and mulesfor the first time By 1954, 71% of all farms had cars, 49% had telephones, and 93%had electricity

Relatively Recent Times

The last several decades were highlighted by crop industries that responded to anincreasingly diverse population and the products that this population demanded.During the 1970s, no-tillage agriculture was popularized, a trend that continued intothe 1980s as more farmers used no-till or low-till methods to curb soil erosion By

1975, 90% of all farms had phones, 98% had electricity

In the 1980s, targeted marketing replaced mass marketing Development of newproducts occurred at a rapid pace during the 1980s and continued during the 1990s.During the 1990s and early 2000s, more farmers began to use low-input sustainabletechniques to decrease chemical applications Also field production of geneticallyengineered crops began

U.S Agriculture and Crop Production

Total U.S agricultural output increased at an average annual rate of 1.88% from

1948 to 1996 (Fig 3.1) In 2002, the total U.S farm cash receipts for agriculture was

in excess of $217 billion (Table 3.1) Crop cash receipts accounted for $97.2 billion

of this amount Feed crops represent the largest crop category in returns

Exports

With the productivity of U.S agriculture growing faster than domestic food and fiberdemand, U.S farmers and agricultural firms rely heavily on export markets to sustainprices and revenues Export revenues accounted for 20% to 30% of U.S farm incomeduring the last 30 years and are projected to remain at this level until 2010 (Fig 3.2).During the early 1990s, U.S exports of nearly all commodities grew (Fig 3.3).After U.S exports peaked in fiscal 1996, export shares of production (in value) formany commodities dropped in the late 1990s, except for nuts, fruits, and vegetables.For some commodities, trade dependency is relatively high Exports of rice, cotton,

18 PART 1 An Overview of the Plant Sciences

Index (1948=100) 250

200 150

100

1948 1960 70 80 90 96

Inputs Output

Productivity

FIGURE 3.1 Growth of agricultural productivity as percentage of 1948 production.

Source: Economic Research Service, USDA.

TABLE 3.1 Value Added to the U.S Economy by the Agricultural Sector

Services and forestry 25.1 24.4 26.2 26.8

Final agricultural sector output 213.0 218.5 227.7 217.2

Source: Economic Research Service, USDA.

and wheat approached 50% shares of U.S production in the 1990s For some fruitsand nuts, the shares exported are even larger

Historically, the bulk commodities (such as wheat, rice, coarse grains, oilseeds,cotton, and tobacco) accounted for most U.S agricultural exports However, in the1990s, as population and incomes worldwide rose, U.S exports of high-value prod-ucts (e.g., meats, live animals, oils, fruits, vegetables, and beverages) expandedsteadily in response to demand for more food diversity (Table 3.2) In 2000, high-value product exports accounted for a 65% share of total U.S agricultural exports,while bulk exports accounted for 35%

Percent 35 30 25 20 15 10 5 0

1960 1965 1970 1975 1980 1985 1990 1995 2000 2005 2010

FIGURE 3.2 Value of agricultural exports as a percentage of gross cash income.

Source: USDA Agricultural Baseline Projections to 2010, February 2001 Economic Research

Service, USDA.

Almonds, shelled Walnuts, shelled Prunes Corn oil Rice Animal hides Grapefruit Pistachios, shelled

Cherries Tobacco, unmfgd.

Raisins Soybeans/meal/oil Sunflower oil Wheat Grapes Dried beans Cotton Sorghum Poultry meat Head lettuce

Percent

avg 1990–1994 1999

FIGURE 3.3 Export share of U.S agricultural products.

Source: Economic Research Service, USDA.