Elevated Carbon Dioxide: Impacts on Soil and Plant Water Relations PDF

Preface

Water and carbon dioxide are the two most important compounds affecting plant growth. In introductory botany textbooks, we have seen the familiar equation for photosynthesis, which shows carbon dioxide (CO₂) joining with water (H₂O), in the presence of light and chlorophyll, to form sugar (C₆H₁₂O₆) and oxygen (O₂), as follows:

Life on earth would not be possible without photosynthesis. We survive because of the oxygen and food (sugars) produced by photosynthesis. Therefore, it is of critical importance to look at the plant water relations under elevated CO₂, because the CO₂ concentration in the atmosphere is increasing.

The CO₂ concentration in the atmosphere was first recorded by Charles D. Keeling (1928-2005) of the Scripps Institution of Oceanography at the University of California at San Diego. He monitored it beginning in 1957 at Mauna Loa in Hawaii and in Antarctica at the South Pole. In the 50 year period between 1958 and 2008, the CO₂ concentration in the atmosphere increased from 316 to 385 ppm. This book aims to put the information in one source as no books document plant water relations under elevated CO₂.

This book was developed from research conducted in the Evapotranspiration Laboratory at Kansas State University between 1984 and 1991 with field-grown sorghum, winter wheat, and rangeland plants under elevated CO₂. Such experiments had not been done before in the semiarid Great Plains of the United States. The rising levels of CO₂ in the atmosphere were of interest to the U.S. Department of Energy, which funded our work. It has been 27 years since we started our first experiments. We can thus make some predictions, based on our early results, about how plants are responding to elevated CO₂, which was 330 ppm in 1984 when we started our studies. I present some of these predictions in this book.

This book is not a literature review. It describes experiments that appear in peer-reviewed journal articles. Thousands of papers have been written on the effects of elevated levels of atmospheric CO₂ on plants. It is impossible to review the entire literature. I have thus picked selected articles as examples and then discuss each one, often providing an illustration from the paper. For each paper that I illustrate, I mention the full name (common and scientific) of the plant under investigation. When this is not mentioned in the original article, I have referred to Fernald (1950) and Bailey (1974). For each experiment, I also provide the type of soil used (if it is given in the original article) and the general conditions of the experiment [greenhouse, growth chamber, opentop or enclosed chambers, or FACE (free-air carbon dioxide enrichment) facility]. All information in the book has been taken from hard copy sources (books and journal articles). I have carefully documented the source of the information. When it comes from a lengthy article or a book, I have provided the exact page on which I found the information. In this way, the interested reader can easily find the source.

This book has much instructive material, which I use to teach my graduate-level class. When a new scientific concept is raised, I provide a detailed explanation.

The book deals only with water and elevated CO₂. It does not deal with temperature, nutrients, or other factors, such as the greenhouse effect (warming of the atmosphere by trace gases), that affect plant growth under elevated CO₂, although in the last chapter I mention temperature briefly.

The book is organized as follows. I start with an introductory chapter (Chapter 1) dealing with drought, because it is predicted that the central Great Plains, where Kansas is located, will become drier as the CO₂ concentration in the atmosphere increases. In this chapter, I provide a preliminary overview of the three types of photosynthesis: C₃, C₄, and Crassulacean acid metabolism. The book then describes water as it moves from the soil through the plant and out into the atmosphere. This is the way that water moves through the soil-plant-atmosphere continuum. Chapters 2 through 5 deal with soil. Chapter 2 discusses the composition of the soil atmosphere. Chapter 3 deals with the interaction of elevated CO₂ in the soil with the physical factors in the soil, such as compaction, that affect root growth. Because oxygen is a key factor for root growth, Chapter 4 deals with variable oxygen concentration of the soil along with elevated CO₂ in the soil. Carbon dioxide can be elevated both in the soil and in the atmosphere. Therefore, I had to distinguish the effects of elevated CO₂ in the soil from elevated CO₂ in the atmosphere. While Chapters 2 through 4 focus on elevated CO₂ in the soil, Chapter 5 deals with soil when the atmosphere above it is elevated with CO₂.

After the discussion of soil and elevated CO₂, I then consider the root. Chapter 6 deals with elevated CO₂ and root growth. In this chapter, studies with roots have been done with carbon isotopes, and I discuss carbon isotope ratios and how they are used in plant science in detail. Chapter 7 deals with the effects of elevated CO₂ on plant water, osmotic, and turgor potentials. Chapters 8 and 9 deal with stomata under elevated CO₂. Chapter 8 discusses stomatal conductance. In this chapter, I present material about the resistances in leaves, the units used to measure stomatal conductance, and the physiological factors affecting stomatal opening and closing. Chapter 9 deals with stomatal density. In this chapter, the geological timescale is presented as we learn from the geological record that stomatal density is affected by CO₂. Next, I take the water out of the plant into the atmosphere and discuss the effects of elevated CO₂ on transpiration and evapotranspiration in Chapter 10. In this chapter, I discuss ethylene, because it is a gas similar to CO₂, and I distinguish the two gases and their effects on plants. I also cover the general principles of evapotranspiration and how it is determined. Next, I discuss water use efficiency in Chapter 11. In this chapter, I present material about the historical aspects of water use efficiency. Chapter 12 compares C₃ and C₄ plants under elevated CO₂ and provides a detailed account of C₄ photosynthesis and its advantages, and how it has evolved. Chapter 13 deals with plant anatomy and focuses on the xylem (including wood)—the tissue that carries water in plants. In this chapter, I discuss the three variations of C₄ photosynthesis, because it is necessary to know this material to understand one of the topics described in this chapter. Chapter 14 deals with phenology and how this is affected by elevated CO₂. In this chapter, I also cover the Q10 value (a value related to the rate of reactions), the degree-day concept, and heat units as these are necessary to know to understand how a plant progresses through its different phenological stages. I also provide sample problems to help students calculate the Q10. Finally, Chapter 15 deals with the growth and yield of many different kinds of plants under elevated CO₂ and well-watered conditions. In any work dealing with plant water relations, some measure of plant growth (e.g., height, biomass, or leaf area) should be provided, because growth integrates all factors affecting a plant. Therefore, I have devoted an entire chapter to this.

Each chapter ends with a summary. Because the humanistic side of science is usually overlooked in scientific books, I have appended to several chapters the biographies of people who have developed the concepts discussed in those chapters.

The units for CO₂ in the different chapters are expressed as ppm, μmol/mol, μL/L (or μl/l-liter can either be capitalized or not capitalized when it is abbreviated), cm³/m³, or in Pascal (Pa). When I worked with the Tri-Societies (American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America) on the book that we (Allen et al. 1997) edited, it suggested the common unit of μmol/mol. Therefore, in this book I have converted the units as much as possible to μmol/mol. I have left the unit of Pascal (pressure unit) unchanged, as one needs to know the temperature and elevation of the place where the research was done to convert it to a concentration unit. As I did not have this information, I retained the unit as Pascal.

A key part of the book are the figures. They have been redrawn from the originals by Eldon J. Hardy, who was my draftsman at the College of Engineering when I was in the Department of Agronomy at Oklahoma State University. He is now retired. This book would not have been possible without his help. His drawings are unparalleled for uniformity, clearness, and precision. I extend my profound thanks to him for working closely with me during the last three years to prepare these drawings.

I am grateful to the publisher, CRC Press, Taylor & Francis, for publishing this book. I would especially like to thank Randy Brehm, editor, Chemical and Life Sciences Group, Taylor & Francis, for her prompt and peerless assistance during all stages of publication of this book. I would like to thank the four reviewers that she contacted and who supported the publication of this book. I would also like to thank John Edwards, project coordinator, Editorial Project Development, Taylor & Francis, for his help during the production of this book.