Water Relations of Plants

493 Pages·1983·9.921 MB·English

by  Paul J. Kramer (Auth.)

Water Relations
of Plants
PAUL J. KRAMER
Department of Botany
Duke University
Durham, North Carolina
1983
ACADEMIC PRESS
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Library of Congress Cataloging in Publication Data
Kramer, Paul Jackson, Date
Water relations of plants.
Includes index.
Bibliography: p.
1. Plant-water relationships. I. Title.
QK870.K7 1983 581.19'212 82-18436
ISBN 0-12-425040-8
PRINTED IN THE UNITED STATES OF AMERICA
83 84 85 86 9 8 7 6 5 4 3 2 1
Preface
The importance of an understanding of the water relations of plants is indi-
cated by the ecological and physiological importance of water. Not only is the
distribution of vegetation over the earth's surface controlled chiefly by the avail-
ability of water, but crop yields are more dependent on an adequate supply of
water than on any other single factor. This book attempts to explain the impor-
tance of water by describing the factors that control the plant water balance and
showing how they affect the physiological processes that determine the quantity
and quality of growth. It is intended for students, teachers, and investigators in
both basic and applied plant science, and it should be useful as a reference to
botanists, agronomists, foresters, horticulturists, soil scientists, and even laymen
with an interest in plant water relations. An attempt has been made to present the
information in terms intelligible to readers in all fields of plant science. If the
treatment of some topics seems inadequate to specialists in those areas, they are
reminded that the book was not written for specialists in soil or plant water
relations, but for those plant scientists, upper-level undergraduates, and graduate
students who need a general introduction to the whole field.
The need for a book summarizing modern views on plant water relations has
been increased by the large volume of publications and the changes in viewpoint
that have occurred in recent years. A number of books on plant water relations
have appeared, but most of them are collections of papers on special topics. This
book attempts to present the entire field of water relations in an organized
manner, using current concepts and a consistent, simple terminology. Emphasis
is placed on the interdependence of various processes. For example, the rate of
water absorption is closely linked to the rate of transpiration through the sap
stream in the vascular system, and it also is affected by resistance to water flow
into roots and by the various soil factors affecting the availability of water. The
rate of transpiration depends primarily on the energy supply, but it also is
affected by stomatal opening and the leaf water supply. Proper functioning of the
physiological processes involved in growth requires a favorable water balance,
which is controlled by relative rates of absorption and water loss by transpiration.
ix
χ Preface
These complex interrelationships are emphasized and described in modern
terminology.
Although the primary objective of this book is to present a survey of modern
concepts in the field of plant water relations, attention is also given to some of
the older work because it constitutes the foundation on which modern concepts
are based. Workers should understand that plant water relations has a long
history of productive research dating back to the beginning of plant physiology as
a science. Quantitative study of water relations began with the measurements of
root pressure and transpiration made early in the eighteenth century by Hales. It
was expanded during the nineteenth century by the research of Dutrochet,
Pfeffer, Sachs, Strasburger, and others. It is interesting to note that by 1860
Sachs was aware that cold soil reduces water absorption by warm season plants
more than absorption by cool season plants, and that late in the nineteenth
century Francis Darwin observed that atmospheric humidity affects stomatal
opening and Dixon proposed the cohesion theory of the ascent of sap. The
concept of water potential was developed during the second decade of the twen-
tieth century under the name of "suction force." In fact, most of the basic
concepts in plant physiology were in existence over 50 years ago, but improve-
ments in research methods have greatly increased our understanding of them.
The large volume of publication in recent years makes it impossible to cite all
of the relevant literature and many good papers have been omitted. Nevertheless,
the bibliography is extensive enough to serve as the primary source of references
in almost any area of plant water relations. Today's lively research activity is
producing significant changes in explanations of various phenomena, and some
long-held views are being reconsidered. For example, the assumptions that
stomatal closure depends chiefly on loss of bulk leaf turgor, that the leaf meso-
phyll is the principal evaporating surface, and that water movement outside the
xylem occurs chiefly in the cell walls are all being questioned. Also, the role of
growth regulators, chiefly abscisic acid and cytokinins, is receiving much atten-
tion in relation to membrane permeability and stomatal closure. Differences in
opinion among various investigators are discussed, and in some instances the
author has indicated his preference, but it is pointed out that in many instances
more research is needed before conclusions can be reached. Readers are re-
minded that so-called scientific facts often are merely the most logical explana-
tions that can be developed from the available information. As additional re-
search provides more information, it frequently becomes necessary to revise
generally accepted explanations, and those that seem logical today may become
untenable next year.
There are some changes from the terminology used in earlier versions of this
Preface xi
book. Osmotic absorption replaces active absorption of water to avoid confu-
sion with active transport, and drought tolerance replaces drought resistance
because tolerance more accurately describes the situation. The bar has been
replaced by the SI unit, the MPa (1 bar = 10 5 Pa or 0.1 MPa), and the millibar by
the kPa (1 mb = 0.1 kPa). There is considerable cross-referencing between
chapters, but there also is some repetition of material in various chapters. For
example, the osmotic properties of cells and stomatal behavior are discussed in
different contexts in different chapters. This makes each chapter a fairly com-
plete unit that can be read without excessive reference to preceding chapters and
facilitates use of the book as a reference.
This book reflects my interactions with many scientists, ranging from the late
Ε. N. Transeau, who first called my attention to the interesting problems in plant
water relations, to R. O. Slaty er who broadened my viewpoint and improved my
terminology, to T. T. Kozlowski who encouraged the writing of this book, and to
scores of other scientists with whom I have discussed problems. I am especially
indebted to the many graduate students and postdoctoral research associates for
their stimulating discussions and valuable suggestions, and to the Department of
Botany of Duke University for providing a good environment in which to work. I
appreciate the cooperation of friends and colleagues who read parts or all of the
manuscript and offered many valuable suggestions. All of the manuscript was
read by M. R. Kaufmann and T. T. Kozlowski, and certain chapters were read
by J. A. Bunce, E. L. Fiscus, A. W. Nay lor, and J. N. Siedow, and various
topics were discussed with other scientists too numerous to list. Their sug-
gestions contributed much to the book, but they should not be held responsible
for any errors that may have crept in during the several revisions. The efficient
assistance of Sue Dickerson and Joanne Daniels in typing the manuscript and
Shirley Thomas in preparing the bibliography also is gratefully acknowledged.
The author also wishes to acknowledge the support provided by various grant-
ing agencies. In the earliest days his research was supported by Duke University
Research Council grants, later by the Atomic Energy Commission, and for the
past 25 years by grants from the National Science Foundation. These grants have
supported many graduate students and postdoctoral research associates and
scores of publications, and have contributed both directly and indirectly to the
production of this book and of the 1969 version which it replaces.
Paul J. Kramer
Water: Its Functions and
Properties
Introduction 1
Ecological Importance of Water 2
Physiological Importance of Water 2
Uses of Water in Plants 5
Constituent 5
Solvent 7
Reactant 7
Maintenance of Turgidity 7
Properties of Water 7
Unique Physical Properties 8
Explanation of Unique Properties 9
Unorthodox Views Concerning Water 14
Isotopes of Water 15
Properties of Aqueous Solutions 15
Pressure Units 16
Vapor Pressure 16
Boiling and Freezing Points 17
Osmotic Pressure or Osmotic Potential 18
Chemical Potential of Water 20
Summary 21
Supplementary Reading 22
INTRODUCTION
Water is one of the most common and most important substances on the
earth's surface. It is essential for the existence of life, and the kinds and amounts
of vegetation occurring on various parts of the earth's surface depend more on
the quantity of water available than on any other single environmental factor.
The importance of water was recognized by early civilizations, and it occupied a
prominent place in ancient cosmologies and mythologies. The early Greek phi-
2 7. Water: Its Functions and Properties
losopher Thaïes asserted that water was the origin of all things, and it was one of
the four basic elements (earth, air, fire, water) recognized by later Greek philoso-
phers such as Aristotle. It was also one of the five elemental principles (water,
earth, fire, wood, metal) of early Chinese philosophers. Today it is realized that
the availability of water not only limits the growth of plants but can also limit the
growth of cities and industries. In this chapter we will discuss the ecological and
physiological importance of water, its unique properties, and the properties of
aqueous solutions.
Ecological Importance of Water
Regions where rainfall is abundant and fairly evenly distributed over the
growing season have lush vegetation. Examples are the rain forests of the trop-
ics, the vegetation of the Olympic peninsula and the northwestern United States,
and the luxuriant cove forests of the southern Appalachians. Where summer
droughts are frequent and severe, forests are replaced by grasslands, as in the
steppes of Asia and the prairies of North America. Further decreases in rainfall
result in semidesert with scattered shrubs, and finally in deserts.
Even the effects of temperature on vegetation are partly produced through
water relations because increasing temperature is accompanied by increasing
rates of evaporation and transpiration. Thus, an amount of rainfall adequate for
forests in a cool climate can only support grassland in a warmer climate where
the rate of évapotranspiration is much higher. This was responsible for develop-
ment of the concept of the rainfall evaporation ratio by Transeau (1905) as an
indicator of the relationship between precipitation and type of plant cover. The
concept was developed further by Thornthwaite (1948), and precipitation and
evaporation are considered in climatic diagrams such as those constructed by
Walter (1979, pp. 25-30).
Physiological Importance of Water
The ecological importance of water is the result of its physiological impor-
tance. The only way in which an environmental factor such as water can affect
plant growth is by influencing physiological processes and conditions, as shown
in Fig. 1.1.
Almost every plant process is affected directly or indirectly by the water
supply. Many of these effects will be discussed later, but it can be emphasized
here that within limits metabolic activity of cells and plants is closely related to
Introduction 3
HEREDITARY POTENTIALITIES ENVIRONMENTAL FACTORS
Depth and extent of root systems SOIL. Texture, structure, depth, chemical
composition and pH, aeration,
Size, shape and total area of
temperature, waterholding capacity,
leaves, and ratio of internal
and water conductivity
to external surface
Number, location, and behavior ATMOSPHERIC. Amount and distribution
of stomata of precipitation
Ratio of precipitation to evaporation
Radiant energy, wind, vapor pressure,and
other factors affecting evaporation
and transpiration
1 '
PLANT PROCESSES AND CONDITIONS
Water absorption
Ascent of sap
Transpiration
Internal water balance as reflected in
water potential, turgidity, stomatal
opening, and cell enlargement
Effects on photosynthesis, carbohydrate
and nitrogen metabolism, and other
metabolic processes
1
QUANTITY AND QUALITY OF GROWTH
Size of cells, organs, and plants
Dry weight, succulence, kinds and amounts
of various compounds produced and
accumulated
Root-shoot ratio
Vegetative versus reproductive growth
Fig. 1.1. Diagram showing how the quantity and quality of plant growth are controlled by
hereditary potentialities and environmental factors operating through the internal processes and
conditions of plants. Special attention is given to factors and conditions affected by water relations.
their water content. For example, the respiration of young, maturing seeds is
quite high, but it decreases steadily during maturation as water content decreases
(see Fig. 1.2). The respiration rate of air-dry seeds is very low and increases
slowly with increasing water content up to a critical point, at which there is a
rapid increase in respiration with a further increase in water content (Fig. 1.3).
Growth of plants is controlled by rates of cell division and enlargement and by
the supply of organic and inorganic compounds required for the synthesis of new
protoplasm and cell walls. Cell enlargement is particularly dependent on at least
4 I. Water: Its Functions and Properties
1400
_ 1200
1000
800
600
400
200
10 15 20 25 30 35 40 45 50
Moisture (%)
Fig. 1.3. Increase in water content and in rate of respiration of oat seeds during imbibition of
water. Note the rapid increase in respiration as the water content rises above approximately 16%. The
water is probably so firmly bound at lower contents that it is unavailable for physiological processes.
(From Bakke and Noecker, 1933.)
Uses of Water in Plants 5
60 30
CORN
0 -0.44 -0.8 -1.2 -1.6
Leaf water potential (MPa)
Fig. 1.4. Relationship among leaf water potential, leaf elongation, and photosynthesis of corn.
Note that leaf elongation almost ceases before there is much reduction in photosynthesis. (From
Boyer, 1970a.)
a minimum degree of cell turgor, and stem and leaf elongation is quickly checked
or stopped by water deficits, as shown in Fig. 1.4 and in Chapter 12. Decrease in
water content invariably inhibits photosynthesis (Fig. 1.4) and usually reduces
the rate of respiration and other enzyme mediated processes.
In summary, decreasing water content is accompanied by loss of turgor and
wilting, cessation of cell enlargement, closure of stomata, reduction in photo-
synthesis, and interference with many basic metabolic processes. Eventually,
continued dehydration causes disorganization of the protoplasm and death of
most organisms. The effects of water deficits on physiological processes are
discussed in more detail in Chapter 12.
USES OF WATER IN PLANTS
The importance of water can be summarized by listing its most important
functions under four general headings.
Constituent
Water is as important quantitatively as it is qualitatively, constituting 80-90%
of the fresh weight of most herbaceous plant parts and over 50% of the fresh
weight of woody plants. Some data on water content of various plant structures