Encyclopedia of Plant Physiology – serie

When WILHELM RUHLAND developed his plan for an Encyclopedia of Plant Physiol­ ogy more than three decades ago, biology could still be conveniently subdivided into classical areas. Even within plant physiology, subdivisions were not too difficult to make, and general principles could be covered sufficiently in the two introductory volumes of the Encyclopedia on the physical and chemical basis of cell biology. But the situation changed rapidly even during the 12-year publication period of the Encyclopedia (1955-1967). The new molecular direction of genetics and structural research on biopolymers had an integrating effect on all other biological fields, including plant physiology, and it became increasingly difficult to keep previously distinct areas separated. RUHLAND'S overall plan included 18 volumes and about 22,000 pages. It covered the entire field of plant physiology, in most cases from the very beginning. But, as each volume appeared, it was clear that its content would soon be outdated.

As plant physiology increased steadily in the latter half of the 19th century, problems of absorption and transport of water and of mineral nutrients and problems of the passage of metabolites from one cell to another were investigated, especially in Germany. JUSTUS VON LIEBIG, who was born in Darmstadt in 1803, founded agricultural chemistry and developed the techniques of mineral nutrition in agricul­ ture during the 70 years of his life. The discovery of plasmolysis by NAGEL! (1851), the investigation of permeability problems of artificial membranes by TRAUBE (1867) and the classical work on osmosis by PFEFFER (1877) laid the foundations for our understanding of soluble substances and osmosis in cell growth and cell mechanisms. Since living membranes were responsible for controlling both water movement and the substances in solution, "permeability" became a major topic for investigation and speculation. The problems then discussed under that heading included passive permeation by diffusion, Donnan equilibrium adjustments, active transport processes and antagonism between ions. In that era, when organelle isolation by differential centrifugation was unknown and the electron microscope had not been invented, the number of cell membranes, their thickness and their composition, were matters for conjecture. The nature of cell surface membranes was deduced with remarkable accuracy from the reactions of cells to substances in solution. In 1895, OVERTON, in U. S. A. , published the hypothesis that membranes were probably lipid in nature because of the greater penetration by substances with higher fat solubility.

In the first part (Part A) of this volume on transport, there was an emphasis on the processes occurring at the membranes bounding the cells. It was convenient to distinguish active and passive processes of transport across the membranes, and to recognize that certain transport processes may be regulated by internal factors in the cells such as cytoplasmic pH, concentrations of ions, of malate or of sugar in the vacuoles, or the hydrostatic pressure. Cells in tissues and organs show the same kinds of properties as individual cells, but in addition there can be cell to cell transport related to the organization of the tissue. Firstly cells within a tissue are separated from the external solutions by a diffusion path comprising parts of the cell walls and intercellular spaces; more generally this extra-cytoplasmic part of the tissue has been called the apoplasm. A similar term is "free space". Secondly, the anatomy of cells in tissues seems to allow some facilitated, local transport between cells in a symplasm. Entry into the symplast and subsequent transport in a symplasmic continuum seems to be privileged, in that ions may not have to mix with the bulk of the cytoplasm and can pass from cell to cell in particular cytoplasmic structures, plasmodesmata. In Chara plants, this kind of transport is found operating across the multi-cellular nodes as the main means of transport between the long internodal cells.

Plant pathology embraces all aspects of biological and scientific activity which are concerned with understanding the complex phenomena of diseases in plants. Physiological plant pathology represents those specialities within plant pathology which focus on the physiological and biochemical activities of pathogens and on the response of host plant tissues. Today there is an increasing recognition on the part of the scientific agri­ cultural community that only through a deeper and more fundamental under­ standing of all the interacting components of the agricultural biota can we expect to improve our capabilities of feeding an expanding world population. It is in this context that physiological plant pathology has assumed new significance within the broader field of plant pathology. No longer are studies on the biochemistry and physiology of pathogens and pathogenesis merely isolated academic exercises; rather, a substantial coherent body of knowledge is accumulating upon which our understanding of the process of disease developmen t and host resistance is being founded. It is from these foundations of knowledge that ultimately new insights into the control of plant diseases may be expected to grow. It seems appropriate, therefore, that at regular intervals those involved in the various subspecialities encompassing the broadest aspects of physiological plant pathology reassess the contributions within the particular specialities in the light of new knowledge and technologies for the purpose of articulating new and productive directions for the future.

The problems associated with the movement of water and solutes throughout the plant body have intrigued students of plants since Malpighi's conclusions in 1675 and 1679 that nutrient sap flows upward and downward in stems through vessels in both wood and bark. Steven Hale's ingenious experiments on the movement of water in plants in 1726 and Hartig's observations of sieve-tube exudation in the mid-19th century set the stage for continued intensive studies on long-range transport in plants. In spite of this interest for more than 200 years in the movement of solutes and water in plants, it has only been within the last 20 to 30 years that extensive research effort has been directed toward a critical evaluation of the interactions among the various cellular organelles. The important roles played by the exchange of metabolites in the control and regulation of cellular processes is now widely recognized, but in most instances poorly understood.

As editor of the two-part Volume V on photosynthesis in RUHLAND'S Encyclopedia, the forerunner of this series published in 1960, I have been approached by the editors of the present volume to provide a short preface. The justification for following this suggestion lies in the great changes which have been taking place in biology in the two decades between these publications, changes which are reflected in the new editorial plan. Twenty years ago it appeared convenient and formally easy to consider photo­ synthesis as a clearly separated field of research, which could be dealt with under two major headings: one presenting primarily photochemical and biochemical prin­ ciples, the other physiological and environmental studies. Such a partition, however, as far as aims and opinions of the authors were concerned, resulted in a rather heterogeneous volume. Today, the tendency in experimental biology is towards a merger of previously distinct disciplines. Biochemists and biophysicists have developed their methods to such an extent that, over and above the analysis of individual reaction sequences, work on the manifold interrelationships among cellular activities has become in­ creasingly possible. Joining them in growing numbers are the physiologists and ecologists with their wealth of information on activity changes in vivo and on the variability and efficiency of the organisms concerned. Furthermore, biochemists, biophysicists and physiologists also now share a lively interest in ultrastructure research, the results and implications of which, through continually improving methodology, have generated important stimuli for the work in the field of cell function.

M. GIBBS and E. LATZKO In the preface to his Experiments upon Vegetables, INGEN-Housz wrote in 1779: "The discovery of Dr. PRIESTLEY that plants have a power of correcting bad air . . . shows . . . that the air, spoiled and rendered noxious to animals by their breath­ ing in it, serves to plants as a kind of nourishment. " INGEN-Housz then described his own experiments in which he established that plants absorb this "nourishment" more actively in brighter sunlight. By the turn of the eighteenth century, the "nourishment" was recognized to be CO . Photosynthetic CO2 assimilation, the 2 major subject of this encyclopedia volume, had been discovered. How plants assimilate the CO was a question several successive generations 2 of investigators were unable to answer; scientific endeavor is not a discipline in which it is easy to "put the cart before the horse". The horse, in this case, was the acquisition of radioactive isotopes of carbon, especially 14c. The cart which followed contained the Calvin cycle, formulated by CALVIN, BENSON and BASSHAM in the early 1950's after (a) their detection of glycerate-3-P as the first stable product of CO fixation, (b) their discovery, and that by HORECKER 2 and RACKER, of the COz-fixing enzyme RuBP carboxylase, and (c) the reports by GIBBS and by ARNON of an enzyme (NADP-linked GAP dehydrogenase) capable of using the reducing power made available from sunlight (via photo­ synthetic electron transport) to reduce the glycerate-3-P to the level of sugars.

It is impossible in a single volume to deal comprehensively with all classes of secondary plant compounds. In the earlier series of this Encyclopedia emphasis was laid on the isoprenoids and plant phenols. While these compounds have not been neglected in the present volume we have attempted to achieve a more balanced presentation by drawing attention to the importance of nitrogenous secondary metabolites such as the alkaloids, amines and non-protein amino acids. Most of the compounds or groups of compounds included in Volume 8 are of restricted distribution within the plant kingdom and wherever possible we have provided information concerning their chemistry, biochemistry, taxonomic signifi­ cance and probable ecological roles. Secondary compounds cannot be defined in terms of restricted distribution, however, nor can they be defined without refer­ ence to the plants in which they occur, as it is possible that a given compound occurring in two species may have a primary role in one and not in the other. As our knowledge of biochemistry increases we shall no doubt find it necessary to revise our ideas concerning the roles of a great many of the compounds which are found in plants.

This is the first of the set of three volumes in the Encyclopedia of Plant Physiology, New Series, that will cover the area of the hormonal regulation of plant growth and development. The overall plan for the set assumes that this area of plant physiology is sufficiently mature for a review of current knowledge to be organized in terms of unifying principles and processes. Reviews in the past have generally treated each class of hormone individually, but this set of volumes is subdivided according to the properties common to all classes. Such an organization permits the examination of the hypothesis that differing classes of hormones, acting according to common principles, are determinants of processes and phases in plant development. Also in keeping with this theme, a plant hormone is defined as a compound with the properties held in common by the native members of the recognized classes of hormone. Current knowledge of the hormonal regulation of plant development is grouped so that the three volumes consider advancing levels of organizational complexity, viz: molecular and subcellular; cells, tissues, organs, and the plant as an organized whole; and the plant in relation to its environment. The present volume treats the molecular and subcellular aspects of hormones and the processes they regulate. Although it deals with chemically distinct classes of hormone, this volume stresses properties and modes of studying them, that are common to all classes.

This is the second of the set of three volumes in the Encyclopedia of Plant Physiology, New Series, that will cover the area of the hormonal regulation of plant growth and development. The overall plan for the set assumes that this area of plant physiology is sufficiently mature for a review of current knowl­ edge to be organized in terms of unifying principles and processes. Reviews in the past have generally treated each class of hormone individually, but this set of volumes is subdivided according to the properties common to all classes. Such an organization permits the examination of the hypothesis that differing classes of hormones, acting according to common principles, are determinants of processes and phases in plant development. Also in keeping with this theme, a plant hormone is defined as a compound with the properties held in common by the native members of the recognized classes of hormone. Current knowledge of the hormonal regulation of plant development is grouped so that the three volumes consider advancing levels of organizational complexity, viz: molecular and subcellular; cells, tissues, organs, and the plant as an organized whole; and the plant in relation to its environment.

R. P. PHARIS and D. M. REID The idea of a separate Encydopedia volume dealing with the "interrelations of plant hormones with factors in the environment of the plant, and its organs and tissues" originated with N. P. KEFFORD, and we are most appreciative of the help and advice provided by Prof. KEFFORD in the formative stages of this volume. We have thus interpreted "environment" very broadly to indude not only factors external to the plant, e. g. , gravity, light, temperature, wind, mechanieal wounding, water, organism s (induding pollen), and magnetic and electric stimuli, but internaI factors as well (e. g. , nutrients, both inorganic and photoassimilate, direction, and time). In our definition of "hormonaI effect", or "hormonaI involvement", we have asked our authors to take a broad ap­ proach, and to examine not only phenomena that are mediated by the known plant hormones, but to discuss as well a wide variety of processes and events where hormonal involvement is implied through more indirect analyses and observations. The volume begins with environmental factors internaI to the plant; R. J. WEAVER and J. O. JOHNSON thus examine "hormones and nutrients", their inter­ relationship in movement, accumulation, and diversion. As one studies a plant during its rapid growth phase, and later as maturation and aging proceed, it becomes apparent that time is an environmental cue of great significance, one which may exert a major influence via hormonal messages.

O. L. LANGE, P. S. NOBEL, C. B. OSMOND, and H. ZIEGLER In the original series of the Encyclopedia of Plant Physiology, plant water relations and photosynthesis were treated separately, and the connection between phenomena was only considered in special chapters. O. STOCKER edited Vol­ ume III, Pjlanze und Wasser/Water Relations of Plants in 1956, and 4 years later, Volume V, Parts I and 2, Die COrAssimilation/The Assimilation of Carbon Dioxide appeared, edited by A. PIRSON. Until recently, there has also been a tendency to cover these aspects of plant physiology separately in most text­ books. Without doubt, this separation is justifiable. If one is specifically inter­ ested, for example in photosynthetic electron transport, in details of photophos­ phorylation, or in carbon metabolism in the Calvin cycle, it is not necessary to ask how these processes relate to the water relations of the plant. Accordingly, this separate coverage has been maintained in the New Series of the Encyclopedia of Plant Physiology. The two volumes devoted exclusively to photosynthesis are Volume 5, Photosynthesis I, edited by A. TREBST and M. AVRON, and Volume 6, Photosynthesis II, edited by M. GIBBS and E. LATZKO. When consider­ ing carbon assimilation and plant water relations from an ecological point of view, however, we have to recognize that this separation is arbitrary.

O.L. LANGE, P.S. NOBEL, C.B. OSMOND, and H. ZIEGLER Growth, development and reproductive success of individual plants depend on the interaction, within tolerance limits, of the factors in the physical, chemical and biological environment. The first two volumes of this series addressed fea­ tures of the physical environment (Vol. 12A) and the special responses of land plants as they relate to water use and carbon dioxide assimilation (Vol. 12B). In this volume we consider specific aspects of the chemical and biological envi­ ronment, and whereas the previous volumes were primarily concerned with the atmospheric interactions, our emphasis here shifts very much to the soil. This complex medium for plant growth was briefly reviewed in Chapter 17, Volume 12A. Since it is difficult to determine the precise physical and chemical interactions in the soil, it is even more difficult to determine the important biological interactions among organisms. Nevertheless there is growing aware­ ness of the significance of these interactions and their effects on physiological processes in the individual plant.

O. L. LANGE, P. S. NOBEL, C. B. OSMOND, and H. ZIEGLER In the last volume of the series 'Physiological Plant Ecology' we have asked contributors to address the bases of ecosystem processes in terms of key plant physiological properties. It has often been suggested that it is not profitable to attempt analysis of complex living systems in terms of the properties of component individuals or populations, i. e. , the whole is more than the sum of its parts. Nevertheless, assessments of ecological research over the last century show that other approaches are seldom more helpful. Although it is possible to describe complex systems of living organisms in holistic terms, the most useful descriptions are found in terms of the birth, growth and death of individ­ uals. This allows analysis of performance of the parts of the whole considering their synergistic and antagonistic interrelationships and is the basis for a synthe­ sis which elucidates the specific properties of a system. Thus it seemsthat the description of ecosystem processes is inevitably anchored in physiological under­ standing. If enquiry into complex living systems is to remain a scientific exercise, it must retain tangible links with physiology. Of course, as was emphasized in Vol. 12A, not all of our physiological understanding is required to explore ecosystem processes. For pragmatic purposes, the whole may be adequantely represented as a good deal less than the sum of its parts.

In 1958, a single volume in the original series of this Encyclopedia adequately summarized the state of knowledge about plant carbohydrates. Expansion into two volumes in the New Series highlights the explosive increase in information and the heightened interest that attended this class of compounds in the interven­ ing years. Even now the search has just begun. Much remains to be accom­ plished; e.g., a full description of the plant cell wall in chemical terms. Why this growing fascination with plant carbohydrates? Clearly, much credit goes to those who pioneered the complex chemistry of polyhydroxylated compounds and to those who later sorted out the biochemical features of these molecules. But there is a second aspect, the role of carbohydrates in such biological func­ tions as host-parasite and pollen-pistil interactions, the mating reaction in fungi, symbiosis, and secretion to name a few. Here is ample reason for anyone concerned with the plant sciences to turn aside for a moment and consider how carbohydrates, so many years neglected in favor of the study of proteins and nucleic acids, contribute to the physiological processes of growth and devel­ opment in plants.

D. BOULTER and B. PARTHIER At the time of the former edition of the Encyclopedia of Plant Physiology, approximately 25 years ago, no complete plant protein amino acid sequences or nucleic acid sequences had been determined. Although the structure of DNA and its function as the genetic material had just been reported, little detail was known of the mechanism of its action, and D. G. CATCHSIDE was to write in the first chapter of the first volume of the Encyclopedia: "There is a consider­ able body of evidence that the gene acts as a unit of physiological action through the control of individual enzymes". No cell-free transcription and pro­ tein-synthesizing systems were available and the whole range of powerful meth­ ods of recombinant DNA technology was still to be developed. Today for the first time with plant systems, it is possible not only to describe their molecular biology but also to manipulate it, i. e. , to move from a description to a technological phase. The properties of living systems are inscribed by those of the proteins and nucleic acids which they synthesize. Proteins, due to their very large size, occur as macromolecules in colloidal solution or associated in supra-molecular colloi­ dal form. The colloidal state confers low thermal conductivity, low diffusion coefficients and high viscosity, properties which buffer a biological system from the effects of a changing environment. Biological systems not only have great stability, but also the capacity to reproduce.

The essential features of constitution, configuration, and conformation in carbo­ hydrate chemistry, so well established in the . first half of this century, had yet to be exploited by those concerned with biochemical and physiological processes in plants when the original Encyclopedia appeared. Two outstanding developments, discovery of sugar nucleotides and the advent of chromatography, brought together the insight and a means of probing complexities inherent in plant carbohydrates. These advances, combined with a modern knowledge of enzymes and cellular metabolism, have provided new horizons of investigation for the student of plant physiology. This volume and its companion (Vol. 13B) present a comprehensive assess­ ment of the current viewpoint in plant carbohydrates with emphasis on those aspects which impinge on physiological processes of growth and development. To accommodate the extensive amount of information to be presented, subject matter has been divided, somewhat arbitrarily, into intracellular and extracellular carbohydrates, with the latter defined as carbohydrates occurring in space out­ side the plasma membrane (plasmalemma). This classification is not exclusive; rather it is intended to lend a degree of flexibility to the way in which subject matter is arranged between volumes. The first section of this volume addresses the occurrence, metabolism, and function of monomeric and higher saccharides of fungi, algae, and higher plants.

With contributions by numerous experts

The first book bearing the title of this volume, Inorganic Plant Nutrition, was written by D. R. HOAGLAND of the University of California at Berkeley. As indicated by its extended title, Lectures on the Inorganic Nutrition of Plants, it is a collection of lectures - the JOHN M. PRATHER lectures, which he was invited in 1942 to give. at Harvard University and presented there between April 10 and 23 of that year - 41 years before the publication of the present volume. They were not "originally intended for publication" but fortunately HOAGLAND was persuaded to publish them; the book appeared in 1944. It might at first blush seem inappropriate to draw comparisons between a book embodying a set of lectures by a single author and an encyclopedic volume with no less than 37 contributors. But HOAGLAND'S book was a compre­ hensive account of the state of this science in his time, as the present volume is for ours. It was then still possible for one person, at least for a person of HOAGLAND'S intellectual breadth and catholicity of interests, to encompass many major areas of the entire field, from the soil substrate to the metabolic roles of nitrogen, potassium, and other nutrients, and from basic scientific topics to the application of plant nutritional research in solving problems encountered in the field.

With contributions by numerous experts

H. F. LINSKENS and J. HESLOP-HARRISON The chapters of this volume deal with intercellular interaction phenomena in plants. Collectively they provide a broad conspectus of a highly active, if greatly fragmented, research field. Certain limitations have been imposed on the subject matter, the most impor­ tant being the exclusion of long-range interactions within the plant body. It is true that pervasive hormonal control systems cannot readily be demarcated from controls mediated by pheromones or information-carrying molecules with more limited spheres of action, but consideration is given in this volume to the main classes of plant hormones and their functions only incidentally, since these are treated adequately in other volumes of this Encyclopedia series (V - ume 9-11) and in numerous other texts and reviews. Similarly, certain other effects, such as those associated with nutrients and ions, are not considered in any detail. Furthermore, we have excluded intracellular interactions, and also consideration of transport phenomena, which are treated in detail in Vol­ ume 3 of this Series. Other aspects of inter-cellular interaction, such as cell surface phenomena and implications of lectin-carbohydrate interactions, and plant-virus inter-relationships, are treated in other sections of this Encyclopedia (Volumes 13B and 14B, respectively). In the volume on physiological plant pathology (Volume 4 of this series) special attention has been given to host­ pathogen interaction. These aspects of our subject will therefore be excluded in the present treatise.

I am honored by the editor's invitation to write a Preface for this volume. As a member of an older generation of plant physiologists, my lineage in plant respiration traces back to F. F. BLACKMAN through the privilege of having M. THOMAS and W. O. JAMES, two of his "students," as my mentors. How the subject has changed in 40 years! In those dark ages B. 14C. most of the information available was hard-won from long-term experiments using the input-output approach. Respiratory changes in response to treatments were measured by laborious gas analysis or by titration of alkali from masses of Pettenkofer tubes; the Warburg respir­ ometer was just beginning to be used for plant studies by pioneers such as TURNER and ROBERTSON. Nevertheless the classical experiments of BLACKMAN with apples had led to important results on the relations between anaerobic and aerobic carbohydrate utilization and on the climacteric, and to the first explicit concept of respiratory control of respiration imposed by the" organiza­ tion resistance" of cell structure. THOMAS extended this approach in his investi­ gations of the Pasteur effect and the induction of aerobic fermentation by poi­ sons such as cyanide and high concentrations of CO , JAMES began a long 2 series of studies of the partial reactions of respiration in extracts from barley and YEMM'S detailed analysis of carbohydrate components in relation to respira­ tory changes added an important new dimension.

The Encyclopedia of Plant Physiology series has turned several times to the topic of photosynthesis. In the original series, two volumes edited by A. PIRSON and published in 1960 provided a broad overview of the entire field. Although the New Series has devoted three volumes to the same topic, the overall breadth of the coverage has had to be restricted to allow for greater in-depth treatment of three major areas of modern photosynthesis research: I. Photosynthetic Elec­ tron Transport and Photophosphorylation (Volume 5 edited by A. TREBST and M. AvRON, and published in 1977); II. Photosynthetic Carbon Metabolism and Related Processes (Volume 6 edited by M. GIBBS and E. LATZKO, and published in 1979); and III. Photosynthetic Membranes and Light-Harvesting Systems (this volume). As we approached the organization of the current volume, we chose a set of topics for coverage that would complement the earlier volumes, as well as provide updates of areas that have seen major advances in recent years. In addition, we wanted to emphasize the following changes in the study of photo­ synthetic systems which have become increasingly important since 1977: the trend toward increased integration of biochemical and biophysical approaches to study photosynthetic membranes and light-harvesting systems, and a renewed appreciation of the structural parameters of membrane organization.