Encyclopedia of Plant Physiology New Series Volume 1 Editors A. Pirson, Gottingen M. H. Zimmermann, Harvard Transport in Plants I Phloem Transport Edited by M.H.Zimmermann and J.A.Milburn Contributors M.J.P. Canny J.Dainty A.F. G.Dixon WEschrich D. S. Fensom D. R. Geiger W Heyser W Holl J.A.Milburn T. R.F. Nonweiler M. V. Parthasarathy J.S.Pate AJ.Peel S.A.Sovonick D.C.Spanner P. M. L.Tammes M. T. Tyree J. Van Die H. Ziegler M. H. Zimmermann With 93 Figures Springer-Verlag Berlin Heidelberg New York 1975 ISBN-13: 978-3-642-66163-1 e-ISBN-13: 978-3-642-66161-7 DOl: 10.1007/978-3-642-66161-7 Library of Congress Cataloging in Publication Data. Main entry under title: Transport in plants I: phloem transport. (Encyclopedia of plant physiology; v. I) Bibliography: p. Includes index.!. Plant translocation. 2. Phloem. I. Zimmer­ mann. Martin Huldrych, 1926~ II. Milburn, John A., 1936~ QK871.T73 582'.041 75-20178 This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically those of translation, reprinting, re-use of illustrations, broadcasting, reproduction by photocopying machine or similar means, and storage in data banks. Under §54 of the German Copyright Law where copies are made for other than private use, a fee is payable to the publisher, the amount of the fee to be determined by agreement with the publisher. © by Springer-Verlag Berlin· Heidelberg 1975 Softcover reprint of the hardcover 1st edition 1975 The use of registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Preface 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. Discussions between the publisher and plant physiologists were therefore initiated to determine if and how the series could be continued. A difficult question concerned the degree of independence of individual fields. Modern biologists, particularly cell biologists, have a tendency to generalize, particularly if their background is primarily in physics or chemistry. Indeed, many basic principles of biology can be considered today well proven for all organisms. On the other hand, nature is extremely diverse; any generalized information that has been obtained in the laboratory with "standard" organisms and in vitro systems has to be compared with results obtained with the wide variety of organisms in nature. In plant physiol­ ogy, as in other fields, this apparent antagonism between general principles and diversity has a stimulating effect on research. Life processes of green plants are being studied with all available methods of modern biology. Electron microscopy, penetrating more and more into the dimen­ sions of molecules, shows increasing concern with the functional aspects of structure. One of the most fascinating ways of studying life processes is the study of their regulation. In this respect, plant physiology is particularly dependent upon the progress made in biochemistry and genetics. It will probably not be long before methods and principles of bacterial genetics can be applied to the much more complex eucaryotic organisms, including green plants with their specific genetic material. Thus, on the one hand, new methods of studying regulation and adaptation break down old barriers between formerly separate fields such as genetics, physiology and ecology. On the other hand, certain areas have remained well defined, particularly those concerning functions of the organism as a whole, such as phloem transport. Today it is impractical or impossible to merely issue supplementary volumes or a revised edition of the Encyclopedia of Plant Physiology. Even though there are still clearly-defined fields, too many of the boundaries have crumbled and new VI Preface combinations of interests are developing. For this reason it was decided not to make subdivisions into predetermined areas in a grand overall publication plan, as it was done in the first edition, but to publish a "New Series" in a much more flexible way. New and expanding fields will be treated separately as the need arises. There will be no introductory volumes to discuss basic principles. Historical concepts, already treated in the old Encyclopedia, will be discussed only if they need to be reconsidered in the light of newer findings. As each volume is to be more or less self-contained, overlapping will become unavoidable in the long run. Such overlap is not too disadvantageous if similar chapters are written by different authors and if the publication dates of the respective volumes are reasonably far apart from each other. Furthermore, it is always desirable that in the case of contro­ versial issues individual representatives can defend their own point of view. The subject matter of larger individual fields will be covered in several volumes, each one however self-contained and complete in itself. Thus the first volumes of the New Series consist ofa set of three, covering transport and exchange phenomena at three levels of organization: the whole plant, tissue and cells, and structures within cells. The New Series differs not only in material from the old Handbook, but also in appearance. The individual volumes will be less extensive, and consequently probably also less costly. They will be written exclusively in English, the language now established as the most suitable for communication in the natural sciences. Moreover, once a manuscript is complete, it will be published within a shorter time than before, probably two or three volumes in the course of each year. These measures should greatly contribute towards distributing the New Series in greater number than the old Handbook, and to making it for years to come one of the most important literary references in plant physiological research. We hope that within the next few years, the New Series will once again cover the whole field of plant physiology, although in a quite different way from the old Handbook. A.PIRSON M.H. ZIMMERMANN Introduction Research on long-distance transport in plants probably began with the work of MALPIGHI in the later 17th century, following the discovery of blood circulation in animals by HARVEY. But the fact that there are two separate long-distance transport channels, the xylem and the phloem, was not recognized until more than 100 years later. The significance of phloem transport remained unclear until the assimilation of carbon dioxide had been discovered by DE SAUSSURE in 1804. Outstanding experi­ mental work early during the 19th century by COTTA, DE CANDOLLE and KNIGHT established the movement of carbohydrates from leaves into stems and roots an~ their storage in the form of starch. THEODOR HARTIG discovered the sieve tube in 1837 and described exudation from both xylem and phloem in 1860. Thus, a reasonably clear understanding of long-distance transport pathways had been reached by the mid-19th century, though the transport mechanisms were unknown. Toward the end of the last century, translocation research suffered from the some­ what dogmatic statement by the great plant physiologist SACHS that diffusion is the mechanism by which assimilates are distributed in plants. SACHS was undoubtedly an outstanding scientist, but his influence was decidedly negative in the field of translocation. Interest in translocation research developed rather slowly at the beginning of the 20th century, possibly because SACHS' influence still lingered on and because interest in plant physiology moved away from whole-plant physiology towards prob­ lems at the tissue and cellular level. During the 1920s, phloem-transport research was re-established in a number of laboratories and for the first time it became quantitative. Mass-transfer studies by DIXON and his students (e.g. MASON) estab­ lished beyond doubt that diffusion was inadequate, by several orders of magnitude, as a mechanism for assimilate distribution over long distances. The search began for a mechanism to explain the phenomenally efficient way in which plants transport solutes rapidly over long distances. The chapters of this volume show that this search has not yet ended to the satisfaction of everyone. In 1926 MiiNCH proposed his pressure-flow hypothesis which was published in greater detail in his book in 1930 by GUSTAV FISCHER in Jena. It had a very profound and lasting effect on virtually all subsequent work. It was considered carefully by MASON and MASKELL and eventually supported by DIXON. Nevertheless, though MiiNCH'S book is unquestionably the most-cited single literature report of phloem transport, it may equally well merit the reputation of the least-read book in the field, at least so far as the English-speaking scientific community is concerned. Interest in the phenomenon of phloem transport has increased steadily during the past 50 years. The number of publications has multiplied annually and several books on the subject have appeared recently.