Auxins and Plant Growth
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A. Carl Leopold
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Auxins and Plant Growth - A. Carl Leopold
AUXINS AND PLANT GROWTH
AUXINS
AND PLANT GROWTH
A. Carl Leopold
UNIVERSITY OF CALIFORNIA PRESS
BERKELEY AND LOS ANGELES 1963
UNIVERSITY OF CALIFORNIA PRESS BERKELEY AND LOS ANGELES I CAMBRIDGE UNIVERSITY PRESS, LONDON, ENGLAND COPYRIGHT, 1955 BY THE REGENTS OF THE UNIVERSITY OF CALIFORNIA THIRD PRINTING, 1963 LIBRARY OF CONGRESS CATALOG CARD NO. 55—5386 MANUFACTURED IN THE UNITED STATES OF AMERICA
Dedicated to Kenneth V. Thimann
PREFACE
It is my strong conviction that the technology or the applied science in a field of knowledge can make efficient progress only when pursued with sound understanding of the fundamental science upon which that technology is based. Progress in technology without a foundation of fundamental science can take only two forms, I believe. First, minor improvements of applications or treatments which are already essentially known can be made by technology alone; and second, occasional (though rare) accidental discoveries of new applications or treatments may be made. Practically every major advance in agricultural technology in the past half century has stemmed from fundamental scientific information.
In view of this strong conviction, it is with considerable alarm that I have become more and more aware that a large proportion of research work, dealing with auxins and growth regulators, is being done without cognizance of what auxins do in plants, how they are formed or destroyed there, or even how one tests for them. This book is written in the hope of providing the agricultural research worker with a brief review of the physiological basis, so far as it is known, upon which the applied technologies rest.
Although several books are currently available in the area of auxin physiology it is my feeling that none of them provide the research man with as complete a general integration of the field as is needed. Especially lacking has been an organized description of the various techniques for obtaining, measuring and identifying auxins, as well as an integration of the fundamental and technological aspects of auxin physiology.
It is my intention to present the general status of knowledge of auxins in plant physiology, and to integrate this fundamental information with each of the applied phases of auxin technology. I have written for the graduate student and the professional research man.
This book does not attempt to review in detail the development of knowledge of auxin physiology, for that is a function of the annual and quarterly reviews. On the other hand, it is not intended that the book should become a manual for the practical applications of auxins.
It is hoped that the book will help to reverse the present tendency to design and carry out research programs involving auxin applications without taking advantage of the great progress that has been made in fundamental physiology. By utilizing the footholds which fundamental discoveries have established, agricultural technology will not be restricted to small consolidations of already known phenomena. Nor will it be restricted only to occasional and accidental discoveries of really new applications. Instead technology may advance in a rapid and orderly manner toward greater and greater efficiency in agriculture at a time, now, when greater agricultural productivity is vital to the peaceful progress of our overpopulated world.
Grateful acknowledgment is made to Drs. J. van Overbeek and F. Skoog for reading all of the manuscript critically, as well as to Drs. J. Bonner, J. C. Crane, E. L. Rice, L. M. Rohrbaugh and G. F. Warren and Professors C. L. Burkholder and R. Klackle for assistance and suggestions concerning individual chapters. Special thanks are due to Dr. S. P. Sen for much of the preparative work on the section dealing with paper chromatography, to Mrs. Frances Scott Guernsey for her abundant and enthusiastic help with the first draft, to Dr. N. K. Ellis for encouragement, and to my wife, Keena, for being so patient.
The literature review for this book was concluded in June, 1954.
Department of Horticulture A. CARL LEOPOLD
Purdue University Lafayette, Indiana
July 14, 1954
CONTENTS 1
CONTENTS 1
CHAPTER I Development of Knowledge of Auxins
CHAPTER II Auxin Extraction and Measurement Techniques
CHAPTER III The Occurrence and Role of Growth Hormones in Plants
CHAPTER IV Developmental Effects of Auxins
CHAPTER V Physiological and Anatomical Effects of Auxin Applications
CHAPTER VI Factors Altering Effectiveness of Auxin Appl cations
CHAPTER VII The Chemical Nature of Growth Regul tors
CHAPTER VIII Theories of the Mechanism of Auxin A tion
CHAPTER IX Methods of Applying Auxins and Their Persistence in Plants and Soils
CHAPTER X Rooting
CHAPTER XI Parthenocarpy
CHAPTER XII Flower and Fruit Thinning
CHAPTER XIII Control of Pre-Harvest Fruit Drop
CHAPTER XIV Flowering
CHAPTER XV Dormancy and Storage
CHAPTER XVI Herbicides
CHAPTER XVII Miscellaneous Uses of Auxins
CHAPTER XVIII Potentials of Auxins and Auxin Research
Bibliography
APPENDIX
Index
CHAPTER I
Development of Knowledge of Auxins
The development of the knowledge of auxins, as in the case of nearly all major scientific advances, originated in experimental inquiries by scientists seeking answers to fundamental questions. If any one worker could be identified as first in the field, perhaps it would be Charles Darwin, who in 1881 published his book The Power of Movement in Plants. His investigations of fundamental questions about plant tropisms opened a minute gate in the dike of the unknown which ultimately led to the flood of information concerning not only tropisms, but the whole general field of plant growth and the growth hormone as well. In the seventy years following this modest beginning, the role of the growth hormone in plants has been clarified to an almost startling degree. The revealed capacity of many chemical compounds to exploit the same mechanisms in plants has led to a situation nearly approaching an agricultural revolution. Auxins and growth regulators promise to have an impact on agriculture as great as the advent of the windmill or perhaps even of the mechanical harvester. At the same time the impact on the science of plant physiology is as great as that of any other single development since the turn of the century.
DEFINITIONS
There has been a considerable confusion among physiologists and agriculturists concerning the terminology of auxins and growth hormones. Before entering into an extensive discussion of these compounds, it will be well to define our terms.
A hormone has been accepted for many years as being a substance which, produced in any one part of an organism, is transferred to another part and there influences a specific physiological process
(Went and Thimann, 1937, p. 3). It may be advisable to emphasize that hormones are produced in the organism, and have the property of serving as chemical messengers, i.e., they are transported from a site of formation to a site of action. In contrast to some compounds which act as substrates for general physiological processes, hormones influence a specific physiological process,
and in each case minute amounts produce a large physiological effect. It is evident that, by this definition, the word hormone cannot apply to chemicals produced exclusively in a laboratory, nor to such indigenous compounds as sugars, amino acids or other substrates for growth. Although these latter types of compounds may be transported, none of them are specific in their control of physiological process, as far as is known. The status of vitamins in this nomenclature is not clear. They are considered to be phytohormones by some (Bonner and Bonner, 1948) in cases where they are transported in the plant.
A phytohormone, or plant hormone, by simple projection of the preceding definition, is a hormone produced specifically in plants. Within this category lie the growth hormone, the still-theoretical flowering hormone, and possibly some vitamins. A few other phytohormones have been proposed from time to time, but these first two are the only ones generally recognized at present. Thimann (1948) has defined a phytohormone as an organic substance produced naturally in higher plants, controlling growth or other physiological function at a site remote from its place of production, and active in minute amounts.
The growth hormone is the phytohormone which is involved in growth. It is normally essential to growth by cellular enlargement, both in length and in width, and is essential to growth of organs— buds, stems, roots, fruits, and so on.
The term auxin was originally suggested to refer to substances which were capable of promoting growth in the manner of the growth hormone (Kogi & Haagen-Smit, 1931). The search for the actual growth hormone in plants led to the isolation of the first pure active compound, not as the indigenous active hormone, but instead as an ingredient of such miscellaneous biological materials as urine, corn oil, and malt extract. The first such compound was named auxin a, the second auxin b. In keeping with the original intent of the term, auxin will be used to refer to an organic substance which promotes growth (i.e. irreversible increase in volume) along the longitudinal axis when applied in low concentrations to shoots of plants freed as far as practicable from their own inherent growth-promoting substances
(Thimann, 1948). This growth promotion may be conveniently measured by any of the standard auxin assay methods described in chapter II. The growth hormone itself is an auxin, and so are any other chemical compounds which can bring about the same growth effect.
The term growth regulator refers to organic compounds other than nutrients, small amounts of which are capable of modifying growth. Included in this category are substances which either stimulate, inhibit, or otherwise alter growth. It is presumed that growth regulators act upon growth by altering the net effect of the growth hormone, though this is speculative in many cases. Included as growth regulators are such substances as auxins, anti-auxins, epinastic agents and other types of materials as discussed in chapter VII.
In the early literature, the term heteroauxin was used to refer to indoleacetic acid (Kogi et al, 1934). Since the time when this term was suggested, a great quantity of evidence has accumulated indicating that indoleacetic acid is indeed the commonest growth hormone, and hence to classify it as an outsider
or heteroauxin seems entirely misleading. In accord with the suggestion by Thimann (1948), we will not use the term in the present discussion.
Unfortunately there has been a considerable usage of the term hormone in referring to synthetic auxins or other growth regulators. This has been done largely to strike an appeal to potential buyers of commercial products containing auxins. Such a misuse of terms is definitely confusing and has no place in scientific writing. The term hormone will be used in the present discussion to refer only to substances identical with those known to be indigenous to the plant, and known to act as hormones in the plant.
TROPISMS AND THE EARLY WORKERS
When Charles Darwin turned his brilliant mind to the study of plant movement and tropisms, the first glimmering of the existence of the growth hormone was revealed. Darwin’s simple and logical experiments, using canary-grass seedlings, a light source and a razor blade, told him that the tip of the shoot is involved in the overall tropic response. Removal of the tip was followed by loss of sensitivity to light in the coleoptile below. He concluded (1881) that when seedlings are freely exposed to a lateral light, some influence is transmitted from the upper to the lower part, causing the latter to bend.
Darwin’s work aroused much interest and discussion, and led eventually to the work of Boysen-Jensen (1913) in Germany, who found that although severing the oat coleoptile tip removed phototropic sensitivity, simply replacing the tip restored the sensitivity again. He concluded that the transmission of the irritation is of a material nature produced by concentration changes in the coleoptile tip.
Careful repetition of Boysen-Jensen’s work in turn by Pal (1919), a physiologist at Budapest, amply confirmed the earlier work and added one simple but crucial point. Paal found that replacement of the severed tip on one side of the coleoptile stump would produce curvatures away from the treated side. This in fact was a replacement of the effect of lateral light by the asymmetrical distribution of some stimulus being produced by the tip. Paal then came to the conclusion that "the tip is the seat of a growth regulating center. In it a substance (or mixture) is formed and internally secreted, and this substance, equally distributed on all sides moves downwards through the living tissue. If the movement of this correlation carrier is disturbed on one side, a growth decrease on that side results, giving rise to curvature of the organ/’
By his careful research, Paal had come essentially to the explanation of phototropism, but more important still, he had demonstrated the existence of a substance
or correlation carrier,
which could control growth processes.
The wider concept of Paal’s correlation carrier
as a growth hormone intimately involved in all plant growth was left for two other minds to grasp almost simultaneously.
EMERGENCE OF THE HORMONE CONCEPT
At the time that Darwin was studying the nature of the phototropic stimulus, Sachs (1880) launched the first theory of substances (which we would now call hormones) controlling plant growth. He envisaged the existence of organ-forming substances moving in various polar patterns, and controlling form and development. Fitting (1909) actually extracted substances from orchid pollen which could cause swelling of the ovary in a manner suggestive of fruit-set. He suggested that these substances were hormones. After Paal’s (1919) deduction that specific substances produced in the coleoptile tip were responsible for phototropism, Soding (1923) established that these same substances were capable of stimulating straight growth as well.
The demonstrations of a correlation carrier in oat tips attracted many new workers to the field. Among these were Cholodny (1927) in Russia and F. W. Went (1928) at Utrecht, who independently extended the correlation carrier theory to both phototropism and geotropism. Each of them then came to the conclusion that all tropisms were mediated by a growth hormone system which was essential to all plant growth. Ohne Wuchsstoff, kein Wachstum
; without auxin there is no growth.
In carrying out his exploration of the role of auxin in growth, Went did two things which opened the field of growth hormones to really systematic study. He discovered that the hormone could be collected in an agar block by diffusion. This was not only the first separation of the hormone from a plant, but it also presented a technique for obtaining the hormone from a great variety of plant materials. Then, utilizing this technique, he worked out a quantitative test for auxin. Using the oat coleoptile, about which so much information had now been accumulated, he established a test so accurate and reproducible that it still stands today as the best auxin assay technique, the Avena test. Thus there arose simultaneously a general theory of the growth hormone, a technique for obtaining the hormone from plants, and a technique for its quantitative assay.
With such a sound basis on which to proceed, it is hardly surprising that a great deal of very productive work followed. In a short eight-year period, from 1928 to 1936, three auxins were isolated, characterized and identified, the quantitative relationships of auxin to tropisms of roots and shoots were established, and at the end of this period half of the major functions of auxin in growth and development as we now know them had been already discovered.
In series of remarkable inquiries, Kogi (1933) and co-workers in Holland found two materials, strongly active in the Avena test, which they named auxin a and auxin b. When isolated and characterized these materials were found to have the molecular formulas given below. Auxin a was first isolated from human urine, and auxin b from corn germ oil. These compounds are principally of historic interest, for they have never been positively isolated from growing plant tissue. For this reason they are not considered to be growth hormones by most physiologists today. There were two lines of evidence which led to the early assumption that auxins a and b were growth hormones. First, their molecular weights are similar to that of the diffusible auxin obtained from oat coleoptiles and some other plant materials (e.g. Went, 1928). Auxin a has a molecular weight of 328, which is quite close to Went’s figure for diffused auxin from Avena, 372. Second, tests for stability show that both auxin a and auxin diffused from coleoptiles are stable to warm acid and break down in warm alkali. The poor reliability of these criteria in the positive identification of growth hormones is discussed in chapter III.
8 Fundamentals of Auxin Action
After some time it was found that not only could auxins a and b not be found in plant materials, but also synthetic analogues of these acids have failed to show auxin activity in conventional growth tests (Kogi and de Bruin, 1950). Consequently it seems reasonable to discard these early substances from the list of known auxins.
Further research by Kgl’s group (1934) identified another auxin compound. Some of the urine samples they worked with exhibited considerably greater auxin activity than could be accounted for as the supposed auxins a and b. They set about isolating this compound and found it to be identical with indole-3-acetic acid. The presence of this auxin was demonstrated in Rhizopus cultures by Thimann (1935), and he showed that it can be differentiated from auxins a and b by its instability in warm acid and stability in warm alkali. This is the opposite reaction to that of auxin a. Auxin b is unstable in either acid or alkali. In the subsequent twenty years indoleacetic acid has been conclusively demonstrated to be the principal growth hormone in seven different species of higher plants, and it is generally accepted now as being the commonest growth hormone in higher plants.
The quantitative relationship of the growth hormone to tropisms was essentially clarified soon after the Avena test was made available. Blaauw (1918) had shown that phototropic movement in Avena was owing to a retardation of growth on the lighted side. Went (1928) now associated this retardation with a decrease in the relative hormone content of the lighted as compared to the unlighted side. Dolk (1929) showed that geo tropic movement in Avena was caused by a simple redistribution of the hormone present. His measurements of diffusible auxin from geotropically stimulated coleoptiles showed that 62.5 per cent was in the lower half. Geotropism in roots was found to be brought about by very similar changes in relative hormone contents of the upper and lower root halves (Hawker, 1932).
With the advent of Went’s Avena test, it was quickly found that different concentrations of auxins may have opposite effects on growth. Where low concentrations of auxin promote growth, higher concentrations inhibit it. Knowledge of the inhibitory capacity soon led to the discovery by Thimann and Skoog (1933) that auxins which are formed at a plant apex inhibit the growth of lateral buds. This explanation of the phenomenon of apical dominance clarified one of the major correlative effects which had puzzled many earlier physiologists.
Further dramatic demonstrations of the physiological roles of auxins came from the experimental treatment of plants with chemical compounds which had been found to possess auxin activity. Went (1934) discovered that auxins can stimulate the formation of adventitious roots (another correlative effect clarified). The full realization of the developmental importance of auxin in morphological differentiation was brought forth much later by Skoog and Tsui (1948), who found that relative auxin levels in plant tissues play a crucial role in determining what sort of growth will take place. Thus auxin content in relation to other plant constituents determines whether growth will be simple cell proliferation, or bud formation, or root formation.
Another role of auxin was discovered by LaRue in 1936. He found that auxins applied to leaves could retard leaf abscission. Later workers have found that abscission of all plant organs (leaves, flowers, fruits, etc.) is correlated with low natural auxin content.
The discovery that auxin plays a key role in parthenocarpy and in fruit-set in general stems from the work of Gustafson (1936).
In a very short time indeed, after the perfection of the Avena test, we see that the essential framework of auxin functions in plants had been brought to light. Auxin was found to be essential for growth, and to be able either to stimulate or to inhibit growth, depending upon the concentration. It was found to be the controlling agent of tropic responses, the major control of apical dominance, and a primary factor in organ formation, in the abscission or non-abscission of plant parts, and in the commencement of fruit development.
Looking at the framework of auxin functions as an assembled structure, the physiologist may see the higher plant as an organism controlled and regulated in its growth and development by generally continuous streams of growth hormones. These hormones, formed in each of the plant extremities, course through the organism from the extremities inward, where they not only control growth itself but mediate the differentiation of new organs and tissues, the shedding of old organs, and the stepwise progression of many of the stages in the life cycle of the organism. In this view, then, the higher plant is not a simple colony of cells but is an entity sychronized in its growth, movement and development principally by streams of the growth hormone flowing through the various plant parts in extremely minute quantities (figure 1).
We should remember that this picture of plant growth is limited in completeness principally by our limitations of knowledge. We are forced to interpret growth phenomena principally on the basis of auxin functions largely because the auxin effects are the ones with which we can experiment most effectively and hence which we know best. This may result in concepts somewhat biased toward auxin, as
Burstrom (1953) has pointed out, but until we know of more subtle controlling influences, these concepts will stand as the most complete descriptions of plant growth and development known.
As time goes on, new research appears to be attributing more and more physiological functions to the growth hormones in plants. The magnitude of their influences on the physiology and anatomy of the plant is undoubtedly still not fully recognized.
THE AGRICULTURAL IMPACT
The development of knowledge of the auxins has had a remarkable effect on the agricultural sciences. Not only has it made possible a much better understanding of many physiological phenomena in plants, but also it has given the agriculturist a set of new tools for use in plant culture. The technological applications of auxins have been expanded extremely rapidly. Unfortunately a very great number of workers have launched into research on the uses of auxins without first obtaining a background in the fundamentals of auxin action in plants. This has led to much confusion, a large amount of misinterpretation of results, and a lack of understanding of what intelligent steps can be taken to obtain desired effects. This confusion, caused in considerable degree by lack of background on the part of research workers, has brought some disrepute to the agricultural uses of growth regulators. Now that the early flare of expansion of agricultural uses is apparently past, it is hoped that further research concerning the uses of these physiologically active compounds will be carried out with a more comprehensive background in physiology.
The use of auxins in agriculture started very modestly. The first application to be discovered was in the rooting of cuttings. In 1934, Went published the results of his experiments which showed that auxins could be used to stimulate root formation. This discovery led to a great many studies of the uses of auxins in plant propagation. At present there are several commercial preparations on the market for this use, and when they are used wisely they are very effective indeed.
In 1935, Zimmerman and Wilcoxon discovered that alpha-naphthaleneacetic acid and some chlorinated phenoxyacetic acids are very strong auxins. The activity of these and particularly of 2,4-dichloro- phenoxyacetic acid (Zimmerman and Hitchcock, 1942) was so much greater than the compounds which had been used previously, that the way was opened for some of the greatest practical uses of auxins.
Discoveries of further practical uses followed in quick succession. In 1936, Gustafson demonstrated that some fruits could be set parthe- nocarpically with auxins. Trials of this technique on commercial fruit crops have led to profitable uses in two crops: tomatoes and figs. In the same year, 1936, the discovery by LaRue that abscission of leaves could be delayed led ultimately to the application of auxins to prevent premature abscission of tree fruits. In 1942, Clark and Kerns found that flowering could be induced in pineapple by auxins. The pineapple industry in Hawaii and in Puerto Rico quickly took up the use of auxin sprays to bring about uniform flowering in each field. During World War II, the herbicidal properties of the phenoxy ace tic acid derivatives were carefully studied under cover of military secrecy both in England and in the United States. These studies were first legitimately published in 1945 and 1947 by four research teams: Slade et al (1945), Blackman (1945), Nutman et al (1945) and Kraus and Mitchell (1947). The herbicidal uses of growth regulators are by far the most widespread. It is estimated that 60,000,000 pounds of 2,4-D are manufactured annually in the United States and perhaps 50,000,- 000 acres of land are treated with this herbicide alone. The dramatic rise in the production of 2,4-D since its first release from secrecy in 1945 can be seen in figure 2.
The development of an understanding of auxins in plant growth has effected startling advances in physiological and technological knowledge. Although the understanding of theoretical physiology has in many respects been outstripped by the technology of the application of auxins, still that is where the foundations of the entire field were laid, and the only sound basis for efficient progress lies in the advances of fundamental physiological understanding.
CHAPTER II
Auxin Extraction and
Measurement Techniques
The majority of studies involving auxins either requires measurements of auxin activity or would be greatly benefited by the inclusion of such measurements. By the carefully worked out techniques of Went and other more recent workers, such measurements are quite easy to make. Techniques are available for which practically no elaborate equipment is needed and the time involved is quite small indeed.
It should be clearly understood that all procedures for the determination of auxin content of plant materials do not measure the same constituents. There are many forms of auxin in the plant. The diffusion technique yields a readily available auxin. Extraction of plant material with cold solvents for a short period of time is thought to yield the same auxin, and in some cases the diffusible auxin has been shown to be quantitatively similar to that obtained by short time extraction. This supply of auxin is commonly termed free auxin.
When extraction is continued over a long period of time there is clearly a production of additional auxin by the plant tissue during extraction and this production can continue over a period of several months of extraction (Thimann and Skoog, 1940). It is assumed that forms of the hormone which are not extractable in a short time are largely unavailable immediately for plant growth, and these forms are spoken of as bound auxin.
The concepts of free and bound auxin, however, are not entirely simple. For instance, Thimann and Skoog (1940) have shown that pure indoleacetic acid added to plant material is not entirely recoverable by short time extraction. Some of it is retained by the plant material and appears in the solvent only after several hours of extraction, in a manner more characteristic of bound auxin than of free auxin. More recently Siegel and Galston (1953) have been able to follow the binding of free auxin onto a protein in vitro. There are many examples of bound auxin being released in the free form during extraction, as mentioned before. Consequently it appears that the free and bound forms are in a dynamic state, and the measurement of one strictly separated from the other is often difficult.
METHODS OF OBTAINING AUXIN
Diffusion
The simplest method of obtaining the growth hormone from plant material is by diffusion into agar. The usual procedure followed is simply to sever the growing tip or other organ to be tested under conditions which discourage transpiration, and place the cut surface for a period of an hour or so on a block of agar, usually 1.5%. This technique yields auxin immediately available for growth and, in etiolated seedlings, it can be shown in many different ways that growth is proportional to the auxin obtained by diffusion.
Three main difficulties can arise in the use of diffusion techniques:
1. The excessive loss of water, or a negative tension in the vascular system—as for example in leaves which have been recently exposed to sunlight—can prevent the accumulation of diffusate in the agar block. This difficulty can sometimes be alleviated by carrying out the diffusion in a high relative humidity, or even with the diffusion source under water. However, many types of leaves and stems will not yield diffusible auxin quantitatively under such conditions.
2. The destruction of auxin at the cut surface frequently interferes with the quantitative yield. Such destruction is apparently enzymatic, in some cases attributed to polyphenol oxidase and in others to peroxidase. Browning of the cut surface may indicate danger of destruction by polyphenol oxidase. Several methods of reducing this destruction have been used. The cut surface may be pressed onto wet filter paper to reduce the amount of enzyme (van Overbeek, 1938). Another method is the incorporation of 10-3 ascorbic acid into the agar as an alternative substrate for the enzyme (Wetmore and Morel, 1949). A promising method appears to be the use of 0.005 M potassium cyanide in the wet filter paper first used to blot the cut surface, and as a droplet on the surface of the agar block as well. Then before the blocks are used for the Avena test, a drop of 0.005 M ferrous sulfate is placed upon the block to precipitate the cyanide ions present. Use of the poison causes no interference with the bioassay (Steeves et al, 1954).
3. The existence of growth inhibitors common in many green tissues will prevent the effective use of diffusion techniques. No means of separating auxins from inhibitors have been worked out for quantities obtained by diffusion, and extraction procedures must be used to this end. Methods of detecting the presence of inhibitors are discussed in the section dealing with the Avena test.
Solvent Extraction
A second method of obtaining the growth hormone from plant tissues is by means of solvent extraction. Early studies using this technique employed chloroform as the solvent (Thimann, 1934). It was found that if the plant material were acidified before extraction, larger quantities of auxin were obtained. A serious drawback to the use of chloroform, however, is the slow accumulation of a toxic substance, perhaps an auxin inactivator, thought to be chlorine (Thimann and Skoog, 1940). The most satisfactory solvent has been found to be diethyl ether (Boysen-Jensen, 1936). The presence of spontaneously-
formed peroxide in the ether will destroy easily oxidizable auxin, so before use the ether should be redistilled over ferrous sulfate and calcium oxide in a small amount of water. The redistilled peroxide- free ether is made much more effective as a hormone solvent if a small amount of water (5%) is added.
Many other solvents have been used for auxin extraction. Ethyl alcohol has been used, but yields smaller amounts than does ether. Water has been used (Gorter, 1932), but inactivation of much of the auxin may take place during the procedure. Terpstra (1953) has nearly eliminated such inactivation during water extraction by adding sodium diethyl di thiocarbamate (100 p.p.m.). Combination of several solvents for complete auxin extraction has been effectively employed by Avery et al (1941).
The relative solubility in ether of indoleacetic acid, with changing pH, has been worked out by Gordon and Nieva (1949). It is evident from figure 3 that most efficient extraction of this auxin from wet plant material into ether can be carried out at pH 2.8. It can also be seen that in slightly alkaline solutions the auxin will move out of the ether and into the water partition, a feature which can be utilized when one wants to take the auxin back into water. A saturated solution of sodium bicarbonate is very effective for this alkaline separation into water.
If the extraction procedure is preliminary to measurement of the free auxin in the plant tissues, the formation of auxin during extraction may be a large source of error. The formation of auxin during extraction has been shown by Skoog and Thimann (1940) to be enzymatic in nature.