Plant Hormones D. H. KILLEFFER
T
OMATOES and watermelons are made seedless, roots are caused to grow from leaves where roots have no business to be, and all manner of other strange and exciting things happen to plants under the influence of potent stuffs called "plant hormones". For plants, like animals and men, are affected by medicines. The effects are not of the same kind and the medicines are different, but a general similarity exists in the responses of both plants and animals to external substances. Lest there be misunderstanding of the subject matter here under consideration, let it be clear that we are at present con cerned with substances which influence physiological processes rather than with remedies for pathological conditions. Both plants and animals supply themselves with healing materials to cure wounds and to form scar tissue of an appropriate kind. In plants these substances are formed in and around wounds to aid the healing process, an intensely interesting subject but not germane to the present discussion. Here we shall concern ourselves with those vital materials variously known as growth hormones, growth regulators, auxins, phytohormones, and growth sub stances. Even with that limitation our subject still possesses a broad sweep likely to include, as research progresses, the whole of the vegetable kingdom and the funda mental facts of its life and growth. Within it fall two distinct groups of sub stances—those which, like hormones, are formed during the metabolism of the plants themselves, and another group of ex traneous foreign substances possessing the power of altering the rate or direction of metabolic processes. The second large group, resembling drugs and medicines in their actions, includes an amazing number of apparently quite unrelated compounds of the widest chemical dis similarity, ranging from such simple compounds as carbon monoxide and ethylene to complex alkyl-aryl derivatives including mukimembered ring systems in their constitutions. Physically, these physiologically potent compounds may act
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in the solid, liquid, and gaseous states in concentrations as dilute as a fraction of κ part per mdlion. Six types of responses of plants to chemical stimuli have been obsewed and studied: anesthesia; prolongation or shortening of dormancy; promotion of cell growth, which may be so biased as to cause leaves or branches to grow up ward (hyponasty) or downward (epinasty) contrary to the plant's usual habit; stimulation of rooting, either normal or abnormal, resulting in the growth of roots at unnatural places on the plant; asexual fruiting produced by artificial stimulation rather than natural pollination; and changes in response to temperature or periods of light and darkness. Obviously these effects are quite dis tinct from those produced by the ordinary plant foods embodied in customary, or even extraordinary, fertilisera. Like the vitamins and hormones in the animal body, the auxins seem to exert a catalytic effect on the life and growth of plants, and like the vitamins, the auxins may be destroyed in the processes which they in fluence. Perhaps the best general char acterization of these phenomena is to class them as broadly catalytic. The minute amounts of stimulants required to produce results, the extraneous nature of many of the active agents, the com plexity of the processes involved, and their general similarity to man}* other catalytic processes, all lead to this view, and so long as no better explanation is available such a one may serve. That plants actually manufacture hor mones has been amply shown by studies of the sensitivity of certain species to vary ing periods of light and darkness. The basic facts are matters of common ob servation. Certain plants flower, for ex ample, in the early spring as soon as the length of day reaches a certain point. Others, notably chrysanthemums, post pone their budding and bfoseo.iing until after the long day periods of summer have passed and the shortening of the day has reached a critical value. Apparently there is set up in the plant a series of reac tions which are controlled by the length of
the day. Others respond to changes of temperature. Still others are less sensitive and seem to flower at times determined by factors so complex as to have baffled analysis. Proof that an actual material is formed in the plant under proper conditions of light, darkness, and temperature has been found in the ability of a plant, sensi tive in these respects, to transfer its sensi tivity to another whose habit is otherwise. For example, a branch from & flowering tobacco plant will stimulate flowering in another vegetative plant of the same species if the cut surface of the graft be held in contact with a cut surface of the vegetative plant. This will occur even though actual union of scion and host is prevented by daily breaking of the con tact between the two. Also striking on this subject were experiments with cocklebur plants (Xanlkium). These plants re main vegetative and refuse to flower so long as the length of day is 16 hours. However, when the period of day is shortened from this length, budding and ultimately flowering are initiated. Thus it is possible, by artificial control of the length of day to which the plants are ac customed, to bring together plants in flower and others which show no such inclination. A contact between cut sur faces on two such plants, even though separated by a thin permeable tissue to prevent the formation of an actual union, will bring about the flowering of the vege tative one. Further, a single leaf or even a part of a leaf of a vegetative plant can be ex pose! to light for an appropriate (critical) time and produce in the whole plant the changes anticipated if the whole plant were so exposed. Obviously tm>, too, requires the transfer from one part of the plant to others of some substance which acts catalytically to produce physiological changes. So far this particular change has not been produced by externally added mate rials. It appears to be a true example of the formation of a hormone of some kind in the plant itself. It is obviously caused by a material formed and transferred within the plant, and one may expect its isolation sooner or later as research is prosecuted to that end. The number of variables involved, which must be con trolled to get at the secret of the process, is large, and the complexity of the mate rial from which infinitesimal amounts of
Test for auxin by applying it in agar to decapitated oat coleoptiles. Curvature results as side below agar grows more rapidly.
395
Propagation of plants showing the effect of root-inducing substances on cuttings of the hemlock (A) and the grape, variety Concord (B), both of which were given a series c( identical tests. Left to right. Control; basal ends immersed for 24 hours in indolebutyric acid solution of 40 mg./liter; basal ends dipped in an alcoholic solution of indolebutyric acid containing 20 mg./cc.; wet basal ends dipped in talcum powder preparation containing indolebutyric acid in ratio of 4 mg./gram active substance must be extracted makes the problem still more intricate. Data available indicate that certain of the vitamins are formed in plants and utilised by them to promote and control growth. In some respects this is a blow to man's self-established role an the acme of creation for which all things were made, but it has been clearly shown that the formation of vitamin Bi, for instance, in plants is far more important to them than prevention of beriberi in man.
Because vitamin Bi has been synthesized tuid is to be had in pure form unoontaminated by extraneous materials, its function with respect to plant life has been studied with vigor and the results obtained are clear and definite. By isolating tips' ot roots alone in nutrient solutions containing the essential mineral salts, sucrose, nitrate nitrogen, and as little as one part in 10,000,000 of vitamin B tt flax roots can be grown in vitro at a rate of approximately an inch a day indefinitely.
A test for auxin by straight growth of sections from oat coleoptile». Section at the left is in
The amount required is extremely minute, but under the circumstance* outlined, this vitamin alone is required by flax roote. Indeed, all root systems so far investigated require B lf which may thus be tentatively considered as the plant root vitamin. Other plant* require others in addition. Roots of pea and radish need nicotinic arid as well, and tomatoes produce their best root growth only when supplied with B« in addition to Bi and nicotinic acid. Other plants have even broader requirements. The formation of vitamin Bi ir. the plant is a function of sunlight on its leave*, as can be shown by growing plants in darkness, and consequently it can be considered a true hormone since it must be formed in one part of the plant and trans* ported to another to be effective. Interesting and important are the results of comparative growth experiments made in nutrient solutions with and without the addition of so little as one part of vitamin Bi ii, 100,000,000 of nutrient solution. Grown thus in quarts sand free from organic matter, cosmos may, in addition to being more vigorous, produce as much as 100 per cent more dry weight when vitaminised, and some grasses yield seven times as much dry weight as controls. As little as one part of crystalline vitamin Bi in 10,000,000,000 parts of nutrient solution causes a detectable increase in yield and vigor of certain plants. To visualize so tiny a concentration, it can be translated into the equivalent of a single five-grain tablet of vitamin (that is, the size of an aspirin tablet) dissolved in a tank of water approximately 22.5 feet in diameter by 100 feet high. Beside that amount a needle in a haystack should be easy to find. Otherwise the equivalent of six or eight five-grain tablets of vitamin might be dissolved in the amount of water required to irrigate an acre of land for a year.
water; at right, in auxin. Sections are held in place on combs and measured under the microscope.
396
May 10, 1940 Carbon Monoxide
α ο
NEWS CUfl—CHt Ethylene
CH*CU=CHt Propylene
CH==CH Acetylene
COCOOH
HrCOOH
Pbenylaoeiic Acid
CHiCOOH
•t-Naphthalenegjyoxalic Acid
J-Indoleaoetic Acid
HCOHCOOH
H=vCHCOOH
Phenylacrylic Acid
HH=CHCOOH
ff -
NO, m-Nitrocinnamic Acid
/YNcKDHrCOOH ^-Napnthoxyaoetic Acid
ω
EDITION
ICHiCHiCOOH
*· Naphthalenegiyeollic Acid
.Mnuolep.-opionic Acid
/\/N>OCHiCONH,
'\
â-Naphthoxyaeetamid
i-Indolebutync Acid
~|(CHi)»C\)H
5/
(XT'
:H(CHI)COOH 1—CHiCOOH
.^-NapbtLoxy-2-Propionic Acid
Flttoreneaeetic Acid
CHtCOOH
o-Napbthaleneaeeiic Acid
|^/\)CH(CsH*)COOH
UJ
h-CHrCOOH
£-Naphthoxy-2-n-Butyric Acid
Anthraceneaeetic Acid
Chemical structures of compounds none urn* physiological activity vary widely as shown by these typical compounds, which were selected as illustrative oy P. W. Zimmerman. Ordinarily organic fertilisers (stable manure and the like) supply vitamin· to the soil· It is a curious fact that while isolated roots appear t o be universally dependent upon supplies of this and other vitamins for their prolific growth, many, if not all, of the great economic crops (corn, wheat, tomatoes, notably) are not particularly benefited by the addition of an excess of this substance t o the soil in which they are grown. The answer is that these plants, probably through the long processes of selection by agricultural ists, have capacities for producing their own vitamins in quantities sufficient for their needs. As vitamin Bi affects the growth of roots, so several other substances have been found to influence and promote growth of other parts of plants. Several specific amino acids, arginine, and a purine, adenine, have been found among others t o promote leaf growth in small concentrations tn wiiro. Furthermore, when added to the soil in which the plants are grown, they also promote leaf growth, and adenine, particularly, pro motes the formation by the plant of vita min R , a e welL At least one substance proved t o be a hormone for the healing of wounds on plants is known. This hormone, found in the aqueous extract of the pods of beans, produces wartlike growths when applied in solution t o growing pods. The same activity characterises 1-deeene-l,· 10-oUcarboxylic acid which has been both isolated from the extract and synthesised.
Growth of cells at an extraordinary rate was the phenomenon which first directed attention to the physiological potency of
397 certain extraneous substance*. Expérimente with ethylene in the atmosphere around plants showed that concentrations so dilute as to be almost undetectable by other means affected the rate of cell growth in a peculiar and specific manner. Leaves of tomato plante, for instance, exposed to atmospheres containing no more than a few parte per million of the hydrocarbon develop a tendency to grow downward instead of upward as normally. This downward curling of leaves is quite distinct from wilting, as the leaves acquire a strength of curl difficult to strnighten out in contrast to the weakness accompanying wilting. Apparently the cause of the change is a differential in the rate of growth between the upper and the lower sides of the leaves. The extreme sensitivity of tomato plants to this reaction has been adopted widely as a method of detecting minute concentrations of ethylene and the effect of this gas on other plants has been carefully studied. Reaction is prompt and definite. N o question remains as to its occurrence after even a short exposure, the time required being inversely related to the concentration of the gas. The ease of recognising differential growth of cells, apparent as curving growth, has made this the usual reaction for detecting physiological activity of unknown substances, for it is not confined to ethylene alone. Ethylene also exert* what seems to be an anesthetic action on plants similar in some respects to its known action on ani-
A. Control p l a n t B. The sensitive plant, Atum+m pudicA. was anesthetixecl by exposing it tor 18 hours to 1.0 per cent carbon monoxide cas. Note the change in normal equilibrium position of the leaves and the disturbance of normal correlation. Unsaturated hydrocarbons and other growth substances cause similar «fleets.
398 mab. The sensitive plant, for example, lose* its sensitivity when treated with ethylene, and other plants similarly be come less sensitive to normally active stimuli. Its important applications are based upon this, for along with anes thesia, there is a distinct stimulation of certain processes in fruits. Thus, the coloring of oranges by ethylene has bt-en widely practiced and it also affects the dormancy of potatoes and other buds, permitting control of this rest period between maturity and new growth. The remarkable fact about extraneous substances which catalyze plant processes is the similarity of the actions of totally different chemical compounds. For in stance, control of the period of dormancy or potato buds can be accomplished by the relatively simple gaseous hydrocarbon ethylene (C\H«) in very much the same way thnt it can be done with a solid, complex aryl-alkyl acid, indolebutyric acid
