Present Status of
PLANT HORMONES P. W. ZIMMERMAN Boyce Thompson Institute for Plant Research, Inc., Yonkers, N. Y.
Phenoxy and benzoic acids substituted in t h e nucleus with halogen, methyl, and nitro groups have opened a new line of attack on plant hormone problems. Phenoxyacctic acid is slightly active for cell elongation b u t does not induce formative effects of growing plants. When substituted i n t h e ortho and para positions, t h e resulting dichlorophenoxyacetic acid is effective, causing cell elongation, adventitious roots, parthenocarpy, and formative effects. p-Chlorophenoxyacetic is less active than t h e dichloro, and t h e ortho compound is less active than the para, Bromo-substituted compounds are slightly less active than t h e chloro. 2,4,6-Tribromophenoxyacetic acid is inactive, b u t 2,3,5-triiodobenzoic acid is very active. The higher homologs, such as chlorophenoxy-or-propionic and chlorophenoxy-a-butyric acids, are active for cell elongation and adventitious roots but not very
active for formative effects. Benzoic acid is physiologically inactive until substituted with halogen or nitro groups. 2-Chloro-5-nitrobenzoic acid is inactive for cell eIongation but is active for modifying t h e pattern of leaves. 2-Bromo-3-nitrobenzoic acid is active for both factors. Considering the increased activity of substituted phenoxy and benzoic acids over phenoxy and benzoic acids, t h e activity of many other substances i n t h e hormone field might be increased by this method. The principal uses for growth substances are to propagate plants from cuttings, prevent preharvest drop of apples, induce seedless fruit (especially tomatoes), and inhibit buds (especially buds of potato tubers). Flowering of plants can be regulated to some extent with t h e most active compounds. Treatment of seeds with growth substances to increase t h e crop has not been effective.
T
HE purpose of this paper is t o list the most important physiologically active substances discovered to date and to discuss some of their practical applications which have been reported from or verified at the Boyce Thompson Institute laboratories. No references will be made t o other workers in this field. Owing to the fact that hormonelike substances induce many varied responses when applied to plants, unwarranted claims have been made, especially for the purpose of exploitation. Vitamin B1 has fallen by the way as a cure-all for plants, although recently attempts were made to revive it for horticultural practice, The vitamin stories printed in the trade journals and advertisements were fantastic but popular, and it took scientists a long time to convince gardeners that they were wasting their time and money when they added B1to fertilizers or sprayed it on their lawns. Although growth substances of the auxin groups have some practical value, they are not cure-alls and must not be considered as fertilizers. They are commonly referred to as plant hormones although the ones in practical use (with the possible exception of the indoles) have never been isolated and chemically identified from green tissue. This, however, should not be taken to mean that these substances do not exist in nature. A variety of physiologically active substances has been extracted from fungi, pollen, seeds, bark, and growing parts of plants. These crude extracts, when applied to growing plants, bring about responses similar to those induced with synthetic compounds. In some cases these substances may be by-products of metabolism, acting like hormones only when applied in relatively high concentrations. For practical purposes the synthetic substances are applied a8 acids, esters, amides, or salts. Wherever a given acid is
active, its derivatives (salts, esters, and amides) have approximately equal activity. Owing, however, to their differences in volatility, solubility, availability, or ease of application, one may be preferred over the other for a particular use. When applied as a vapor, the methyl or ethyl esters are preferred because they are more volatile than the acids; when used as a lanolin preparation, the acid may be preferred; and when used as a water solution, the salts or acids are preferred. The amides and esters are nearly insoluble in water but can be used for certain purposes in water solutions because only small amounts of the growth substances are required to induce responses in plants. EFFECT OF VARIOUS GROUPS
The various groups of compounds have different practical uses although there is considerable overlapping. For convenience and brevity the groups are considered separately. Naphthalene Compounds. When properly applied, a-naphthaleneacetic acid and its derivatives inhibit buds, induce roots on cuttings, and prevent abscission layers from forming. It is frequently important to prevent buds of tubers, bulbs, corms, cuttings, and trees from growing. For this purpose naphthalene compounds are very effective. After treatment with a-naphthaleneacetic acid, potatoes can be stored indefinitely without much shrinkage, and fruit trees can be prevented from flowering (although methods have not been fully perfected) until the danger of frost has passed. The same treatment may be important t o delay flowering of trees and thus to spread the fruit crops over a longer period for harvest. This would apply particularly to tropical species such as mango and avocado. Also, the flowering period for 596
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ornamental shrubs can be regulated to some extent. Wherever inhibition is desired, the a-naphthalene compounds appear to be most effective. The beta isomers as well as the higher homologs of a-naphthaleneacetic acid are inactive. An important recent use of a-naphthaleneacetic acid spray solution is for prevention of preharvest drop of apples. There is considerable variation in effectivenessof naphthaleneacetic acid for different varieties of fruit, but on the whole the reports from both scientists and practical growers are favorable. Indole Compounds. Indolebutyric acid appears to be of most practical importance in this group. Its main use is for propagation of plants by cuttings (Figure 1). It was one of the first found t o be generally effective when applied t o cuttings of many species of plants. Recent tests with mixtures of indolebutyric acid, naphthaleneacetic acid, and chlorophenoxy compounds indicate that they will be an improvement over a single substance. When a lanolin preparation of indolebutyric acid is applied to the stigma of an open flower, it induces parthenocarpy and fruit set without pollination. The result is a seedless tomato with a delicious and slightly sweeter than normal flavor.
Figure 1.
597
phenoxyacetic acid, 133"; 2,4-dichlorophenoxy-a-propionic acid, 117" to 118"; 2,4-dichlorophenoxy-a-butyric acid, 83" to 84"; 2,4,5-trichlorophenoxyaceticacid, 157" to 158"; 2,4,5trichlorophenoxy-a-propionicacid, 177" to 178"; 2,4-dichlorophenoxyacetamide, 155" to 156"; 2,4dichlorophenoxy-a-propionic amide 85" to 86"; and 2,4-dichlorophenoxya-butyric amide, 110" to 111' C. Other substitutions are shown in Table I. (Substances are said to be active if they
Showing Roots of Celastrus Cuttings Induced with Growth Substances
Left to r i g h t : controls without treatment; basal end treated with 2-chlorophenoxyNaphthoxy Compounds. The folacetic acid; basal ends treated with 2,4-dichlorophenoxyaceticacid; basal ends treated lowing naphthoxy compounds synthewith indolebutyric acid. sized in this laboratory are active: pnaphthoxyacetic acid, melting a t 155" C.; 8-naphthoxy-a-propionic acid, 106" C. ; p-naphthoxy-a-butyric acid, 125" C. The corresponding cause unusual cell elongation within an hour or two after 1-naphthyl isomers and p-naphthoxy-@-propionic acid are local application to the epidermis of stems or leaves. Some substances have a formative influence on new organs which inactive. P-Naphthoxyacetic acid and its active higher homologs grow after the plant is treated; this response is referred to as modification of organs. A substance may be active for induce adventitious roots, cause plants t o grow with modified organs, and induce seedless fruit. They are not so favorable both or either one of these responses.) Bromo-substifor propagating plants from cuttings as indolebutyric acid, tuted phenoxy compounds showed approximately the same but for some species they can be used effectively. For inducactivity as the corresponding chloro-substituted coming parthenocarpy and seedless tomatoes b-naphthoxyacetic pounds. acid is twenty to thirty times as effectiveas indolebutyric acid Phenoxyacetic acid as such is practically inactive, but but not so active as dichlorophenoxyacetic acid. From a halogen-substituted phenoxy compounds are activated acscientific point of view, the naphthoxy compounds are intercording to the position and the number of substituted groups esting because they induce formative effects when applied in the nucleus of the molecule. For example, o-chloroto growing plants. Growth which develops after treatment phenoxyacetic acid is slightly active. The substitution of frequently shows leaves, stems, flowers, and fruit modified in the chloro group in the para position increases the activity size, shape, venation, and pattern. ten to twenty fold. However, a substitution of the chloro To induce seedless tomatoes, the flower clusters are sprayed groups in both the ortho and para positions increases the with a water solution of 50 to 150 mg. per liter of P-naphactivity still further. A substitution of chlorine in the 2,4,5 thoxyacetic acid or P-naphthoxypropionic acid or a mixture of positions also brings about a very active compound, approxithe two. Vapors of the esters of P-naphthoxyacetic acid may mately equal in activity to 2,4-dichlorophenoxyacetic acid. be used to set fruit in an entire greenhouse a t one time. For Tetrachlorophenoxyethyl chloride with chloro groups in the 2,3,4,6 positions was synthesized and found inactive. The this purpose methyl- or ethyl-P-naphthoxyacetate is volatilized over a hot plate, and the air is stirred with an electric higher homologs of substituted phenoxyacetic acid are also fan. Twenty-five milligrams are sufficient for a greenhouse very active. 2,4-Dichlorophenoxy-a-propionicacid is active, with a volume of 2500 cubic feet. The house is kept closed Nitro substitutions did not act the same as halogen groups. for several hours after the treatment is started. 2,4-Dinitrophenoxyacetic acid was inactive. m-NitropheSubstituted phenoxyCompounds. substituted phenoxy noxyacetic acid was active. mAminophenoxyacetic acid was also active. Methyl groups substituted in the 3,5 positions compounds have received very little attention, but they are now proving to be, perhaps, the mosteffective of all growth were inactive for cell elongation but induced modifications showing a formative influence on the Plants. ManY other substances. The following are some of the physiologically active substituted phenoxy Compounds synthesized in this groupings have been used, bringing about varying degrees of laboratory: 2-chlorophenoxyacetic acid, melting point 148" activity in the compounds. However, the most active substituted phenoxy compounds for cell elongation were the 2,4to 149" C.; 4-chlorophenoxyacetic acid, 159" to 160"; 2,4dichlorophenoxyacetic acid, 140" to 141"; 2,4dibromodichlorophenoxyacetic acid and the 2,4,5-trichlorophenoxy-
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CONFIGURATION OF HALOGEX-SUBSTITUTED PHENOXY COMPOUNDS IN RELATION TO ACTIVITY TABLE I. MOLECULAR Phenoxy or Substituted Phenoxy Acid CHzCOOH
Activity Involving Induced: Cell elongation Modification and curvature of organs
I
Phenoxy or Substituted Phenoxy Acid CHLCOOH
Activity" Involving Induced : Cell elongation Modification and curvature ,of organs
I
Phenoxyacetio
Active
Inactive
CHsCHCOOH
I
Active
Active
Active
Inactive
Active
Active
2,4-Dichl,orqphenoxya-propionic
Active
Inactive
2,4-Dichlorophenoxya-butyric
Active
Inactive
2,4,5-Trichlorophenoxya-propionic
Active
Inactive
CHaCH2CHCOOH
I
0
0 Active
Active 4-Chlorophenoxyor-butyric CI
CHaCHzCHCOOH
I
CHzCOOH
0 Active
Active
CH2COOH Cl
1
0
CHsCHCOOH '
2-Chlorophenoxyacetic
Active
Active
CHiCHCOOH
I
n
0 I
n
C1
c1
2-Chlorophenoxya-propionio
Active
Inactive
CHaCHzCHCOOH
3
c1
CHaCHaCHCOOH
I
c1
0
.3"' (jC1
2-CMorophenoxya-butyric
Active
Inactive
CHiCOOH
I
0
CHzCOOH
I
3-Chlorophenoxyacetic
Active
... CHaCHCOOH
I
CHaCHCOOH
I
3-Chlorophenoxya-propionic
Active
Inactive
6
c 1 c1
CHsCHzCHCOOH
I
CHaCHzCHCOOH
0
I
Inactive Active Cl CHzCOOH
CHaCHCOOH
I
I
0 c1
0 4-Chlorophenoxya-propionic
Active
Inactive
ClAcI
2,4,6-Trichlorophenoxy- Inactive
*..
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Figure 2. Modified Leaves, Flowering Habit, and Correlation Phenomena of Tomato Shoots Induced with 2,3,5Triiodobenzoic Acid Control untreated shoot in u per left Others from plants treated on soil of $inch p i t with 5 mg. of ehemioal in 50 co. of watcr. Note terminal shoots ending in flower clusters and auxiliary buds growing flower cluaters instead of leafy shoots.
---t
acetic acid. It is possible that with other combinations still greater activity could be obtained with substituted phenoxy compounds. Though not fully tested, it is evident a t this time that substituted phenoxy compounds will be important for propagating plants, for preventing preharvest fruit drop, for inhibiting buds, and for inducing seedless fruit. For the latter response, dichlorophenoxyacetic acid is far more effective than any other compound yet discovered. For example, 10 mg. per liter of water as a spray is approximately optimum for inducing parthenocarpy and seedless fruit of tomatoes. This is in contrast with 3000 mg. per liter for indolebutyric acid and 100 mg. per liter for naphthoxyacetic acid. One application of the spray to a cluster with two or more open flowers causes development of the ovaries without pollination of all but the smallest buds. When used as a vapor in a greenhouse with a volume of 2500 cubic feet, 5 mg. of methyl dichlorophenoxyacetate were sufficient to induce fruit set throughout the house. For propagation of plants, halogen-substituted phenoxy compounds were effectivewhenused alone (Figure 1)orina mixture of other well-known substances. 2,4-Dichlorophenoxyacetic acid and 2,4dichlorophenoxy-a-butyric acid were particularly effective when mixed with indolebutyric acid and naphthaleneacetic acid. It appears possible that a mixture can be made to include several of the active substances and thus extend the effectivenessof a preparation to include more species of susceptible plants. So far, no one substance used alone has been found effective on all species. Not all of the substituted chlorophenoxy compounds have the same formative influence on plants. Phenoxyacetic acid is inactive for modifying organs, whereas both phenoxy-apropionic and phenoxy-a-butyric are active for this type of response. Dichlorophenoxyacetic acid is active for both cell elongation and organ modification. Dichlorophenoxy-apropionic and -or-butyric acids, although active for cell elongation and induction of roots, are inactive for modification of organs. It has not yet been possible to determine what part of the molecule i s responsible for a given response. Table I shows graphic formulas of a number of substances which have the capacity to induce both formative effects and cell elongation. The table also shows formulas of substances which lack the capacity for inducing one or the other of the two responses. Substituted Benzoic Acids. This group has not been investigated so thoroughly as the substituted phenoxy compounds, but enough has been done to show that, when various combinations of halogen and nitro groups are substituted in the nucleus, the molecule is activated. Benzoic acid is physiologically inactive, but 2,3,5-triiodobenzoic acid has a pronounced formative influence on plants. 2-Bromo-3nitrobenzoic acid is mildly active for cell elongation but very active for modification of organs. 2-Chloro-5-nitrobenzoicacid is inactive for cell elongation bub modifies organs. Triiodobenzoic acid has a pronounced effect on the flowering habit and correlation phenomena. For example, treated tomato plants produce flower clusters at the end of the main branches (Figure 2) instead of or in addition t o those along the stem,
Axillary shoots are modified in form and produce abnormally long internodes (Figure 2). The possible practical applications for substances of the substituted benzoic group have not been thoroughly investigated. It would seem, however, that any substance as active as the substituted benzoic group for regulating the growth of plants is certain to be found useful. As a matter of fact, the morphogenetic effects which can be induced by this group make these substances appear more like hormones than the well-known auxins which are effective for causing quick responses through cell elongation. The animal biologists are not concerned with cell elongation but rather with morphogenetic effects in their work with animal hormones in relation to growth. ACTIVITY AND METHODS OF APPLICATION
The activity of growth substances is usually detected by curvatures resulting from induced cell elongation or by formative effects on later growth (Figure 3). The former response occurs within a comparatively short period of time, considered in terms of minutes or hours. Morphogenetic (formative) effects appear within days or weeks after the plant has had time to grow and produce new organs. The first evidence of
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Fkure 3.
Vol. 35, No. 5
Resaonse of Tomato Plants to Tests
Ahore: (:ontrol (IrJt) and three plants responding to local treatment with lanolin preparation of methyl indolepropionate 0.05, 0.025, and 0.01 per cent. Below: Control ( l r f t )and plnnt which grew in a glass cage previously unrd with vapors of ethsl dichlorophenox>acotate.
Above: Control ( l e f t )and plant which grew after 5 mg. of triiodobenaoic acid had been applied to the soil. Below: Control ( l e f t ) .plant sprayed with o-chlorophenoxyacetic acid 300 mp./liter ( m i d d l e ) , and plant sprayed with trichlorophenoxyethyl chloride approximately 500 mg./Iiter ( r i g h t ) .
formative effects appears on the new leaves which are modified in size, shape, pattern, and texture, differing considerably from the normal organs of the species. Later the effects appear on flowers, fruit, gron-th habit, and correlation phenomena of organs (Figure 2 ) . If a chemical compound does not induce curvatures, cell elongation, or formative effects, it is considered inactive. By way of definition, the active ones are called gronth regulators, growth substances, auxins, etc. They are incorrectly called “plant hormones” since the synthetic substances, with the exception of cinnamic acid, ethylene, and indoleacetic acid, are not known to be identical with naturally occurring hormones. Methods of treating plants with hormones are varied. To detect activity of unknown substances on green tissue test objects, lanolin preparations are convenient (Figures 3 and 4). To induce roots on cuttings to propagate plants, the basal ends are dipped into powder preparations or solutions of the substance. The concentration requirements vary with the species, and for general use more than one strength is necessary. To prevent preharvest apple drop, a water solution containing 10 to 50 mg. of naphthaleneacetic acid per liter is sprayed on the entire tree a t the time apples begin to fall prematurely. To inhibit growth of buds on tubers, bulbs, corms, rhizomes, trees, shrubs, etc., the plants or plant parts are exposed t o vapors, dipped into solutions, sprayed with solutions, dusted with powder preparations, or otherwise treated with naphthaleneacetic acid or its derivatives. For inhibition by vapor treatments, methyl and ethyl naphthaleneacetate are particularly effective. The esters of the halogen-
substituted phenoxy compounds are very volatile and can be used effectively for inhibiting growth by way of vapor treatments. The same substances which accelerate growth with low concentrations will inhibit growth when the concentration is great enough. Several methods are used for inducing seedless tomatoes and eggplants. The two most commonly used are treatment of the excised stile of open flowers with a lanolin preparation or spraying the open flowers with water solutions or eniulsions. According to Howlett, 0.3 per cent lanolin preparation with indolebutyric acid is near the optimum. In water solution the most effective concentrations for five chemicals are as follows: indolebutyric acid, 3000 mg. per liter; pnaphthoxyacetic acid, 100; P-naphthoxypropionic acid, 50 to 100; p-chlorophenoxyacetic acid, 50 to 100 (Figure 5 ) ; dichlorophenoxyacetic acid, 10. Approximately 100 per cent fruit set without pollination is assured when open flowers are treated with these recommended concentrations, One treatment of a tomato cluster with dichlorophenoxyacetic acid when two or more flowers are open causes set, of fruit on buds as well as open flowers. To apply vapors in a greenhouse, the esters are warmed over a hot plate and an electric fan circulates the air. For vapor treatments the esters of pnaphthoxyacetic, P-naphthoxypropionic, p-chlorophenoxyacetic, dichlorophenoxyacetic, and trichlorophenoxyacetic acids are most effective. The milligrams per 1000 cubic feet required vary with the condition of a greenhouse, the amount of heat applied to the chemical, etc. One to ten milligrams of the dichlorophenoxyacetic acid ester or 25 to 50 mg. of the
May, 1943
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INDUSTRIAL AND ENGINEERING CHEMISTRY Figure 4. Modified Growth Habit and Correlation Phenomena of Kalanchoe Plants Induced with 4-Chlorophenoxyacetic Acid Normal plant a t upper left. Other plants were treated around the stem with a preparation containing 5 mg. of chemical per gram of lanolin. The modification occurred approximately 6 inches above the treated part.
&aaphthoxyacetic acid ester should be sufficient. Indolebutyric esters are only slightly volatile and cannot be used successfully as vapors. Seedless fruit have been induced by spraying the entire plant with substituted phenoxy compounds or by adding the chemical to the soil, but the methods have not been perfected to the point where they can be safely recommended. To date it has been possible to produce successfully seedless tomatoes, peppers, eggplant, cucumbers, and squash. Seedless watermelons have not been produced for practical purposes by any method although the idea is popular. UNPROVED CLAIMS
Above we have discussed only the responses which can be readily duplicated by scientists and growers. It is necessary to mention a few claims which do not find good scientific backing. Cut flowers have not lasted longer after treatment with plant hormones. However, some flowers last longer when treated and remain attached to the plant. Treatment of seeds of grass and crop plants in this laboratory with a number of the most effective substances has not improved the crop. However, reports on the effect of seed treatment with growth substances are conflicting. Some extravagant claims in favor of this method have been made. Based on personal experience and the scientific reports of others, recommendation of seed treatments with growth substances for agricultural use would be premature and unwarThe practice of adding vitamins and plant hormones t o ferranted. The seedlings can be readily inhibited or otherwise tilizers has more sales than practical value. If any substances affected by this method, but there is not yet any convincing to be discovered in the future should stimulate growth of the evidence of stimulation that improves or increases the harentire plant, there may be something in favor of applying it vest. with fertilizers. There are many substituted phenoxy and Since growth substances induce adventitious roots, it benzoic compounds which have not been fully tested, and would seem that treatment of the root system at transplantamong them may be some which can be used profitably with ing time should be favorable for re-establishing the plants. fertilizers. At present, however, this method should be used Favorable reports along this line have been made, but no only for experimental purposes. convincing evidence has been presented to show that trees or other plants have benefited by treatment of the root system CONCLUSION with vitamins or hormones during transplanting. We now have under observation 13,000 tree seedlings growing on the The plant hormone field is still new and holds much for the institute grounds; they have been treated in a variety of future. Much more has been accomplished than could have ways with the best known growth substances, but so far no been predicted even five years ago. The new method of subfavorable results are in evidence. It is well known, however, stituting halogen and other groups in the nucleus to activate that roots as well as stems respond to treatment with the molecule makes possible many new substances for regrowth substances, and it would be premature to rule search, and some of them are certain to be of considerable out future possipractical imbilities along this o o- r t a- n c- e- f- o- -r line. It would be premature also to recommend the t r e a t m e n t of transplants for horticultural p r a c t i c e . The subject may . . still. serve as the basis the 104th Meeting Figure 5. Cross Sections of Seedless Fruit Induced on Tomato Plants of the AMERICAN of an extensive in a Garden by Treating a Flower Cluster with a Spray Containing CHBMICAL SOCIETY, field project. 100 Mg. of 4~-ChlorophenoxyaceticAcid per Liter of Water Buffalo, N. Y . I
