HORMONES-PLANT
GROWTH REGULATORS1
P. W. ZIMMERMAN Boyce Thompson Institute for Plant Research, Inc., Yonkers 3, New York
THE
subject of this paper involves one of the newer horizons in science--chemicals which regulate the growth of plants. As a concept lying along the frontiers of science, plant hormones appeared on the horizon with the publication of a book in 1880 by Charles Darwin called "The Power of Movement in Plants." Very little progress was made during the first fifty years, but startling advances occurred during the last twenty years. Since the discovery by BoysenJensen that phototropic curvatures in plants were induced by an influence of a chemical nature, rapid progress has been made. The term "hormone" for plants is not well defined. From a popular point of view the word means any influence of a chemical nature which regulates the growth of plants. The term "growth snbstance" or "auxin" is often used to mean the same as "hormone." Interest in the subiect of ~ l a n hormones t from the standpoint of fundamkntal science is still uppermost in the minds of many workers. It is increasingly evident, bou.ever, thnt a n ntttlck on prnctienl problcms in t h e field of nlnnt hormone8 brine.; a b o u t resdts that contribute io both science and oractice. Laboratow curiosities pointed the way to horticultural applications, and we now use plant hormones to propagate plants, prevent preharvest drop of apples, inhibit buds to prevent potatoes from sprouting, increase fruit set and induce seedless tomatoes, regulate flowering of pineapples, kill broad-leaved species without killing grass, and defoliate plants without killing the stems. Groups of chemicals best known as growth regulators are as follows: indole compounds involving p-indoleacetic and p-indolebutyric acids; naphthalene compounds involving a-naphthaleneacetic acid and its derivatives; p-naphthoxyacetic acid and higher homologues; substituted phenoxyaliphatic acids, esters, and salts, especially 2-chlorophenoxyacetic acid, 4chlorophenoxy-
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1 Based on a paper presented at the Twelfth Summer Conference of the New England Association of Chemistry Teachers, University of Connecticut, Storrs, Connecticut, August, 23, 1950.
acetic acid, 2,4-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, and their higher homologues; and substituted benzoic acids involving especially 2,3,6trichlorobenzoic acid, 2,3,5-triiodobenzoic acid, and 2,5-dichlorobenzoic acid. To these should be added unsaturated hydrocarbon gases, especially ethylene. There are many natural hormones which have not been isolated and identified but, nevertheless, are known to exist. Indoleacetic acid and ethylene, mentioned above, are made by plants and have been identified by chemical methods. Many workers are trying to find how hormones work and how they are synthesized by plants. Physiological activity is found with simple chemical structures like ethylene as well as the more complex structures like
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Root mducing P O ,
of Ind~lebutyricAcid when Applied t o Camellia iaponiea
LB~+-NO~treated.
Right-Treated
113
with indolebutyrio acid in talcum powder.
JOURNAL OF CHEMICAL EDUCATION
114
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view~t waterwayin L O U ~ S Infested ~ ~ . -aith water nyacinth enaphthaleneacetic acid and substituted phenowaliphatic acids. New physiologically active compounds are being discovered almost daily, and i t is likely that the most effective ones are yet to come. Extensive uses of plant hormone-like substances became evident during the past year when the sale and use of 2,4-D (2,4dichlorophenoxyacetic acid) practically equaled that of the insecticide DDT which has been leading the list of all organic agricultural chemicals. This new selective herbicide is now being used to control weeds in farm crops, yards, golf courses, and marine growth in swamps and waterways of the south. New uses are being found for 2,4D, and its sales may have exceeded that of DDT in 1950. Weed killing is by far the most important practical use of hormone-like chemicals. This alone has a potential annual market of more than 100,000,000 pounds. The second in importance is the use of naphthaleneacetic acid and its derivatives to prevent p r e h a ~ e s tdrop of apples and inhibit bud growth of potatoes.
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There are also other achievements with plant hormones, and some of these illustrate the varied l i e s of attack from the scientific and practical point of view. In Hawaii several kinds of plant hormones are being used to force pineapples into flower or to increase fruit size. Still other hormones prevent premature flowering, and finally vegetative propagation of pineapple is facilitated with plant hormones. Induction of roots on cuttings was one of the early practical uses found for plant hormones. The same chemicals which induce roots to form may also inhibit growth of other parts of the plant. Let us try to look beyond the present horizons for plant hormones and predict some of the things to come. We shall have hormone-like selective weed killers for every conceivable use--the corn fields, the wheat fields, the orchards, the gardens, the forests, the waterways, the swamp lands, the hedge rows, and the yards. Any hormone which has specificity in its effects is likely to find special uses. Perhaps the forest can be sprayed to kill all but pines or other desired species. With a little stretch of the imagination we can picture a lawn without crabgrass, a bayou in Louisiana without water hyacinth and alligator weeds, pastures without thistles, onion fields not weeded by hand, and helicop ters available for spraying a t 25 cents an acre. Bud-inducing chemicals are seriously needed. They would facilitate propagation of plant parts where buds have not appeared or where they have been lost. Day lilies, Gloriosa lilies, and dahlias are often lost because the storage organ does not have a natural bud. Internodes which do not normally produce adventitious buds could be used for propagation purposes. A shootinducing substance should cause new shoots to arise where desired on the plant to make possible propagation of budless parts or improve the shape and appearance of intact plants. There is no end to the conceivable uses of bud- or shoo&inducing substances. Flower-forming substances which are thought to exist in nature may be isolated, identified, and used as a common tool. If this becomes a reality it should be possible to force longday types to flower during short days or shortday types to flower during the long days. In short, it should be possible to induce flowering of plants a t will and to force flowers to grow at unusual places. We can imagine plants with flowers on internodes, on leaves, and even on roots. At the present time there appears to be considerable variation in the time of ripening of fruit. Under the influence of chemicals the time of ripening should fit into our needs. During the past season i t has been shown that apples treated with certain hormone-like chemicals ripened prematurely. In a lesser degree this has been noticed for tomatoes. Tomato flavors are not affected under the influence of the chemical. Apples, however, change h v o r and consistency, but the very fact that modifications in time of ripening have been demonstrated offers encouragement for practical methods applicable to all or many fruits. t Since it has been demonstrated that fruit buds can be
FEBRUARY, 1891
delayed through treatment with growth substances, one tions, progress has been more rapid than in fundamental is led to the assumntion that fruitine of tronicnl s ~ e c i e s research, and the rem~ltsare so spectncular that interest and flokering of piants in general c k be stnggeied to in the suhject is now sweeping the world. Re~eatch extend throughout the entire season. Mangoes, for holds much for the future and more important applicaexnmple, flower nnd ripen fruit a t very definite periods tions are sun: to be made. There is much competition in the fields covered by of thc yenr. During the rest of the year tlmy nre not avnilable as food. Since this is an importnnt tropical this paper Perhaps that is why progress has been food it would be desirable to extend it throu~houtthe rapid. Rewnrch in some phases of the subject can he season. This should lxcome a renlity by the proper done by nmnteura and prnrtirnl horticulturists as well hormone applications to growing buds. The iden is as by trnincd scientists. It is well to recognize the particulnrly npplicnhlc to tropicnl plnnts, because the fact that severnl trchniqurs have played n pnrt in tempernture nnci other growing condit,ions would not solving diffirult prohlcms. W e take new courage when limit the time of fruiting. We ~hould,however, be also we rememl~rrthat this is the nge of rescnrch. \Vc must able to stagper flowering or spring shrubs in the north follo\v where research leads. Thousl\nds of researchers so that we could enjoy them over longer periods. arc looking into the unknown or fields that have no Without mnking further pmlictions, it appennt that other limits thnn mnn's imagination. Rcsenrch is not the field of plant hormones presents a chnllrnge for confined to the lnborntory. Its origin is in the individscientists with vnriecl nttncks. As fur as fundnmental unl. All of us are resenrehers no matter what our research is concerned, it might be snid that the work is jobs, and we must surpass what has already heen acjust beginning. We have found only a few natural complished to keep a h a d . We must senrch for new hormones, and we still know littlc about how hormones and better methods, for even thst which we now d o well work. I+om the standpoint of horticultuml applicn- must br done hettrr tomorrow.
