THE FUNCTION OF ESSENTIAL OILS IN PLANTS

III. THE FUNCTION OF ESSENTIAL OILS IN PLANTS


When the plant organism is alive and in process of development, external substances are constantly absorbed and transformed into "building stones." This reshaping of the foreign substances and their incorporation into the plant system, known as assimilation, requires energy, which is obtained by a series of reactions, whereby a part of the assimilated products is oxidized. The balance of these two series of reactions appears in the growth of the plant. Therefore, while some of the plant material is in a continuous flux, undergoing degradation and rebuilding, another important part of the reaction products can be expected not to take part in an uninterrupted chain of reactions.
Some of these products such as cellulose will be deposited in cell walls, the plant thereby acquiring a more rigid structure. Other substances such as starch are stored as energy and organic material sources, to be drawn upon when circumstances arise which cause the re-entrance of these substances into the reaction chain. We can thus assign a certain function in the plant to these particular compounds; but we find it much more difficult to do this with a number of other substances, such as alkaloids, anthocyanins and flavones, essential oils, resins and rubber latex.
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126 Gaponenkov and Alcshin, /. Applied Chem. U.S.S.R. 8 (1935), 1049.
126 Haagen-Smit, unpublished results.

It is a well-known fact that some plants emanate, besides carbon dioxide, a considerable amount of organic material, chiefly the carriers of the smell of the plant. In some rare cases so much oil is excreted that the oil can be set afire, as in Ruta graveolens and Dictamnus. At the same time, relatively large amounts of these essential oils are deposited in the plant, and our only evidence for the assumption that such compounds are unimportant sources of energy to the plant is the fact that, prior to leaf abscission, the oils are not transferred to the stem, as is the case with a large part of the carbohydrates.
The question has, therefore, repeatedly been asked: Does the plant derive any specific benefit from these oils?127 Opinions in this field are based on the observations that some oil-bearing plants are attractive to certain animals, whereas others are repellent. In individual cases, therefore, a contribution is made toward more effective pollination through insect visits.128 In a number of other cases a degree of protection against the depredations of animal129 and plant130 parasites may be afforded by the irritating effect of many oils. Some observers maintain that the oils function as reserve food, as a means of sealing wounds, or as a varnish to prevent excessive evaporation of water (cell fluid). These opinions are not beyond question, and do not appear to be supported by experimental evidence often having their origin merely in a teleological approach to the subject. Most investigators, including Tschirch, the famous resin chemist, hold the view that the functions attributed to those substances are more often of accidental, than of essential, importance to the plant. Those who consider these products a result of phenomena accompanying the growth process have used the term "waste product." This, however, rather underrates the value of these secretion products, which, through their formation, may contribute to syntheses important in the continued existence of the plant. Some carbohydrate precursors may serve as hydrogen acceptors, and in doing so may become unusable for further synthesis. Their function, therefore, may arise in their formation, and not in a later stage. Thus we might compare the oils to "Hobelspane," the shavings of a plane. As reason for their disposal, the opinion has been expressed that substances such as terpenes, mostly hydrocarbons, are so far remote in their chemical and physicochemical conduct from the properties of the living substances that they are excreted as "korpcrfremde" or alien materials.
127 Czapek, "Biochemie dcr Pflanzen." Ill Auflage, G. Fischer, Jena (1925). Dctto, Flora 92 (1903), 146. Gerhardt, Naturwiss. 8 (1920), 41.
12 v. Frisch, Verh. Zool. Bot. Ges. Wien 65 (1915), 1;.$8 U918), 129.
129 Stable, "Pflanzen und Schnecken," Zeitsch. Natur. Medicine 22, NFXV, Jena Preycr, Flora 103 (1911), 441. Haberlandt, "Physiologische Pflanzenanatomie," 4 Aufl.
130 Verschaffeit, Kgl Ak. Amsterdam (1910), 536. Gertz, Jahr. Wis. Bot. 56 (1915), 123.

There are others Avho refuse to believe in the waste product origin of the essential oils, and Lutz suspects that these oils have constituents which can be hydrogen donors in oxido-reduction reactions. Although they are thereby transformed into neutral compounds as far as catalysis is concerned, they might re-enter the reaction scheme through a reduction in the presence of light. To prove this theory, experiments were carried out on a fuLgus belonging to the Hymenomycetes on which Lutz deternrned the antioxidant or hydrogen-donor action of oil constituents. Phenols were found to be excellent donors, as is well known from the investigations of Moureu, but secondary and tertiary alcohols and aldehydes also showed strong activity. Hydrocarbons are inactive in the dark, but become active in the light. On the other hand, primary alcohols, terpene oxides like cineole, and ketones are inactive, and perhaps are from this viewpoint the real waste products. Lutz131 considers the oil components as moderators in intracellular oxidation to protect against the action of atmospheric agents. He also includes the possibility that some of the components may be used as an energy source during a deficiency state caused by an interruption of the normal assimilation of carbon dioxide
It has also been suggested that plants which emanate a considerable arnount of oils are prevented from becoming too warm since heat is absorbed in the vaporization of the oils. In this way the oils wrould function as a water-sparing mechanism. However, measurements of the relatively large amounts of water and small amounts of oil involved, show clearly that such a contribution would be negligible. In the search for some useful function for the terpenes, Teodoresco132 was one of the few who carried out experiments on the effects of oils on plants. He showed that the absorption of sun radiation by the essential oil atmosphere around the plant was negligible and certainly did not have any influence on the water evaporation. This oft-debated point, based on a misinterpretation of TyndalPs work, was solved by admitting oil vapor around the plant without, however, making direct contact, and determining the loss of weight through transpiration. Neither direct weighing of the loss of water, as shown in Illustration 2.8, nor transpiration measurements with a potometer demonstrated any heatscreening effect. If, however, the oil was allowed to come in contact with the plant, a considerable reduction in transpiration was evident. Although the damaging effect of prolonged exposure to the oils had been observed before, Teodoresco showed that when the vapor is removed soon enough, recovery follows in a few hours. This action is not confined to the oils obtained from the same plant, but is a more or less nonspecific effect for the volatile oils in general.

181 BuU. soc. chim. biol. 22 (1940), 497.
i Audus and Cheetham, Ann. Botany (N.S.) 4 (1940), 465.

It would, therefore, appear that a number of essential oils exercise directly or indirectly a definite action on the transpiration in plants. However, experiments carried out on partial saturation of the atmosphere surrounding the plant, simulating more closely the outside conditions, showed that the concentration of essential oil vapor would rarely be high enough to cause any significant decrease in transpiration.
Vapor of  Rosmarinus officinalis on Dracocephalum moltlavica.1
ILL. 2.8. Vapor of  Rosmarinus officinalis on Dracocephalum moltlavica.1
l Teodoresco, Rev. gen. boian. 35 (1923), 382. Refutations also by Gnjns, Arch, neerland. physiol. 3 (1919), 377. Detto, Flora 92 (1903), 147. Xicol, Compt. rend. 489 (1929) 289; Biochem. J. 26 (1932), 658.
The oils inside the plant, although enclosed by special tissues, might have an influence on the transpiration and other important functions of the plant rather than the vapor of the excreted oil, since there is reason to believe that cell walls would not be an insurmountable obstacle for the oil. This effect would result in a general retardation of a number of the plant activities. Teodoresco mentions specifically a decrease in the nyctmastic, seismonastic, phototropic and gcotropic movements. The oils inhibit also the formation of chlorophyll in etiolated plants when exposed to light, and cause a decrease in permeability. Continued exposure to the oil vapors causes damage to the living substance, producing a greater permeability, which is in turn followed by death. 133,134,135 General toxic action on plants has been observed by Bokorny136 with oil of turpentine in concentration of 1 : 50,000.
The action of some essential oils is similar in certain respects to that of anaesthetics on animal cells. This problem of anaesthesia is one of the fundamental problems in general physiology, and the results obtained in these studies might well contribute to the understanding of similar effects in plants. The first effect of fat solvents, narcotics and stimulating agents is identical, 137 and it may be assumed that they cause a reversible lowering of the permeability for water and water-soluble substances, in harmony with the findings of Teodoresco and others on the pronounced inhibition of transpiration in plants.
The inhibiting and damaging effect of the oils on many life processes has been turned to our advantage in the use of these compounds as bactericidal and fungicidal agents. However, from the diversity of the compounds in essential oils, it is clear that we have to regard with suspicion any general statement on the bactericidal action of the essential oils. From specific cases which have been studied, it can be concluded that the terpene derivatives, while possessing action bactericidal to certain organisms, are not able to inhibit growth in all of the numerous types of microorganisms.
It is, therefore, not astonishing to see aqueous infusions of, for example, lavender, peppermint and juniper drugs fall victim to putrefaction after several days of standing.138
On account of their bactericidal action, a number of volatile oils have been employed in the past for the treatment of urogenital infections. The simultaneous irritating effect on animal tissue applied in measured degree may stimulate repairs of tissue, and assist in the removal of mucous from trachea and bronchia and relieve tension of the stomach and colic.
Other toxic effects are reported on cultures of fibroblasts, and further examples of their inhibitory effect on life processes can be seen in the anthelmintic effect of different oils, especially chenopodium oil. The effect
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33 Burgerstein, Verhandl Zool. Bot. Ges., Wien, 1884.
34 Heller, Flora (1904), 1.
35 Vandervelde, Chem. Zentr. I (1900), 481; II (1901), 440.
36 Pflag. Arch. 72 (1899), 555.
37 Heilbron, "An Outline of General Physiology," 37 (1938), 437. Davson and Danielli, "The Permeability of Natural Membranes," (1944).
188 Kliewe and Hutmacher, Dent. Apoth. Ztg. 53 (1938), 952. DC Potter, Compt. rend. 8oc. 6ifti 131 (1939), 158.

of this American wormseed oil on roundworms, hookworms and intestinal imoebas is very similar to that of santonine, a sesquiterpene lactone present on Semen cinae. The toxicity of these oils for certain organisms cannot be neasured simply by their bactericidal action. Thymol, for example, is much stronger antiseptically but is much less active as an anthelmintic than jantonine or ascaridole, the active components of wormseed oil. The toxic effect of some essential oils and oil components is not limited to the organisms which have to be destroyed, and excessive use in higher animals and man causes depression of the higher centers followed by convulsions. A few cases are known where an apparent stimulating effect is observed. This is the case with terpene compounds such as camphor and menthol, which are used as circulatory stimulants in cases of collapse. It is assumed that when the action of the heart muscle is depressed, camphor may improve the cardiac condition and remove arrhythmia.
On the basis of recent investigations, these effects seem also to be due to in inhibitory action on nerve fibers which counteract other fibers belonging to the sympathetic nervous system. Through this effect on the inhibitors, sertain muscles are stimulated. A similar explanation might well hold for the acceleration and strengthening of the peristaltic movements of the small intestines of rabbits, according to Haffner,139 and Sone and Shiro. 140
In general, we observe a definite toxic effect on the important life processes, and excessive doses, because of depression and paralysis of the central nervous system, are followed by death. The essential oils probably interere with delicate mechanisms, through their chemical and physical eiiects, either by entering and disturbing colloidal systems or by taking part in certain reactions. The oils themselves are at the same time exposed to many influences, which may change them in such a way that removal though the kidneys is possible. This so-called detoxication process takes many forms, and may consist of esterification, oxidation, reduction, or conjugation with compounds such as glycuronic acid and amino acids. When borneol is fed to dogs, it appears as glycuronide in the urine; when vanillin is ingested, oxidative processes are responsible for the excretion of vanillic acid. A combination of both processes is evident when camphor is removed in oxidized form. When such a removal is not possible, as, for example, by accumulation of the oils through injection, the organisms react by walling off the foreign material and tumors and sterile abscesses are reported. animals. A considerable part of the metabolites which are not immediately taken up in further reactions or are not removed by evaporation will have to remain in or near the secretion cells. The interfering action of the oils may then cause lysis of the surrounding cells and changes in the normal metabolism, resulting in the formation of cork and mucilagenous layers, with low permeability for the oil. We may safely conclude that once removed from the continuous chain of reactions, these compounds are a potential danger to all living tissues, and both plant and animal react by walling off the oil from the other tissues. If this is not possible, reactions will take place until the compounds are so transformed that they can be excreted, or until they have become harmless from the point of view of the surrounding tissue.
141 This reaction is in principle similar to what happens in a plant lacking ;he elaborate detoxication and excretion mechanisms present in the higher
138 Arch, exptl. Path. Pharmakol. 186 (1937), 621.
140 Tfihoku J. exptl. Med. 30 (1937), 540.
141 Saito, Folia PharmacoL, Japan 23 (Breviaria 2) (1936), 6.

From a general viewpoint, essential oils, alkaloids, resins, rubber, anthocyanins and many other secreted substances may have in principle a similar history. Their precursors, linked with essential processes in the organism, undergo secondary and further changes when exposed to the medium in which they are left behind.
Due to their commercial importance, our chemical knowledge of these end products exceeds by far our knowledge of the processes from which they have boon derived. It is hoped that more fundamental studies are being carried out in this direction which, in turn, will lead to our more rigid control over their formation in the plant.

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