The Function of Essential Oils in Plants

            A living plant is a dynamic organism.  There is continuous absorption and transformation of external substances that allow the plant to thrive and grow.  This process, called assimilation, requires energy, produced by oxidation reactions within the plant itself.  Of the substances absorbed and produced by the plant, some are constantly being used and transformed, allowing growth and the continuation of the metabolic activity of the plant.  Some are products which are stored.  Products like cellulose is deposited in the cell walls, allowing for the rigid structure of many plants.  Starches are stored as a energy source.  Other substances function, such as alkaloids, anthocyanins, flavones, essential oils and resins are more difficult to  determine. 

            Many plants emit a considerable amount of organic material, mostly the carriers of the plant's odor, called essential oils.  Additionally, large amounts of these essential oils are deposited within the plant itself.  It has been deduced that these essential oils do not provide an energy source for the plant.  We can assume this, because they remain in the leaves in plants that lose their leaves.  Starch, or carbohydrate stores, are moved into the stem before the leaves drop.

            Therefore, what is the function of the essential oil?  One idea is that in some cases the oils produce a scent attractive to certain animals and insects, aiding in pollination.  In other cases the scent is noxious, acting as a repellent; or irritating, also functioning as a repellent.  Other ideas are that, the oils act as an energy reserve, act to seal wounds, or as a varnish to prevent evaporation of water.  Attract & Defend!

            Many researchers are of the opinion that the oils are a byproduct of the plant's metabolic processes, and are of accidental benefit to the plant.  Some, however, refuse to believe that the essential oils are merely waste products.  Lutz believes that the essential oils can act as an antioxidant or donate hydrogen in oxidative reactions, especially in the presence of light.  In an experiment with a fungus, he determined the antioxidant properties of some essential oil constituents.  Previous work has shown that phenols are excellent hydrogen donors, Lutz showed that secondary and tertiary alcohols, as well as aldehydes also performed as antioxidants.  Hydrocarbons were active in the light, but not in the dark.  He also found that primary alcohols, terpene oxides (such as cineole) and ketones are inactive.  Ultimately, Lutz proposed that the essential oil components moderate oxidative reactions to protect the plant.  He also feels that it is possible that some of the constituents may be used as a back-up energy source.

            It has been proposed that essential oils act to keep water in the plant, or to prevent them from getting too warm.  Laboratory experiments have shown that some essential oils may prevent transpiration (plant sweating) However, outside the lab it has not been demonstrated that the essential oil prevent a significant amount of water loss. 

            Some essential oil components have toxic effects.  There are some that function similarly to anesthesia on animal cells.  These qualities are what make it possible to use some essential oils as bactericides and fungicides.  Terpene derivatives act on certain bacteria.   Others have effects on fibroblasts, and others work on some types of parasitic worms.  Although some essential oil components work on bacteria and the like, others have significant effects on higher  functioning organisms.  Camphor and menthol, terpene compounds, are used as circulatory stimulants.  Investigations have determined that these effects seem to be due to an inhibitory action on the parasympathetic nerve fibers.  Although it is not know how the essential oils work, it is thought they either disrupt some metabolic systems or interfering or participating in some reactions.  The oils are transformed in the body for excretion through the kidney.  The detoxification reactions can consist of esterification, oxidation, reduction or conjugation.  Many substances are transformed and excreted by the body.  Others may be isolated or walled off, resulting in a sterile abscess or tumor.

            Plants do not have elaborate detoxification pathways, so will wall off offending substances.  This brings us back to why many researchers are of the opinion that the essential oils, resins, alkaloids and other plant compounds are simply waste products.  The components are now harmful, although they may have been useful in another form, and have to be isolated in order to prevent damage to the plant tissue itself.

Bibliography

Guenther, Ernest.  The Essential Oils, Vol. 1.  Krieger Publishing Co.  Florida , 1948
Rose, Jeanne. 375 Essential Oils  and Hydrosols.  Herbal Library, 415/564-6785. 2000.