Why Do Plants Produce Essential Oils?
Found in aromatic plants, an essential oil is a concentrated liquid containing the volatile molecules of a plant. Not only do these organic chemicals play a very important role in communication between plants and in messages from plants to animals, but they solely contribute to a plant’s fragrance and taste - and many of the medicinal values. Think of the oil as the essence, or spirit, of the plant. Currently, there are about 400 to 500 essential oils being commercially produced depending on what you include or exclude, from grapes and guavas to peppermint and pineapple! I personally count about 100 medicinal essential oils, although all essential oils are in their own respect.
Of course, over the millions of years, plants have evolved much like us, just not as verbally talkative! While essential oils are in the plant, they are constantly changing their chemical composition, helping the plant to adapt to an ever-changing internal and external environment.
For the plant, essential oils serve to:
Attract or repel insects! In producing essential oils, animal species are either attracted or repelled by the fragrance - aiding in pollination or defense. Did you know that insects have been pollinating flowers for over 200 million years? Like humans, they are attracted to specific plants for one of three possible reasons: its aroma, color, or physical structure. Scent appears to be more ancient than flower color as an attractant and various insects, including bees, butterflies, and even beetles, are known to be attracted by the aroma of a plant. If, for example, a plant becomes infested with herbivores, then they’ll change their chemistry to attract beneficial insects like parasitic wasps with the help of a scent signal from their leaves. A well articulated move by the plant, not only in altering its own chemistry, but in doing so it faces a dilemma as it can’t simultaneously emit both attracting and detracting aromas.
Fight competition and/or conserve resources! Some plants release chemicals to prevent competing vegetation from growing within its area or zone. By emitting allelochemicals, toxic compounds that inhibit respiration and photosynthesis in other plants, it prevents non-symbiotic plants from germinating in close proximity - either for competitive reasons and/or as a protection tactic as it has an advanced knowledge of the limited resources of the area.
Protect! Plants, like all living things, need to protect themselves from various types of predators. Plants use terpenoid compounds to deter not only insects, but animals from approaching them. A shining example is that very, very rarely will insects be found on plants of mainly the Lamiaceae (mint or deadnettle), Lauraceae (laurel) or Rutaceae (citrus) families - as over 200 species of plants produce linalool, a naturally occurring terpene alcohol.
Fun fact: You can identify a plant belonging to the Lamiaceae (mint or deadnettle) family if the flowers come in fives, they’re (usually) aromatic, the leaves are opposite of each other and the stem is square.
Provide defense + immunity! Resins and complex combinations of terpenes are released by some plants to act as antimicrobial, antifungal, and antibacterial agents against a wide range of organisms that may threaten the survival of the plants. Compounds such as sesquiterpenoid lactones from Asteraceae, the daisy family, have been found to not only play a very significant antimicrobial role as protection from herbivores, but have been found to greatly benefit humans as well - both as part of a balanced diet and as pharmaceutical agents in aiding the treatment of cardiovascular disease and cancer. One outstanding study of Douglas fir trees revealed that they release a complex mixture of terpenes from their needles to defend against the spruce budworm. Each year, the trees would and will vary the composition and production of their terpenes, thus decreasing the ability of the budworm to develop widespread immunity to specific compounds.
Also... In some harsh climatic conditions, the essential oils of certain species collect on their leaves to protect against water loss and many waxy species that you may come across when out hiking are built in direct response to that foreseeable problem.