Primary and Secondary MetabolitesAs you probably learned many times in school, plants grow by way of photosynthesis. Sunlight hits the leaves, and through a sophisticated chemical process, the plant cells combine carbon dioxide and water to produce new plant material. Thankfully for us, one of the byproducts of this process is the oxygen we need to survive. However, the structural plant materials (the primary metabolites) are not the only products of photosynthesis. Plants also produce secondary metabolites that do not necessarily become part of the structure of the plant, but rather are released into the atmosphere. Why would this occur? Why would a plant expend energy producing substances that aren’t part of its growth? As evolution would dictate, there must be some benefit to this. These secondary metabolites must help the plant survive and, more importantly, reproduce. Let’s put ourselves into the position of a plant in the forest or the plains and consider its main concern when it comes to survival.
Poisonous to PredatorsOne of the primary "concerns" of a plant is that it not get eaten. There is an incredible number of herbivorous animals that like to nibble essentially every plant. These predators are not picky, and yet different plants get eaten at substantially different rates. Why is this? One rather obvious explanation is that the plant could produce a chemical that makes it taste bad to predators, or even causes physical illness in the predators. Interestingly, many of the secondary metabolite poisons are very strong alkaloids that we use in medicine. Some can be used directly as they come from the plant, while others are modified by pharmacologists. Consider cocaine as produced by the coca plant, morphine as produced by the opium poppy, and atropine as produced by deadly nightshade. These are all examples of alkaloids that plants produce naturally as a poison to ward off predators. This happens pervasively throughout the plant kingdom, but it is not the only method that plants use to protect themselves.
AdaptationIt has been shown that essential oil production within a plant helps plants to adapt to factors in the environment such as drought, high temperatures, radiation, and pollution.
PollinationAromatic compounds can also help plants attract pollinators, like bees and butterflies. These insects are critical to the plants' reproduction.
Co-EvolutionThere is another way that plants have evolved to protect themselves more indirectly. Co-evolution is the process by which two or more species evolve dependently on one another. Essentially all herbivores have predators - animals that eat the herbivore. For example, insects (the most threatening herbivore from most plants' perspectives) are prey for frogs and birds. Imagine this: a plant is eaten by a particular type of insect. Predatory relationships (habits) between particular insects and plants develop due to proximity and evolutionary learning on the part of the insects. In order to protect itself from these insects, the plant emits a fragrance. This fragrance is appealing to birds, but the birds don't eat the plants - they eat the insects. As a result, the plants which emit fragrances that are pleasant to birds have fewer insects eating them! This type of evolutionary learning occurs over hundreds, if not thousands of years according to the survival of the fittest, which is essentially lots and lots of trial and error.
The fragrant substances responsible for this type of co-evolution are “aromatic”, and they comprise the foundation of aromatherapy. The secondary metabolites above fall into a variety of categories, including the alkaloids (which tend to be strong and poisonous), antibiotics, phenols, terpenes, and terpenoids. Terpenes are one of the largest classes of organic compounds (compounds produced by living organisms). They are readily produced and released into the air by most plants, but not readily produced by many animals. There are tens of thousands of different terpenes. Some of them have names which might appear familiar, such as limonene, geraniol, eucalyptol, pinene, citronellol, and others. These terpenes are what give the plants which often share their names the fragrances they emit. Interestingly enough, Vitamin A is actually a terpene! To read more about terpenes and the role they play in aromatherapy and in your life, check out our other blog posts: You Evolved To Breathe Plants What Is Aromatherapy And Where Did It Come From? Are You Terpene Deficient? Probably. Could Your Environment Be Contributing To Your Stress?