While essential oils are in the plant, they are constantly changing their chemical composition, helping the plant to adapt to the ever-changing internal and external environment. It is obvious to understand that the plants are producing essential oils with a variety of purposes, including:
To attract pollinators and dispersal agents
We all know that plant reproduction (and then their survival) is definitely linked to the action of pollinating insects. And to enhance this process, plants have developed many stratagems to attract them. We, as human, are attracted by plants and flower for various reasons such as their odour, colour or special shape. So do the insects. Here is one of the first purposes of the essential oil production by the plants. As we do, insects are attracted to specific plants for one of three possible reasons: its aroma, its color, or its morphology or physical structure. Various insects, including bees or butterflies are known to be attracted by the aroma of a plant.
To play a role in allelopathy, a type of plant-to-plant competition
Another purpose of the essential oil production is the competition. Indeed, as human, plants are fighting for the protection of their living pace. To do so, a plant will releases chemicals to prevent competing vegetation from growing within its living area. This phenomena is called “allelopathy”.
Eucalyptus has been extensively studied for its allelopathic effects of eucalyptus. Phenolic acids and volatile oils released from the leaves, bark and roots of certain Eucalyptus spp. have deleterious effects on other plant species which are trying to grow nearby.
To serve as defense compounds against insects, animals and other pathogens such as bacteria and fungus.
Plants, like other living things, need to protect themselves from various types of predators. Plants use terpenoid compounds to deter insects and other animals from approaching them. In his book “The biological role of essential oils”, Shawe described several intereing way used by the plants to protect themselves from potential pradators. For example, he stated that the presence of linalool in the citrus peel confers resistance to attack by the Caribbean fruit fly. He also wrote that the absence of instect in the peppermint is due to the essential oils. Another amazing example is provided by the Douglas fir tree. To defend itself against the spruce budworm, it releases a complex mixture of volatile oils, from their needles and even more fascinating, the plant understood that pathogens and predators could develop resistances. So, to avoid this, the Douglas fir trees never produce the same mixture (in terms of composition) every year and by doing so, this “smart” tree will decrease the ability of the budworm to develop widespread immunity to specific compounds.
Plants and trees will also release resins and oils 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 sesquiterpene lactones found in plants such as feverfew, yarrow, and blessed thistle, have been found to play a strong antimicrobial role as well as a protective role from herbivores.