Most of our understanding and general knowledge about the language of scent comes from the animal world. Most people know that dogs and other animals mark their territories with urine or other stinky substances. We take advantage of this behaviour, either through harvesting the musk of civet cats for perfumes or training dogs to smell out bombs, drugs or bodies. However, did you know that plants have also evolved strategies that utilize the sense of smell?
Some (not all) flowers are very fragrant, and use the unique signature scent of their species to attract pollinators like bees and other animals. As a visual species, we tend to put more emphasis on the colour and structure of flowers in this role, but the perfume of the flowers is just as important. Plants maximize their scent, constructed from volatile compounds like C6 alcohols, aldehydes and acetates, when their sexual organs are fully mature and ready for the transfer of pollen, and they use volatiles that only attract the species of pollinator suited to their purposes. But this isn’t the only message a plant can send by scents made from volatile compounds.
Plants have a whole language of scents that they use under different circumstances. Most phytophagus insects and animals have a small range of plant species they can consume; koalas and pandas are good examples of animals with an extremely limited diet. Both species can identify by smell what plants they can eat safely. In the case of koalas, the gum leaves that koalas eat are quite poisonous, and so koalas are especially adapted to detoxify the injurious part of their diet. If they ate the wrong sort of leaves, they would get sick and die, so it is very important for a koala to sniff the leaves it is planning to consume.
However, the plants green leaf volatiles are not just a passive response to herbivores. And there is more chemistry going on in the smell of a fresh-mown lawn than you realize. That delicious fragrance is a cry for help!
When a very hungry caterpillar chews a blade of grass or some such food source, it releases the scent of green leaf volatiles. It wafts through the air until it reaches the olfactory receptors of an insect that preys on that species of caterpillar. The insect will now seek the source of the scent, and find the very hungry caterpillar. Of course, if there aren’t any predators around to smell the green leaf volatiles, the caterpillar just munches away. But, usually, enough caterpillars are killed to prevent the complete consumption of the plant. In a way, the plant was signalling the predator, in a system that benefits both the plant and the predator.
Other plants release green leaf volatile compounds that act like a warning to other plants of the same species. When the volatiles are absorbed via plant respiration, it triggers an increased production of tannin. Tannin is astringent and so makes a plant less palatable to herbivores. This is just one of the strategies plants use to reduce grazing. Parasitic plants can also use this same strategy to locate their hosts.
It is the tender tendrils of plants that are especially sensitive to airborne volatile organic compounds. When a parasitic seedling encounters the chemical signature of a host plant, it displays a positive growth response towards the source of the chemical signature. In this case, it means the parasitic plant grows towards a host plant, eventually making contact and able to commence the strangulation of the host.
I was tempted to call this article ‘The Secret Life of Plants’, but now you are in on the secret. Next time you are strolling through a park and enjoying the fresh smells of the greenery, you will know you are witnessing chemical warfare!