Plants are immobile, unlike animals which can move around. Yet, plants are still able to interact with other organisms and the environment. Through a series of biosynthetic processes, simple matter (such as carbon dioxide from the atmosphere) can be converted to complex compounds. Volatiles are then released in different ways to communicate messages with other organisms or their neighbours, in order to tackle environmental stress or even resist attack from others.
Many plants require the help of animals to propagate pollen and seeds for reproduction. Volatile substances are essential. Specialised organs of flowers and fruits, such as epidermal cells and oil glands, are used to store or emit volatiles. The most widely known example is the fragrance released by flowers to attract insects for pollination. Several volatiles are generated by fruits, to inform animals of the different stages of their maturity. This way, the animals know the most suitable time to feed on the fruits, which increases the efficiency of the propagation process.
Calling for help
Apart from reproduction, plants also utilise volatiles for defence. In nature, plants have to face attack from numerous herbivores including insects. When we pick a plant, we often encounter the smell of “grass”. The smell is actually from the green leaf volatiles (GLVs), which are released at the broken part of the plant. GLVs do more than preventing the wounded part from infection; they are also a warning to nearby plants. Their neighbours will enter a “prime state” after receiving the GLVs as a signal to prepare for potential attack. Research has shown that maize seedlings exposed to GLVs can rapidly accumulate compounds such as jasmonic acid as a self-defence mechanism.
Amazingly, some plants will “call for help” through volatiles! When Tobacco Hornworm (Manduca sexta) launches a feast on Wild Tobacco (Nicotiana attenuate), the plant emits volatiles that attract the natural enemy of the caterpillars - Big-eyed Bugs (Geocoris spp.), to help them resolve the danger.
Still other plants have evolved more sophisticated survival strategies. Broad-leaved Helleborine (Epipactis helleborine), for example, besides producing nectar, releases volatiles that mimic the presence of wasps' prey, hence luring wasps to come and pollinate them.