Plant Awareness is an ever growing subject within the academic community and has over the last few decades uncovered some remarkable findings. In part 1 I briefly discussed plant senses. In this presentation, I will discuss the ability of plants to communicate via airborne chemicals and what purpose this serves for their survival. Part 3 will discuss the ability of plants to communicate through their root system.
Plant communication is far more common than originally thought. The continuous scientific inquiry of this rather small scientific field keeps unearthing fascinating features of plant communication, despite the harsh criticisms faced by fellow academics. Many in the scientific field are still caught at a crossroad between knowing if the communication is done on purpose, or if it is simply a result of environmental conditions and occurrences, causing automated responses within the plant that neighbouring organisms eavesdrop on. Whichever is true, let us look at what we currently know about airborne plant communication.
Plants communicate using a variety of what is known as volatile organic compounds or VOCs. These VOCs are categorised into four groups namely, terpenoids, fatty acid derivatives, amino acid derivatives, phenylpropanoids/benzenoids. These chemical packages serve as a communication method not only to plants of the same species but also other plant species, soil microbes, fungi, insects and animals. This is not always beneficial to the plant as it might attract unwelcome guests. However, for the most part, it does play a critical role in plant survival for not only the individual but neighbouring plants also.
VOCs are used for a wide variety of purposes that aid in the defence of the organism. Such as alerting the entire plant of danger in order to increase defensive chemicals throughout the plant which also alert neighbouring plants of danger so that they too preemptively respond defensively. Also, to attract insects that deter or parasitise on unwelcome herbivore attackers. Some cases claim that even non-insect animals may be attracted by VOC's to rid the plant of attacking herbivores, but the evidence of this is sparse as of yet. As mentioned before, the debate continues as to if this is an automated response or not. It is generally believed that this form of communication evolved secondarily from plant responses when under attack.
Plant to plant communication through VOCs is well documented. For example, Vachellia and Senegalia, formerly known as the Acacia genus have provided ample evidence of chemical defence against large herbivores that feed on their protein-rich leaves. Since most of these plants are highly palatable, they have an array of defensive tactics. Including thorns, mutualistic relationships with attacking insects and of course chemical defence. When a large herbivore, such a giraffe begins to browse on the leaves of the tree. The plant senses this and begins to increase the tannin level within the leaves. This bitter tannin decreases the palatability of the leaves and in high doses are deadly. The plant also releases VOCs such as ethylene which warns other plants in the area of impending danger. In one extreme case, a South African zoologist found that this defence mechanism was responsible for the death of more than 3000 antelope in particularly over-browsed game ranches in north-eastern South Africa.
Plant to animal communication is also well documented and of growing interest to the scientific and agricultural communities in particular. The primary known use of plant-animal communication is for defence. Most plants studied in this fashion have shown that when under attack by a herbivore, the applicable chemical packages are released which attract species that prey, parasitise or simply deter the attackers. This is an impressive feature as communication thus occurs from the first to the third trophic level. A large variety of plants have been studied in regards to this method of communication. For example, some tree species are in a mutualistic relationship with particular ant species that live in the tree and respond aggressively when the chemical alarm of the plant is activated. Other plants, when for example are being attacked by caterpillars will release corresponding chemicals that attract applicable wasp species that parasitise on the caterpillar. This is by no means the only example of such a plant-insect relationship.
Even though this topic and examples thereof can be discussed for hours on end, this presentation has covered the basics of airborne plant communication. It is evident that plants have the ability to communicate with other living species through the use of chemical signals. This may lead to amazing breakthroughs in conservation and agricultural practices in the future and is certainly a field that will have many more fascinating discoveries to look forward to.