We have only just started to translate the secret language of plants.
What does the plant say?
Klassekampen, 12 Apr 2019
Plants don't talk, everyone knows that, right? Actually, that's not entirely true. Of course, you will never hear plants gossip with each other about the scandals which that one rebellious bush around the corner has ended up in, but they do communicate with each other. Plants can 'smell' each other and in this way transfer information. A secret language that can even influence our climate.
The smell of freshly cut grass will never be the same again, once you know of the existence of this language: in reality, it is one big cacophony, the screams of all those damaged blades of grass – in the form of a group of molecules we call volatile organic compounds – with which they warn other plants of danger. The lawn mower is hard to avoid, but if, for example, a small beetle starts to gnaw at a leaf, the plant emits the same substances. These can be picked up by nearby plants, allowing them to deploy their defense mechanisms. In this way, they ensure that they are less appetizing to insects – and they avoid being eaten.
Plants can 'smell' each other and thus transfer information
This may all sound too fantastic, which was also the reaction of most scientists when it was suggested a few decades ago. Back then, it was already known that plants emitted volatile substances. Why they did this was more unclear, but communication was not thought possible for passive plants. Yet, this was precisely the conclusion of an American study from 1983. That study examined willows that were affected by insects and how they defended themselves by changing the composition of their leaves. This was compared to the leaves of other willows close by and far away, which the insects had not yet eaten.
It turned out that the trees that were nearby, but unaffected, showed the same change in their leaves as the trees attacked by the insects. The trees that were far away showed no change. So, there had to be something that would make the untouched tree know that its neighbor was fighting off an insect attack, thereby triggering its defense mechanism. The trees had no contact through their roots, and the researchers suggested that pheromone-like substances (fragrances that are normally released by animals to send signals to other members of their species) could be the answer.
However, this study was quickly shot down by other, more prominent scientists. Correlation (that two things occur simultaneously) is not the same as a causal relationship, and other factors could not be excluded. Perhaps both trees had an illness that could explain the reaction? Therefore, it took until the next decade before follow-up investigations confirmed the mechanism in a laboratory. If eaten leaves were placed in an enclosed space near a healthy plant, the healthy plant's defense mechanism was triggered.
Nowadays it is generally accepted that plants can use volatile organic compounds to give off signals. And not only to protect against insects, but also to attract species that are needed for pollination. It is one of the main reasons why flowers smell so good. Also, plants have a major influence on the composition of our atmosphere through these substances.
In recent years, a great deal of research therefore focused on a small group of volatile organic compounds that are very common and can have an impact on our climate. These substances can clump together in the air, which eventually leads to the formation of aerosols – microscopic particles in the air. These particles not only reflect sunlight, but water droplets can also attach to them which influences the properties of clouds. More particles make clouds whiter so that they reflect more sunlight back into space, which leads to a cooling – but it’s very uncertain how big this influence is.
It is still uncertain how important the complex interplay between plants, aerosols and clouds is for global warming. Our own emissions of aerosols (from fossil fuels) also play a major role in this. By putting all these processes in climate models, we can better understand them. We already know that plants emit more volatile organic compounds at higher temperatures. Whether climate change is going to change something in that interaction seems therefore certain.
Although we have only just begun to translate the secret language of plants, and much is still unclear, I hope that now that you know this, you will experience nature in a slightly different way during your next walk in the forest – and smell that the forest also has something to say.
This text first appeared in Klassekampen on 12 Apr 2019