Plants have developed surprisingly complex communication networks which allow them to communicate with each other about what's happening on the surface.
Graphical illustration of above ground interactions between neighboring plants by light touch and their effect on below-ground communication. Image credits: Elhakeem et al.
Despite their immobile lifestyle, plants are actually more active than you'd think. Aside from all the biochemical reactions that enable them to go about their day-to-day lives, plants can also communicate complex messages underground. Essentially, these messages take the form of chemicals secreted by roots into the soil which are then detected through the roots of nearby plants.
These chemical "messages in a bottle" can tell plants whether their neighbors are relatives or strangers and help them direct their growth accordingly.
Touch is one of the most common stimuli in higher plants and is well known to induce strong changes over time. Recent studies have demonstrated that brief touching among neighboring plants can be used to detect potential competitors. As plants grow in close proximity to other plants, they constantly monitor any cues that happen above ground -- but they do the same below ground as well.
To better understand how this happens, as well as to learn more about the ways above ground factors influence what happens below the surface, a team of scientists from the Swedish University of Agricultural Sciences “stressed” corn seedlings and then looked for growth changes in nearby plants. Essentially, they brushed the corn leaves to simulate the touch of a nearby plant leaf and then monitored what chemicals the plant root secreted. The team then took those chemicals and transferred them to other plants to see how they react. They found that plants exposed to the chemicals responded by directing their resources into growing more leaves and fewer roots than control plants.
"Our study clearly shows that roots of very young maize seedlings pose an extraordinary capacity to quickly detect changes in cues vectored by growth solution directing roots away from neighbours exposed to brief mechano stimuli. In this way, roots may detect the changed physiological status of neighbours through the perception of cues they release, even if chemical analyzes did not show significant changes in metabolite composition."
Basically, the team showed that what happens above ground influences what happens beneath the ground surface of a plant -- and the way through which they communicate this is more complex than we thought. This makes a lot of sense since the ability of plants to rapidly detect and respond to changes in their surrounding environment is essential for determining their survival.
Lead author Velemir Ninkovic concludes:
"Our study demonstrated that changes induced by above ground mechanical contact between plants can affect below ground interactions, acting as cues in prediction of the future competitors."
Journal Reference: Elhakeem A, Markovic D, Broberg A, Anten NPR, Ninkovic V (2018) Aboveground mechanical stimuli affect belowground plant-plant communication. PLoS ONE 13(5): e0195646. https:/