Plants, like other living organisms, have a full set of responses to external stimuli. Flowers open when the sun comes up and close when it goes down, shoots grow towards light and roots avoid light, and some plants even move their leaves when touched. The responses are slow, but are clearly observable via time-lapse photography.

New research by Ben-Gurion University, however, indicates that plants not only respond to respond to stimuli as it happens – but can also anticipate problems based on the warning signals emitted by other plants in the neighborhood. The study was conducted by a group led by Professor Ariel Novoplansky of Ben Gurion’s Swiss Institute for Dryland Environmental and Energy Research of The Jacob Blaustein Institutes for Desert Research, and was published recently in PLoS ONE, a publication of the Public Library of Science.

Scientists have long suspected that plants could pick up “stress signals” from other plants, relating to predators like insects or rodents, drought, excess heat, etc. – with the requisite responses. In the study, Novoplansky subjects a row of garden pea plants to drought conditions, choosing plants whose roots were intertwined on one side, and no root connections on the other side. Thus, the study would check to see the level of response in plants that could communicate through their roots (called “shared” neighbors in the study), as well as others that could only communicate via shoots (“unshared” neighbors).

In addition, the “shared” root volumes were connected by the roots to plants further afield, in order to see whether the plants whose immediate neighbors were under stress would communicate their discomfort further down the line to plants that were not directly connected to the stressed plants.

The study showed that fifteen minutes following the imposition of drought conditions, there was significant stomatal closure  in both the stressed plants and their nearest unstressed “shared” neighbors, with the  microscopic pores on the surface of the plants closing up in order to avoid losing moisture to evaporation, etc. Within one hour, all shared neighbors  – including those far away from the original “victim” – closed their stomata. But no stomatal closure was observed in the unshared neighbors. Bottom line: Unstressed pea plants were able to detect the stress in their immediate neighbors and take defensive measures – and that “fear factor” was communicated to other plants down the line, until they all closed up their stomata to protect themselves.

“The results demonstrate that unstressed plants are able to perceive and respond to stress cues emitted by the roots of their drought-stressed neighbors and, via ‘relay cuing’, elicit stress responses in further unstressed plants,” said Professor Novoplansky. “Further work is underway to study the underlying mechanisms of this new mode of plant communication and its possible adaptive implications for the anticipation of forthcoming abiotic stresses by plants,” he added.