Researchers have for the first time found evidence of plants making airborne emission of ultrasound screams when subjected to stresses. The sound contains information that can reveal the state of the plant emitting it. The researchers experimented with tomato and tobacco plants and subjected them to two different stresses — drought and cutting of stems.
The ultrasound emitted is in the range 20-100 kHz and can be detected from a distance of 3-5 metres. The team led by Lilach Hadany from the School of Plant Sciences and Food Security at Tel-Aviv University which carried out the research speculates that if stressed plants can emit ultrasound, then neighbouring plants should be equipped to hear these distress sounds too.
“We don't know the mechanism of response in the plants yet. We have recently shown that plants' response to pollinator sounds involves their flowers, but we expect that receptors to ultrasound, if such exist, would be in the vegetative parts,” Prof. Hadany says in an email to The Hindu.
Many moths, which use tomato and tobacco plants as hosts for their larvae, are already known to hear and react to ultrasound at frequencies emitted by the plants. “These moths may then potentially avoid laying their eggs on a plant that had emitted stress sounds,” the write.
The results have been posted on bioRxiv preprint server; the manuscript is yet to be peer-reviewed.
The researchers suspect that the sounds are generated by a process called cavitation — where air bubbles form, expand and explode in the xylem causing vibrations. “Cavitation explosions have been shown to produce vibrations similar to the ones we recorded,” they write.
The sounds were first recorded in the laboratory conditions using boxes that are acoustically isolated and then verified in greenhouses that were not acoustically isolated. They found that on average, tomato plants subjected to drought stress emitted 35 sounds an hour compared with tobacco that made just 11. However, when the stems were cut, the average number of sounds emitted by tomato plants dropped to 25 per hour while it increased slightly to 15 per hour in the case of tobacco. Controls emitted fewer than one sound per hour.
In greenhouse, the researchers studied how the plants emitted sounds in response to lack of water. Watered tomato plants were placed in the greenhouse for 10 days without watering.
Very few sounds were emitted in the first three days when there was sufficient water. But the number of sounds per day increased on the fourth to sixth day and decreased as the plants dried.
Tomato and tobacco plants emitted ultrasound at different mean peak frequencies when subjected to the two stresses. In general, the two plants emitted sounds at a higher peak frequency when the stem was cut than when under drought. Under water-stressed conditions, the mean peak frequency was nearly 50 kHz for tomato and about 55 kHz for tobacco. When the stems are cut, the mean peak frequency was 57 kHz for tomato and nearly 58 kHz for tobacco plants.
Plants belonging to two different taxa also emitted ultrasound when subjected to the same stresses. “But the stresses were not identical, and we did not run a fully controlled experiment of them. We are sure that they emit sounds, and the peak frequency of the recorded sounds was in the same range, but all the rest would require additional experiments,” she says.
“We are currently extending the work to additional species and conditions. We are also in the process of examining the potential agricultural applications of the findings, which would likely be more diverse than the original research,” Prof. Hadany says.