According to a new study that researchers describe as a global breakthrough, plants aren’t as quiet as we thought.
They make sounds, especially when injured or stressed, but not the ones we can hear with our ears.
Researchers at Tel Aviv University recorded and analyzed the sounds made by plants and found that they make clicking sounds that emanate at a volume similar to that of human speech. but too loud for our ears to pick up.
“Apparently, an idyllic flower field can be quite a noisy place,” said Lilach Hadany, a professor in the Wise College of Life Sciences’ School of Plant Sciences and Food Safety. he said in a press release. “It’s just that we can’t hear the sounds!”
Overall, the researchers found that plants made far more sounds when under stress, and that the sound varied depending on the plant and the type of stress they were experiencing.
This means that plants not only make sounds, but those sounds contain information that animals and other plants can understand.
This isn’t the first time scientists have investigated whether or not plants emit sound, but recording and analyzing it is a novelty, the researchers say.
“From previous studies, we know that vibrometers attached to plants record vibrations. But do these vibrations also become sound waves in the air, that is, sounds that can be recorded at a distance?” Hadany said: “Our study addressed this question, which researchers have been debating for many years.”
TALKING WITH PLANTS
To isolate the plants and ensure that any sounds picked up by the recording equipment only come from the plants and not from the surrounding atmosphere, the researchers placed plants in a speaker box in an environment with no background noise.
They used recording equipment capable of picking up frequencies between 20 and 250 kilohertz, reaching more than 15 times the maximum frequency an average adult can hear.
Plant types used in this study included wheat, corn, cactus, and henbit, with most of the research focused on tomato and tobacco plants.
“Before placing the plants in the acoustic box, we subjected them to several treatments: some plants had not been watered for five days, some had had their stems cut, and others were intact,” Hadany said. “Our intention was to test whether plants make sounds, and whether these sounds are affected in any way by the plant’s condition.”
In general, the plants made sounds within the range of 40 to 80 kilohertz, which means that they made ultrasonic sounds. Plants that had a good day were generally quieter, making less than one sound per hour on average.
But plants that were dehydrated or injured made dozens of sounds every hour, according to the researchers.
These sounds would be loud enough to be heard by any creature within three to five meters that could hear within the proper frequency.
In a video released by Tel Aviv University along with the study, researcher Yehuda Anikster played a recording of sounds made by tomato plants and vines, which had been manipulated by researchers to reproduce them at a frequency that human ears could pick up. The tomato plants created heavier, thicker clicks, while the recording from the grape vines had lighter, shorter clicks.
The differences in the recordings were clear enough to tell the type of plant and the type of stress it was experiencing just by listening to the sound, the researchers found.
THE SOUND INCREASES AS THE STRESS
After the researchers recorded the isolated plant sounds, they re-recorded them in a greenhouse, surrounded by other plants and sounds, and found that they were still able to recognize the distinct plant sounds.
In the greenhouse, they also recorded a group of tomato plants as they became increasingly dehydrated and found that their sound output point steadily increased for the next four to five days after they were watered, and then decreased.
Essentially, the thirstier the plants got, the louder they complained, until they reached a certain critical stage of dehydration, at which point their voices quieted as they dried up.
“In this study we resolved a very old scientific controversy: we proved that plants do make sounds!” Hadany said: “Our findings suggest that the world around us is full of plant sounds, and that these sounds carry information, for example about water shortages or injuries.”
Because many animals, such as bats and rodents, are capable of hearing frequencies much higher than ours, the researchers surmised that these animals probably knew the symphony of forests and plants long before we did.
Animals have probably learned to interpret the information communicated by the sounds that plants make, the researchers said, and it’s possible that other plants get information from these sounds as well.
“We think humans can use this information too, with the right tools, like sensors that tell growers when plants need watering,” Hadany said.
So how do plants make these sounds? That’s something researchers don’t yet know. They theorized that changing pressures within the stem of the plants could be creating the actual noise, with different stem sizes corresponding to the different frequencies observed between plant types, but this is something that will need to be investigated further.