Plants do not have vocal cords or lungs. Hadany says the current theory for how plants make noises centers on their xylem, the tubes that transport water and nutrients from their roots to their stems and leaves. Water in the the xylem is held together by surface tension, just like water sucked through a drinking straw. When an air bubble forms or breaks in the xylem, it might make a little popping noise; bubble formation is more likely during drought stress. But the exact mechanism requires further study, Hadany says.

The team produced a machine-learning model to deduce whether a plant had been cut or was water stressed from the sounds it made, with about 70% accuracy. This result suggests a possible role for the audio monitoring of plants in farming and horticulture.

To test the practicality of this approach, the team tried recording plants in a greenhouse. With the aid of a computer program trained to filter out background noise from wind and air-conditioning units, the plants could still be heard. Pilot studies by the authors suggest that tomato and tobacco plants are not outliers. Wheat (Triticum aestivum), corn (Zea mays) and wine grapes (Vitis vinifera) also make noises when they are thirsty.

Chattering grasses?

Previously, Hadany’s team has also studied whether plants can ‘hear’ sounds, and found that beach evening-primoses (Oenothera drummondii) release sweeter nectar when exposed to the sound of a flying bee.

So are plant noises an important feature of ecosystems, influencing the behaviour of plants and animals alike? The evidence isn’t yet clear, according to Graham Pyke, a retired biologist at Macquarie University in Sydney, Australia, who specializes in environmental science.

He’s sceptical that animals listen to the moans of stressed plants. “It is unlikely that these animals are really able to hear the sound at such distances,” he says. He thinks the sounds would be too faint. Further research should shed more light on the matter. But Pyke says he’s perfectly willing to accept that plants ‘squeal’ when stressed.

This article is reproduced with permission and was first published on March 30, 2023.