How Much Can Plants Hear?

Plants may possess more complex senses than previously thought, according to a new study led by the University of Western Australia that has revealed that plants have the ability to both detect and respond to sound.

Talking to plants has always been fairly common practice among many aboriginal tribes – and to this day, the most devoted gardeners will claim that playing classical music or providing words of positive reinforcement can help encourage healthy plant growth. Taking this one step further, Leigh Joseph, an ethnobotanist with Canada’s Squamish First Nation, is starting her Ph.D. this fall to examine the use of plant foods and medicines to treat type 2 diabetes.

To support her research, Joseph has had to gather wild plants used to create traditional medicines – and the protocol she was taught requires her to provide a verbal offering to the plants through a prayer, a thanks, a song, or by simply introducing herself.

“When you go out into the forest, into an estuary, or a different ecosystem, where you’re going to be harvesting a plant, food, or medicine, is you want to get yourself into a space of respect and understanding that we are interacting with the plants who are considered our second oldest ancestors that we have after the rocks, which are called the grandfathers and the plants are referred to as the grandmothers,” Joseph said. “Those offerings are meant for the plant and meant also for the ancestors who walk beside us when we go out and harvest on the land.”

Thanks to new research by Dr. Monica Gagliano and her team, published earlier this year in Oecologia, there seems to be evidence that the plants might really be listening. A research associate professor adjunct in Evolutionary Ecology at the University of Western Australia, Gagliano worked with a group of researchers to prove that plants can hear.

“We know they are really good at looking for water by sniffing it using the humidity gradients, but sometimes a plant is just too far away from the water source. There are pretty much infinite possibilities of where the plants could direct their roots, and they can’t afford to just direct them at random,” Gagliano said. “So how do they know which one is the right direction to go for?”

To examine this behavior, Gagliano and her colleagues used maze-like pots shaped in an upside-down Y to grow pea seedlings. One arm was placed in dry soil, and the other in a tray of still water or a coiled plastic tube with running water. After five days of undisturbed growth, the plants were removed from their enclosures to allow the researchers to see how the root systems developed.

When the water was hidden inside the coil and difficult to access, the seedlings still grew toward the source. More interesting was that the plants continued to grow toward the sound – even when they could only hear a recording of running water.

“They just knew the water was there, even if the only thing to detect was the sound of it flowing inside the pipe,” Gagliano said. “It indicates that the invasion of sewer pipes by tree roots may be based on the plants ‘hearing’ water, and shows that their perception of their surroundings is much greater and far more complex than we previously thought.”

In 2014, a study revealed that the rock cress Arabidopsis was able to not only distinguish between the sounds of a caterpillar chewing its leaves and the branches simply rustling in the wind, but produced more chemical toxins when it was played a recording of feeding insects – in an attempt to drive away the predators. Another study showed that when plants are exposed to a specific frequency of bee buzzing, they will begin to release more pollen.

“We tend to underestimate plants because their responses are less visible to us,” said Heidi M. Appel, an environmental scientist at the University of Toledo who authored the study. “But leaves turn out to be extremely sensitive vibration detectors.”

In fact, Jack C. Schultz, a professor in the Division of Plant Sciences at the University of Missouri who has studied plant interactions for the past four decades, describes these living creatures as “very slow animals.” Time-lapse footage demonstrates the precise movements and behaviours of plants, showing similarities to both animals and even humans – hunting for food, vying for territory, evading predators, and even trapping prey.

“Plants move with purpose,” Schultz said. “To respond correctly (to their environment), plants also need sophisticated sensing devices turned to varying conditions.”

According to a biologist at Germany’s University of Greifswald, Michael Schöner, plants may be equipped with organs that allow them to perceive noises, which could take the form of any fine, hairy structure that is able to function like a membrane.

But no matter how plants are able to hear, this research shows that we may need to start examining how noise pollution could impact plant life as well as animal life. Excess noise in the environment may block information channels between plants, Gagliano said, preventing them from communicating effectively when they need to warn each other of insects or other concerns.

“We begin to see the complexity of plant interactions with sound in using it to make behavioural decisions,” Gagliano noted. “In the animal world, there is a strong call to understand how acoustic pollution adversely affects populations, but now we know plants also need to be part of these studies.”

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