Could that noise be the silverbeet (chard) screaming? It sounds like a cry of agony coming from the board as the knife slides through the leaves. Those leaves are fresh from the garden, severed from the plants a few minutes ago – the knife cuts through living plant tissue. Can plants feel pain? Can they feel themselves being cut, damaged or eaten?
Those questions must have occurred to most gardeners as they snipped, cut or pulled plants in the garden. The thought of causing pain to a person or animal makes most people feel uncomfortable. So, what about plants, whether we eat them as food or just tend them in the garden? Does a plant suffer pain when we prune it, crush a leaf to capture its scent, or eat it? Click To Tweet
Can Plants Feel Pain?
Some people claim that plants DO feel pain; others believe it’s not possible. Let’s see what some of the scientific studies show.
The answer depends partly on how we define “pain”.
Wikipedia offers this definition: “Pain is a distressing feeling often caused by intense or damaging stimuli,…Pain motivates the individual to withdraw from damaging situations, to protect a damaged body part while it heals, and to avoid similar experiences in the future.”
So what we usually call pain is an unpleasant sensation, courtesy of our nervous systems, brains and specific pain receptors called nociceptors. Pain warns us that something is not right: we have been cut and need to stop the loss of blood; we touch a very hot surface and a minor burn warns us not to go there again; a pain in the head could be a warning of imminent illness. Our bodies use pain as a warning system or a danger signal.
Do Plants Have a Nervous System?
We always think of pain in terms of a feeling that arises from the brain and nervous system. But plants don’t have brains, neither do they have nerves. So they don’t have a nervous system. But they are able to respond to stimuli in a way that has no need for a brain or a nervous system. In fact, they have some surprising responses that are similar in many ways to some of our pain reactions.
Surprising Plant Responses
The Sound of Being Eaten
At the University of Missouri (MU), Rex Cocroft and Heidi Appel showed that plants can detect the sounds made when caterpillars chew them.What? Plants can hear themselves being eaten? Click To Tweet
In response to that gruesome sensation, the plants then defend themselves from attacks by releasing chemicals. Cocroft used a special vibration microphone with laser technology to record the sound of a caterpillar eating. He then played back the recording to an Arabidopsis – a small cabbage-like plant used in the study. The plant reacted to the sound by producing extra mustard oil, which is toxic to caterpillars. You can watch the video here.
As you saw in the video, the Arabidopsis can recognise the sounds of an attack as a warning of danger and defend itself with a poison that it makes itself.
During a second experiment, Cocroft used groups of plants. He recorded several sounds – chewing vibration, insect songs, and wind vibrations – and played a different sound to each group.
To test for reactions, he measured levels of anthocyanins (the chemical producing dark red and purple in plants – also the red in wine) in the groups of plants, looking for increased levels. The plants in the group that had been played the chewing vibrations had an increase in anthocyanins, which showed a reaction to those vibrations.
Let’s anthropomorphize for a moment. The plants’ stress (which we shall call anxiety) caused the anthocyanins (chemicals which produce a red colour) to increase when they sensed the sound of danger. Then we could say that the plants flushed or blushed when anxious. It’s not an exact plant/human equivalent but I find it a quirky way of remembering the reaction.
There was no response from the other groups to the other sounds/vibrations.
Plants That Screamed
At the Institute for Applied Physics in Bonn, Germany, scientists have discovered that plants emit ethylene gas when damaged or stressed. In a complex process Frank Kuehnemann, a scientist at the Institute, observed what happened when part of a plant, such as a leaf was cut off. The plant released ethylene gas over its entire surface.
Researchers collected ethylene gas from stressed plants and bombarded the molecules with special lasers, causing them to vibrate. They then used microphones to pick up the resulting sound waves. They found that the more stress the plants were under, the louder sound. In effect, the plants sensed the damage and screamed.
Based on this research, I can say that the silverbeet probably cried out when cut, but that that wasn’t the noise I heard.
What do These Studies Show? Can Plants Feel Pain?
To answer those questions we need to recall the comments about how we define pain. We always think of pain as a feeling that comes from the brain and nervous system. Plants have no brains or nervous systems like those found in the animal world.
The answer then must be that plants do not experience the feelings of pain that animals would in similar circumstances. The responses that the experimenters observed were reactions to some other sensation.
Biologist Daniel Chamovitz, Dean of Life Sciences at Tel Aviv University in Israel explained this clearly, saying that “plants have pressure receptors, which record when the plant is touched, but they don’t have pain receptors.” [My italics and emphasis]
And there’s a key point: plants are aware of forms of pressure such as being touched… or being eaten. For example, plants like Mimosa curl up their leaves when touched. As Chamovitz said: “We know that when aphids attack leaves, it elicits an electric signal in plants that goes from leaf to leaf to signal it to start protecting itself. It’s propagated very similarly to the way it’s propagated along a nervous system.”
And he added: “The take-home message is that neural systems are one way to process information, not the only way.”
Then What Were Those Cries?
If plants don’t feel pain, what were those cries that Kuehnemann’s microphones recorded?
The plants had emitted ethylene gas as a stress response. As the gas molecules vibrated, the microphone system picked up the movement as sound. The cries then came from the vibrations of the ethylene gas molecules, which had been emitted by the plants. The difference is that the plants’ “cries” were a response to pressure, not pain. Remember, plants have no pain receptors. It may seem like a small difference but biologically it’s a large one.
Research is continuing and further studies could perhaps reveal a closer link between the sensory systems in plants and animals. However, to date, the evidence says that plants cannot feel pain.
In answer to the question: “Can plants feel pain?” we define pain as being a product of a neural system, which is specific to animals. Humans and other animals feel pain because we are equipped with pain receptors, called nociceptors.
Plants do not have nervous systems, brains or nociceptors. Therefore, they are unable to feel pain. They do, as Chamovitz noted, have pressure receptors that send signals through the plant. Those signals cause the plant to respond in one or more ways, including those that have been described above.
Thus plants feel pressure: they do not feel pain.
Despite the similarities between animal and plants in the ways they recognise damage and danger, the resulting sensations they respond to are not the same.