Do plants experience pain? A brief introduction to the concept of plant neurobiology.

Few things capture the essence of summer like the fragrance of freshly mowed grass. For many, it's a welcome sign that warm weather is finally here. But for the grass itself, this scent tells an entirely different story.

The fresh scent of cut grass is, in fact, a chemical alarm, a distress signal from plants calling on nearby creatures for help when under attack (often by insects, but sometimes due to the sharp blades of a lawnmower). This raises the question: Do plants feel pain?

The situation is a bit nuanced because, while plants don't experience pain in the same way humans do, some plant researchers suggest they may feel it in their own unique manner. Let's explore some plant neurobiology to understand how these multicellular organisms might sense pain.

Nervous Systems

Pain perception is usually linked to living beings with a nervous system, which consists of sensory receptors, neurons, and brain regions that process sensory data. However, plants lack a brain or nervous system—but they do have intricate signaling and communication mechanisms that help them react to their surroundings.

Plants rely on an array of chemical and electrical signals to detect changes in light, gravity, temperature, and touch. They can also adjust their growth by moving toward or away from stimuli, altering their root and shoot growth, and producing defensive chemicals to fend off predators. These responses are controlled through complex biochemical processes and plant-signaling molecules like hormones.

Chemical Defenses

Though plants don't communicate in the same way humans do, they rely on chemical signals to safeguard themselves. When faced with danger—be it from landscaping tools, a ravenous caterpillar, or other threats—plants can't simply uproot and escape. Instead, they must stand their ground and defend themselves using a variety of molecular reactions.

These chemical signals can be used to poison predators, warn nearby plants of impending danger, or even attract beneficial insects to provide assistance [source: Krulwich].

In some cases, a plant's chemical defenses serve more than one purpose. For example, plants that produce caffeine use it to fend off threats, while simultaneously giving bees a caffeine high. The bees, treated like regular patrons at a coffee shop, keep coming back, offering their pollination services in return.

A Complex Biological Network

As they grow, plants can change their paths to avoid barriers or reach out for support using their tendrils. This behavior is governed by a sophisticated biological network spread across the roots, leaves, and stems, aiding them in growth and survival. In a forest, for instance, trees can signal to their kin about insect invasions.

A researcher once injected fir trees with radioactive carbon isotopes, and within just a few days, the carbon traveled from tree to tree, linking every tree in a 30-meter-square area. The scientist discovered that mature trees "communicated" within the network, sharing nutrients through their root systems to nourish nearby seedlings until they were strong enough to absorb light independently [source: Pollan].

Screaming Cucumbers

Now that we've touched on some unique methods of communication, let's explore some of the more unsettling discoveries scientists have made in this area.

At the University of Bonn's Institute for Applied Physics in Germany, researchers found that plants emit gases that could be likened to cries of distress. Using a laser-powered microphone, they detected sound waves produced when plants release gases due to injury or being cut.

Though these sounds are beyond the human ear's perception, the hidden voices of plants have revealed fascinating truths, like how cucumbers scream when they are ill, and flowers whimper when their leaves are cut [source: Deutsche Welle]. It turns out, cucumbers aren't the only ones vocalizing in this way.

Tomato Plants Under Stress

In a 2019 study published in the journal Cell, researchers from Tel Aviv University used microphones placed near dehydrated or damaged tomato and tobacco plants. They successfully detected ultrasonic sounds emitted by the plants from about four inches away. These sounds, ranging from 20 to 100 kilohertz, could potentially be picked up by certain organisms several meters away.

Unfortunately, you won't hear these distress calls while relaxing near your basil plant, as they occur at ultrasonic frequencies beyond the range of human hearing. When adjusted to frequencies humans can hear, these stress-induced sounds resemble the noise of tap dancing on bubble wrap.

Although these ultrasonic sounds are inaudible to humans, they could potentially be heard by mammals, insects, and other plants in their natural surroundings, triggering appropriate responses.

Sounds of Chewing Trigger Plant Defenses

In a strange and unsettling discovery, it appears that plants can hear the sounds of themselves being consumed. Researchers at the University of Missouri-Columbia have uncovered that plants can detect and react to the chewing noises made by caterpillars feeding on them. Upon hearing these sounds, the plants activate various defense mechanisms [source: Feinberg].

For some scholars, the evidence of intricate communication systems — where plants release gases when in distress — suggests that plants might experience a form of pain. However, others argue that without a brain or nervous system to process such sensations, pain cannot truly exist. But before you reconsider your vegetable choices, rest assured that these plants are probably not feeling pain in the same way as animals or sea creatures do.

Nevertheless, many scientists propose that plants can display intelligent behavior even without a brain or conscious awareness [source: Pollan].

Plant Consciousness May Be a Real Thing

Recent research has uncovered fascinating revelations about plant behavior, challenging long-held beliefs about their abilities. Certain plants, such as the Mimosa pudica, can be put into a state of anesthesia with substances like ether or lidocaine, halting their reactions to stimuli and suppressing their electrical activity.

This has raised intriguing questions about whether this 'sleep' state could indicate some level of awareness or consciousness in plants. A small group of researchers, including Paco Calvo from the University of Murcia, are exploring this concept seriously.

Plants demonstrate remarkable abilities, perceiving and responding to various environmental factors, while also engaging in communication and complex interactions with other species. While some of these behaviors are instinctual, others may point to a form of cognition. Calvo's research focuses on identifying characteristics that suggest cognitive behavior in plants, such as adaptability, foresight, and purposefulness.