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Plants wither away when they don鈥檛 receive enough water and sunlight. Can they also feel pain as a result?
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Matthew Stevens
Sydney, Australia
The short answer is no, plants cannot feel pain. Pain is an experience that provokes an instant response to avoid the cause of the pain. This evolved in animals because it gives a survival advantage: remove yourself from the cause of the pain and live to see another day.
If plants ever evolved the capacity to feel pain, they would also need the capacity to avoid the cause of their distress. For plants that are rooted to the spot, that is nigh impossible. A plant species that devoted resources to building pain receptors and developing avoidant behaviours could, for example, be outcompeted by plants that don鈥檛 feel pain and put the same resources into making more seeds.
This isn鈥檛 to say that plants can鈥檛 feel sensations at all: numerous observations and experiments show that plants respond to touch, such as Mimosa pudica and tendril vines. When nibbled, many plants release volatile chemicals that alert other parts of themselves and other plants. This triggers them to produce defensive chemicals, which are expensive to make in the absence of predators.
In short, there is no survival advantage for a plant to be able to feel pain if it can鈥檛 take action to avoid the harm. Developing tough, spiky or distasteful features is a much more likely advantage than developing motility.
Mike Follows
Sutton Coldfield, West Midlands, UK
Researchers have long recognised the survival value of pain, with many animals experiencing it and responding accordingly. While plants don鈥檛 have nervous systems like animals do 鈥� meaning they cannot feel pain in the way we understand it 鈥� they do have mechanisms that allow them to respond to environmental stress.
For example, while plants cannot move when they detect a lack of water, they may close their stomata (the pores on their leaves) in order to retain more moisture.
Plants can also release chemicals to warn nearby plants about threats like herbivores. The compound we recognise as the scent of freshly cut grass, for instance, is part of a larger group of chemicals called green leaf volatiles. These are volatile organic compounds that diffuse easily through the air and provide a chemical function somewhat similar to the pheromones used by many animals.
It is important to recognise that pain isn't merely a detected signal; it is sensed by the nervous system, which plants lack
Recently, scientists have uncovered the existence of extensive mycorrhizal networks, symbiotic associations between plants and fungi that are a key part of what has come to be called the 鈥渨ood wide web鈥�. Fungi form symbiotic relationships with the plants鈥� roots, creating connections between different plants and trees that facilitate the exchange of nutrients, water and chemical signals between them.
When plants are disturbed, whether by a bug bite or a mechanical injury, they release glutamate, an amino acid that activates receptors and triggers a 鈥渃alcium wave鈥�. This ripple of calcium spreads throughout the plant. Intriguingly, this is similar to the role that calcium plays in humans, signalling between neurons during stress or pain.
The way that plants respond to threats or damage is understood as a survival mechanism based on biochemical processes, but it doesn鈥檛 include the emotional or sensory experiences we associate with pain. For a long time, it was thought that insects didn鈥檛 experience pain either, and while it is now accepted that some do, this doesn鈥檛 mean we should think this will automatically extend to plants as research advances.
Pain isn鈥檛 merely a detected signal; it must be sensed by the nervous system, which plants lack. Pain evolved to alert us of potential damage, but since plants can detect damage without sensing pain, they aren鈥檛 significantly disadvantaged.
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