Can plants learn?

by Nathaniel Virgo

This post is about an idea I’ve had for a long time, about an experiment to test whether plants can learn. I’m very far from being a plant biologist, so I’m unlikely to ever be in a position to do this experiment, but it’s an interesting thing to think about.

Learning isn’t something we normally associate with plants, of course. We don’t normally think of plants as behaving at all – but of course they do, it’s just that they generally do it much more slowly than animals. Anyone who’s ever pitched a tent for a few days at a time will have noticed that the grass underneath grows tall and thin and pale. It does this in an attempt to find some light. If the tent were instead a small rock or fallen tree then the grass would be able to grow its way out from underneath and get a source of energy again.

But is this learning? No – it’s an evolutionarily in-built response, equivalent to what in animals we could call an instinctive or reflex response. You could say it’s a kind of learning that’s taken place on an evolutionary time scale (some the grass’ ancestors grew in this way an survived, so grass as a whole “learnt” to do it), but we’re interested in learning on the time scale of an individual.

So what makes a behaviour “learning”? There are probably a lot of different definitions depending on exactly what question you want to ask, but the one I’m interested in here is classical, or Pavlovian, conditioning. Pavlov noticed that dogs salivate in preparation for eating, and discovered that if he rang a bell every time he fed them, they would salivate whenever he rang the bell. The point is that the bell is otherwise an arbitrary thing – it doesn’t cause salivation unless the dog has learned to associate it with food.

So could we do the same thing with plants? It seems to me that it wouldn’t be too hard. Plants don’t salivate, but they do have their own set of “reflex” responses, including the response to low light that I mentioned above. Another is the response to grazing by animals, which usually involves growing tougher leaves, and perhaps producing some bitter-tasting or toxic compounds in order to deter further grazing.

The basic idea of the experiment would be something like this: we grow a whole load of plants at the same time, under identical conditions. These plants are divided into a test group and a control group. For the test group, at some randomly determined time we expose them to some arbitrary stimulus. This has to be something the plant is able to detect, but it shouldn’t be something that normally produces a strong response – I guess it could be something like a small change in soil pH, or blocking out part of the light for a while. In classical conditioning terms, this is called the “conditioned stimulus” (CS).  Then, some time later (perhaps the next day) we apply a different stimulus, one that the plant will respond to (but which will not kill it). For example, we could block out all the light for a couple of days, or we could clip the leaves to simulate grazing. This is called the “unconditioned stimulus” (US). Some time later we do this again – apply the conditioned stimulus, then apply the unconditioned stimulus. This is repeated several times. Finally, we apply the conditioned stimulus but not the unconditioned one and observe the results.

For the control group we apply both stimuli, but we apply both stimuli at randomly determined times. So the plants have been exposed to the same conditions overall, except that there’s no point in learning an association between the conditioned stimulus and the unconditioned stimulus, because they’re uncorrelated to one another.

A successful result from this experiment would be that the test group responds to the conditioned stimulus as if it were the unconditioned one. So we change the pH slightly and the plants start to grow tall and thin, or we block out part of the light and they produce anti-grazing toxins. But crucially, it’s not a successful result unless the control group doesn’t react in this way. This shows us that the response to the US is a learned one, and not just something that all plants of that species would do anyway.

Of course there are a lot of subtleties involved in running such an experiment, most of which I’m probably not aware of. An unsuccessful result wouldn’t be very interesting – it would only show that we were using stimuli that that species of plant can’t respond to. If plants can learn, some are probably better at it than others. The experiment would be easier to perform with a fast-growing weed (I was considering trying it myself with watercress at one point), but I suspect that trees are probably more likely to be smart, since they have more invested in staying alive for a long time.

This would be a very interesting experiment to try. Although it might seem unlikely to be successful, it probably isn’t all that hard to carry out, on the general scale of plant experiments. If any experimental plant biologist ever reads this who would like to give it a try, please feel free to get in touch!

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