Can plants learn?

Are plants conscious? A new opinion paper doesn't beat about the bush: "no" is the answer...

"So what?" I hear you ask! "Surely everybody knows that plants are not conscious?" In fact, the question of whether plants can think, learn, and intentionally choose their actions has been a subject of debate since the establishment of plant neurobiology as a new scientific field more than a decade ago. But this week, an opinion article published in the journal Trends in Plant Science, argues against plant consciousness.

“They are arguing that where another paper has suggested that plants can have innate consciousness and intelligence, that this is incorrect,” says Howard Griffiths, Professor of plant sciences at Cambridge University.

One of the studies cited in the Trends paper claims to have demonstrated that plants are capable of learning the association between the occurrence of one event and the anticipation of another event. This is known as Pavlovian learning, after the Russian physiologist Ivan Pavlov who famously made dogs' mouths water just by ringing a bell. He arrived at this situation by sounding his bell whenever he gave his dogs their food. Eventually the dogs began to associate the ringing of the bell with getting fed, so they salivated whenever the bell rang, whether or not it was accompanied by something in their dish.

The more modern plant scientists had replicated this study in pea plants, exposing them to two different stimuli during the training phase: light and wind. Their results seemed to suggest that the plants could be trained to associate the presence - or absence - of wind with the presence of light. But the authors of the new Trends paper were left unconvinced.

In the training stage, pea seedlings were grown in a Y-shaped ‘maze’, with the peas in the bottom branch of the Y. For some of the pea seedlings the two stimuli came from the same arm of the Y-maze, and for others the two stimuli came from different arms. But for both sets of peas the stimuli were swapped, randomly, from the left to the right arm at various times during the training phase, which lasted three days.

The fourth day was test day. A control group of peas were left undisturbed, with no stimuli. All of this control group grew in whichever direction the light had last arrived from. This is the expected behaviour. “We know that plants can remember because we have plants that track the sun.... they start the day ready to face the sun and then they will follow the sun round,” says Griffiths.

The plants in the test group were exposed to wind only in the test, so they had a choice to make: grow in the direction that the light had last been, or grow in the direction that would be predicted due to the direction of the wind, which was actually the opposite direction to where the light had last been. 65% of the test group chose the latter option. That means that the peas which were trained with the light and wind coming from the same arm grew towards the wind, and peas trained with the wind and light in opposite arms grew away from the wind.

Nevertheless, the critics are arguing that the pea study needs to be repeated with more stringent controls in place, before a clear conclusion can be drawn. That said, we know that some cereals have "ears", so why not pea plants too...