Learning by association is when a neutral stimulus is associated with a stimulus that evokes some kind of behavior. The classic example is the work by physiologist Ivan Pavlov in the 1890's on associative learning in dogs. If the dogs heard the clicking of a metronome and fed shortly thereafter, they began to associate the clicking of the metronome with getting food. Pavlov found a significant increase in the dogs production of saliva after they heard the metronome but before they saw or smelled the food. At the time, this was a remarkable discovery.
The conventional wisdom is that learning by association is limited to animals since they have the required central nervous system, including of course, a brain. However, researchers have recently found evidence of this same ability in plants. Since plants have no central nervous system, this would be a revolutionary discovery.
Phototropism is a plant's natural tendency to lean toward light, which it needs for its basic metabolism and growth. This tendency has been extensively studied and analyzed. Light is a stimulus that evokes a measurable behavior in plants.
Dr. Monica Gagliano and her co-workers designed an elegant experiment to test the theory that plants, like Pavlov's dogs, could learn to associate a neutral stimulus with one that results in a behavior of some kind, such as phototropism. Their results are published in the journal Scientific Reports.
They took the seedlings of pea plants* and put them in Y-shaped cones that fit over their pot so the plant could receive a stimulus in one side or the other.(see figure). The neutral stimulus was a small fan placed on one side of the "Y", the other stimulus was a grow light which caused the plant to lean toward it due to phototropism. In one group of plants (i) the "training period" consisted of the plants being exposed first to the fan, then to the light on the same side of the "Y". In the control group (ii), the fan and light were on different sides. After 3 days, when the fan was turned on the plants in group (i) began to lean toward it before being exposed to the light. They were responding just like Pavlov's dogs. In the control group of plants (ii), the plants leaned toward the light but did not lean toward the fan when it came on; they had no reason to since they never received light there.
Dr. Gagliano found a statistically significant effect of pea seedlings associating the fan with the light compared to controls. The seedlings anticipated the timing and direction of light and used their innate phototropism to lean toward it. Dr. Gagliano states: "Whilst the possibility that plants also learn by association has been considered by earlier studies, our current study provide the first unequivocal evidence."
These experimental results beg the question: since plants have no central nervous system, in fact, no nervous system at all, how can they learn by association? Does the ultimate mechanism of learning in plants reside in the cellular, or even molecular level of its physiology? Anything that improves plant's capture of light would improve growth, reproduction, and survival. Is responding to a neutral stimulus an evolutionary adaptation to allow plants to develop and survive as a species.? The questions generated by this study are almost endless; it's a scientist's dream.
The excitement surrounding this study has now tempered somewhat. Whenever experimental results indicate new and unexpected findings, the scientific community will demand more studies to reproduce and collaborate the results. Such a study to replicate Dr. Gagliano's experiment was recently published in the journal eLife by Dr. Kasey Markel. With some minor differences, he repeated Dr. Gagliano's work and did not see the same learning by association in his plants. He recommended more studies and ones where he or other researchers collaborate with Dr. Gagliano to ensure consistent materials and methods.
Regardless of the results of future studies on plant's ability to learn, Dr. Gagliano's creative and innovative thinking is to be celebrated. She has opened the door to our thinking about plants and even life itself. Maybe higher life forms with a nervous system are not the only ones that can adapt to their environment by associative learning. Maybe the fundamental elements of life at the cellular and molecular level are responsible for some of observed behaviors and adaptations. What wonderful concepts to contemplate. We will see what the future holds.
*For more information about pea plants, including how to grow and care for them, see https://happydiyhome.com/pea-plant/.