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In recent years, plant physiologists have examined the physiology of plant behavior and intelligence. The concepts of learning and memory are relevant in identifying how plants respond to external cues, a behavior necessary for survival. Monica Gagliano, an Australian professor of evolutionary ecology, makes an argument for associative learning in the garden pea, Pisum sativum. The garden pea is not specific to a region, but rather grows well in cooler, higher altitude climates. Gagliano and colleagues’ paper aims to differentiate between innate phototropism behavior and learned behaviors.

Plants use light cues in various ways, such as to sustain their metabolic needs and to maintain their internal circadian rhythms. Circadian rhythms in plants are modulated by endogenous bioactive substances that encourage leaf-opening and leaf-closing and are the basis of nyctinastic behaviors (Ueda and Nakamura).

Gagliano and colleagues constructed a classical conditioning test in which pea seedlings were divided into two experimental categories and placed in Y-shaped tubes. In a series of training sessions, the plants were exposed to light coming down different arms of the tube. In each case, there was a fan blowing a light wind down the tubes in either the same or opposite arm. The unconditioned stimulus (US) was the predicted occurrence of light and the conditioned stimulus (CS) was the wind blowing by the fan. Comprehensive previous experimentation shows that plants respond to light by bending and growing towards it through differential cell growth and division on either side of the plant stem (Liscum et al).

During the testing phase, the pea seedlings were placed in different Y-pipes and exposed to the fan alone and their direction of growth was subsequently recorded. The ‘correct’ response by the seedlings was deemed to be growing into the arm where the light was “predicted” from the previous day. The majority of plants in both experimental conditions grew in a direction consistent with the predicted location of light based off of the position of the fan the previous day. For example, if the seedling was trained to associate the fan and light as coming down the same arm of the Y-maze, the following day the seedling grew towards the fan in the absence of light cues despite the fan being placed in the opposite side of the Y-arm. Plants in the control group showed no preference to a particular arm of the Y-pipe. The percentage difference in population behavior observed between the control and experimental groups is meant to distinguish innate phototropism from associative learning.

While the physiological mechanism of associative learning in plants is not known, Telewski et al. describes a hypothesis that describes photoreception as the basis of mechano-perception in plants. One mechanism for mechano-perception in plants includes MS ion channels and calcium channels.

Mechanosensory proteins in cell lipid bilayers, known as MS ion channels, are activated once they are physically deformed in response to pressure or tension. Ca2+ permeable ion channels are “stretch-gated” and allow for the influx of osmolytes and calcium, a well-known second messenger, into the cell. This ion influx triggers a passive flow of water into the cell down its osmotic gradient effectively increasing turgor pressure and causes the cell to depolarize. A downstream cascade causes plants to respond in various ways to increased turgor pressure. Gagliano hypothesizes that the basis of associative learning in Pisum sativum is the coupling of mechanosensory and photosensory pathways mediated by auxin signaling pathways. The result is directional growth to maximize a plant’s capture of sunlight.

Gagliano et al. published another paper on habituation behaviors in the mimosa pudica plant whereby the natural behavior of the plant was diminished by repeated exposure to a stimulus. (Gagliano 2014). There has been controversy around the topic of plant cognition by several critics. Charles Abrahmson, a psychologist and behavioral biologist, says that part of the issue of why people disagree about whether plants have the ability to learn is that researchers do not use a consistent definition of learning or cognition (Abrahmson). Similarly, Michael Pollan, an author and journalist, says in his piece An Intelligent Plant that researchers do not doubt Gagliano’s data but rather her language and specifically her use of the term “plant learning” (Pollan). A direction for future research is testing whether circadian rhythms in plants modulate learning and behavior and also surveying researchers on their definitions of “cognition” and “learning.”