Transfer plants from the side compartment with LL to the compartment with HL and vice versa. Test the plants after 28 days using complete training protocol mentioned above under training 2.
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The Mimosa Habituation apparatus is designed to facilitate controlled mechanical disturbances, such as simulated drops, to study the rapid leaf closure response of Mimosa plants as a defense mechanism against predation. Each disturbance triggers the folding and drooping of Mimosa leaflets along the stem.
This setup comprises an acrylic container affixed to a marked steel rail, with a soft foam base providing support. Potted plants are elevated to a designated 15-cm height and allowed to descend by sliding along the rail. A shallow depression in the foam base minimizes bouncing upon impact.
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Mimosa pudica, originally identified in Brazil, is a perennial shrub now classified as a pantropical invasive weed. This plant exhibits two distinct types of movement in response to environmental stimuli. Nyctinastic movements, known as sleeping movements, react to changes in light, while seismonastic movements respond to mechanical, thermal, chemical, or electrical stimuli (Sanberg, 1976). Mimosa pudica demonstrates both types through leaflet closure and petiole dropping.
Wallace investigated environmental factors influencing Mimosa pudica’s seismonastic movements, finding optimal sensitivity at 40°C, with responses observable between 14°C and 60°C, unaffected by humidity levels. Under artificial lighting with a 14-hour light and 10-hour dark cycle, peak sensitivity occurs around 5 AM and is lowest at 7 PM. Leaf sensitivity also diminishes with age.
Mimosa pudica employs inducible defenses, triggered in response to threats, such as temporarily closing its leaves upon touch—a behavior known as hiding time (Reed-Guy et al., 2017). This rapid leaf-folding response deters predators by startling them, reducing plant visibility, minimizing exposed surface area, and highlighting defensive thorns (Jensen et al., 2011). However, these defensive responses incur energy costs for reopening leaflets, reducing photosynthetic rates by up to 40%, a significant trade-off (Reed-Guy et al., 2017).
Plants employ foraging strategies based on resource availability and distribution, balancing risks and rewards per the optimal foraging theory (Simon et al., 2016). In resource-poor environments, plants increase foraging efforts, raising predation risks compared to those in higher-quality settings.
Habituation, a form of learning, involves decreasing response to repeated stimuli. Mimosa pudica exhibits short-term and long-term habituation, persistently folding leaflets initially but gradually ignoring repeated physical disturbances, akin to animal habituation (Gagliano et al., 2014). This adaptive behavior saves energy by filtering out irrelevant stimuli.
Sensory adaptation and effector fatigue can reduce plant responsiveness. Sensory adaptation results from prolonged sensory organ stimulation, necessitating longer inter-trial intervals to rule out. Effector fatigue, where response mechanisms falter, can be assessed through dishabituation tests reintroducing stimuli to gauge response restoration (Abramson et al., 2016).
Habituation was initially documented in 1887 by George and Elizabeth Peckham in their study on the ‘Mental Powers of Spiders’ (Christoffersen, 1997). In the plant kingdom, the first habituation experiment dates back to 1873 when Pfeffer studied Mimosa pudica. Pfeffer repeatedly stimulated the leaflets, initially causing them to close and the petiole to drop. Over time, however, the leaflets ceased responding and remained open despite continued stimulation. Bose later replicated Pfeffer’s findings in 1906, demonstrating habituation using both mechanical and electrical stimuli. He also showed that leaflet closure could resume if sufficient rest time was provided between stimulations.
Holmes and Gruenberg expanded on this research in 1965, exploring Mimosa habituation in the context of stimulus discrimination. They distinguished between water droplets and finger touches as stimuli, showing that the plant could discriminate between the two. This differentiation indicated that leaflet closure in response to finger touches was not due to fatigue, as it responded even after habituation to water droplets (Abramson et al., 2016).
By the 1960s, enough research had accumulated to establish an operational definition of habituation. In 1966, Thompson and Spencer formulated a definition based on nine behavioral characteristics, providing a framework for subsequent studies (Gagliano et al., 2018; Rankin et al., 2009).
In 1972, Applewhite investigated whether training variables known to influence habituation in animals also affected Mimosa. Using detached leaflets immersed in water, he employed dishabituation controls. More recently, Gagliano et al. studied short-term and long-term habituation in Mimosa pudica, examining the effects of light intensity on leaflet closure responses. Their findings corroborated earlier studies by Holmes and Grunberg.
Set up a growth room of approximately 5.30 m² for the experiments. Divide the room into three compartments using black plastic sheets. Conduct training trials in the central compartment and test trials in the side compartments. Equip each side compartment with fluorescent lights: one with low-intensity lights (LL) and the other with high-intensity lights (HL). Measure the light intensity just above the plants in each compartment before beginning the experiment. Grow individual Mimosa pudica plants of similar heights (6-8 cm) in 10 cm round acrylic pots filled with a standard mix of loamy soil and organic compost in a 1:1 ratio. Fertilize and water the plants as needed. Maintain a 12-hour light/12-hour dark cycle, with 60%-70% relative humidity and a temperature range of 21-24°C.
Randomly assign individual potted plants to either HL or LL conditions. Leave them undisturbed for 5 days until training day.
Training 1
Position a potted plant in the acrylic container and release it from a height of 15 cm in the morning. Repeat this drop from the same height after 8 hours. Perform this procedure for all plants in each side compartment.
Training 2
Place a potted plant in the acrylic vessel and drop it from a height of 15 cm. Perform this action 60 times, with intervals of 5 to 10 seconds between each drop. After completing this set, repeat the process six more times consecutively, increasing the intervals between each set of 60 drops. This procedure should be conducted for all plants in each side compartment.
Place the individual plant in a close fitted foam container that is attached to a shaker plate. Turn on the shaker plate and set it to 250 revolutions per minute for 5 seconds. Observe the leaf folding behavior.
Test each of the trained plants from both side compartments six days after 1-day training using 60 consecutive drops from a set height of 15 cm.
Transfer plants from the side compartment with LL to the compartment with HL and vice versa. Test the plants after 28 days using complete training protocol mentioned above under training 2.
After each drop, randomly select three leaves from each plant. Measure the width of each leaf (in mm) from tip to tip using digital calipers. Calculate the average of these three measurements. Represent the response as the maximum leaf width immediately after the drop relative to the maximum leaf width before the drop. Ensure that observations are taken promptly from the three leaves both before and after the drops.
Mimosa pudica is an ideal plant for habituation experiments due to its ease of maintenance, well-documented natural history, and noticeable leaflet-closing response to external stimuli (Abramson et al., 2016). Habituation experiments with Mimosa pudica are generally straightforward and require minimal equipment.
However, Mimosa pudica’s long recovery time of up to 15 minutes poses challenges. Key training variables, such as the inter-stimulus interval and the time between a response and its outcome, need to be brief for effective association formation. This slower response rate complicates comparisons between plant and animal behavioral studies, as animal responses are typically much quicker. To advance plant learning research, developing automated methods for stimulus presentation and response recording is essential. Understanding the rate, duration, and temporal pattern of the target response is crucial before studying habituation. Additionally, a control group that undergoes the training environment without receiving habituation training should be included (Abramson et al., 2016).
Some researchers contend that Mimosa pudica possesses a neural capacity for learning behavior. If this perspective is validated, it would enhance our understanding of nervous system functions and the specific mechanisms behind Mimosa’s responses. An advantage of using Mimosa for studying hormonally induced nervous system activities is that its cells are relatively large, accessible, and excitable compared to the smaller, less accessible cells typically involved in such processes. This characteristic offers new opportunities to explore the interactions between electrical and chemical control mechanisms.
Studies on plant learning through habituation provide valuable data for comparisons with animals, as habituation is a universally observed phenomenon across the animal kingdom. Despite being considered the simplest form of learning, habituation shares several properties with more complex learning processes, such as the ability to recover responses over time, the formation of new behavior patterns, and improved performance with repeated sessions. These properties, along with the plant’s sensitivity to variables like stimulus intensity, frequency, and pattern, present further areas for investigation.
Wallace RH (1931). Studies on the Sensitivity of Mimosa pudica III. The Effect of Temperature, Humidity, and Certain Other Factors Upon Seismonic Sensitivity. American Journal of Botany 18(4), 288-307. DOI: 10.2307/2435904
Thompson RF, Spencer WA (1966). Habituation: a model phenomenon for the study of neuronal substrates of behavior. Psychological Review 73(1), 16-43. DOI: 10.1037/h0022681
Applewhite PB (1972). Behavioral plasticity in the sensitive plant, Mimosa. Behavioral Biology 7(7), 47-53. DOI: 10.1016/S0091-6773(72)80187-1
Sanberg PR (1976). “Neural capacity” in Mimosa pudica: a review. Behavioral Biology 17(4), 435-52. DOI: 10.1016/S0091-6773(76)90811-7
Christoffersen GR (1997). Habituation: events in the history of its characterization and linkage to synaptic depression. A new proposed kinetic criterion for its identification. Progress in Neurobiology 53(1), 45-66. DOI: 10.1016/S0301-0082(97)00031-2
Rankin CH, Abrams T, Barry RJ, Bhatnagar S, Clayton DF, Colombo J, Coppola G, Geyer MA, Glanzman DL, Marsland S, McSweeney FK, Wilson DA, Wu CF, Thompson RF (2009). Habituation revisited: an updated and revised description of the behavioral characteristics of habituation. Neurobiology of Learning & Memory 92(2), 135-8. DOI: 10.1016/j.nlm.2008.09.012
Jensen EL, Dill LM, Cahill JF Jr (2011). Applying behavioral-ecological theory to plant defense: light-dependent movement in Mimosa pudica suggests a trade-off between predation risk and energetic reward. The American Naturalist 177(3), 377-81. DOI: 10.1086/658343
Gagliano M, Renton M, Depczynski M, Mancuso S (2014). Experience teaches plants to learn faster and forget slower in environments where it matters. Oecologia 175(1), 63-72. DOI: 10.1007/s00442-013-2873-7
Abramson CI, Chicas-Mosier AM (2016). Learning in Plants: Lessons from Mimosa pudica. Frontiers in Psychology 7, 417. DOI: 10.3389/fpsyg.2016.00417
Simon FW, Hodson CN, Roitberg BD (2016). State dependence, personality, and plants: light-foraging decisions in Mimosa pudica (L.). Ecology & Evolution 6(17), 6301-9. DOI: 10.1002/ece3.2340
Reed-Guy S, Gehris C, Shi M, Blumstein DT (2017). Sensitive plant (Mimosa pudica) hiding time depends on individual and state. PeerJ 5:e3598. DOI: 10.7717/peerj.3598
Gagliano M, Abramson CI, Depczynski M (2018). Plants learn and remember: lets get used to it. Oecologia 186(1), 29-31. DOI: 10.1007/s00442-017-4029-7
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