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Mimosa pudica was first identified in Brazil. It is a perennial shrub which is now considered a pantropical invasive weed. Many plant species show two types of movements of the anatomical parts. Nyctinastic movements also known as sleeping movements, occur in response to changes in visible light. Seismonastic movements occur in response to other stimuli like mechanical, thermal, chemical or electrical (Sanberg 1976). Mimosa pudica shows both types of movements in the form of leaflet closure and petiole dropping. Wallace conducted research on the environmental factors related to seismonastic movements of Mimosa pudica. He found that the optimum sensitivity is seen at a temperature of 40oC with an apparent response still shown within the range of 14oC to 60oC, with no change in sensitivity seen due to relative humidity in the environment. Under artificial lightning, in a 14 hour light: 10 hour dark cycle, the sensitivity is highest near 5 AM and lowest at 7 PM. The sensitivity of the leaves also decrease with age.
Mimosa pudica show inducible defenses which are described as behavioral responses initiated only in the presence of a threat. Such behaviors when used by the plant at appropriate times provide greater resistance to attack from herbivores and increase chances of survival. Mimosa pudica temporarily closes its leaves when touched. The time the leaves remain closed is referred to as the hiding time (Reed-Guy et al., 2017). It is believed that the leaf folding behavior shown by Mimosa pudica reduces predation risk by scaring away predators due to rapid closing movement of the leaves, decrease visibility of the plant due to closed leaves, decrease surface area exposed to predators and making thorns more visible to the predators (Jensen et al., 2011). Such defense responses inflicts energy costs required to reopen the leaflets and also opportunity costs as this causes the photosynthetic rate of the plant to decrease by up to 40% which represents a substantial cost associated with such antipredator behavior (Reed-Guy et al 2017). Plants decide to forage based on the availability and distribution of resources. Optimal foraging theory states that an organism faces trade-offs during foraging and optimal strategy will be naturally selected during foraging that maximizes fitness by balancing risk and reward. When plants are in a poor quality environment, they will put more effort into foraging which will expose the plants to a higher risk of predation in order to gain resources as compared to the plants in a good quality environment (Simon et al., 2016).
Habituation is the simplest form of learning which refers to a decrease in a response to a repeated stimulus. Habituation can be either short-term or long-term depending upon the length of recall. Mimosa pudica shows habituation behavior by folding its leaflets initially but when they are repeatedly disturbed physically the plant learns to ignore the stimuli. Plants growing in environments that require a lot of energy show more marked leaf-folding habituation which can last even for a month if the plant is left undisturbed. Such persistent behavioral change of long lasting habituation is similar to the habituation observed in animals. Learning of habituation response allows the individuals to pay attention to important stimuli and save energy by ignoring harmless or irrelevant stimuli (Gagliano et al., 2014).
A decrease in the sensitivity of the plant to respond can be due to sensory adaptation or effector fatigue that must be ruled out. Sensory adaptation occurs due to intense periods of stimulation of sensory organs causing a decrease in responsiveness. To rule out sensory adaptation, a longer inter-trial interval should be selected between stimuli. A test trial can also be used in which habituation is tested after training with a long enough time interval between training and the test trail. Effector fatigue is the inability of the effector response mechanisms to function properly. A test trial is given to the subject using a second stimulus that triggers the target response. If then reintroduction of the original stimulus produces a response, effector fatigue can be ruled out. This procedure is referred to as dishabituation which is used as a control in most of the habituation experiments (Abramson et al., 2016).
Habituation was first described in 1887 by George and Elizabeth Peckham in a publication on the ‘Mental Powers of Spiders’ (Christoffersen 1997). In plants, the first habituation experiment was reported in 1873 by Pfeffer using Mimosa pudica. Pfeffer used repeated mechanical stimulation of leaflets, which initially caused leaflets to close and petiole to drop but eventually the leaflets stopped responding and stayed open despite continuous stimulation. Later on in 1906, Bose conducted another study using Mimosa which confirmed the findings of Pfeffer. Bose also used electrical stimulation along with mechanical stimulation, both of which resulted in habituation of leaflets. He also demonstrated that leaflet closure can be deployed again if sufficient rest period is allowed in between stimulation. Holmes and Gruenberg in 1965 then studied Mimosa habituation in context of stimuli discrimination. They used a drop of water and finger touch as stimuli to show that the plant is able to discriminate between the two stimuli because the leaflets close due to finger touch after they have habituated to water drops. Also the leaflet closure was not due to fatigue otherwise it would not have responded to stimuli from finger touch (Abramson et al., 2016).
By 1960’s, enough studies had been conducted to form an operational definition of habituation, so in 1966 Thompson and Spencer defined habituation based on the list of nine behavioral characteristics that are till date used as frame work for future studies. The list of characteristics have been confirmed by most of the studies and have been expanded over the years by many researchers (Gagliano et al., 2018, Rankin et al., 2009).
Applewhite in 1972 studied whether the training variables that are known to influence habituation in animals also influence habituation in Mimosa. He used detached leaflets placed in water for the experiment along with using a dishabituation control.
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