Mimosa pudica Response to Varying Stimuli

By Amber Mooney and Rachel Leslie

Mimosa pudica is one of only a few plants that can respond to the environment with a relatively large and vigorous movement in a matter of seconds. How? M. pudica plants have special cells at the base each of their leaflets and petioles called pulvini. Pulvini have large vacuoles on the dorsal and ventral sides of the leaves that fill with water depending on the ion concentration in that vacuole. Pulvini control the ion concentration within these vacuoles by actively transporting potassium ions out of the cell, and transporting chlorine ions into the cell. When a leaf is stimulated, the membranes of the vacuoles on the ventral side of the leaf becomes permeable to the ions and the gradient is disturbed or depolarized which causes water to leave the ventral vacuoles. Simultaneously, the dorsal vacuoles are filled with ions causing water to rush in. The pressure from these vacuoles causes the leaflets and petioles to fold up and droop down. This response is much the same as what happens in the muscle cells of most animals.

Movement such as the folding response of the Mimosa plant is called thigmonastic movement. This is the movement of plants in response to a stimulus that is independent of the direction that the stimulus comes from. In Mimosa pudica the response of folding the leaflets and dropping the petioles can move from one leaf to the next throughout the whole plant if the stimulus is large enough. In this experiment, we set out to determine the effect that different stimuli have on the action potentials and response times of the Mimosa pudica plant. We also wanted to determine whether or not the strength of the stimulus made a difference in the amplitude of the action potential and recovery period of the leaves. We obtained 3 mature M. pudica plants, and cared for them in the same environment for approximately 5 days prior to running the experiments. We then attached a volt meter to seven different leaflets on the various plants. Using three types of stimuli, fire, water, and touch, we caused each leaf to respond to a given stimuli. We also tested the difference between a strong and a weak touch stimulus. Using the volt meter and a video camera, we recorded the action potential and response of each leaf during a 15 minute time interval.

Unfortunately, we only obtained a clear action potential for one of the experiments, the first fire stimulus. The other three trials did not produce a clear action potential even though they folded up as expected. This could be due to problems with the equipment setup, not enough water in the soil, gaps in the electrode connections, or the absence of a strong stimulus, thus it does not prove that an action potential did not occur. As expected, the response time appeared to be shorter for the stronger stimuli (water and the heavy object) than for the weaker stimuli (light object). However, response time for the fire was longer in both fire trials. Surprisingly, the folding time was longer for the stronger stimuli. We would have expected it to be a quicker response and faster folding. However, it did take longer to recover after the stronger stimuli, which is what we expected. There are several sources of error in this experiment that must be acknowledged including differences in age, quality, and environmental conditions for each of the plants. Some were smaller and had been shipped prior to the experiment. Our experiment shows that our methods are a good method for further study with a few modifications to reduce error and more trials are necessary for any strong conclusions to be drawn.