Mimosa Action & Humidity's impact on its reaction to touch

Mimosa Action!

Hi there ! We are students from the “Frontiers of Life” bachelor, created and hosted by the Centre of Research and Interdisciplinarity ! The Biosensors seminary has slowly come to an end, and the project we worked on and are about to present, is the last one out of 4.

For those who do not know what the Biosensors are and missed our first blog posts, it’s an interdisciplinary seminar during which we develop and design in small groups several one-week scientific research-projects connected to each other by various notions such as light, forces and chemical gradients.

The aim of these projects is to observe and compare biological (plants, bacteria, insects, vertebrate, human ….) and electronic sensors (light, movement, conductivity, temperature sensors…).

This week, the comparison wasn't necessary, and we were free to develop any project, as long as it involved sensing (biological or electrical).

For our last project, we decided to study one of the most fascinating phenomena in the plant kingdom : thigmonasty. Thigmonasty consists in the response of a plant to touch or vibrations.  Mimosa pudica appears in scientific literature as a common example capable of responding to various stimuli (rain drops, wind …)

We got inspired by a project that had been conducted by former FDV students in 2013, named “Electrical response of Mimosa pudica to an external stimulus”. Mimosa pudica is a herb that has leaves that fold inward when touched. The project consisted in comparing the voltage response of Mimosa pudica  (in terms of electrical potential) according to the nature of the stimulus. They could observe that the response differs whether the stimulus was mechanical or electronic.

After more bibliographic research, we realised that water was the basis of the thigmonastic reaction.

Indeed, the thigmonastic movement is produced by the main pulvinus (sort of articular surface from a plant) at the base of the petiole (stalk that attaches the leaf to the stem) and «results from the explosive loss of water from specific cells in the pulvini, causing the cells temporarily to collapse and inducing very quick curvature in the organ where they are located»

(http://plantsinaction.science.uq.edu.au/edition1/?q=content/8-2-5-nastic-movements) . This reaction also causes a variation in the electrical potential of the plant.

Combining these informations, we asked ourselves this question:

Do different water conditions impact Mimosa pudica’s electric response to a stimulus?

According to previous studies, the efflux of potassium ions (chemicals that own an electrical charge) from the pulvinar cells of Mimosa Pudica was shown to increase substantially during the seismonastic reaction,  generating an electric current which can be measured by means of a voltmeter or an oscilloscope.

We supposed that the plant’s cells watered with different amounts of water would release various volumes of electrolytes (ie. fluids carrying electric charges), and thus that we could measure a variation in the electric response of the plants according to the humidity of the earth the mimosa growing in.  

What did we compare?

To answer this question, we used 9 Mimosa that grew in the same conditions. We divided them into 3 different humidity conditions,  planning to take 3 measures (repetitions) per plant:

• 3 of them were dried using a dehumidificator (“dry”)
• 3 of them were left as they were (“normal”)
• 3 of them were watered with 20 cL of water (“wet”)

One of us was then chosen to touch each plant so as to minimize the variation in “force” used.

What did we measure?

We then had to acquire our data. We placed 2 electrodes on our Mimosa, one in its soil as a reference and one on the node of the branch stimulated.

After that, we connected it to a voltmeter and tested our protocol. That was when we encountered our first difficulty: the electrode would detach each time the plant was stimulated!
We thought about other types of electrodes, but they would've broken our stems and we didn't want to hurt our Mimosa. We were quite stumped.

That is when we had an idea: why not use an oscilloscope! The oscilloscope’s electrodes are easy to place and with their hook-like shape wouldn't fall of!

We found our physics teacher and asked him if he could lend us an oscilloscope and its electrodes. He did more than that! He showed us many types of electrodes, helping us choose the most adapted, and helped us install the oscilloscope, showing us how to set it up, how to acquire the data directly on a USB key (instead of filming the screen and try to transform it into data) and testing its usefulness on Mimosa with us.

Thanks to this new tool we were able to design our final precise protocol that we will use to acquire data. So we repeated a simple experience for all the conditions that we described above. Indeed we created a mechanical stimulus during an electrical tension acquisition with the oscilloscope to observe the response on the drawn graph.

We knew that amplitude of the signal after the stimulus and the time it takes to go back to “normal” (return time) were two good indicators to quantify the response. So, as you might expect it, we measured both for each of our 27 graphs.

What are our results?

Then, we decided to plot the averages values per plant (repetitions) as one bar but the different plants (replicates) separately. Thus we obtained those two graphs (the first one represents the amplitude of the signal’s peak and the second one the return time, both depending on water conditions).

In both case, we seem to see an increase of both the amplitude of the perturbation and the return time as we expected. This finding would mean that water would have a direct impact on the electric response of the plant.

But this trend, is not measurable. Indeed we have been constrained to name the conditions “arbitrary” because our soil humidity sensor appeared to be unworking.    

Anyway, we have also notices that in each case, the return time and the amplitude of the peak are directly correlated. Indeed we can see exactly the same relative differences between all the replicates on the two graphs.

   

Finally, this project was for us a chance to develop our background knowledge on a particular subject and to train our project leading skills. Hopefully, it might brings you [readers] something on one of this two aspects of research!

Curious? Interested? Want to know more? Have a look at the links :

• Want to read scientific articles about Mimosa pudica and its thigmonastic reaction? Have a look here or there!
• If you want more information, but easier to understand than scientific articles, you might prefer reading this, that and what about this instead!
• Or maybe you want to grow out your own Mimosa pudica? You can find information here!