If I asked you for an example or proof that plants can feel, your first option to come to mind would probably be a fly trap plant. To list some more, you also presumably know that vine tendrils start coiling when they feel there is a fence or something to grab onto. Trees know when their branches are being swayed in a wind and respond to this with stronger roots. In fact, plants are probably 10 times more sensitive than we are – to have a complete idea: a human can notice about two micrograms on their skin, a cucumber plant can feel the weight of only a quarter of a microgram. However, with a basic background in physics, it should be no surprise that both systems work almost the same way.
Our somatic sensory system is very complex, but there are three main parts: the receptors, which sense touch, pain or temperature, the neurons, which transmit these signals to the brain, and the brain, which processes this information and responses to it. Every input into this system starts with an electro-chemical reaction called a depolarization. We will not be going into details, but if you would like to know more about this, there are plenty of educational videos on the internet. From various tests and comparisons between human and plant system, we can notice similar trends. If we go back to the fly trap plant, we can make it close by simple electrode signal, which is similar to flexing your muscle. Even more, with certain inhibiting drugs, which stop human nerve signaling, we can stop the plant from closing. If we inhibit aquaporins, which are a type of channels that let water in and out of cells, we also prevent the plant from closing. This leads to a simple conclusion that electricity is the signal for leaf movement and that water is essential for this mechanism too. The basic cellular mechanisms are therefore conserved between plants and humans.
All of this is happening based on an environment around a cell. The cell can regulate the size of a vacuole by pumping ions in or our and thereby causing water to come in or out of the vacuole. If you have a cell in an isotonic solution (an ion concentration both inside and outside the cell is equal), there will be a free flow of water into and out of the cell, and size of the cell will not change. If you put the cell in a hypertonic solution (a salt solution, where the concentration of solutes outside the cell is higher than within the cell), water will go from the vacuole to the outside, causing a shrink of the vacuole, in attempt to equalize the concentrations. It is important to notice that cell wall will remain the same size. Logically, in the exact opposite situation of a hypotonic solution (for example a distilled water), the water will go into the cell, primarily into the vacuole, trying to enlarge the cell. However we know that the protoplast is bound by the cell wall, and therefore the result is a condition called turgidity – it becomes erect because of the pressure on the cell wall and provides the plant its structure this way.
To visualize all of this, let’s take a look at a mimosa plant in this video. When you touch it, you cause a loss of potassium and because it goes out together with water, you are lowering the inner pressure, the cell becomes flaccid and all the leaves close. This reaction is called thigmonasty – a plant movement in response to touch, and these movements are always only in one direction. On the other hand, we also have a reaction called thigmomorphogenesis, which is a permanent change in the structure in response to mechanical stimulation. This is the case I have mentioned before. If you compare a tree on top of a mountain and in a valley, they will look very different in certain ways, for obvious reasons of an adaptation to the environment. Scientist discovered that plant cells contain a number of genes and each of these genes respond to different stimulus. Some genes are transcribed only in a blue light, some genes are transcribed when it is cold, and some genes are transcribed after a touch. These actions then result in different behavior of cells and therefore different physical condition of the whole plant.