Do you ever pet your plant? Turns out, they not only feel touch, but also react to touch. Aside from the Venus fly trap, every plant species is capable of detecting touch, albeit their reaction may not be as noticeable. There are a variety of responses plants exhibit in response to mechanical stimuli that may surprise you. Plants can direct their growth in a specific direction through a phenomenon known as thigmotropism! There are two kinds of thigmotropisms – positive, where the plant grows toward the stimulus and negative, where the plant grows away from it. All plants exhibit negative thigmotropism when their roots come across obstacles such as rocks. Similarly, plants that are dependent on external support such as climbing beans rely on positive thigmotropism to grow toward a support. In the amazing saga of plant evolution, some species have evolved to become “touch specialists'' through an extreme sensitivity to certain mechanical stimuli such as a touch or vibrations.
Some plants respond to touch by performing a specific movement in a phenomenon called thigmonasty. An interesting example is Mimosa pudica, a member of the legume family. When M. pudica is touched, the plant will rapidly fold its leaves inward and when it is touched with a greater force or the plant is shaken, it droops its stem! How does the plant do this? After sensing the touch stimulus through mechanoreceptors, turgor pressure within the plant cells is released! The pulvini (sing. pulvinus) are a thickened area at the base of a leaf stalk in certain plants that require changes in turgor pressure to move leaves. When the leaves are open, the water pressure within the cells are high which pushes on the cell walls causing the cell to be rigid, keeping the leaves upright. When the plant is touched, chemicals are released causing the cells within the pulvini to lose turgor pressure by rapidly moving the water out of the cell. This causes the cell to collapse and the leaves fold in. Another great example of thigmonastic movement is the Venus flytrap (Dionaea muscipula). In this case, the movement is even more rapid as the reaction involves action potentials. Much like animals, there are electrical impulses which allow the modified leaves (commonly known as the trap) of the Venus flytrap to close via similar mechanisms.
Finally, here’s why you shouldn’t pet your house plant or move it around frequently. Thigmorphogenesis occurs when a plant experiences regular mechanical perturbation or environmental stressors, such as wind, physical barriers, predation, and humans petting them. Mechanical stimulus causes the plant to undergo morphological and physiological changes that would not occur otherwise. This response enables the plant to survive in its environment. Plants that are touched daily show stunted growth, a thicker shoot diameter, and stabler root system. This is why your tomato seedlings may get leggy if grown indoors without a grow fan and also the reason why most industrial greenhouses and growth chambers have fans – to prevent the plant from falling over when planted outside where wind is a legitimate concern. There have been attempts to use mechanical stimulus in growing operations in other novel ways, but more research is required before this can be implemented on a mass scale. The method of applying mechanical stress to plants has also been used to create ornamental plants by purposefully causing stunted growth, allowing the plant to stay small, but increase its diameter. If you would like to keep your plants small, give them a soft pat on the “back” everyday. Otherwise, don’t touch them!