Fluttering leaf to quantify internal water stress

Water stress in plants is a predominant factor that directly affects essentials such as growth, photosynthesis, and crop yield. It is always crucial to evaluate the water stress in plants and supply adequate treatments, including cases of food problems and drought. However, many traditional methods such as using a sensor insertion or soil moisture test have a chance to be inaccurate or injure the plant. Here, we propose a noninvasive way to measure the water stress of plants by observing the bending and twisting motion of leaves. Experimentally, we observe the frequency and amplitude of leaf movement by dropping small balls on the tip and sides of the lamina and adapt beam theory to predict the movement. As a result, our theoretical model matches with experimental measurements regarding the frequency and maximum amplitude of both bending and twisting. Moreover, a strong correlation between the water stress and the natural frequency of the leaf is characterized. This result implies a connection of the stiffness of leaves according to water stress, so it could be utilized as an overall indicator of plant performance.