``Streamside plants'' that grow along rivers sometimes become submerged when the water level of the river rises due to heavy rain or typhoons, but it has become clear that they can avoid and tolerate water flow stress. There is. "Narrowing", in which the main part of the leaf, the leaf blade, becomes thinner is known as the main trait for reducing water flow stress, but narrowing alone is not considered effective in reducing water flow stress. This has been difficult, and analysis of other organs has been required.

　In this research, we focused anew on the petiole, which is the part of the stalk that connects the leaf blade and stem, and attempted a mechanical approach. We measured the mechanical properties (flexural modulus, bending strength, and breaking strain) of the petiole of the Japanese cypress, a plant growing along a mountain stream, using a three-point bending test, and compared it with a closely related species, the Japanese cypress, which grows inland.

　As a result, although no statistical difference was observed in the bending elastic modulus, the bending strength and breaking strain were significantly higher for the Yasha mainspring, indicating that the leaf petioles of the Yasha mainspring were stronger and more flexible than those of the main spring. It was suggested that this species evolved to reduce water flow stress along mountain streams.

　In addition, through comparative anatomical analysis, we found that the size of thick-walled cells in the leaf petiole of Yaschaspring was significantly larger than that of the mainspring. As a result, the distance between the fulcrums of the cells increases relative to the force exerted on the petiole of the Yashaspring, allowing for large displacements, which is thought to result in the petiole being highly flexible.

　This study shows that the differentiation into plants along streams may be explained not only by narrowing of the leaves, but also by changes in petiole traits and the entire aboveground part, including other organs, and a mechanism that reduces water flow stress during high water levels. A new discovery was made about this. In the future, they will continue to investigate whether similar evolution has occurred in other plants along mountain streams, and are also looking forward to progress in research at the genetic level.

Paper information:[Scientific Reports] Rheophytic Osmunda lancea (Osmundaceae) exhibits large flexibility in the petiole