Is Genetic Differentiation Involved in the Morphological Adaptation of <i>Adenophora triphylla</i> var. <i>japonica</i> (Camanulaceae) to Water Flow Stress Along Rivers?

Plants along rivers have narrow lanceolate leaves, flexible stems, and petioles to avoid the water flow stress caused by flooding. This study aimed to determine whether the adaptation of <i>Adenophora triphylla</i> (Thunb.) A.DC. var. <i>japonica</i> (Regel) H. hara (Campanulaceae) with narrow leaves in riverside habitats was achieved through phenotypic plasticity or genetic morphological changes, where we conducted comparative morphological analyses through cultivation experiments. Our cultivation experiments revealed that the morphology of radical leaves in the riverside population had significantly smaller and narrower laminas and shorter and thicker petioles than those in the inland population, and that the former had significantly more radical leaves than the latter, suggesting that the former brings the total leaf area closer to that of the latter by increasing the number of radical leaves. The cauline leaves were significantly thinner and smaller in the riverside population than in the inland population, and the stems of the former were significantly shorter and thicker than those of the latter. In addition, a significant difference was observed between the riverside and inland populations in the number of rosette leaf branches from the rhizome, with the former having significantly more rosette leaf branches. Our results reveal that populations of <i>Ad</i>. <i>triphylla</i> var. <i>japonica</i> with genetically distinct leaf and stem morphologies have become established along rivers, where flooded water imposes strong selective pressure. In these riverside populations, thicker and shorter petioles and stems appear to reduce bending moments without breaking, while narrower and smaller laminas of both radical and cauline leaves further contribute to this reduction.