ETNIER, S.A.; VILLANI, P.; Butler University; Butler University: Changing mechanical properties of lily pad petioles (Nymphaea odorata) across life history stages

Lily pads (Nymphaea odorata) are common in still and slow-flowing freshwater. During early growth, lily pad leaves are completely submerged until the petiole grows long enough for the lamina to reach the surface. In some cases, the petiole will continue to grow until the lamina is held above the water�s surface. The physical demands on the petiole change during these life history stages as a leaf transitions from a fully aquatic to an aerial environment. When the leaf is submerged, the petiole is exposed to tensile forces because the lamina is positively buoyant. In surface leaves, petiole length typically exceeds water depth; hence, the petiole experiences tensile loads only under conditions of wind or waves. The petiole of an aerial leaf must support the weight of the lamina, loading the petiole in compression. We investigated the mechanical properties of petioles with submerged, surface and aerial lamina. Using uniaxial tests, we determined elastic modulus, tensile breaking strength, and extension ratios in freshly collected petioles. Elastic modulus and breaking strength of petioles increased through ontogeny, with the highest values associated with aerial leaves. In contrast, average maximum extension ratios decreased from 1.15 (n=12) to 1.12 (n=13) to 1.03 (n=13) in submerged, surface, and aerial leaves, respectively. Interestingly, these changes were independent of petiole cross-sectional area, suggesting that mechanical differences were due to material, rather than structural changes. These mechanical results will be coupled with a descriptive characterization of the morphology of petioles at different life history stages based on standard histological techniques.