Solanales Stem Biomechanical Properties Are Primarily Determined by Morphology Rather Than Internal Structural Anatomy and Cell Wall Composition.

Self-supporting plants and climbers exhibit differences in their structural and biomechanical properties. We hypothesized that such fundamental differences originate at the level of the material properties. In this study, we compared three non-woody members of the Solanales exhibiting different growth habits: (1) a self-supporting plant (potato, Solanum tuberosum), (2) a trailing plant (sweet potato, Ipomoea batatas), and (3) a twining climber (morning glory, Ipomoea tricolor). The mechanical properties investigated by materials analyses were combined with structural, biochemical, and immunohistochemical analyses. Generally, the plants exhibited large morphological differences, but possessed relatively similar anatomy and cell wall composition. The cell walls were primarily composed of hemicelluloses (~60%), with α-cellulose and pectins constituting ~25% and 5%-8%, respectively. Immunohistochemistry of specific cell wall components suggested only minor variation in the occurrence and localization between the species, although some differences in hemicellulose distribution were observed. According to tensile and flexural tests, potato stems were the stiffest by a significant amount and the morning glory stems were the most compliant and showed differences in two- and three-orders of magnitude; the differences between their effective Young's (Elastic) modulus values (geometry-independent parameter), on the other hand, were substantially lower (at the same order of magnitude) and sometimes not even significantly different. Therefore, although variability exists in the internal anatomy and cell wall composition between the different species, the largest differences were seen in the morphology, which appears to be the primary determinant of biomechanical function. Although this does not exclude the possibility of different mechanisms in other plant groups, there is apparently less constraint to modifying stem morphology than anatomy and cell wall composition within the Solanales.