Growth strategy of climbers depends on external support, as opposed to erect self-supporting plants. Self-supporting plants and climbers exhibit numerous differences in their morphology, vascular anatomy and biomechanical properties. Fundamental functional differences supposedly should originate at the tissue and cell walls levels.
In this study we compared three non-woody evolutionarily close Solanales species with different growth habits: a self-supporting plant (potato, Solanum tuberosum), a trailing plant (sweet potato, Ipomoea batatas) and a twining climber (morning glory, Ipomoea tricolor). Several biomechanical analyses were combined with structural and immunocytochemical approach, always comparing the uppermost mature stem internode.
The stems showed large morphological differences, with the climber morning glory stem being the most slender and least circular. Stem anatomy was somewhat similar in all three species- being herbaceous, mostly unlignified with inner phloem and collenchyma at the stem periphery. Potato stems were hollow and xylem was organized in small separate vascular bundles, while in morning glory and sweet potato the xylem was continuous around the stem. Cell wall composition was very similar. The highest fraction of the cell wall were the hemicelluloses (~60%), α-cellulose constituted around 25% of the cell wall, and pectins constituted 5-8%. In morning glory hemicellulose a content was a bit lower, and α-cellulose content was slightly higher. Immunohistochemistry of specific cell wall components also showed little variation among the species, with some differences in hemicelluloses allocation. According to tensile and flexural tests, potato stems were significantly the stiffest and the morning glory stems the most compliant, while Elastic modulus values were not sufficiently different. Stiffness parameter takes into account the morphology of the organ, while the elastic modulus depends mainly on the material properties of measured stem. Apparently, mainly morphology influences the biomechanical function.
In this case, closely related species diverged into different stem types, while generally preserving their ancestral traits of anatomical structure and cell wall composition. Function was changed by simple changes in stem morphology. However, in other taxonomic groups the biomechanics could be changed by other factors.
Immunolocalisation of pectins in Solanum tuberosum stem.
