Adaptive spatiotemporal changes in morphology, anatomy, and mechanics during the ontogeny of subshrubs with square-shaped stems.

Abstract

Plant stems can be regarded as fiber-reinforced structures characterized by anatomical heterogeneity, mechanical anisotropy, and adaptability to changing internal and external constraints. Our study focused on adaptive spatiotemporal changes in morphology, anatomy, and mechanical properties during the ontogeny of Leonurus cardiaca L. (Lamiaceae) internodes, proving considerable functional adaptability. Four-point bending tests and torsional tests were carried out on the same internodes to measure flexural and torsional stiffness. Axial and polar second moments of area for entire cross sections and for individual tissues were determined from transverse stem sections immediately after testing. Based on these data, additional relevant mechanical parameters such as bending elastic modulus, torsional modulus and twist to bend ratio were calculated. Leonurus cardiaca is characterized by a square-shaped hollow stem in transverse section with an outer frame of various strengthening tissues and an inner ring of parenchyma. Statistical analyses of axial and polar second moment of area, flexural stiffness, torsional stiffness, bending elastic modulus, and torsional modulus revealed significant differences for all comparisons with respect to spatial resolution (two adjacent internodes) and temporal resolution (in June before flowering and in September after fruit formation). The twist to bend ratios of the internodes, however, always remain in the same range. With respect to spatiotemporal development, stems of the subshrub L. cardiaca show a marked increase in flexural and torsional stiffness during ontogeny. Strikingly, changes in stem mechanics are more influenced by variations in mechanical tissue properties than by changes in relative proportion of different tissue types.