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Hierarchical super-structure identified by polarized light microscopy, electron microscopy and nanoindentation: implications for the limits of biological control over the growth mode of abalone sea shells
Background: Mollusc shells are commonly investigated using high-resolution imaging techniques based on cryo-fixation. Less detailed information is available regarding the light-optical properties. Sea shells of Haliotis pulcherina were embedded for polishing in defined orientations in order to investigate the interface between prismatic calcite and nacreous aragonite by standard materialographic methods. A polished thin section of the interface was prepared with a defined thickness of 60 μm for quantitative birefringence analysis using polarized light and LC-PolScope microscopy. Scanning electron microscopy images were obtained for comparison. In order to study structural-mechanical relationships, nanoindentation experiments were performed. Results: Incident light microscopy revealed a super-structure in semi-transparent regions of the polished cross-section under a defined angle. This super-structure is not visible in transmitted birefringence analysis due to the blurred polarization of small nacre platelets and numerous organic interfaces. The relative orientation and homogeneity of calcite prisms was directly identified, some of them with their optical axes exactly normal to the imaging plane. Co-oriented "prism colonies" were identified by polarized light analyses. The nacreous super-structure was also visualized by secondary electron imaging under defined angles. The domains of the super-structure were interpreted to consist of crystallographically aligned platelet stacks. Nanoindentation experiments showed that mechanical properties changed with the same periodicity as the domain size. Conclusions: In this study, we have demonstrated that insights into the growth mechanisms of nacre can be obtained by conventional light-optical methods. For example, we observed super-structures formed by co-oriented nacre platelets as previously identified using X-ray Photo-electron Emission Microscopy (X-PEEM) [Gilbert et al., Journal of the American Chemical Society 2008, 130:17519–17527]. Polarized optical microscopy revealed unprecedented super-structures in the calcitic shell part. This bears, in principle, the potential for in vivo studies, which might be useful for investigating the growth modes of nacre and other shell types.
Structure-property relationships in mechanically stimulated Sorghum bicolor stalks
Mechanical properties of plants and underlying structure-property relationships are important for agricultural purposes as well as for biomimetic concepts. In this study, the effect of mechanical stimulation on morphology and bending properties of the stalk was investigated for Sorghum bicolor (Poaceae), a widely used drought-tolerant biomass grass. An experimental set-up allowing for defined growth and mechanical perturbation (flexing) during a defined growth period was designed. Mechanical properties of individual internodes of the stalk were determined by three-point bending tests. We found that the three investigated lines showed differences in their general bending strength in the non-stimulated condition. However, similar high range of bending strength values was measured for all plant lines after they underwent the mechanical stimulation procedure. The anatomy of internode cross-sections was examined to evaluate structure-property relationships. An increased thickness of the outer sclerenchymatous tissue was observed for internodes with higher bending strength values. Dried internodes fail under lower strains but showed higher bending strength. These findings show that mechanosensitivity in sorghum is dependent on genetic as well as environmental factors. The experimental system presented here offers new straight-forward possibilities for S. bicolor line selection for applications requiring mechanical strength of the stalk.