Markus R"uggeberg1, Thomas Speck2, Ingo Burgert1

1 Max-Planck-Institute of Colloids and Interfaces, Department of Biomaterials, Potsdam, Germany
2 Plant Biomechanics Group Freiburg, Botanic Garden, University of Freiburg, Germany

The hollow culms of the Giant Reed are stiffened by numerous vascular bundles accompanied by sclerenchymatous fibres and by a ring of sclerenchymatous fibres in the periphery of the culm. These stiffening elements are embedded in parenchymatous tissue. We have found differences in stiffness between sclerenchymatous and parenchymatous tissue of more than one order of magnitude. This puts some constraints on the structural and mechanical design of the interface of these tissues. In case of mechanical loading, high stresses are generated in the stiff fibres, whereas stresses in the relatively soft parenchyma cells stay rather low. Stress discontinuities are likely to occur at the interfaces under these conditions making such structures prone to failure.
Micromechanical tests, image analysis, synchrotron X-ray diffraction and UV-microspectrophotometry were used to investigate mechanical properties, cell parameters, cellulose microfibril orientation and lignification of sclerenchyma and parenchyma in the vicinity of their interface with high spatial resolution. The investigations revealed a lowering of the difference in stiffness between both tissues at their interface, which was mainly due to gradual alterations of cell parameters. The stiffness profile across the interface was interpreted as a compromise between providing sufficient stiffness to the slender culm and lowering stress discontinuities at interfaces.