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Functional principles of baobab fruit pedicels - anatomy and biomechanics.

BACKGROUND AND AIMS
Fruit pedicels have to deal with increasing loads after pollination due to the continuous growth of the fruits. Thus, they represent interesting tissues from a mechanical as well as developmental point of view. However, only few studies exist on fruit pedicels. In this study, we unravel the anatomy and structural-mechanical-relationships of the pedicel of Adansonia digitata reaching up to 90 cm in length.


METHODS
Morphological and anatomical analyses included the examination of stained cross sections from various positions along the stalk as well as X-ray micro-tomography (µ-CT) and scanning electron microscopy (SEM). For mechanical testing, fiber bundles derived from the mature pedicels were examined via tension tests. For establishing the structural-mechanical relationships, density of the fiber bundles as well as their cellulose microfibril distribution and chemical composition were analyzed.


KEY RESULTS
While in the peduncle the vascular tissue and the fibers are arranged in a concentric ring like way, this organization shifts to the polystelic structure of separate fiber bundles in the pedicel. The polystelic pedicel possesses five vascular strands that consist of strong bast fiber bundles. The fiber bundles reveal a Young´s modulus of up to 5 GPa, a tensile strength of up to 400 MPa, a high density (>1g/cm-3), and a high microfibril angle of around 20°.


CONCLUSIONS
The structural arrangement as well as the combination of high density and high microfibril angle of the bast fiber bundles are most likely optimized for bearing considerable strains in torsion and bending while at the same time allowing for carrying high tension loads.