Meeting Abstract
The seed pods (follicles) of many Banksia species, native to Australia, store their seeds on the plant for years before they open upon the heat of a bushfire and release their seeds after rain. For this purpose, the follicles must stay intact and functional. We could show in the past that opening temperatures depend on the local climate and can change even within one species. However, follicle properties are only partly understood and hardly anything is known about the 2nd water triggered opening step. Here we determine structural, mechanical, and hygroscopic properties of selected follicle tissues (endocarp and mesocarp) for a fundamental understanding of both initial opening and seed release. Within the endocarp, cellulose fibrils are oriented rather along the longitudinal fiber axis, in the mesocarp their orientation is perpendicular. Experiments on the hygroscopic behavior of the pericarp layers show gradually increasing swelling behavior from the follicle inside to the outside. The maximum shrinkage of 20 % for mesocarp tissue close to the exocarp is exceptional for lignified fibers. The tensile stiffness of both dry and wet tested tissue strips revealed large differences between endo- and mesocarp and between different moisture-states. These values largely exceed those of other lignocellulosic materials such as wood. Nanoindentation showed differences in indentation moduli depending on the pericarp layers and their water content. We hypothesize that the drying-induced increase in stiffness and the softening upon wetting enables the seed pod to perform the two-step opening which is essential for the reproduction and survival of Banksia species.
