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Schlagwörter:
Banksia; follicle; dehiscence; fiber; mechanics; Banksien; Follikel; Dehiszenz; Faser; Mechanik
Zusammenfassung:
Plants of the Australian Banksia genus can retain mature seeds for more than a decade and release them after bushfire and rain. Their mechanism of reproduction evolved according to the conditions of their habitats: nutrient-poor soils and Mediterranean climate with hot summers, seasonal rainfalls, and frequently occurring bushfires. Despite the hostile environment, life in these regions is highly diverse and so are the strategies plants developed to prevail in evolution. The seed dispersal of Banksia is only one out of many fascinating examples. Throughout the long-term seed retention, Banksia’s seed pods (“follicles”) ensure stability and protectiveness. At the same time, they maintain their motion potential for the delayed seed release after a heat trigger and subsequent rainfall. The cause for these contrary properties, stability and movability, lies in the material of the follicle valves (“pericarp”) itself, as the infructescence remains without metabolism on the plant after maturation. The present work reveals sophisticated interactions between chemical composition, structure, and physical properties of the pericarp layers, based on detailed histological, spectroscopic, microscopic, and micro-mechanical investigation. The thick middle layer of the pericarp, the “mesocarp”, shows a continuous cellulose tilt angle perpendicular to the longitudinal fiber axis, which leads to remarkable swelling and shrinkage upon wetting and drying. A gradient in lignification causes more pronounced dimensional changes towards the outside of the follicle. Shrinkage of the mesocarp fibers upon maturation is restrained in closed follicles by the sealed junction of the pericarp valves, by the stiffness of the endocarp, and possibly by the resistance of the brittle yet hard exocarp. In consequence, mesocarp fibers develop drying stresses, as measured by a newly designed experimental setup. These stresses increase further when heat-dried. During the first fire-triggered opening step of the seed pods, the largest proportion of the mesocarp shrinkage potential is still retained. Subsequent wetting leads to pronounced swelling, while stiffness and hardness of the follicle tissue diminish to a larger extent than for wood fibers of trees. This “flexibilization” and “softening” increases towards outer mesocarp regions. In comparison, dry mesocarp shows homogeneous mechanical properties over the layer thickness. Alternation of wetting states, of the related mechanical properties, and of the hygroscopic deformation in accumulating moistening cycles increases the drying shrinkage of the mesocarp against the resistant endocarp. It leads to progressing deflection of the follicle valves for seed release. Gradual transitions of chemical composition in the mesocarp and variable cellulose tilt angles in the endocarp cause successive changes of the physical behavior over the thicknesses of the layers. On the opposed edges of meso- and endocarp high deformability or high stiffness is reached, respectively. At the same time, stresses towards the interface of the layers are reduced, so the valves remain intact throughout bending. Based on the measurements, a comprehensive picture can be drawn of the principles underlying the Banksia seed retention and release. The insights can moreover serve as an inspiration for biomimetic applications, requiring both motion and long-term stability.
