Climate‐Dependent Heat‐Triggered Opening Mechanism of Banksia Seed Pods

Abstract

Heat‐triggered fruit opening and delayed release of mature seeds are widespread among plants in fire‐prone ecosystems. Here, the material characteristics of the seed‐containing follicles of Banksia attenuata (Proteaceae), which open in response to heat frequently caused by fire, are investigated. Material analysis reveals that long‐term dimensional stability and opening temperatures of follicles collected across an environmental gradient increase as habitats become drier, hotter, and more fire prone. A gradual increase in the biaxial curvature of the hygroscopic valves provides the follicles in the driest region with the highest flexural rigidity. The irreversible deformation of the valves for opening is enabled via a temperature‐dependent reduction of the elastic modulus of the innermost tissue layer, which then allows releasing the stresses previously generated by shrinkage of the fiber bundles in the adjacent layer during follicle drying. These findings illustrate the level of sophistication by which this species optimizes its fruit opening mechanism over a large distribution range with varying environmental conditions, and may not only have great relevance for developing biomimetic actuators, but also for elucidating the species' capacity to cope with climatic changes.

Structural Orientation and Anisotropy in Biological Materials: Functional Designs and Mechanics

“…Differing from hydro‐driven actuations, seed release can additionally be stimulated by heat, e.g., after exposure to fire, for plants in fire‐prone ecosystems where the anisotropic structures also play a key role. The Banksia attenuata is a good example which demonstrates preferred alignments of cellulose fibrils along the orthogonal and length directions of the follicle valves in its meso‐ and endocarp layers (Figure c) . The longitudinal shrinkage of the mesocarp is thus restricted by the endocarp during the drying process of the follicle, creating considerable residual stresses in the valves.…”

“…b) The opening of the chiral seedpod and the preferred alignments of cellulose microfibrils (indicated by dashed lines) in the inner and outer layers of the valve . c) Microcomputed tomography image of the longitudinal section of Banksia attenuata follicle and X‐ray scattering images of the exocarp (Ex), mesocarp (Me), and endocarp (En) layers in the follicle valve . d) Variations in the axial stress at constant length, the axial strain in the absence of constraint, and the effective Young's modulus generated by swelling as a function of the microfibril angle for a plant cell represented using a hollow cylinder without considering the torsional deformation .…”

“…Copyright 2011, American Association for the Advancement of Science. (c) Adapted with permission . Copyright 2017, Wiley‐VCH.…”

“…In situ small/wide‐angle X‐ray scattering (SAXS/WAXS) techniques demonstrate less strict constraints than in situ SEM on the sample dimension, hydration state, loading condition, and electrical conductivity. By capturing the information of characteristic structural features, these approaches are effective for recognizing the structural orientations and their changes in biological materials during deformation, and even quantifying the local strains of individual constituents . For example, SAXS and WAXS can be used to detect, respectively, the staggered spacing between collagen molecules (≈67 nm) and the crystalline lattice spacing of minerals in mineralized collagen fibrils, and thereby have been widely utilized in probing bones, fish scales, and skins .…”

Manche mögen's heiß!

“…Pflanzen, die dort überleben wollen, müssen über besondere Strategien verfügen. So nutzen etwa Pflanzen aus der Gattung Banksia Buschbrände gezielt für die Vermehrung: Ihre Samenkapseln öffnen sich erst durch die hohen Temperaturen des Brandes . Diese Strategie ist risikoreich, denn da die Pflanzen der Elterngeneration durch das Feuer in der Regel vernichtet werden, hängt der Fortbestand der Population davon ab, dass die Samen zur richtigen Zeit freigesetzt werden.…”