Abstract
As a physical response to water loss during drought, inner Selaginella lepidophylla stems curl into a spiral shape to prevent photoirradiation damage to their photosynthetic surfaces. Curling is reversible and involves hierarchical deformation, making S. lepidophylla an attractive model with which to study water-responsive actuation. Investigation at the organ and tissue level has led to the understanding that the direction and extent of stem curling can be partially attributed to stiffness gradients between adaxial and abaxial stem sides at the nanoscale. Here, we examine cell wall elasticity to understand how it contributes to the overall stem curling. We compare the measured elastic moduli along the stem length and between adaxial and abaxial stem sides using atomic force microscopy nano-indentation testing. We show that changes in cortex secondary cell wall development lead to cell wall stiffness gradients from stem tip to base, and also between adaxial and abaxial stem sides. Changes in cortical cell wall morphology and secondary cell wall composition are suggested to contribute to the observed stiffness gradients.
Highlights
Nature is a perpetual source of inspiration for biomimetic and actuating devices[1]
Transverse sections from a set of regions along the length of inner S. lepidophylla stems were scanned with atomic force microscopy (AFM) to generate topographical images of the adaxial and abaxial cortical cell walls (Fig. 2A,B; Supplementary Fig. 1)
Abaxial cell walls show very distinct secondary cell wall layers, while those in the adaxial region appear relatively smooth. This pattern is observable in sections along the length of the stem and is visible in cortical cell walls imaged with transmission electron microscopy (Supplementary Fig. 2)
Summary
Current biomimetic research involves a multi-scale approach to investigate how structural and mechanical properties at various length-scales determine organism function[2]. Juxtapositions of different tissues and/or cell wall structures and compositions can drive organ movements in response to water or humidity. A thin epidermal layer and an amphicribral vascular bundle are present; these tissues do not significantly contribute to the differential swelling and shrinking that drives stem curling/uncurling. We use AFM indentation to locally measure the nanoscale elastic properties of cell walls, as well as to assess the level of inhomogeneity and stiffness gradients of the tissue across representative transverse sections of the plant, both longitudinally (stem tip to base) and between adaxial and abaxial stem sides. We preliminarily investigate the viscoelastic behaviour at given loading rates
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