Twist and lock: nutshell structures for high strength and energy absorption

Abstract

Nutshells achieve remarkable properties by optimizing structure and chemistry at different hierarchical levels. Probing nutshells from the cellular down to the nano- and molecular level by microchemical and nanomechanical imaging techniques reveals insights into nature's packing concepts. In walnut and pistachio shells, carbohydrate and lignin polymers assemble to form thick-walled puzzle cells, which interlock three-dimensionally and show high tissue strength. Pistachio additionally achieves high-energy absorption by numerous lobes interconnected via ball-joint-like structures. By contrast, the three times more lignified walnut shells show brittle LEGO-brick failure, often along the numerous pit channels. In both species, cell walls (CWs) show distinct lamellar structures. These lamellae involve a helicoidal arrangement of cellulose macrofibrils as a recurring motif. Between the two nutshell species, these lamellae show differences in thickness and pitch angle, which can explain the different mechanical properties on the nanolevel. Our in-depth study of the two nutshell tissues highlights the role of cell form and their interlocking as well as plant CW composition and structure for mechanical protection. Understanding these plant shell concepts might inspire biomimetic material developments as well as using walnut and pistachio shell waste as sustainable raw material in future applications.

The walnut shell network: 3D visualisation of symplastic and apoplastic transport routes in sclerenchyma tissue

“…Sclereids with multiple pit channels can be found in shell tissues of different species (Sebaa and Harche 2014 ; Hammami et al 2013 ; Kaniewski 1965 ). Also our previous studies of the walnut shell report a very dense channel network between neighbouring cells (Huss et al 2021 ; Xiao et al 2021 , 2020 ; Antreich et al 2019 ). With a detailed SFB-SEM study, we reconstructed for the first time the cell connections in 3D during the sclerification process in the shell.…”

“…However, biophysical models predict a significant reduction in effective symplastic permeability when PD are clustered into pit fields compared to a random distribution (Deinum et al 2019 ). Furthermore, thin pit channels present in large numbers increase the contact surface between cytoplasm and cell wall which could be beneficial for faster incorporation of lignin as the maximal diffusion distance decreases (Xiao et al 2021 ).…”

“…Reis et al ( 2004 ) showed similar fillings into pit channels in cherry pericarp and Sebaa and Harche ( 2014 ) in Argania spinosa . This ingrowth into the pit channels led to a Lego © -brick-like interlocking of the lamellae (Xiao et al 2021 ) and caused the nubby appearance of cracked cells because cracks propagated along the ingrowth of the cellulose layers. Schmier et al ( 2020 ) found similar features in cracked coconut endocarp and assumed that pits deflect cracks into the cell.…”

“…The shape is defined during morphogenesis prior to sclerification due to cellulosic cell wall thickenings that restrict the expansion of the cell wall on different loci (Antreich et al 2021 ). With the onset of sclerification, a secondary cell wall is deposited in helicoidally arranged lamellae and lignin is incorporated during maturation (Xiao et al 2021 , 2020 ). These processes and the 3D puzzle shape give the shell tissue greater strength and stiffness compared to other nut shells with non-lobed cell shapes (Huss et al 2020 ).…”

“…PDs can be clustered in pit fields, where no cellulose is deposited during cell wall thickening and sclerification. This leads to the presence of rather long channels particularly in greatly thickened cell walls, visible in sections and cracked surfaces of various nut shell tissues (Xiao et al 2021 ; Antreich et al 2019 ; Flores-Johnson et al 2018 ; Schüler et al 2014 ; Hammami et al 2013 ; Kaniewski 1965 ). To prevent the deposition of cellulose material during sclerification a regulating mechanism has to exist.…”