By: Nannan Xiao, Sebastian Johannes Antreich, Martin Felhofer & Notburga Gierlinger
Notburga Gierlinger is Associate Professor at the Institute of Biophysics at the University of Natural Resources and Life Sciences (BOKU) in Vienna, Austria. She leads the research group "Biological Materials at the Micro- and Nano-Level" and the focus is on nanomechanical and chemical imaging of plant tissues with the aim to gain a better understanding of natural materials. The last five years the research group focused on nutshells within the ERC consolidator grant 681885 “Scattering and tapping on soft-hard-open nuts”.
Many nuts protect their tasty kernels by robust shells made-up of sclerenchyma cells. These have typically thick and lignified cell walls and come along in various shapes: from isodiametric to polylobate to fibrous. The polylobate sclereids or puzzle cells are the secret behind the remarkable mechanical performance of walnut and pistachio shells. The two species rely only on this cell type throughout the whole shell (Fig. 1), while others (e.g. macadamia or hazelnut) build layers based on fibers and “normal” isodiametric sclerenchyma cells. Sclerenchyma cell with lobes increases contact surfaces and slow down crack propagation in walnut and pistachio shells. The two species grow 3D-jigsaw puzzles, in which each cell interlocks on average with fourteen neighboring cells. Pistachio brings this interlocking to perfection by balljoint-like structures. These connections keep the shell tissue assembled even when lignin, an aromatic glue between the cells, is removed. Lignin plays a role in gluing the puzzle cells together, but also within the cell wall material between the cellulose microfibrils. It adds hydrophobicity to the shell and contributes to shell mechanics. Pistachio shells come along with three times less lignin than walnut shells. Through the optimization of cell shape and interlocking pistachio seems not to need such a strong interface and can save lignin. On the nano level the cell wall is also optimized for high mechanical performance by a layered arrangement. This is achieved by twisting the orientation of the cellulose microfibrils from layer by layer. By changing the angle of the twist and the thickness of the layers mechanical performance can also be tuned.
For understanding the packaging concepts of nutshell, a detailed view on the micro-and nano scale is necessary. In pistachio shells the cell shape with numerous lobes, the interlocking into intrinsically tied 3D-structures as well as cell wall molecules and their arrangement helped to explain the remarkable mechanical performance. This knowledge can be of interest when it comes to cracking of nuts for kernel extraction as well as using the nutshell waste in future bioeconomic approaches.
Pistachio nutshell tissue structure, cell shape (ball-joint structures) and cell wall organization: Scanning electron microscopy gives an overview of the tissue structure and confirms the same cell type across the entire shell. Raman imaging (blue inset) highlights lignin as glue between the cells and in the cell wall. Chemical isolation of single cell opens the view on the cell shape and highlights the numerous lobes and tremendous surface area. A zoom with electron microscopy reveals their interlocking by ball joint structures as well as the layered cell wall. Atomic force microscopy confirms the layering based on twisted cellulose fibrils.