The rapid progress of genome sequencing of Juglans species was reviewed recently and will pave the way for functional genomics research ( Chen et al., 2019). In a sustainable economy we furthermore can benefit from an optimized utilization of the nut waste-the shell and the husk-and for this, in-depth knowledge of these materials is of vital importance. Studies on nuts have proven their nutritional importance ( Martinez et al., 2010), through antioxidant activities ( Jahanbani et al., 2016 Panth et al., 2016). World production of walnuts exceeds three million tons since 2012 and took over almond and hazelnut ( Bernard et al., 2018). The walnut ( Juglans regia L.), also known as English or Persian walnut, is the most widespread tree nut in the world, and is thus an economically important tree species ( De Rigo et al., 2016). This remarkable design can be attributed to natural selection in the course of evolution ( Sallon et al., 2008). The hardened endocarp of the nut provides a physical barrier around the seed and protects the embryo against biotic and abiotic factors in the natural environment ( Dardick and Callahan, 2014). The nut is commonly defined as a dry, indehiscent, usually one-seed fruit with a hard and tough endocarp (shell) enclosing the seed, which develops from a simple ovary. Understanding the walnut shell and its development will inspire biomimetic material design and packaging concepts, but is also important for waste valorization, considering that walnuts are the most widespread tree nuts in the world. This accumulation of aromatic components is reminiscent of heartwood formation of trees and is suggested to improve protection properties of the mature walnut shell. In the final mature stage, fluorescence increased throughout the cell wall and a fluorescent layer was detected toward the lumen in the inner part. Furthermore, Raman imaging of fluorescence visualized numerous pits as a network of channels, connecting all the interlocked polylobate walnut shells. Focusing on the cell wall level, Raman imaging showed that lignin is deposited first into the pectin network between the cells and cell corners, at the very beginning of secondary cell wall formation. With maturation secondary cell wall thickening takes place and the amount of all cell wall components (cellulose, hemicelluloses and especially lignin) is increased as revealed by FTIR microscopy. The cells toward this outer green husk have the thickest and most lignified cell walls. Walnut shell development takes place beneath the outer green husk, which protects and delivers components during the development of the walnut.
Structural changes at the macro level are explored by X-ray tomography and on the cell and cell wall level various microscopic techniques are applied. For a better understanding of the interlocked walnut shell tissue, we investigate the structural and compositional changes during the development of the shell from the soft to the hard state. The shell is based on only one unit cell type: the polylobate sclerenchyma cell. The walnut shell is a hard and protective layer that provides an essential barrier between the seed and its environment. 2Division of Structural and Functional Botany, Department of Botany and Biodiversity Research, University of Vienna, Vienna, Austria.1Institute of Biophysics, Department of Nanobiotechnology, University of Natural Resources and Life Sciences, Vienna, Austria.Antreich 1, Yannick Marc Staedler 2, Jürg Schönenberger 2 and Notburga Gierlinger 1 * Nannan Xiao 1, Peter Bock 1, Sebastian J.
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