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

Abstract

Main conclusion High symplastic connectivity via pits was linked to the lignification of the developing walnut shell. With maturation, this network lessened, whereas apoplastic intercellular space remained and became relevant for shell drying. Abstract The shell of the walnut (Juglans regia) sclerifies within several weeks. This fast secondary cell wall thickening and lignification of the shell tissue might need metabolites from the supporting husk tissue. To reveal the transport capacity of the walnut shell tissue and its connection to the husk, we visualised the symplastic and apoplastic transport routes during shell development by serial block face-SEM and 3D reconstruction. We found an extensive network of pit channels connecting the cells within the shell tissue, but even more towards the husk tissue. Each pit channel ended in a pit field, which was occupied by multiple plasmodesmata passing through the middle lamella. During shell development, secondary cell wall formation progressed towards the interior of the cell, leaving active pit channels open. In contrast, pit channels, which had no plasmodesmata connection to a neighbouring cell, got filled by cellulose layers from the inner cell wall lamellae. A comparison with other nut species showed that an extended network during sclerification seemed to be linked to high cell wall lignification and that the connectivity between cells got reduced with maturation. In contrast, intercellular spaces between cells remained unchanged during the entire sclerification process, allowing air and water to flow through the walnut shell tissue when mature. The connectivity between inner tissue and environment was essential during shell drying in the last month of nut development to avoid mould formation. The findings highlight how connectivity and transport work in developing walnut shell tissue and how finally in the mature state these structures influence shell mechanics, permeability, conservation and germination.

The shape and volume of air, kernels and cracks, in a nutshell

“…Examining the relative air volume compared to airless relative content of shell and packing tissue, the correlations suggest that as a walnut grows, packing tissue develops at the expense of shell tissue. This could be explained by the fact that the shell undergoes a careful biochemical balance between insulation and permeability of air and water as the walnut develops (Antreich et al, 2022) (Figure 4). This nuanced relationship between air, shell, packing tissue during shell development is further highlighted when we consider their individual influence over ease of kernel removal.…”

Experimental Study and Thermodynamic Analysis of Carbon Dioxide Adsorption onto Activated Carbons Prepared from Biowaste Raw Materials

“…During the adsorption process, these pores serve as transport pores for adsorptive molecules. In general, the heterogeneous surface with open pit channels of the carbon adsorbents is inherited from the precursors, walnut [73] and hazelnut [74] shells, which consist of polylobate cells, forming a 3D structure with thick lignified cell walls. All changes in the surface morphology and the porous system of the nutshells occurring upon the carbonization and activation processes can be attributed to the disruption of the original macromolecular network structure formed by the cellulose and lignin units, the removal of volatile organic compounds, and the subsequent rearrangement of a new matrix structure with micropores [73,74].…”