Arrangement of Cell-Wall Constituents in Chemically Treated Norway Spruce Tracheids

Tanja Zimmermann, Klaus Richter, Nico Bordeanu, Jürgen Sell

Abstract


The cell-wall of tracheids in conifer wood has evolved to provide both water conduction and mechanical strength to the standing tree. However, its structure at the nanometer level is not yet accepted beyond doubt, and little is known about the interactions between the cell-wall components. In the present study, the fracture pattern of the S2 layer of Norway spruce tracheids was observed by field emission scanning electron microscopy (FE-SEM) after pretreatment of the cell wall with various alkali solutions, acetic and nitric acid, and ASAM delignification. The resulting cell-wall arrangements were also studied in ultra-thin sections of unfractured samples with transmission electron microscopy (TEM). In the case of untreated samples (reference), radial fracture patterns—perpendicular to the compound middle lamella—were regularly observed. A treatment with 10% and 18% NaOH or 24% KOH at room temperature—associated with a slight decrease of glucomannan—resulted in the disappearance of these radial fracture formations. As the severity of the alkali treatment increased and acid and ASAM delignification was applied, concentric alignments in the cell wall became more and more discernable. The increasing loss of hemicelluloses and lignin therefore led to distinct changes in the fragmentation patterns of the cell walls. In addition, reduction in strength and stiffness were determined for all chemically treated cell walls. It is concluded that even slight changes in cell-wall constitution influence the interactions of the cell-wall components and thus fracture mechanics and ultrastructural appearance of wood cell walls.

Keywords


Cell-wall structure;interactions of cell-wall components;S2 layer;chemical treatment;FE-SEM;TEM

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