Study of Norway spruce cell wall structure with microscopy tools

Abstract

The distribution and orientation of wood cell wall polymers play an important role in its physical, chemical and mechanical properties, and thus in the transformation into final products. Specifically, the orientation of cellulose elementary fibrils (EF) controls the performance of wood in almost every end use. Moreover, lignin is covalently linked to many of the cell wall polysaccharides, which imposes a serious technical challenge during the degradation of cellulose into value added products. Therefore, the deep understanding of the organization of the cell wall materials is imperative. In this study, normal Norway spruce wood was studied with a high-resolution cryo-transmission electron microscope (cryo-TEM). Both, two- (2D) and three-dimensional (3D) imaging techniques within TEM were applied on ultrathin wood sections to understand the wood structure. The defibration mechanisms in high-temperature thermomechanical pulping (HT-TMP) was also studied with a conventional TEM. Furthermore, the accessibility of cell wall lignin was studied with TEM and Raman microspectroscopy by analysing fresh and solvent extracted ultrathin sections of Norway spruce branch wood. The results showed that the organization of EFs varies from layer to layer and also within a single layer. In addition to the well-adopted concept of longitudinal EF angle in tangential plane, this study showed the presence of an out-of-plane EF angle relative to the cell wall plane. The S1 layer had a transverse EF orientation with a predominant radial lamellar structure of EF bundles. Both crossed and parallel EF orientations were detected in the S1-2 transition layer, which was supported by the defibration mechanisms in HT-TMP. EFs in the outer-S2 layer had a relatively high longitudinal EF angle and a large out-of-plane angle with respect to the tangential plane, which continued to decline inward and became almost axial in the inner-S2 layer. A transverse, out-of-plane EF orientation in the S3 transverse sections was observed. The models of the wood cell wall summarize most of the findings regarding the wood ultrastructure. Study of the lignin extracted ultrathin sections showed the change of lignin concentration in all cell wall layers during the extraction process. However, lignin obtained after extraction consists mainly of secondary wall lignin as this area contains most of the total cell wall lignin in conifer tracheids. The new observations on the wood cell wall structure may lead to a better understanding of the reactivity of cellulosic fibers in biochemical, chemical and mechanical treatments.