Open access publications in the SLU publication database

Abstract

Wood from Norway spruce and other conifers consists mainly (>90%) of one cell type called tracheids. The morphology of these tracheids has a profound effect on wood industrial processes and products made from wood. A more in-depth understanding of tracheid morphology is therefore required in order to improve wood industrial processes and products derived from conifer wood. The aim of this thesis was to study the morphology of Norway spruce (Picea abies (L.) Karst) tracheids with emphasis on cell wall organisation, especially microfibril orientation in the secondary cell wall. A literature review provided an overview of the micro- and ultrastructure of Norway spruce tracheids. The use of cavities from soft rot fungi was discussed and evaluated as a way of determining microfibril angles (MFA) in the secondary cell wall of tracheids. Soft rot fungi were shown to be a useful tool for ultrastructural research and results correlated well with polarisation confocal microscopy when measuring microfibril angles of the middle layer of the secondary cell wall (S2). Both methods showed a decreasing trend in S2 microfibril angle, from the beginning of earlywood (MFA ca. 30°) and towards the end of earlywood (MFA ca. 5°). The low microfibril angles continued into the latewood. No direct correlation was found between tracheid microstructure (i.e. cell length, cell width and cell wall thickness) and the microfibril angle in the S2 layer. Several methods were used to investigate the structural organisation of the outer layer of the secondary wall (S1). It was concluded that the S1 layer is rather homogenous layer being primarily comprised of microfibrils oriented approximately perpendicular to the tracheid axis. No evidence could be found for a crossed fibrillar structure with microfibrils in alternate S and Z helices. The transition of microfibril orientation from the S1 to the S2 layer appeared abrupt. Cell wall models illustrating microfibril orientation in the cell wall layers were discussed from a historical perspective. Three new cell wall models, based on results obtained in this thesis and from the literature were presented in order to visualise the microfibril orientation in the cell wall layers of Norway spruce tracheids.