Structural studies of single wood cell walls by synchrotron X-ray microdiffraction and polarised light microscopy

Wood is a complex material, from which different structural factors are extensively studied. Despite decades of work, the question on how cellulose chains are oriented in a mature wood cell wall is not yet fully answered. The orientation is defined as the angle between the chains and the cell axis (microfibril angle, MFA). In this work tracheids of Norway spruce (Picea abies [L.] Karst.) were studied with a micrometer-size beam of synchrotron radiation to probe the single cell walls through bordered pits. The obtained MFA distributions are asymmetric and contain mostly either left- or right-handed helices. The asymmetry most likely arises from the structure of the S2-layer of the secondary wall. The mean values and the positions of the maxima of the distributions obtained by X-ray diffraction were compared to values of MFA obtained by polarisation microscopy from the same parts of the cell walls. The results by polarisation microscopy were different; possible reasons for this are discussed.