Microfibril orientation across the secondary cell wall of Radiata pine tracheids

Abstract

Polarised light microscopy and transmission electron microscopy were used to measure the variation in cellulose microfibril orientation across the secondary cell wall of Pinus radiata D. Don tracheids using oblique sections. Variations in orientation for the S1, S2 and S3 layers were determined for radial and tangential cell walls from the juvenile and mature earlywood at the base of the stem and at 5 m height. Microfibrils in the S1 layer are usually arranged in an S-helix (>90°) varying from 79° to 117° among tracheids. Within individual tracheids the microfibril orientation in the S1 region can be quite variable, sometimes changing from an S to Z-helix but without a well-defined crossed structure. Microfibrils in the S2 layer form a Z-helix (<90°) with average orientation varying from 1° to 59° among tracheids, while the S3 layer is also often a Z-helix varying from 50° to113° in orientation. The S2 layer shows significant variation among sample points within the tree related to age and height, while the S1 and S3 layers show small random variations within the tree. Microfibril orientation in the S2 layer is very uniform, although when variation does occur the trend is exclusively for an increase towards the S1 region. Orientation in the S3 layer often varies continuously from the S2 boundary to the lumen. The S3 layer becomes much thinner with increasing age and height, making measurement by light microscopy more difficult. The innermost lamellae of microfibrils at the lumen surface sometimes show a distinctive clustering, forming macrofibrils of varying orientation but this may be the result of delignification treatment.