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Ultrastructure of Transwall Fracture Surfaces in Radiata Pine Wood using Transmission Electron Microscopy and Digital Image Processing

Transwall fractures in tangential cell walls of Pinus radiata D. Don tracheids were examined by transmission electron microscopy. Transwall fracture in the S 2 region occurred at angles less than 90° to the microfibril orientation with fractures occurring across the microfibrils producing a distinctive sawtooth fracture face and exposing broken ends of microfibrils at the fracture surface. Little or no apparent disruption to the cell wall adjacent to the fracture indicates a brittle type fracture in the secondary wall region. Detailed examination revealed evidence for plastic deformation of the compound and cell corner middle lamella regions, resulting in a thin, low density boundary layer, 5-50nm thick, adjacent to the fracture surface. These features were enhanced using image analysis and processing techniques including densitometry, and a fast fourier transform filter to enhance the periodic structures within the secondary wall. This study provides evidence for differences in the physical properties of the secondary wall and middle lamella regions of the cell wall at the ultrastructural level. Transwall fractures in both regions produce irregular surfaces in comparison to the smooth interlamella surfaces which occur in intrawall fracturing. This is discussed in relation to the known molecular architecture of these cell wall regions.