Ultrastructure of tracheid cell walls in radiate pine (Pinus radiata) mildcompression wood

The structure of tracheid cell walls in mild compression wood of Pinus radi ata D. Don was examined by confocal laser scanning fluorescence microscopy (CLSM) and transmission electron microscopy (TEM). Fluorescence microscopy using lignin autofluorescence revealed cell to cell variations in lignifica tion of the cell corner middle lamella (CCML) and outer S2 (S2(L)) regions supporting observations made at higher resolution with TEM. The mild compre ssion wood sample examined lacked the characteristic intercellular spaces o f most grades of compression wood and thus represents the least severe grad e of compression wood. TEM observations revealed evidence for radial striat ions in the 52 region probably representing regions of alternating high and low lignin concentration, which may be the precursor to the helical caviti es found in severe compression wood. These radial striations typically show ed a sinuous and branched structure, extending partly into the S2(L) region , where the orientation of lignin lamellae was predominantly tangential fol lowing the arrangement of the cellulose microfibrils. At high magnification lignin occurred in a patchy distribution overlaying the typical lamellate arrangement associated with normal wood. Significant variability in the lig nification of the CCML region was also observed sometimes giving a patchy o r granular appearance. The lignification of the CCML and the S2(L) regions was generally comparable, while S1 and S2 layers were less lignified. A res idual S3 layer was sometimes present in the form of a very thin layer sligh tly more lignified than the adjacent S2 layer.