Lignin distribution in mild compression wood of Pinus radiata

Lignin distribution in the tracheid cell wall of mild compression wood in P inus radiata D. Don was examined by interference microscopy, confocal fluor escence microscopy, and ultraviolet (UW) microscopy. Two anatomically diffe rent samples of mild compression wood were compared with a sample of normal wood using quantitative interference microscopy and microdensitometry comb ined with confocal fluorescence microscopy to estimate the quantitative or semiquantitative lignin distribution in the S2 and S2(L) regions of the sec ondary cell wall and of the cell corner middle lamella (CCML). One of these samples was briefly examined by UV microscopy for comparison. Quantitative interference microscopy provided information on lignin concentration in di fferent regions of the cell wall with values of 26, 46, and 57%, respective ly, for the S2, S2(L), and CCML regions of sample 1 and 20, 29, and 46%, re spectively, for the same regions of sample 2. Microdensitometry of confocal fluorescence images provided semiquantitative information on the relative lignin distribution based on lignin autofluorescence. Comparison between th e two compression wood samples using autofluorescence gave results that wer e in partial agreement with interference microscopy with respect to the rel ative lignification levels in the S2, S2(L), and CCML regions. Some improve ment was achieved by using calibration values for hemicellulose rather than holocellulose for interference data in the S2, region. Results for UV micr oscopy performed on sample 1 indicated that the lignification of the CCML r egion was comparable with that of the S2(L) region in this sample but with some variation among cells. All three techniques indicated significant vari ation in lignification levels of the S2(L) and CCML regions among adjacent cells and a significant reduction in the lignification of the CCML region c ompared to normal wood.