Comparison of anatomy and composition distribution between normal and compression wood of Pinus bungeana Zucc. revealed by microscopic imaging techniques.

The anatomy and topochemistry in normal and compression wood tracheid cell wall of Pinus bungeana Zucc. were investigated by fluorescence microscopy and confocal Raman microscopy. Using fluorescence microscopy, the severity of compression wood was classed as a mild type for the reason that it did not contain all compression wood features. Chemical imaging by confocal Raman microscopy was used for analyzing the distribution of lignin and cellulose, as well as the functional groups of lignin in tracheid cell walls. By comparison with normal wood, highly lignified outer S2 layer [S2(L)], thicker S1 layer, and obviously reduced lignification in the middle lamella were characteristic of compression wood. In addition, smaller microfibril angle was observed in the S2(L) region. The distribution of coniferyl alcohol and coniferyl aldehyde in normal and compression wood was enriched in S1 and S2 layers but lack in cell corner and/or S2L regions, which showed an opposite pattern to lignin distribution. Confocal Raman microscopy with high spatial resolution contributes to a further understanding of the differences between normal and compression wood in polymers distribution and molecules orientation in situ.