Distribution of lignin and cellulose in compression wood tracheids of Pinus yunnanensis determined by fluorescence microscopy and confocal Raman microscopy

•Lignin and cellulose distribution was examined in situ by confocal Raman microscopy.•The lignin concentration was highest in the CCML followed by the S2L.•The cellulose concentration in the S2L was between that in the CCML and the S2.•The cellulose microfibril angle in the S1 was higher than that in the S2L and the S2.

A detailed understanding of the topochemistry of compression wood tracheid walls is important from the perspectives of plant biochemistry and commercial utilization. The present study aimed to determine the anatomy and distribution of lignin and cellulose in situ in compression wood tracheids of Pinus yunnanensis by fluorescence microscopy and confocal Raman microscopy. Anatomical observation by fluorescence microscopy revealed that P. yunnanensis can be classified as mild compression wood and the lignin distribution in tracheid walls was heterogeneous. Confocal Raman microscopy was used to examine the distribution of lignin and cellulose within morphologically distinct tracheid wall regions. The histochemical observations indicated that the highest level of lignification occured in the cell corners middle lamella (CCML) some what less in the outer S2 layer (S2L), while the lowest lignin concentration was found in the S2. In contrast, cellulose distribution showed the opposite pattern with the highest cellulose concentration occurring in the S2 layer. The Raman analysis also indicated that the cellulose microfibril angle (MFA) in the S1 was high and thus the cellulose fibrils and the CC, CO stretching occur at a large angle.