Raman Imaging of Lignin and Cellulose Distribution in Black Spruce Wood (Picea mariana) Cell Walls

A detailed understanding of wood cell wall structure and organization is important from both fundamental and practical point of views. A state-of-the-art 633-nm laser based confocal Raman microscope was used in situ to investigate the cell wall organization of black spruce wood. Chemical information on lignin and cellulose from morphologically distinct cell wall regions was obtained and Raman images of their distribution were generated. The initial findings were that lignin and cellulose concentration in tracheid cell walls differed significantly both within and between the distinct morphological regions. For instance, secondary wall region of some cells showed lignin concentration variation by as much as 100%. Although, normally the concentration of lignin was higher in the compound middle lamella region (CmL) compared to secondary wall (S2 and S3), there were numerous instances when the magnitudes of the two concentrations were similar or were even reversed. Some of the true cell corner middle lamella (CCmL) regions had the highest lignin concentration; however, even in this morphological region, the lignin concentration varied. Compared to lignin, cellulose distribution was more uniform; nevertheless, in specific limited areas, cellulose concentration varied significantly. At the present time the intensity maps of lignin and cellulose distributions are interpreted solely in terms of concentration differences but effect of orientation needs to be carefully considered in order to truly reveal the organization of the wood cell walls.