The cell wall composition of Norway spruce earlywood and latewood revisited

The xylem of gymnosperm trees consists mainly of tracheids, which generally have larger radial diameters and thinner walls in earlywood than in latewood. Information on possible chemical differences is limited, but according to a few studies, earlywood normally contains more lignin relative to cellulose than latewood. However, for methods giving results per gram dry matter, this result is expected since a larger part of the earlywood cell wall material consists of lignin-rich middle lamella. In this study, we used Raman microspectroscopy to assess relative biopolymer composition of earlywood and latewood secondary cell walls from mature sapwood of Norway spruce (Picea abies (L.) Karst.). In addition, infrared spectroscopy of isolated earlywood and latewood from the same growth rings as well as additional growth rings was performed. Both methods showed significantly higher relative lignin content in the earlywood which confirms results from earlier studies.