A lignin-like polymer in the cuticle of spruce needles: implications for the humification of spruce litter

Information on the type and amount of refratoryy biopolymers produced by plants is still insufficient. The aim of the present work was to determine the chemical (structural) composition of spruce cuticles as a source material for humification. Intact cuticles were isolated by conventional techniques from fully developed needles of Norway spruce (Picea abies (L.) Karst.). The cuticles were subjected to a series of selective treatments to remove different types of polymers. Extraction with organic solvents, to remove lipids and waxes, was followed by saponification to remove the cutin polyester. Finally, the cuticle residues were hydrolyzed to remove polysaccharides. Through investigations combining CP/MAS ¹³C-NMR spectroscopy, analytical pyrolysis, and wet chemical methods (CuO oxidation), the chemical composition of the polymer was determined in the cuticle, and in the residues obtained by the selective chemical treatments. These data show that the isolated spruce cuticles consist of extractable lipids, polysaccharides, and cutin, biopolymers commonly found in plant cuticles. In addition, a lignin-type polymer was identified, which was selectively isolated after the treatments described above, from the final residue. In conjunction with results from a microscopic survey of the cuticles, these investigations provide evidence for the presence of a lignin-like polymer as a component of intact cuticles of Norway spruce. The consequences of this finding for the humification process of spruce litter are discussed.