Research articleStructure of lignins in developing xylem of Norway spruce

The developing xylem in a Norway spruce (Picea abies) clone was investigated during a growth season and compared to lignin from sapwood of the same tree clone. Klason and acid-soluble lignin contents were determined as well as the carbohydrate monomer distribution and protein content. By analyzing lignin thioacidolysis products, it was shown that only guaiacyl units could be detected in the materials, and the relative amount of β-O-4′ bonds was assessed. Monomeric and selected dimeric lignin products were identified by mass spectrometry. The specimens were embedded and thin sections examined by microscopy to determine the state of cell differentiation in the samples. In the spring and early summer, growth was very rapid and the intention was to collect tissue in which exclusively the middle lamella/primary cell wall had begun to lignify. Combining data regarding Klason lignin, protein content and carbohydrate monomer distribution with microscopy, it was found that the developing xylem sample from mid-June contained lignin from exclusively middle lamella/primary wall. The Klason lignin content in the developing xylem during the growth season was 20%, 5% and 10% in April, June and August, respectively. Thioacidolysis showed that the lignin had more condensed structures than lignin from the reference Norway spruce clone wood. Mass spectrometry showed that the developing xylem specimens from June and August contained more lignin structures with end-groups than the reference sample. These results suggest that lignification in the cambial layer and early developing xylem may take place more in a bulk fashion during the summer.