Distribution of lignin, pectins and hemicelluloses in tension wood fibers of European ash (Fraxinus excelsior)

ABSTRACT Although there is considerable information on the chemistry of gelatinous (G) layers in tension wood (TW) fibers consisting of S1 + S2 + G cell wall structure (poplar), little is known on the chemistry of G-layers in TW fibers organized with S1+ G structure. This study investigated the distribution of lignin and non-cellulosic polysaccharides in ash TW fibers (S1+ G) using histochemistry and immunolocalization methods. TW fibers studied were fully developed (mature fibers) and obtained from two (TW-1, TW-2) mature European ash trees (Fraxinus excelsior L.). Based on differences in microfibril angle (MFA) and TW trees used, TW fibers were mainly classified into three types; 1) Type-1 fibers with MFA almost parallel to the fiber axis that were found in TW-1, 2) Type-2 fibers with 12° MFA that were abundant at the end of growth rings of TW-1 and 3) Type-3 fibers with 10° MFA that were found in TW-2. The S3 layer was absent in all TW fibers. In this study, the secondary cell wall structure of Type-1 and Type-2/Type-3 fibers were defined as G and GL (gelatinous-like) layers, respectively. Lignin with syringyl (S) units was detected in G/GL-layers, in which intensity and patterns of lignin staining likely related to the difference in MFA between G- and GL-layers. With hemicelluloses, heteroxylan and heteromannan epitopes were detected in G/GL-layers but these were much less abundant than those in S2 layers of normal wood (NW) fibers. Like lignin, distribution patterns of heteromannan epitopes in G/GL-layers likely related to differences in MFA between fiber types. Sparse xyloglucan epitopes were also detected in G/GL-layers. Homogalacturonan epitopes were absent in G/GL-layers. All fiber types showed abundant a-1, 5-arabinan epitopes in G/GL-layers. Overall results indicate that the chemistry of ash TW fibers studied differs significantly from that of other species reported previously, specifically TW fibers composed of S1 + S2 + G structure.