Bark decay by the white-rot fungus Lentinula edodes: Polysaccharide loss, lignin resistance and the unmasking of suberin

The chemical composition of oak bark during growth of Lentinula edodes was studied to assess the transformation of lignin, suberin, tannin and structural polysaccharides. Oak logs (Quercus alba) were decayed by L. edodes over 8 years, during which time they were sampled at five intervals (30, 40, 66, 77, 101 months). Elemental analysis (C, H, N and O), solid-state 13C NMR as well as off-line thermochemolysis in the presence of tetramethylammonium hydroxide (TMAH) with gas chromatography-mass spectrometry were used in the characterization of fresh and degraded barks. Solid-state 13C NMR analysis showed that cellulose and xylans were the main structural components of fresh bark but that L. edodes caused a 46% decrease in polysaccharide content and that loss of crystalline and non-crystalline cellulose regions occurred in parallel. The decrease in cellulose content was accompanied by a relative increase in the proportion of methylene carbon from suberin. The aromatic region of the spectra revealed a loss of a shoulder at 145 ppm attributed to tannins after 30 months decay; in contrast the aromatic content remained unaltered, suggesting that highly structured tannins were degraded in preference to lignin. Chemolytic treatment with TMAH confirmed moderate changes in guaiacyl and syringyl acid-aldehyde ratios with growth indicating that the fungus had not caused extensive oxidative side chain alterations. The ratio of syringyl to guaiacyl units (S/G) in fresh oak bark was lower at 0.6 than that previously reported for oak sapwood (S/G>1) and a slight decrease in S/G values was observed with fungal decay. The resistance of bark lignin as compared to sapwood lignin is attributed in part to the inhibiting effect of tannins and suberin on fungal growth.