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Observations by electron microscopy of the micromorphological alterations caused in wood by fungal strains in which the cellulase system had been repressed or inhibited showed a substantial destruction of the cellulose network. The question thus arises which agents, other than enzymes, could be responsible for the modification of polysaccharides during fungal attack. Among the small diffusible agents possibly involved, activated oxygen species generated from hydrogen peroxide have been suggested. In particular the powerful oxidizing hydroxyl radical (· OH), which can be generated by reductive cleavage of H 2 O 2 or by reduction of oxygen, has been reported to take part in degradation of lignin by white- and brown-rot fungi. Manganic ions which are generated by manganese-dependent peroxidase (MnP) represent another diffusible oxidizing agent. To investigate whether these agents, produced by lignin-degrading enzymes, could also participate in wood polysaccharide degradation, the action of hydroxyl radicals and of some Mn(III)-organic acid complexes on carbohydrate and polysaccharide model compounds and also on wood and pulps, was studied. The action of ·OH, generated by the iron-catalyzed Fenton's reagent, on the glucuronoxylan from birch wood resulted in an extensive degradation of the hemicellulose (about 50% loss of carbohydrate after 180 min). A study of the degraded products suggests that internal cleavage occurs in the xylan chain accompanied by sugar ring degradation, the latter becoming predominant on the soluble oligomeric products. In a parallel study, the susceptibility of carbohydrates to oxidation by manganic ions showed that monosaccharide degradation occurred rapidly in strongly acidic medium. The mechanism which was studied with a Mn(III)-pyrophosphate complex, involves, as an initial step, the oxidative cleavage of C 1 with the formation of a lower carbohydrate, and then proceeds by a recurrent series of oxidations. However, the reaction seems to be controlled by the formation of a Mn(III)-pyrophosphate complex with the sugar, which prevents further oxidation. The two oxidizing agents were applied to birch wood wafers. The hydroxyl radicals had only a relatively weak degradative action on the cell walls. However, when the radicals were generated in situ, the extent of degradation as visualized by electron microscopy, gave images comparable to those obtained on fungal-degraded wood. Samples treated with manganic pyrophosphate and manganic organic acid complexes also showed typical degradation indicating that lignin was primarily degraded and also that hemi-celluloses had also been partly damaged.

During screening of basidiomycetes for wheat straw delignification, considerable lignin degradation with a limited attack to cellulose was attained with Pleurotus eryngii. Straw solid-state fermentation (SSF) was optimized, and the enzymatic mechanisms for lignin degradation were investigated. No lignin peroxidase was detected under liquid or SSF conditions, but high laccase and aryl-alcohol oxidase levels were found. The latter enzyme has been fully characterized in PI. eryngii and it seems to be involved in a cyclic redox system for H202 generation from aromatic compounds. Results obtained using homoveratric acid suggest that Pleurotus laccase could be involved in degradation of phenolic and non-phenolic lignin moieties. Histological and ultrastructural studies provided some general morphological characteristics of the fungal attack on wheat straw. Whereas a simultaneous degradation pattern was observed in straw treated with Phanerochaete chrysosporium, PI. eryngii caused partial degradation of middle lamella and separation of individual sclerenchymatic fibers. When these straw samples were subjected to refining tests, energy saving after biological treatment was the highest in the case of straw treated with PI. eryngii, which also produced the lowest substrate loss. From these results, a correlation between preferential removal of lignin, separation of sclerenchymatic fibers and pulping properties was provided during fungal treatment of wheat straw.