Chemical Changes in Wood Components and Cotton Cellulose as a Result of Brown Rot: Is Fenton Chemistry Involved?

The most destructive form of wood decay, brown rot, is caused by a relatively small number of species of Hvmenomycetous basidiomvcetes. These basidiomycetes are unique among cellulose destroyers because they are the only known microbes that can degrade the cellulose in wood without first removing the lignin (Cowling, 1961; Liese, 1970). Brown-rot fungi leave a brown residue—hence their name—that has been partially o-demethylated (Kirk, 1975). Furthermore, brown-rot fungi degrade cellulose in an unusual manner that differs from that of other cellulolytic organisms. Hyphae of these ubiquitous fungi invade wood cells and bring about a rapid depolymerization of the cellulose with low losses in total wood substance (Cowling, 1961; Kayama, 1962b). The average number of glucosyl residues per cellulose molecule (degree of polymerization, DP) is thereby reduced from about 104 (Goring and Timell, 1962) to about 200 (Cowling, 1961). The resulting fragments correspond to the size of the cellulose “crystallites.” This effect is thought to be brought about by cleavages within the amorphous regions of the cellulose that separate the crystallites (Cowling, 1961). Similar depolymerization of cellulose to the “limit DP” (to the crystallites) is effected by acid hydrolysis (Battista, 1950) and by chemical oxidants (Koenigs, 1972a, 1974a,b, 1975; Highley, 1977; Kirk et al., 1991). As a result of the initial attack by brown-rot fungi and the depolymerization of the cellulose, wood strength collapses. How this rapid depolymerization occurs is a perplexing biochemical question: as Cowling and Brown (1969) recognized over two decades ago, even the smallest cellulases (approximate diameter 25 _, length 140 _) are too large to penetrate the pores of wood (median pore diameter approximately 10 _; maximum 35–100 J. Also, cellulases do not mimic the action of brown-rot fungi in generating cellulose crystallites (Chang et al., 1981; Phillip et al., 1981). Our own examination of the change in pore structure of wood as it is decayed by a brown-rot fungus suggests that the depolymerizing agent is between 12 and 38 in diameter (Flournoy, 1991; Flournoy et al., 1991).