Changes in the Chemical Composition of Wood Caused by Six Soft-Rot Fungi

Abstract

Standardized blocks of alder, poplar, and pine wood were hemicellulose (xylan). Lignin, which was analyzed by the decayed by six soft-rot fungi (Graphium sp., Monodictys sp., “sulfuric acid” method, was depleted by all the fungi. Pine Paecilomyces sp., Papulospora sp., Thielavia terrestris, and was not decayed significantly by three of the fungi, and only Allescheria sp.), all of which had been isolated from pulp chip to low weight losses (15% or less) by the other three storage piles. Samples of the woods at different weight losses ( Paecilomyces sp., Papulospora sp., and Thielavia terrestris). were analyzed for lignin, glucan (cellulose), xylan, and Analysis of blocks that were decayed showed that lignin was mannan (hemicelluloses) to allow calculation of the depletion depleted faster than the cellulose or the hemicelluloses by in these major components caused by the fungi. Paecilomyces sp. and Allescheria sp. Carbohydrates were depleted faster than lignin in the alder and poplar, cellulose usually faster than the major Phytopathology 65:473-476 Additional key words: Alnus rubra, Populus balsamifera, Pinus monticola, wood decay, thermophilic fungi. Of the three major types of fungal decay of wood (soft­ rot, brown-rot, and white-rot) the least studied is soft-rot, which is caused by numerous members of the Fungi Imperfecti and Ascomycetes ( 1 ) . Research has been especially deficient in elucidating the chemistry of the soft-rot of wood. In fact, the chemical effects of only one fungus, Chaetomium globosum, on one kind of wood, European beech (Fagus sylvatica), have been investigated (5). Chaetomium globosum causer, extensive loss in weight of beech, depleting the cellulose and pentosans (mainly xylan) considerably faster than the lignin (7, 9, 10). Working with isolated polysaccharides rather than whole wood, Keilich et al. (4) and Takahashi and Nishimoto (11) found that C. globosum was capable of degrading xylans and mannans, the former being degraded at a slightly greater rate than the latter. Soft-rot fungi, including Chaetomium globosum, have been isolated in large numbers from pulp chip storage piles in recent years (2). In laboratory decay studies, these isolates caused considerable loss of wood substance, particularly in hardwood chips, thereby emphasizing the importance of soft-rot fungi as destructive organisms. However, it has not been determined whether all or most soft-rot fungi cause chemical changes similar to those caused by Chaetomium globosum, or whether for example, some attack the lignin, the cellulose, or one of the hemicelluloses preferentially. Such information is needed for a full assessment of the destructive capabilities of these organisms, particularly those known to be prevalent in chip storage piles and capable of causing substantial weight losses in wood. The purpose of this research, therefore, was to determine quantitatively the changes in the major structural components of wood during decay by several wood-chip-inhabiting soft-rot fungi. MATERIALS AND METHODS.-Wood samples and decay.-Outer xylem blocks, 25.4 × 12.5 × 6.3 mm (1 × 1/2 × 1/4 in.), with the 6.3 mm dimension in the fiber direction, were cut from single boards of red alder (Alnus rubra Bong.), balsam poplar (Populus balsamifera L.), and western white pine (Pinus monticola Dougl.). The blocks were numbered, conditioned to constant weight a t 27 C and 70% RH, and then weighed. Eight replicate blocks from each wood species were strung together, sterilized, and then subjected to decay by the serial-block method (3). Following 12 weeks of incubation at 27 C for mesophilic test fungi, and at 45 C for thermophilic test fungi, the blocks were removed, reconditioned, weighed, and their weight losses calculated. The small size of the blocks, coupled with their relatively large cross-sectional area, permitted rapid penetration by the test fungi and uniform decay throughout the blocks. The test fungi used were all isolated from wood chip storage piles, and had been shown in previous laboratory tests to be capable of causing appreciable weight losses in wood. Mesophilic isolates included Graphium sp. (MEBC-11), Monodictys sp. (ME-GC-18), and Papulospora sp. (ME-PC-19). Thermophiles included were Paecilomyces sp. ( M E T B C 1 0 ) , Thielavia terrestris (Apinis) Mallock & Cain = Allescheria terrestris (H-63­ 1), and Allescheria sp. (ME-T-BC-9). The decayed blocks of each species were roughly grouped according to weight losses caused by each test fungus, and blocks of desired weight loss were removed for individual chemical analysis. (The variation in weight loss within a wood species by the same decay fungus reflects primarily the intentional variation in the moisture contents among blocks in a test (3).) A block of poplar and one of pine, well decayed by each of the test fungi, were sectioned transversely and tangentially and examined microscopically to determine whether each of the test fungi caused typical soft-rot in both hardwoods and softwoods. Analyses.-Decay and control blocks were ground to pass a 0.50-mm (40-mesh) screen and the meal dried thoroughly at 45 C under high vacuum. The samples were analyzed, without extraction, for “sulfuric acid lignin” by an adaptation of a published procedure (12) and for total reducing sugars in acid hydrolysates (determined colorimetrically as glucose) (8). Relative amounts of glucose, xylose, and mannose in the hydrolysates were determined colorimetrically after paper chromatographic separation (8). From these values, the glucan, xylan, mannan, and lignin, and the losses of each during decay were calculated (6). Angiosperm wood is composed mainly of cellulose, a hemicellulose ofthe O-acetyl-4-O-methylglucuronoxylan type (approximately 75% xylan) and lignin, plus a smaller