1 Triangle/Lab/2819/WHITE.htm

fungal protoplasm but also for the swelling of the substrate and the diffusion of the digestive enzymes into the substratum through pits and pores.

The decay of timber is monitored by a chemical extraction procedure. Lignin is quantitatively extracted by chlorite-acetic acid. Hemicelluloses are extracted as alkali-soluble material, whereas cellulose is insoluble in alkali. Extractions and estimations of these materials show that the various wood-decay fungi differ in their abilities to digest lignin, cellulose and xylan (Table 12.1). The hyphae grow in intimate contact with the cell wall, enzymatically fragmenting it and transforming the decayed plant material into humus with a high phenol content. Fungi cannot use lignin as the sole carbon source; the presence of a metabolizable carbohydrate, from which hydrogen peroxide can be generated, is essential for the growth of fungus and for depolymerization of lignin.

12.1.1 White-Rot and Brown-Rot Fungi

The presence in wood of lignin—a complex phenylpropanoid polymer that is the most recalcitrant of organic compounds—renders wood decay a very slow process. The fungi referred to as the white-rot fungi produce polyphenol oxidase and laccase that oxidize phenol compounds, giving the wood a bleached or pale appearance and transforming it into a fibrous mass. Electron microscopy of decaying wood using KMnO4 as a fixative and stain shows the greatest electron density in the middle lamella. Once lignin is removed, the middle lamella between cells is degraded and the cells separate (see Blanchette, 1991). In contrast, the brown-rot fungi by some undiscovered mechanism circumvent the lignin barrier and utilize hemicelluloses and celluloses, leaving lignin essentially undigested. The wood breaks into pieces that crumble into a brown powder. A method of distinguishing between the brown and the white rot fungi is the Bavendamm test, based on the formation of a brown color in the medium when phenolics are oxidized. The fungi are grown on an agar medium containing phenolic compounds. The white rot fungi produce polyphenol oxidase that shows a dark zone around mycelium. The brown rot fungi do not produce polyphenol oxidases and do not show any coloration.

12.1.2 Litter Decomposers

The microorganisms bringing about decomposition of litter are identified by plating decaying tissue macerates on suitable nutrient agar or by the direct observation method of the tissue incubated in damp atmosphere and identifying these on the basis of morphology. Among the soil-inhabiting fungi—Penicillium (Fungi Anamorphici), Humicola (Fungi Anamorphici), Trichoderma (Fungi Anamorphici), Mucor (Straminipila), Collybia (Basidiomycotina), Hydnum (Basidiomycotina), Marasmius (Basidiomycotina), Mycena (Basidiomycotina) and others—are typical litter-decomposing fungi. Scanning electron microscopy of tissue surfaces allows the identification of fungi that are actually growing on the plant litter. The colonization appears to be largely superficial. The rate of decomposition of leaf litter is markedly dependent upon the rate at which litter fragments, a result of the activity of small soil animals such as earthworms, slugs, millipedes and mites that ingest plant litter (Dix and Webster, 1995). Fragmentation increases the surface area for microbial attack, though little is known of the role of individual species. In temperate regions the agarics are among the most active agents of decomposition. They are active producers of laccases or polyphenol oxidases which detoxify litter phenolics. However, litter decomposition in tropical forests has been little studied.

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