Ultrastructural investigations were carried out to obtain information on the cell wall distribution of CCA elements during soft rot decay of 4% K33 CCA preservative treated birch (Betula verrucosa) wood. TEM observations on fibres at various stages of S2 cavity formation by mutabilis showed a distinct pattern of electron dense materials to occur within the S2 cell wall region. These materials (granular and fibrillar) were observed either concentrated around existing or previously existing cavity hyphae or more loosely associated within intervening hyphal S2 regions. TEM X-ray microanalytical studies on these materials showed high levels of CCA to be present particularly with the granular materials (thought to be melanin type material) associated with fungal hyphae; levels which greatly exceeded that recorded in neighbouring, but undegraded S2 cell wall regions. Lesser CCA levels were also found associated with non-hyphal electron dense fibrillar-like materials produced as a result of either true soft rot cavity formation or erosion attack within S2 fibre cell walls. X-ray studies on cavity hyphae has so far shown only significant levels of copper and possibly arsenic to have been taken up, and to be located in both the ground cytoplasm (only Cu) and in electron-dense bodies (Cu and As). The distribution of CCA in the degraded wood cell walls is discussed with respect to both the decay pattern, metal resistance of the fungus and the known biochemical nature of the attack process by soft rot fungi. Aspects of extracellular CCA detoxification by Phialophora mutabilis and other soft rot forming Phialophora species are further considered.

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