| Summary: | Specificity in mushroom compost was demonstrated to be due to nutrient deficiency. Growth of micro-organisms was inhibited by a lack of suitable nutrients which can be induced by microbial leaching. Growth inhibition could not be explained by the presence of antibiotics or volatile compounds. Addition of readily utilisable nutrients resulted in increased microbial activity in compost, this being correlated with reduced growth of A.bisporus. Specificity was maintained by a number of mechanisms including the production of volatile inhibitors by mushroom mycelium. The products of reactions between phenolic compounds and laccase from A.bisporus to produce inhibitors might also be important in this context. Availability of substrates to micro-organisms and their C:N ratios were important in restricting growth of compost inhabiting fungi. However, A.bisporus was able to synthesise enzymes for substrate degradation at otherwise restrictive C:N ratios. This was attributed to marked biochemical and structural changes within the mycelium. A large number of extracellular enzymes could be induced in A.bisporus suggesting that a wide range of substrates are suitable for degradation. The selective nature of compost was not explained by differences in inoculum potentials of micro-organisms. When considering the effects of competitive saprophytic ability on specificity it was apparent that substrate degradation was of major importance. A microbiological and microscope study of straw through composting and cropping provided some information on the nature of changes in the substrate with time. During composting a significant increase in microbial biomass occurred on straw, this being markedly reduced during spawn run and corresponding with a reduction in numbers of thermophilic micro-organisms isolated from spawn run compost. Quantitative extraction and chromatographic assay revealed that a major proportion of compost was polysaccharide in nature, the greater proportion of this probably being microbial in origin. This appeared to act as a major nutrient source for A.bisporus during spawn run. Close association with humic acid was suggested to explain lack of degradation of this polysaccharide by other compost inhabitants. These results suggest that operations aimed at producing express substrates might consider specificity in the light of the mechanisms proposed in the present study and benefit from the knowledge that bacterial extracellular polysaccharides may serve as efficient nutrient sources for mushroom growth.
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