Influence of the water potential of the casing soil on crop yield and on dry-matter content, osmotic potential and mannitol content of the fruit bodies ofAgaricus bisporus

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Abstract
In the culture of the mushroom, Agaricus bisporus, significant amounts of water are taken up from the casing soil. The water potential of the casing soil was lowered by adding sodium chloride. A lowering of the water potential reduced crop yield, slowed fruit-body development and increased the dry-matter content of sporophores as a percentage of wet weight. Values for dry-matter content of the fruit bodies ranged from 6.7 to 11.5%. Regulation of dry-matter content is therefore possible by adjusting the concentration of salt in the casing soil. The total dry weight of all harvested fruit bodies of the first two flushes (breaks) from cultures with a low water potential in the casing soil (high salt) was as high as that of the crop of cultures with a high water potential (no salt added). The total concentration of osmotically active substances in the pressed juice of caps of fruit bodies increased with decreasing water potential of the casing soil. The increase in mannitol concentration of the press juice contributed significantly to this increase. A positive correlation was found between the concentration of osmotically active substances in the pressed juice and the increase in dry-water content of fruit bodies. Mechanisms of water uptake and movement from the casing soil to the growing fruit bodies are discussed.