The mushroom (Agaricus bisporus) has a\ud requirement for a ‘‘casing layer’’ that has specific\ud physical, chemical and microbiological properties\ud which stimulate and promote the initiation of primordia.\ud Some of these primordia then may develop\ud further into sporophores, involving differentiation of\ud tissue. Wild and commercial strains of A. bisporus\ud were cultured in axenic and nonaxenic microcosms,\ud using a rye grain substrate covered by a range of organic\ud and inorganic casing materials. In axenic culture,\ud A. bisporus (commercial strain A15) was capable\ud of producing primordia and mature sporophores on\ud charcoal (wood and activated), anthracite coal, lignite\ud and zeolite, but not on bark, coir, peat, rockwool,\ud silica or vermiculite. Of six strains tested, only\ud the developmental variant mutant, B430, produced\ud rudimentary primordia on axenic peat-based casing\ud material. However, none of these rudimentary primordia\ud developed differentiated tissues or beyond 4\ud mm diameter, either on axenic casing material in the\ud microcosms or in larger-scale culture. In larger-scale,\ud nonaxenic culture, strain B430 produced severely\ud malformed but mature sporophores in similar numbers\ud to those of other strains. Typically, 3–6% of primordia\ud developed into mature sporophores, but significant\ud differences in this proportion, as well as in\ud the numbers of primordia produced, were recorded\ud between 12 A. bisporus strains
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