Cadmium and manganese in contrast to calcium reduce yield and nutritional values of the edible mushroom Pleurotus pulmonarius

Pleurotus pulmonarius is a species of the oyster mushroom which has become the second most popularly cultivated mushroom in the world. In this study, we show that renewed fruiting from excised stipes can be used as a simple and rapid in vitro bioassay system to detect fruiting modulators. We used this and conventional cultivation techniques to examine the effects of cadmium, a potential contaminant from industrial sources, calcium, which is an ingredient in mushroom compost, and manganese, which has been claimed to improve the yield of P. ostreatus. All the three metallic salts did not affect sporulation. Calcium chloride addition shortened the time taken for the mushroom to fully cover the cultivation compost and improved yield. Insoluble calcium salts at higher concentrations had a similar though less pronounced effect. The calcium and total amino acid contents of fruit bodies also increased. Compost supplementation with calcium is desirable for cultivation of the oyster mushroom but not indispensable since the straw-based cultivation substrate is itself able to provide the required minerals. By contrast, manganese chloride retarded mycelial growth and decreased yield but increased the total amino acid content in the stipe whilst manganese sulphate did not enhance accumulation of manganese into fruit bodies. Excess manganese induced browning of vegetative tissues. Cadmium ions did not kill the oyster mushroom at 4.5 mM but reduced yield by 50%. At this concentration cadmium decreased the total amino acid content and affected the amino acid profile but did not affect the form and shape of the fruit bodies. Pleurotus pulmonarius concentrated cadmium to such an extent that consumption of as little as 20 g (D.W.) of the most contaminated samples would exceed the weekly limit tolerated by humans and thus pose a health hazard. Monitoring the heavy metal contents of mushrooms marketed for food is, therefore, advised as the source of the substrate for cultivation is usually not known.