Some potential inaccuracies of the p-nitrophenyl phosphomonoesterase assayin the study of the phosphorus nutrition of soil borne fungi

The p-nitrophenol phosphomonoesterase assay (pNPPase) is commonly used to m easure cell-wall-associated and extracellular phosphatase activity of soil fungi. pNPPases are usually assayed in the context of fungal nutrition, whe re inorganic P supply might be enhanced by the mineralisation of organic P sources in the soil. We report here on a series of experiments with the ect omycorrhizal basidiomycete Hebeloma cylindrosporum that highlight component s of accepted methodology that might impinge on the reliability of the assa y. These include the loss of pNPPase after filtration, inaccuracies in meas uring wall-associated enzyme and the ample pool of intracellular pNPPase ca n be mistakenly measured as external pNPPase if cells are accidentally dama ged.