Amanita muscaria collected from a forested area in northern Poland in 2015 were analysed for activity concentrations of 137Cs and 40K. Total K concentration values were calculated from 40K data. Mushrooms were grouped in six fruiting bodies size classes regarding to their developmental stage. The 137Cs activity concentrations declined in A. muscaria as the fruiting bodies maturated. The contents of 40K/K activity remained constant in caps at different developmental stage, while for stipes an increase was around twofold (40K from 925 ± 55 Bq kg-1 dry biomass in the baby individuals and 1600 ± 63-1700 ± 53 Bq kg-1 dry biomass in two oldest classes). A. muscaria is a weak accumulator of 137Cs, while a steep drop in activity concentrations of this nuclide in the fruiting bodies as they maturate has not been reported earlier for mushrooms. Clearly, a fate of 137Cs in A. muscaria is highly different from that of 40K/K that is an essential element to fungi. In parallel, an observed increase in the content of 40K/K in stipes of fruiting bodies with developmental state can be related to its 'hardening' and more fibrous nature with age and basic function to support the cap, but this has not been studied.
Keywords: Caesium-137; mushrooms; natural radioactivity; potassium-40; radioactive pollution.