Regulation of heavy metal concentrations in cereal grains from uranium mine soils

AIMS: Along a gradient of diminishing heavy metal (HM) concentrations formed by local inclusions of uranium mine soils into non-contaminated cropland, duplicate 1-m² plots of 3 winter wheat cvs. (Akteur E, Brilliant A, and Bussard E) were established at 3 positions within a winter rye (cv. Visello) culture. It was the goal to determine permissible soil HM concentrations tolerated by cereal cvs. with variable excluder properties, and regulatory mechanisms which optimize the concentrations of essential minerals and radionuclide analogues in viable seeds from geologically related soils with diverging HM content. METHODS: Total metal concentrations / nitrogen species in soils, shoots, and mature grains were determined by ICP-MS / spectrophotometry, and Kjeldahl analyses. RESULTS: No non-permissible concentrations in grains of the 4 cereal cvs. were caused by elevated but aged total soil resources (mg kg⁻¹ DW) in As (156); Cu (283); Mn (2,130); Pb (150); and in Zn (3,005) in the case of Bussard although CdCuZn elicited phytotoxicity symptoms. Uranium (41) contaminated grains of Akteur and Brilliant but not of Bussard and Visello due to their excluder properties. The concentration in Cd (41) had to be reduced to 20/2 mg kg⁻¹ for the production by excluder cvs. of fodder/food grains. Cultivars excluding both HM and radionuclide analogues such as BaCsSr synchronously were not identified. Whereas plant tissue concentrations in the metalloprotein-associated elements CdCoCuMnNiZn rise and fall generally with Nₒᵣg, grains of the wheat cvs. differed too little in Nₒᵣg to designate variations in their metal acquisition rates solely as protein-regulated. Wheat grains confined nevertheless the concentrations in Cu to 11–14 mg kg⁻¹ although the respective soil concentrations varied by factor 19. Grain deposition in CaFeMn(Zn) and in nuclides followed the same rules. CONCLUSIONS: It is hypothesized that cereals down-/up-regulate grain:soil transfer rates from soils with excessive/deficient trace metal resources to equip viable seeds with an optimum but not maximum in essential minerals. Positive correlations between metal concentrations in planta to those in soil can thereby be lost.