Retention, loss and translocation of radionuclides applied to foliar surfaces of wheat

An experiment to determine the fates of five fission and activation products (⁸⁵Sr, ¹⁰⁶Ru, ¹³⁷Cs, ⁵⁴Mn and ⁶⁰Co) when applied to the foliar surfaces of wheat plants is reported. The retention, loss and translocation of these radionuclides were determined following contamination events at three intervals over a typical summer cropping season in southern Britain. Retention and loss of each radionuclide were strongly time-dependent and could be described by a simple exponential model. Biological half-lives for each radionuclide are presented. A slightly more complex model was applied to the case of the translocation of ¹³⁷Cs, ⁵⁴Mn and ⁶⁰Co. This model relied on an exponentially declining input of each nuclide from foliage to grain; rate constants of radionuclide loss in this model were found to agree with those derived using the simple exponential loss model above. ⁸⁵Sr and ¹⁰⁶Ru were not found to be translocated from foliage to grain. Field loss of radioactivity was most strongly correlated with the mean daily decline in crop relative growth rate following contamination. No obvious correlation existed between the rate of field loss and the cumulative rainfall during the experiment. Maximum translocation of radioactivity to grain was found to coincide with maximum relative growth rate. It is suggested that little actual loss of radioactivity occurs from grain once translocation has taken place. Plant growth rate is identified as a key parameter which should be integrated into models predicting the dynamics of radionuclide transfer in agricultural and other ecosystems. (author)