Effect of application time of radionuclides on their root uptake by Chinese cabbage and radish

For investigating the effect of radionuclide deposition time on root uptake of Mn-54, Co-60, Sr-85 and (CS)-C-137 by Chinese cabbage and radish, a mixed solution of the radionuclides was applied to the soil surfaces in culture boxes placed in a greenhouse at 4 different times during plant growth. Soil-to-plant transfer factor (TF) was determined on the basis of radioactivity in unit area of soil. In Chinese cabbage and, roughly speaking, in radish, TF of Sr-85 decreased with increasing time between sowing and the radioactivity application. TFs of the other radionuclides were the highest at the third or last application. Concentrations of the radionuclides in soil were examined after harvests. They decreased exponentially with increasing soil depth and most radioactivity remained near the surface. TFs of the radionuclides applied 2 d before sowing were higher than those aged for I year by factors of 3-10 depending on radionuclide and plant species. (C) 1998 Elsevier Science Ltd. All rights reserved.