Experiments on the biokinetics of tritium in plants

The uptake of tritium from the atmosphere has been studied experimentally in wheat plants under field conditions. During the growth phase of the grains the plants have been exposed to tritiated water vapour for 1 h in a box at different times of the day. The diffusion processes leading to uptake into tissue water and clearance after exposure as well as a part of the metabolic processes leading to synthesis of organically bound tritium are dependent on light. The total amount of generated organically bound tritium did not change significantly until harvest, but the tritiated assimilates were translocated from the photosynthesizing organs to the grains. The tritium concentration in organic matter of harvested grains was about 3 times lower after an exposure at night-time compared to an exposure at day-time because of reduced uptake into tissue water. The light dependency of the photosynthetic processes did not influence the final concentrations in grains because of the longer residence time of tritium in tissue water during darkness. (orig.)