For estimations of the ingestion dose of tritium the dynamic behaviour of organically bound tritium (OBT) is studied in the framework of safety considerations for the nuclear fusion technology. In diet relevant plants, uch as wheat, the formation of OBT and the subsequent translocation into the seeds, till the time of harvest have been investigated in chamber experiments. Sets of field data on photosynthesis, transpiration and stomatal resistances at individual plants during several vegetation periods have been collected by gas exchange measurements. These data were used to test the recently developed model ‘Plant-OBT’. The paper analyses the results of comparisons between calculated and observed tritium concentrations in wheat plants after short-term exposures to atmospheric tritiated water (HTO). While the final OBT concentrations in the grains can be Simulated sufficiently, the modelling of the OBT formation and turnover processes in the leaves seems unsatisfactory so far. The unsolved problems in the recent OBT modelling approach are discussed.