There is a need for a better understanding of biological effects of radiation exposure in non-human biota. Correct description of these effects requires a more detailed model of dosimetry, particularly for plants. In this paper, we propose a simple model for dose calculations in roots and shoots of Arabidopsis thaliana seedlings exposed to radionuclides in a hydroponic exposure setup. This model is used to compare absorbed doses for three radionuclides, Am-241, Sr-90, Ba-133. Using established dosimetric calculation methods, dose conversion coefficient values were determined for each organ separately based on uptake data from the different plant organs. These calculations were then compared to the DCCvalues obtained with the ERICAtool under equivalent geometry assumptions. When comparing with our new method, the ERICAtool appears to overestimate internal doses and underestimate external doses in the roots for all three radionuclides, though each to a different extent. These observations might help to refine dose-response relationships, The DCCvalues for Sr-90 in roots are shown to deviate the most. A dose-effect curve for Sr-90 beta-radiation has been established on biomass and photosynthesis endpoints, but no significant dose-dependent effects are observed. This indicates the need for use of endpoints at the molecular and physiological scale.