Abstract Comparison of experimental data obtained from short (SDEF) and long duration exposure flights (LDEF) recently led to results, which will contribute for the estimation of genetic risk for long and/or repeated stay of man in space. Under orbital conditions biological stress and damage are induced in test subjects by cosmic radiation, especially the high energetic, densely ionizing component of heavy ions. Plant seeds were successful model systems for a biotest in studying the physiological damages and mutagenic effects caused by ionizing radiation in particular stem cells. In this article we present an overview of our space experiments with Arabidopis thaliana seeds. We present first results of investigations on certain damage endpoints (seed germination, plant survival, mutation frequencies), caused by cosmic ionizing radiation in dry dormant plant seeds of Arabidopsis thaliana after different short term (e.g. IML-1 and D-2) and long term (e.g. EURECA and LDEF-1) space exposures. Total dose effects of heavy ions and the other components of the mixed radiation field on damage endpoints and survival after space exposure and gamma-ray preirradiation were obtained. A new method of total dose spectrometry by neutron activation has been applied.