The biosphere is constantly exposed to ionising radiation, due to cosmic radiation and the presenceof natural and anthropogenic radionuclides in the environment. The biological effects of the resultingradiation exposure are currently poorly understood in plants, due to a large influence of parameters suchas species, cultivar, experimental setup and plant age on the response. In this study, 7-, 10- and 14-day-old Arabidopsis thaliana seedlings were exposed to 96 h and 168 h of 100 mGy h−1chronic _-radiation ina hydroponic setup. The response of growth, photosynthesis and the transcription of genes involved inDNA repair, cell cycle and signalling were measured. The results show that there is a difference in growthresponse between plants of different ages and decreasing radiosensitivity with increasing seedling age.This is linked to differences in regulation of DNA repair and cell cycle control at the transcriptional level.