Dose-responses of germination and seedling growth of plants namely Amaranthus blitum, Cajanus cajan, Eleusine coracana, Guizotia abyssinica, Lathyrus sativus and Lycopersicon esculentum obtained following γ-irradiation of seeds at a wide range of doses established G. abyssinica (LD50 = 0.32; D50 = 0.17 kGy) as highly radio-sensitive species. Seed-irradiation caused oxidative stress to the emerging seedlings that was evident by the dose-dependent increase in the generation of reactive oxygen species (ROS: H2O2, O2 •−, •OH), which in turn also showed good correlation with the dose-dependent induction of cell death, lipid peroxidation, protein oxidation, protein and non-protein thiols, and antioxidant enzymes activities, in the leaf tissue of G. abyssinica seedlings. The above biochemical endpoints could, therefore, serve as biomarkers of the previous exposure of seeds to ionizing radiation. Furthermore, the highest ineffective threshold (HITD = 0.019 kGy) and the lowest effective threshold (LETD = 0. 021 kGy) doses of ionizing radiation were determined on the basis of induction of cells with chromosome aberration (CA) or micronucleus (MN) in root meristems of the germinating irradiated seeds. The present G. abyssinica genotoxicity assay thus validated the non-linear threshold (NLT) dose-response to ionizing radiation as opposed to the prevailing linear no-threshold (LNT) dose-response concept.