Abstract Following nuclear events, risk assessment and protection of plant communities in contaminated ecosystems require in-depth understanding of radiosensitivity of plants. However, the physiological and molecular factors defining differential sensitivity to chronic ionising radiation exposure are poorly understood. In this study, we compared early molecular events associated with protection, repair, and stress responses as well as phenotypic and cellular effects in gamma-irradiated seedlings of the radiosensitive conifer Norway spruce ( Picea abies ) and the radiotolerant herbaceous Arabidopsis thaliana . After 48-h of irradiation, Norway spruce showed reduced growth at 290 mGy h -1 and severe organelle damage at ≥ 1 mGy h -1 whereas A. thaliana showed unaffected development, minor organelle damage at ≥ 100 mGy h -1 only and significantly less DNA damage at all dose rates. Comparative transcriptomics revealed that Norway spruce mobilized transcription of DNA damage repair and antioxidant genes at ≥ 40 mGy h -1 only while A. thaliana showed massive activation of genes related to DNA damage repair, antioxidants, and other stress responses as well as growth-promoting hormones and cell wall components at ≥ 1 mGy h -1 . Adverse effects on chloroplasts and mitochondria from low dose rates on and comprehensive downregulation of photosynthetic genes and activation of respiration genes at ≥ 40 mGy h -1 in Norway spruce but not in A. thaliana may reflect the higher energy demand in Norway spruce to simultaneously maintain its far larger genome and engage protection and repair systems. Hence, the absence of transcriptional response at lower gamma doses and activation of repair and protection at high dose rates only, when accumulated damage is high, is consistent with the high radiosensitivity of Norway spruce. Conversely, the more massive transcriptional activation of crucial repair and protection pathways even at low dose rates complies with the high radiotolerance of A. thaliana .

High radiosensitivity in Norway spruce (<i>Picea abies</i>) is rendered by less comprehensive mobilisation of protection, repair and stress responses compared to the radiotolerant<i>A. thaliana</i>

High radiosensitivity in Norway spruce (Picea abies) is rendered by less comprehensive mobilisation of protection, repair and stress responses compared to the radiotolerantA. thaliana