Developmental, morphological and physiological effects of chronic low doses of ionising radiation on plants on earth and in space

Conclusions in the current literature are essentially unanimous regarding what is known about the effects of ionising radiation on plants. It is agreed that acute high-dose effects (primarily from laboratory tests) are well-documented and understood but that the same cannot be said for lower doses. ‘Low-dose’ research is itself contentious until there is sufficient understanding to define what constitutes a ‘low’ dose. Studies using lower doses typically (but with some notable exceptions) feature dose-effect relationships at doses to plants orders of magnitude lower than high-dose studies (i.e. they examine doses measured in μGy-mGy as opposed to kGy-Gy). A compilation of data reported here emphasises not only the lack of studies that utilise low and environmentally-realistic doses of ionising radiation (μGy-mGy) but also the particular lack of such experiments that used a controlled environment. Additionally, the compilation revealed a lack of transgenerational studies. In research reported here five generations of Arabidopsis thaliana were grown in soil contaminated with Cs-137 at low field-relevant dose rates (35 μGy/h). The developmental stages defined by a long-established phenotypic model (Boyes et al., 2001) were charted over entire life cycles. A detailed analysis of leaf morphology in generations of radioactively-exposed plants was undertaken with semi-autonomous image analysis software (LAMINA- Leaf shApe deterMINAtion). The same leaves were analysed for antioxidant changes via an assay for glutathione. No transgenerational trends were identified in any of the endpoints examined. Significant changes were detected in development and morphology in some treatments in some generations and are in line with stochastic effects expected to occur over generations in the conditions. No significant differences were found in glutathione concentrations. No significant differences in root length were found between treatments or between generations. Lastly, seeds from two species of crop plants Eruca sativa and Solanum lycopersicum (rocket and tomato) that had flown aboard the International Space Station, and that had received a cosmic radiation dose in low-Earth orbit of similar magnitude to the Arabidopsis, were grown in Cs-137 contaminated soil. Previous exposure to cosmic radiation as seeds did not change uptake characteristics of either species or total growth or root length. In conclusion, while some effects have been observed, nothing was found to suggest that doses of ionising radiation at ‘low’ doses has a significant, negative impact on generations and populations of plants throughout these experiments. This contrasts with many reports from the field at similar dose rates and provides evidence that: a) current radioprotection limits are satisfactory at protecting plant populations, b) many effects on plants of low-dose radiation reported from the field are due to either past exposures to populations or other environmental factors and c) what constitutes a low dose of ionising radiation to plants could be redefined