Abstract
Simple SummaryThe pale grass blue butterfly has been used to monitor the biological impacts of the Fukushima nuclear accident because this butterfly is sensitive to low-dose radioactive pollution in the field. However, the butterfly has been shown to be highly tolerant to radioactive cesium (137Cs), the major radionuclide in Fukushima, in an artificial diet in laboratory experiments. This field-laboratory paradox may be explained by the field-effect hypothesis; the host plant may change its nutrient contents in response to radiation stress. Leaves from Tohoku (mostly polluted areas including Fukushima) showed significantly lower sodium contents than those from Niigata. In Tohoku samples, an increase in the radioactivity concentration of cesium (137Cs) in leaves or in the ground radiation dose was accompanied by a decrease in the sodium content. The sodium content appeared to be related to other nutrient factors. Thus, the sodium imbalance of the plant may be caused by radiation stress, and this imbalance may be one of the reasons that this monophagous butterfly showed high mortality and morphological abnormalities in the field shortly after the accident in Fukushima.The pale grass blue butterfly Zizeeria maha is sensitive to low-dose radioactive pollution from the Fukushima nuclear accident in the field but is also highly tolerant to radioactive cesium (137Cs) in an artificial diet in laboratory experiments. To resolve this field-laboratory paradox, we hypothesize that the butterfly shows vulnerability in the field through biochemical changes in the larval host plant, the creeping wood sorrel Oxalis corniculata, in response to radiation stress. To test this field-effect hypothesis, we examined nutrient contents in the host plant leaves from Tohoku (mostly polluted areas including Fukushima), Niigata, and Kyushu, Japan. Leaves from Tohoku showed significantly lower sodium and lipid contents than those from Niigata. In the Tohoku samples, the sodium content (but not the lipid content) was significantly negatively correlated with the radioactivity concentration of cesium (137Cs) in leaves and with the ground radiation dose. The sodium content was also correlated with other nutrient factors. These results suggest that the sodium imbalance of the plant may be caused by radiation stress and that this nutrient imbalance may be one of the reasons that this monophagous butterfly showed high mortality and morphological abnormalities in the field shortly after the accident in Fukushima.
Highlights
Upon the Fukushima nuclear accident in March 2011, a massive amount of radioactive materials was released from the Fukushima Dai-ichi Nuclear Power Plant to the surrounding environment in the Tohoku district, Japan [1,2,3,4,5,6,7,8,9,10]
Comparison of the coefficient of variation (CV) among the nutrient factors indicated that the sodium content was by far the most variable factor when all samples from the three groups were used (Figure 7a)
Considering that sodium and potassium were positively correlated and that sodium responded to the radioactive factors in the present study, it is possible that the Oxalis plant may lower the sodium content to increase antioxidants and to cope with the high background noise of reactive oxygen species (ROS) when exposed to ionizing radiation
Summary
Upon the Fukushima nuclear accident in March 2011, a massive amount of radioactive materials was released from the Fukushima Dai-ichi Nuclear Power Plant to the surrounding environment in the Tohoku district, Japan [1,2,3,4,5,6,7,8,9,10]. One of the important fieldbased experiments was the so-called internal exposure experiment, in which larvae in Okinawa without previous exposure to radionuclides from the Fukushima nuclear accident were fed contaminated leaves from Fukushima, resulting in high mortality and abnormality rates [38,60] This experiment was repeated in additional studies using the same species [61,62,63], and a similar but more rigorous experiment was conducted using the cabbage white butterfly [45], confirming the results of the previous internal exposure experiments. Our “laboratory-based” data have revealed that the pale grass blue butterfly was highly tolerant to radioactive cesium (137 Cs), the major radioactive contaminant in leaves from Fukushima, when fed an artificial diet [72] This result is reminiscent of the case of gall-forming aphids [39,40].
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