The transfer of radiocesium released in the 2011 Fukushima Daiichi Nuclear Power Station accident to petioles of wild butterbur (Petasites japonicus)

Following the Tokyo Electric Power Company's Fukushima Daiichi Nuclear Power Station accident in March 2011, shipping restrictions were imposed on more than 10 Japanese edible wild-plant species (July 31, 2017) in which high levels of radiocesium (sup(134+137)Cs) were detected. However, few studies have examined radiocesium transfer from the environment to edible wild plants and the factors that affect this transfer. We sampled the petioles of fuki, (wild butterbur: Petasites japonicus), an edible deciduous perennial, from 100 habitats in four municipalities in Fukushima Prefecture, Japan, in June 2016. The amounts of sup(137)Cs in litter and soil to a depth of 5 cm were each significantly associated with fuki petiole sup(137)Cs concentrations (P < 0.001) and with air dose rates (ADRs, P < 0.01). Regression analysis between ADRs and fuki petiole sup(137)Cs concentrations was performed and verified using a petiole dataset that was obtained in April 2016 from 28 habitats with a wide range of ADR values. The slopes of the regression equations for June and April were not significantly different (P = 0.494). However, their Y-intercepts were significantly different (P = 0.0002) and the June sup(137)Cs concentration was approximately five times that for April at the same ADR, suggesting an increase in sup(137)Cs concentrations between April and June. We determined the 95% prediction interval of sup(134+137)Cs concentration per fresh petiole weight for June 2016, based on petiole water content and sup(134)Cs/sup(137)Cs concentration ratio. The prediction interval of fuki petiole sup(134+137)Cs concentrations were distributed between about 1/8 and 8 times the predicted values. This dramatic seasonal variation in sup(137)Cs concentrations suggests that further examination of temporal shifts in fuki sup(137)Cs concentrations is required.