137Cs Distribution in Mulberry(Morus alba) after the Fukushima Dai-ichi Nuclear Power Plant Accident and Effect of Spray Application of a Liquid Potassium Fertilizer onto Trunk Surface

After the Fukushima Dai‐ichi Nuclear Power Plant accident in March 2011, contamination of mulberry-leaf tea with over 100Bq kg－1 137Cs was found. We therefore investigated 137Cs distribution in mulberry trees(Morus alba) and soil in Nihonmatsu City from 2012. As a result, mulberry leaves near the end of branches(0 - 30cm), which are usually used as edible leaves, were shown to contain higher 137Cs concentration than the others. Trunk showed highest 137Cs concentration among the plant parts of mulberry investigated, while root 137Cs concentration was relatively low. Autoradiographic studies for leaf, bark and cross-sectional trunk suggest that radioactive substances were deposited onto bark and leaf radioactive contamination could be due to translocation of 137Cs from trunk. Spray application of 0.5% KH2PO4 solution onto trunk surface significantly reduced 137Cs concentration in edible leaves emerged about one month after the application. However, the effect was limited and insufficient to solve the problem of radioactive contamination in edible mulberry leaves.