Radiocesium Accumulation in Koshiabura (Eleutherococcus sciadophylloides) and Other Wild Vegetables in Fukushima Prefecture
Wild vegetables naturally grow in the mountains, and their new buds and leaves are routinely eaten by local residents. In Fukushima, wild vegetables are more contaminated than agricultural products because most forests have not been decontaminated and radiocesium still remains in the forest soil. Radiocesium concentrations in wild vegetables can vary depending on the species, and in the case of koshiabura (Eleutherococcus sciadophylloides), it was found to have the highest concentration among wild vegetables. To acquire basic knowledge about radiocesium accumulation in koshiabura, we collected young trees which had been grown in the forest of Date City, Fukushima and investigated the radiocesium concentration in each part and its seasonal transition.
KeywordsKoshiabura Radiocesium Wild vegetable
8.1 Monitoring and Examination of Agricultural Products
A large amount of radioactive substances, especially radiocesium (134Cs and 137Cs), were spread throughout the environment by the Fukushima Daiichi Nuclear Power Plant Accident, and forests, croplands, and residential areas were contaminated. Agriculture is an important industry in Fukushima Prefecture, and it produces various agricultural products in its warm climate and rich natural environment. Since the accident, several measures in the production process such as deep plowing, stripping, and applying potassium fertilizer have been performed to prevent agricultural products from absorbing radiocesium.
8.2 Wild Vegetables and Local People
Wild vegetables naturally grow in the mountains of Japan, and it is the new buds and leaves of these plants that are commonly eaten. These plants have been an important part of peoples’ lives in the mountain villages, not just providing nutrition but also allowing the local people to experience more intimately the changing of the seasons. These edible plants are sometimes preserved by drying or salting. Collecting wild vegetables is a great pleasure for local people. The locations of rare wild vegetables are so precious that they are sometimes kept a secret from other family members. Wild vegetables have been closely related to Japanese people’s lives for a long time, and have no doubt contributed to enriching Japanese culture. In addition, they have supported the local traditions in communities by creating a unique food culture in each region as well as being a food source for the local people. In Fukushima Prefecture, 70% of the land area is forested, and the Fukushima citizens have a strong connection with the forests. Unfortunately, most forests near Fukushima Daiichi Nuclear Power Plant were contaminated by the nuclear accident, and high concentrations of radiocesium have been found in wild vegetables in many regions.
8.3 Reasons for High Radiocesium Concentration in Wild Vegetables
The forests where wild vegetables grow are too expansive to decontaminate – cleaning up contaminated fallen leaves and other debris on the ground was performed only within 20 m from residential areas. Radiocesium still remains in forest soil, and wild vegetables growing there are thought to absorb radiocesium from the soil. In addition, the application of potassium fertilizer which is used as a countermeasure to prevent crops from absorbing radiocesium has not been performed in forests, as wild vegetables are not cultivated but grow naturally. Also, because the ratio of organic matter is high and clay mineral is low in the forest soil, the sorption of radiocesium to the soil is mild and thus transfer of radiocesium to wild vegetables will be higher than in cultivated soil. Finally, because wild vegetables are perennial plants or trees, it is considered likely that these plants absorbed and accumulated radiocesium which fell directly on their leaves and barks at the time of the accident in 2011.
8.4 Radiocesium Concentration of Each Category of Wild Vegetables
Koshiabura is a deciduous tree belonging to the Araliaceae family and found in every part of Japan from Hokkaido to Kyushu region. Although it is a popular wild vegetable with tasty and nutritious new buds, its sale continuous to be regulated in many regions of Fukushima Prefecture 6 years after the accident. Therefore, to acquire the basic knowledge about radiocesium accumulation in koshiabura, we collected young trees which had been grown in the forest of Date City, Fukushima and investigated the radiocesium concentration in each part and its seasonal transition.
8.5 The Seasonal Transition of Radiocesium Concentration in Koshiabura
Factors such as the symbiosis with mycorrhizal fungi of koshiabura have been implicated as the reason for its high radiocesium accumulation potential (Sugiura et al. 2015), further research is needed considering that the concentration was the same for another species (i.e., Q. serrata) in the current investigation in late September. On the other hand, newly grown leaves in early May showed the highest radiocesium concentration. The monitoring investigation was performed on the edible parts of each wild plant when its taste was optimum. Therefore, one reason why radiocesium concentration in koshiabura was so high was because the monitoring investigation coincided with the stage of newly grown leaves in early spring. Though further research is necessary to clarify why high radiocesium concentration occurs in newly grown leaves in early spring, one of the possible factors is that the behavior of radiocesium is similar to that of potassium. In the investigation in Date City, the concentration of potassium in leaves showed a similar seasonal transition as potassium. Several studies reported that Japanese cedar (Cryptomeria japonica) (Tubouchi et al. 1996) and butterbur (Kiyono and Akama 2015) also showed a high concentration of potassium in leaves in spring and the gradual decrease after spring. Potassium is an essential element for growth and photosynthesis, and it moves into growing cells quickly at the time of foliation. Cesium is also a group-1 alkali metal and its movement is considered to be similar to that of potassium and thus it is easily accumulated in new leaves. Although high concentrations of manganese have been reported to accumulate in the leaves of koshiabura (Mizuno 2008), the relationship between cesium and manganese is still unclear and the potential effect of manganese accumulation on cesium accumulation needs to be further investigated.
Seven years have passed since the accident, and agriculture is gradually resuming in the regions where the evacuation order has been cancelled. To help the agricultural industry recover while ensuring the production of safe agricultural products, we hope to continue the monitoring investigations as outlined in this chapter.
Wild vegetables are an irreplaceable pleasure for people residing in mountain villages. However, consuming wild vegetables is not completely without risk because high concentrations of radiocesium in spring corresponds to the season of collecting wild vegetables. Although we have performed some countermeasures such as the application of potassium fertilizer and the removal of the soil surface, it is assumed that it will take several years to find out whether these measures will suppress the absorption of radiocesium in koshiabura. The movement of radiocesium in wild vegetables is still unclear, and further research and continuous monitoring are necessary.
The authors would like to thank Nobuhito Ohte (Kyoto University) and Riona Kobayashi (The University of Tokyo) for their technical advice, and to Junnichiro Tada (Radiation Safety Forum) and Takashi Kurosawa (Date City) for koshiabura experiment at Date City.
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