Translocation of Radiocesium Released by the Fukushima Daiichi Nuclear Power Plant Accident in Japanese Chestnut and Chestnut Weevil Larvae
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This article is an Advance Online Publication of the authors’ corrected proof. Note that minor changes may be made before final version publication. The Horticulture Journal Preview e Japanese Society for doi: 10.2503/hortj.MI-132 JSHS Horticultural Science http://www.jshs.jp/ Translocation of Radiocesium Released by the Fukushima Daiichi Nuclear Power Plant Accident in Japanese Chestnut and Chestnut Weevil Larvae Yoshito Sasaki*, Yasuo Ishii, Hironobu Abe, Katsuaki Mitachi, Takayoshi Watanabe and Tadafumi Niizato Fukushima Environmental Safety Center, Japan Atomic Energy Agency, Fukushima 963-7700, Japan To examine the translocation of radiocesium scattered by the Fukushima Daiichi nuclear power plant accident that occurred in March 2011 to the Japanese chestnut, we investigated the autoradiography and radiocesium concentration in each part of Japanese chestnuts. The Japanese chestnut fruit has a thin skin between the kernel (cotyledons) and shell; the kernel of the fruit is edible. The 137Cs concentration in each part of the fruit was found to be almost the same at about 1.0 × 104 Bq·kg−1 DW, as well as leaves. The radiocesium concentration in chestnut weevil larvae found on the fruit was approximately one-seventh of that in the kernel. Key Words: cesium, chestnut, fruit, Fukushima Daiichi, insects. trunks. The chestnut fruit is a forest product eaten in Introduction Europe, China, and Japan (Korel and Balaban, 2009). A large amount of radionuclides was released into After the CNPP accident, it was reported that chestnut the environment by the accident at the Fukushima fruits grown in Italy had a relatively high radiocesium Daiichi nuclear power plant (FDNPP) of the Tokyo transfer rate (Monte et al., 1990). Radiocesium released Electric Power Company, as a result of the Great East in the FDNPP accident detected from Japanese chest- Japan Earthquake on March 11, 2011 (Chino et al., nuts (Castanea crenata Sieb. et Zucc.) (Ministry of 2011). Radionuclides with short half-lives, such as 131I Agriculture, Forestry and Fisheries, http://www. (8.04 days half-life) decayed in a few months, whereas maff.go.jp/j/kanbo/joho/saigai/s_chosa/H25gaiyo.html, many of those with relatively long half-lives, such as January 4, 2016). Kusaba et al. (2015) also reported that 134Cs (2.06 years half-life) and 137Cs (30.2 years half- the radiocesium concentration of chestnut fruit has de- life), are still present in the environment. Potassium and creased year by year. We examined radiocesium con- cesium are congeners; cesium behavior is similar to that centrations in each part of the fruit of the Japanese of potassium, and these elements are easily incorpo- chestnut and the ratio of radiocesium concentrations in rated into plants. If a plant containing radiocesium is in- soil and chestnut fruit [i.e., the transfer factor (TF)]. gested by humans, then there is a possibility of internal Radiocesium transfer from chestnut fruit to insects exposure. Mushrooms, wild vegetables, and fruits, as parasitic to the fruit was also investigated in the study. well as wood, are important forest products. Elucidation It was reported that there was a decrease in the survival of radiocesium translocation to these products could re- rate and high malformation rate in aphids (Akimoto, duce exposure. At the time of the Chernobyl nuclear 2014) and pale grass blue butterfly (Hiyama et al., power plant (CNPP) accident (April 26, 1986), because 2012) in radiocesium-contaminated areas due to the the deciduous trees were budding, radioactive materials FDNPP accident. The web spider in the class of arach- were caught in the tree canopy. At the time of the nida (Ayabe et al., 2014) and aquatic insects FDNPP accident, because the deciduous trees were in a (Yoshimura and Akama, 2014) also incorporated radio- dormant stage and their leaves were falling, released cesium. However, little is known about the transfer of radioactive material was caught in the branches and radiocesium from food (primary producers) to insects (consumer) in the food web. The chestnut weevil [sci- entific name: Curculio sikkimensis Heller (Coleoptera: Received; November 11, 2015. Accepted; May 17, 2016. First Published Online in J-STAGE on August 11, 2016. Curculionidae)] is a parasitic insect found on the chest- * Corresponding author (E-mail: [email protected]). nut fruit. Chestnut weevils lay eggs in the fruit of the © 2016 The Japanese Society for Horticultural Science (JSHS), All rights reserved. 2 Y. Sasaki, Y. Ishii, H. Abe, K. Mitachi, T. Watanabe and T. Niizato chestnut, and the hatched larvae feed on the chestnut crispula Blume), and Japanese Chestnut (Castanea fruit. After the mature larvae exit from the fruit, they crenata Siebold et Zucc), as well as partially distributed burrow into the soil and grow in the soil until they be- Japanese red pine (Pinus densiflora Sieb. et Zucc.) in come adult worms (Imamura et al., 2004). The chestnut the ridges and forest edge. The average tree height and weevil larvae are used as fishing bait and a commodity tree density of the canopy layer are 11.6 m and 9.2 trees (Umeya, 2004). Since the larvae that crawl out of the per 100 m2, respectively. About 5% of the forest floor is chestnut fruit have eaten only the fruit prior to that covered by Dwarf bamboo (Sasa nipponica Makino et point, it is easy to evaluate the transition rate of radio- Shibata). The thickness of the litter layer is roughly cesium to the larvae from the chestnut fruit. We also ex- 1 cm, and the soil is classified as a brown forest soil amined the radiocesium concentration in the chestnut (IUSS Working Group WRB, 2015). The samples of the weevil larvae. Japanese chestnut tree (bark and wood), leaves and fruits were collected from the same tree. The sample Materials and Methods tree had a height of 12.3 m, a diameter at breast height Sampling of 21.8 cm, and a canopy projection area of 11.5 m2. Samples were collected in Yamakiya Kawamata Chestnut fruits were separated from the ripe spiny cu- Date-gun in Fukushima Prefecture (Fig. 1). The study pule and dropped to the ground in October 2013. Fallen area is located in a mountain forest on a crest slope leaves were collected immediately after defoliation in (Tamura and Takeuchi, 1980) of a valley-head area that November 2013. Chestnut tree samples were obtained faces west, and does not face the FDNPP. The study from branches of the canopy at a height of approxi- area is a mixed forest mainly composed of Konara oak mately 7 m in February 2014 (Sasaki et al., 2016). (Quercus serrata Murray), Mizunara oak (Quercus Fruits were dried to a constant weight at 90°C, and Fig. 2. Picture of chestnut weevil larvae (Curculio sikkimensis). A shows chestnut weevil larvae exiting the chestnut fruit. B shows the chestnut weevil larvae. Fig. 1. Air dose rates (μSv·h−1) at 1 m above the ground surface on Fig. 3. Distribution of radioactivity in the fruits and leaves of April 29, 2011 (http://ramap.jmc.or.jp/map/eng/map.html) and Japanese chestnuts. Optical photograph (A, C, and E) and auto- location of the sampling site in the Yamakiya area, Kawamata radiograph image (B, D, and F). A and B are the outer shell of town, Fukushima Prefecture. The symbol ▲ indicates the sam- the Japanese chestnut. C and D are the inner skin and kernel. E pling site. The GPS coordinates of the field are 37°35'17.7"N, and F are the leaves. The image toward the left in A and B is 140°42'08.3"E. FDNPP indicates the Fukushima Daiichi the inside of the shell and the one toward the right is the outside nuclear power plant. of the shell. Hort. J. Preview 3 the outer shell, inner skin, and kernel (cotyledons) were laser scanner (Typhoon FLA7000; GE Healthcare, separated. Each part of the fruit was placed in an 80 mL United Kingdom). capacity polystyrene container (V7 container, sanoya- sangyo, Japan) and subjected to radiocesium concentra- Transfer factor (TF) from the soil to chestnut tion measurements. Chestnut litter leaves were dried to Transition of radiocesium to plants was evaluated a constant weight at 90°C. The dried samples were using TF. TF was expressed by the ratio of the radio- placed in V7 containers to measure radiocesium con- cesium concentration in the fresh weight of the plant centrations. The soil surface at 5 cm depth was col- to the radiocesium concentration in the dry weight of lected in one location using a shallow soil sampler soil at a depth of 20 cm from the surface (Bq·kg−1) (sampling tube ϕ110 × 50 mm) at the base on the down- (International Atomic Energy Agency, 2010). Cp repre- stream side of the chestnut tree. Collected soil was sented the radiocesium concentration of the plant −1 dried to a constant weight at 105°C. After the soil was [Bq·kg FWplant], and Cn represented the radiocesium −1 evenly mixed, it was placed in a 100 mL polystyrene concentration of the soil [Bq·kg DWsoil-20 cm depth]. container (U8 container, Umano Kagaku Youki, Japan) (2) and subjected to radiocesium concentration measure- Cp = TF × C n ment. Collected chestnuts were placed at room tempera- Radiocesium concentration in soil at a depth of 20 cm ture for about a month. Chestnut weevil larvae were was one-quarter of the average radiocesium concentra- collected after they crawled out of the chestnut fruit tion in soil at a depth of 5 cm. Radiocesium concentra- (Fig. 2). Collected larvae were dried to a constant tion in the fresh weight of chestnut was calculated using weight at 90°C and then placed on a 0.45 μm pore size the moisture content of the chestnut fruit.