Cesium uptake and translocation from tea cutting roots (Camellia sinensis L.)

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Highlights

  • 137Cs transfer factor by tea cutting roots in hydroponics were determined.

  • 69% of 137Cs remained in roots and 31% was transported from roots to shoots.

  • 137Cs was preferentially translocated to new shoots rather than mature leaves.

Abstract

To estimate the uptake of radiocesium (137Cs) by tea plant roots, 1-year-old rooted tea cuttings (Camellia sinensis L. cv. Yabukita) at the time of bud opening were cultivated hydroponically for 27 days in pots containing nutrient solutions with or without 137CsCl (600 Bq mL−1). Total 137Cs radioactivity of whole tea plants were 6.1 kBq g−1 dry weight. The plant/solution 137Cs transfer factors of different tissues were in the range of 2.6 (in mature leaves) to 28.2 mL g−1 dry weight (in roots), which were lower than those reported in wheat and spinach. In total, 69% of 137Cs remained in roots and 31% was transported from roots to shoots. The results indicated that 137Cs was preferentially translocated to new shoots, which are used for manufacturing tea, over mature leaves.

Introduction

Radioactive cesium (i.e., 134Cs and 137Cs) released by Tokyo Electric Power Company Holdings's (TEPCO's) Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident caused by the Great East Japan Earthquake and tsunami of March 11, 2011, has been detected at levels that exceed the provisional permitted value in tea leaves around the east area in Japan. In a report by the Ministry of Health, Labour and Welfare (2011), the first crops of tea harvested in Ibaraki, Chiba, Gunma, Kanagawa and Shizuoka Prefectures after the accident were contaminated with radiocesium at a higher level than the provisional regulation value for raw food materials set by the Food Safety Commission of Japan, that is 500 Bq kg−1 (sum of 134Cs and 137Cs activities). Therefore, as the shipment of tea leaves in these areas was stopped and workers suffered significant economic losses. During the 2 years following the TEPCO's FDNPP accident, Hirono and Nonaka (2016) reported a exponential decrease in the concentration of 134Cs and 137Cs in new shoots of tea plants in Shizuoka, Japan, approximately 400 km southwest from the FDNPP. In a previous study, we showed that radiocesium was primarily absorbed from the lower surface of tea leaves through the stomata, and that the greater part was transported to newly emerged tea tissues, especially new shoots, during the new shoot growth period (Ikka et al., 2018). New shoots harvested in the first flush of 2011 were indirectly contaminated by FDNPP's radiocesium fallout. However, because tea plants are a perennial crop, there is concern about the long-term effects of indirect contamination from the soil or source organs other than the new shoots that are used to make tea products (Tagami et al, 2012, 2020). To reduce radiocesium amounts in tea plants, pruning was most effective measure (Hirono and Nonaka, 2016).

In the FDNPP accident, the soil of tea fields was contaminated by radionuclides (Takeda et al., 2013). In particular, long-lived 137Cs in the soil will be a source of radiocesium contamination in tea plants for the long term through root uptake. In general, Cs is not an essential element for plant growth and therefore its absorption mechanism and use by plants has not been studied in depth. As Cs is an alkali metal like K, the two have similar chemical properties and K transport systems also function in Cs uptake by roots (Avery et al, 1991, 1993; Zhu and Smolders, 2000). Previous studies showed that 137Cs activities in the primary and secondary roots of tea plants after the Chernobyl Nuclear Power Plant accident in 1986 greatly decreased in 1987, but then gradually increased up to 1993 (Topcuoğlu et al., 1997). Ertel and Ziegler (1991) reported that 20% of the 137Cs translocated into new leaves of larch (Larix decidua Mill.) and about 50% of that into sycamore maple (Acer pseudoplatanus L.) resulted from root uptake during the 2.5 years after the Chernobyl accident. These reports suggest that tea plants might take up 137Cs from contaminated soils over the long term. However, the rate of 137Cs uptake by tea roots has not previously been estimated.

The 137Cs uptake activities of plants is estimated as the soil-to-plant transfer factor (TF) (Ehlke and Kirchner, 2002). However, in the literature, for a number of long-lived radionuclides, reported soil-to-plant TFs showed variations that exceeded three orders of magnitude (Coughtrey and Thorne, 1983; Frissel, 1992). The soil-to-plant TF is a macroscopic parameter that integrates various soil chemical, soil biological, hydrological, physical, and plant physiological processes, each of which shows its own variability. Therefore, in several plant species, TFs have been estimated in hydroponic culture by determining the solution-to-plant TFs, enabling more precise measurement of the 137Cs uptake activities of plant roots (Smolders et al., 1997; Smolders and Shaw, 1995).

In this study, we examined radiocesium uptake into tea roots by applying 137Cs into hydroponic solution supplied to cultured tea plants, and determined the TFs of 137Cs in several plant parts during the development of new shoots.

Section snippets

Plant materials and hydroponic experiments

One-year-old rooted tea cuttings (Camellia sinensis L. cv. Yabukita) were carefully washed with tap water to remove soil and then transplanted to a/5000 Wagner pots supplied with continuously aerated hydroponic solution (volume 3 L) in a greenhouse in Shizuoka University (Shizuoka, Shizuoka, Japan). The hydroponic solution was prepared according to Konishi et al. (1985), adjusted to pH 4.2 with 1 M H2SO4 and renewed every week until the start of treatment.

In 2014, we conducted the following

Results and discussion

Each of five parts and total dry weights (DW) at the end of treatment are shown in Table 1. The DW of new shoots was 0.9 g plant−1; two or three new shoots per plant emerged during 27 d of cultivation in the draft chamber with radiocesium application, meaning that it is possible to evaluate the translocation of the radiocesium from tea roots to new shoots.

137Cs radioactivity and TFs from tea roots are shown in Table 1. Total 137Cs radioactivity of whole tea plants was 6.1 kBq g−1 DW. Comparing

Conclusion

In conclusion, we investigated the uptake of 137Cs radioactivity and its transfer from tea roots to other plant parts in model hydroponic conditions. The plant/solution 137Cs TFs among different tissues of tea plants were in the range 2.6 (in mature leaves) to 28.2 mL g−1 (in roots), which were lower than values previously reported in wheat (Smolders and Shaw, 1995) and spinach (Smolders et al., 1997). In total, 69% of 137Cs remained in the roots, and 31% was transported from the roots to

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This study was supported by radioactivity projects Development of Radioactive Materials Removal and Reduction Technology for Forests and Farmland in 2012 and Development of Decontamination Technologies for Radioactive Substances in Agricultural Land in 2013–2014. We thank Huw Tyson, PhD, from Edanz Group (www.edanzediting.com/ac) for editing a draft of this manuscript.

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