Reduction of Radiocesium in Internal- and Surface-Contaminated Komatsuna (Brassica rapa var. perviridis) during Washing and Processing.

Radiocesium dynamics data during food processing are required for the realistic estimation of internal radiation content in food. Radiocesium contamination of leafy vegetables can occur externally due to the adhesion of fallout and/or resuspension from the air, and internally from soil via the root transport. Information regarding the dynamics of both surface and internal radiocesium contamination during food processing is required; however, such information for leafy vegetables is limited compared to other major agricultural products. In this study, the effect of washing on the removal of surface radiocesium contamination by resuspended materials and that of cooking (grilling, boiling, and microwave heating) on internal radiocesium contamination were investigated using komatsuna (Brassica rapa var. perviridis), a leafy vegetable. The surface-contaminated samples were experimentally grown in a difficult-to-return area in Fukushima Prefecture, which has not yet been decontaminated. The internally contaminated komatsuna were obtained after experimental cultivation in a greenhouse with soil containing 137Cs and no surface contamination. The concentration of 137Cs in surface-contaminated komatsuna was reduced to approximately half (processing factor: 0.55) after washing with water. However, the annual processing factor ranged from 0.12 to 0.95, suggesting that the growing environment and climatic conditions may affect the removal rate of radiocesium by washing. Internal contamination of 137Cs was removed by 23% and 14% by boiling and grilling, respectively, but no effect was observed for microwaving. Moreover, the concentration of 137Cs decreased by 0.66-fold after boiling, while it increased by 1.19- and 1.20-fold after grilling and microwaving, respectively. Therefore, boiling was found to be preferable than grilling or microwaving for radiocesium removal.

The ratio of plant 137Cs to exchangeable 137Cs in soil is a crucial factor in explaining the variation in 137Cs transferability from soil to plant.

To mitigate radioactive cesium from soil to plant, increasing and maintaining the exchangeable potassium (ExK) level during growth is widely accepted after Tokyo Electric Company's Fukushima Dai-ichi Nuclear Plant accident in Japan. This is because the antagonistic relationship between soil solution K and 134Cs + 137Cs (RCs) concentrations changes the transfer factor (TF: designated as the ratio of radioactivity of plant organ to soil) of RCs. As the relationship between ExK and TF depends on the soil types, crop species, and other environmental factors, the required amount of ExK should be set to a safe side. Eleven years after the accident, as the activity of 134Cs was almost negligible, 137Cs became the main RCs in most of the agricultural fields in Fukushima Prefecture. We propose a new indicator, the concentration ratio of plant 137Cs to soil exchangeable 137Cs (Ex137Cs), instead of TF, which showed a better correlation with ExK even among soils with different properties (or mineralogy).

Effectiveness of non-exchangeable potassium quantified by mild tetraphenyl­boron extraction in estimating radiocesium transfer to soybean in Fukushima.

Non-exchangeable K released from soil minerals can reduce radiocesium transfer to plants, as well as exchangeable K. We investigated the effect of non-exchangeable K on radiocesium transfer to soybean, and the non-exchangeable K extraction method most suitable for estimating the transfer risk. In Fukushima Prefecture, Japan, 106 soils were collected from 89 soybean fields during 2014-2018 to analyze non-exchangeable K contents using three methods: boiling nitric acid extraction, tetraphenyl­boron extraction, and mild tetraphenyl­boron extraction. The non-exchangeable K contents quantified by the former two methods were dependent on the amount of micas, which are K-bearing minerals. The non-exchangeable K content by mild tetraphenyl­boron extraction depended on the amount of K fertilizer application and K-fixing minerals but not on micas, indicating that it reflects fertilizer K fixed by the minerals. The soil-to-plant transfer factor of radiocesium was most correlated with the non-exchangeable K content by the mild extraction (rs = -0.67). This correlation was also stronger than that between exchangeable K and the transfer factor (rs = -0.40). As non-exchangeable K content increased, the exchangeable radiocesium fraction decreased, indicating that radiocesium was fixed together with K. Additionally, multiple regression analysis indicated that non-exchangeable K by the mild extraction significantly decreased the transfer factor even if the exchangeable radiocesium fraction was kept constant. Thus, the fixed K was considered to repress radiocesium transfer to soybean through both radiocesium fixation and K supply. With the criterion of total extracted K, the sum of exchangeable and non-exchangeable K, as 65 mg K2O 100 g-1 by the mild extraction, fields with high and low transfer factors were able to be differentiated more effectively than with a current criterion of exchangeable K as 50 mg K2O 100 g-1. The results revealed that mild tetraphenyl­boron extraction is effective for estimating radiocesium transfer to soybean.

Variations in radioactive cesium accumulation in wheat germplasm from fields affected by the 2011 Fukushima nuclear power plant accident.

Decreasing the transfer of radioactive cesium (RCs) from soil to crops has been important since the deposition of RCs in agricultural soil owing to the Fukushima nuclear power plant accident of 2011. We investigated the genotypic variation in RCs accumulation in 234 and 198 hexaploid wheat (Triticum spp.) varieties in an affected field in 2012 and 2013, respectively. The effects of soil exchangeable potassium (ExK) content to RCs accumulation in wheat varieties were also evaluated. A test field showed fourfold differences in soil ExK contents based on location, and the wheat varieties grown in areas with lower soil ExK contents tended to have higher grain RCs concentrations. RCs concentrations of shoots, when corrected by the soil ExK content, were positively significantly correlated between years, and RCs concentrations of shoots were significantly correlated with the grain RCs concentration corrected by the soil ExK content. These results indicated that there were genotypic variations in RCs accumulation. The grain to shoot ratio of RCs also showed significant genotypic variation. Wheat varieties with low RCs accumulations were identified. They could contribute to the research and breeding of low RCs accumulating wheat and to agricultural production in the area affected by RCs deposition.

Influence of the nonexchangeable potassium of mica on radiocesium uptake by paddy rice.

A pot cultivation experiment was conducted to elucidate the influence of the nonexchangeable potassium (K) of mica on radiocesium ((137)Cs) uptake by paddy rice (Oryza sativa L. cv. Koshihikari), and to evaluate the potential of mica application as a countermeasure to reduce radiocesium transfer from soil to paddy rice. The increase in the exchangeable K concentrations of soils, measured before planting, due to mica (muscovite, biotite, and phlogopite) application was negligible. However, in trioctahedral mica (biotite and phlogopite)-treated soil, the release of nonexchangeable K from the mica interlayer maintained the soil-solution K at a higher level during the growing season in comparison to the control, and consequently decreased the (137)Cs transfer factor for brown rice (TF). The sodium tetraphenylboron (TPB)-extractable K concentration of the soils, measured before planting, was strongly negatively correlated with the TF, whereas the exchangeable K concentration of the soils, also measured before planting, was not correlated with the TF. Therefore, we conclude that TPB-extractable K is more reliable than exchangeable K as a basis of fertilizer recommendations for radiocesium-contaminated paddy fields. Phlogopite-treated soils exhibited higher TPB-extractable K concentrations and lower TF values than biotite-treated soils. We thus conclude that phlogopite application is an effective countermeasure to reduce radiocesium uptake in paddy rice.

Variations for Fusarium head blight resistance associated with genomic diversity in different sources of the resistant wheat cultivar 'Sumai 3'.

Fusarium head blight (FHB), caused by Fusarium graminearum, is a serious disease of wheat (Triticum aestivum L.) associated with contamination by the mycotoxin deoxynivalenol (DON). The FHB-resistant wheat cultivar 'Sumai 3' has been used extensively around the world. The existence of variation in FHB resistance among 'Sumai 3' accessions has been discussed. In this study, genetic variation among 'Sumai 3' accessions collected from six countries were identified using SSR markers; our results demonstrate unique chromosome regions in Sumai 3-AUT and Sumai 3-JPN ('Sumai 3' accessions from Austria and Japan, respectively). Field evaluation indicated strong resistance to FHB in Sumai 3-AUT. The polymorphic rate (number of polymorphic markers/number of available markers × 100) based on a DArT array was 12.5% between the two 'Sumai 3' accessions. Genotyping for DNA markers flanking FHB-resistant quantitative trait loci (QTLs) revealed genetic variations for the QTL regions on 5AS and 2DS; however, no variation was observed for the QTL regions on 3BS and 6B. Thus, the variation in FHB resistance among 'Sumai 3' accessions in the field is due to genetic diversity.

Distribution of photoperiod-insensitive allele Ppd-A1a and its effect on heading time in Japanese wheat cultivars.

The Ppd-A1 genotype of 240 Japanese wheat cultivars and 40 foreign cultivars was determined using a PCR-based method. Among Japanese cultivars, only 12 cultivars, all of which were Hokkaido winter wheat, carried the Ppd-A1a allele, while this allele was not found in Hokkaido spring wheat cultivars or Tohoku-Kyushu cultivars. Cultivars with a photoperiod-insensitive allele headed 6.9-9.8 days earlier in Kanto and 2.5 days earlier in Hokkaido than photoperiod-sensitive cultivars. The lower effect of photoperiod-insensitive alleles observed in Hokkaido could be due to the longer day-length at the spike formation stage compared with that in Kanto. Pedigree analysis showed that 'Purple Straw' and 'Tohoku 118' were donors of Ppd-A1a and Ppd-D1a in Hokkaido wheat cultivars, respectively. Wheat cultivars recently developed in Hokkaido carry photoperiod-insensitive alleles at a high frequency. For efficient utilization of Ppd-1 alleles in the Hokkaido wheat-breeding program, the effect of Ppd-1 on growth pattern and grain yield should be investigated. Ppd-A1a may be useful as a unique gene source for fine tuning the heading time in the Tohoku-Kyushu region since the effect of Ppd-A1a on photoperiod insensitivity appears to differ from the effect of Ppd-B1a and Ppd-D1a.

Distribution of photoperiod-insensitive alleles Ppd-B1a and Ppd-D1a and their effect on heading time in Japanese wheat cultivars.

The genotypes of photoperiod response genes Ppd-B1 and Ppd-D1 in Japanese wheat cultivars were determined by a PCR-based method, and heading times were compared among genotypes. Most of the Japanese wheat cultivars, except those from the Hokkaido region, carried the photoperiod-insensitive allele Ppd-D1a, and heading was accelerated 10.3 days compared with the Ppd-D1b genotype. Early cultivars with Ppd-D1a may have been selected to avoid damage from preharvest rain. In the Hokkaido region, Ppd-D1a frequency was lower and heading date was late regardless of Ppd-D1 genotype, suggesting another genetic mechanism for late heading in Hokkaido cultivars. In this study, only 11 cultivars proved to carry Ppd-B1a, and all of them carried another photoperiod-insensitive allele, Ppd-D1a. The Ppd-B1a/Ppd-D1a genotype headed 6.7 days earlier than the Ppd-B1b/Ppd-D1a genotype, indicating a significant effect of Ppd-B1a in the genetic background with Ppd-D1a. Early-maturity breeding in Japan is believed to be accelerated by the introduction of the Ppd-B1a allele into medium-heading cultivars carrying Ppd-D1a. Pedigree analysis showed that Ppd-B1a in three extra-early commercial cultivars was inherited from 'Shiroboro 21' by early-heading Chugoku lines bred at the Chugoku Agriculture Experimental Station.