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In surrounding areas influenced by the Fukushima Dai-ichi Nuclear Power Plant accident, the radiocesium uptake by crops was evaluated based on soil exchangeable K. As risk evaluation does not function effectively in several fields, K buffering was characterized using quantity/intensity (Q/I) relationship analysis to evaluate the availability of exchangeable K. The results of the analysis clearly explain the difficulty in the risk evaluation: i) difficulty in increasing exchangeable K, owing to retention as non-exchangeable K by vermiculite; ii) difficulty in retaining exchangeable K due to low K buffering capacity, reflecting the lack of K-selective clay minerals; and iii) difficulty in increasing soil solution K intensity due to high K buffering capacity, reflecting the abundance of K-selective clay minerals. Among the parameters calculated from the Q/I relationship analysis, β values, corresponding to the retention rate of loaded K to the soil, for total K (βt), exchangeable K (βe), and non-exchangeable K (βn) would be useful as simple indicators of soils with a predicament in the risk evaluation of radiocesium uptake by crops through exchangeable K. These soils showed distinctively low βt values (0.18 or lower), high βe value (0.37 or higher), or high βn/βt ratio (0.53 or higher). [ABSTRACT FROM AUTHOR]

Root exudates are derived from plant metabolites and the composition is affected by plant nutrient status. Acid phosphatase and organic acids are well-known root exudates in soils with low phosphorus (P) status. We applied metabolite profiling technique to investigate rice root exudates under P deficiency. Oryza sativa (cv. Nipponbare) was grown in solution culture at P concentrations between 0 to 258 µM for several weeks. Root exudates and root extracts were collected and their metabolites were determined by capillary electrophoresis / time-of-flight mass spectrometry (CE-TOF MS). Shoot P concentration and dry weight of rice plants grown at low P levels were lower than those at high P levels. Twenty-nine anionic and 51 cationic compounds and 15 anionic and 32 cationic compounds were identified in root extracts and in root exudates, respectively. More than 80% of metabolites in the root exudates showed higher concentration at low P levels than at high P levels. On the other hand, only 20% of metabolites in the root extracts showed higher concentration at low P levels than at high P levels. These results suggest that rice roots exude many metabolites in response to P deficiency.

The presence of low concentration of inosine in solution in soil has increased the root growth as shown in our previous work. Thus the aim of this work was to clear the mechanism of the inosine-induced enhancement of root growth with emphasis on plant hormone and metabolites profile. In the first experiment the rice plant (Oryza sativa L. cv Nipponbare) was precultivated in nutrient solution for 3 weeks. Then seedlings with the same size were transferred to pots with the following treatment solutions (0 N + 0 inosine, 0 N + 20 mg L-1 inosine, 4 mg L-1 N + 0 inosine and 4 mg L-1 N + 20 mg L-1 inosine). Two weeks after the treatment started, xylem sap was collected for determining cytokinin and auxin concentration by ELISA, then fresh and dry weight of shoot and root, and root length were measured. In the second experiment, rice seedling were prepared as Experiment 1 then samples were collected after inosine treatment (0, 1, 2, 24, 72, and 96 hours) for the analysis of metabolites profiling by using GC-MS. Dry weight (shoot and root), and root length and number of lateral root were measured. As a result, inosine application increased root length and concentrations of trans-zeatin riboside and indole-3-acetic acid content in xylem sap significantly. The allantoin was the most up regulated compound in the metabolite profile. Thus, it is suggested that inosine application increased concentration of plant hormones and allantoin, possibly resulting in growth enhancement of plant root.

This study was to elucidate the factors contributing to organic nitrogen (N) acquisition in different plant species, pak-choi (Brassica rapa L. var. chinensis) and tomato (Solanum lycopersicum L.). Two hypotheses were proposed and examined: 1) higher protease activity in rhizosphere contributes to the ability to utilize organic N in soils; 2) the differences in ability to absorb amino acids are responsible for the different characteristics of organic N utilization. Seedlings were grown in soils mixed with vermiculite with different N sources (no N (-N), ammonium sulfate (AS), and cattle farmyard manure (CM)). Plant growth and N accumulation were the best in the AS treatment in both species. When comparing the growth and N accumulation between -N and CM treatments, those of pak-choi were significantly enhanced in the CM treatment, but no difference was found in tomato. In the rhizosphere soils, the highest protease activities were detected in the CM treatment irrespective of plant species. The correlation analysis between rhizospheric protease activity and plant total N accumulation in the -N and CM treatments showed that a significant positive correlation only in pak-choi. The determination of amino acid absorption rate in excised roots indicated that glycine was taken up at significantly higher rate in pak-choi than in tomato. Taken together, two possible factors for the acquisition of organic N was suggested from this study: protease activity in rhizosphere and ability to absorb amino acids.

To evaluate the effectiveness of potassium (K) application in mitigating137Cs transfer from soil to plants, several vegetable species were cultivated under field and pot experiments. In the field experiment, squash, sweet potato, turnip, potato, and carrot were examined in 2020 and 2021 in two different areas of Hamadori (coastal region in Fukushima Prefecture). Transfer factor (TF) was calculated by dividing harvest radioactivity (Bq kg−1 dry or fresh) to soil radioactivity (Bq kg−1 dry) and was negatively correlated with the amount of exchangeable K (ExK) at harvest, regardless of the species, year, and location. In the pot experiment, edamame (immature soybean seed), spinach, turnip, and komatsuna were cultivated, and it was confirmed that ExK was the most powerful factor in regulating TF. Based on the relationship between ExK and TF for each vegetable species, the amount of ExK required to keep the 137Cs concentration lower than a certain level (standard limitation value and one-quarter of that value) was calculated. [ABSTRACT FROM AUTHOR]