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内容記述 |
1 Introduction To mitigate radioactive cesium (RCs) transfer from soil to plant, it is essential to understand the dynamics of cesium in plant rhizosphere. White lupin (Lupinus albus L.) is known to form cluster root particularly under phosphorous (P) deficient condition, which releases explosive amount of rhizodeposition (e.g. citric acid, malic acid, etc.) and contributes to nutrient acquisition [1]. Moreover, white lupin is known to have a high capacity of RCs uptake especially under potassium (K) deficient conditions. One possible reason is that rhizodeposition from white lupin may contribute to the mobilization of RCs adsorbed to soil. Here, a pot experiment and a rhizobox experiment were conducted to investigate the possibility of rhizodeposition from white lupin to mobilize cesium (Cs) in soil. 2. Pot experiment White lupin ‘Kievskij mutant’ was grown in 4 L pot with K deficient soil (7.1 mg of K2O kg-1 soil) from Fukushima prefecture in a greenhouse. Five treatments of K fertilizer application rates (0, 5, 15, 30, 60 mg of K2O kg-1 soil) were established (n=3). After 42 days of cultivation, shoots and roots of the plants were harvested. Rhizosphere soil was collected by shaking roots in the air to separate the attached soil, while bulk soil was collected from the soil left inside the pot. Dry weight and contents of K and 137Cs of the plants were measured. The soil was extracted with 1.0 M of ammonium acetate solution to measure exchangeable K and 137Cs. Although the dry weight did not significantly differ among the K fertilizer application rates, the 137Cs content in shoot significantly decreased with increasing application rates of K fertilizer. The concentration of the soil exchangeable 137Cs significantly decreased with increasing application rates of K fertilizer. In addition, the concentration of the soil exchangeable 137Cs was higher in planted treatments compared with unplanted treatments, suggesting that white lupine solubilized 137Cs adsorbed to the soil in the rhizosphere. 3. Rhizobox experiment Eleven-day-old white lupin ‘Energy’ was grown in a rhizobox, which consisted of a square nylon 48-µm mesh bag and a pair of soil boxes. Each soil box was 160-mm high × 110-mm wide × 14.5-mm deep and filled with K deficient soil from Fukushima prefecture. The test plants were placed in a growth chamber (n=3). After 20 days of cultivation, 11CO2 (with a half-life of approximately 20 minutes) was administered, and the translocation of 11C-labeled photosynthates, assimilated in the leaves, to the roots was visualized for 80 minutes using Positron Emitting Tracer Imaging System (PETIS). Immediately after imaging, roots and soil were separated, and the distribution of 11C in the soil alone was imaged for 5 minutes to obtain a two-dimensional visualization of 11C labelled carbon metabolites secreted from the roots into the soil. Subsequently, the rhizobox was divided into 2 × 2 cm sections, and the soil was collected to measure the concentrations of exchangeable elements. In all rhizoboxes, white lupine formed cluster roots. PETIS imaging of 11C distribution in the soil revealed that, in each rhizobox, certain cluster roots formed localized “hotspots” of carbon metabolite secretion into the surrounding soil, which was considered the rhizosphere soil. These results indicate that even under K deficient conditions, white lupine forms cluster roots and actively secretes carbon metabolites into the soil. In one of the rhizoboxes, a positive correlation was observed between 11C intensity in the soil and concentration of the soil exchangeable Cs, which implies that carbon metabolites secreted from cluster roots of white lupin may have mobilized Cs previously adsorbed to soil particles and increased its availability. 4. Conclusion Based on the results of cultivation experiments under K deficient conditions, white lupine was found to form cluster roots even under K deficiency, and some of these cluster roots actively released carbon metabolites. This suggests that the release of rhizodeposition from cluster roots may have contributed to the elevated exchangeable 137Cs concentrations in the rhizosphere soil of white lupine. On the other hand, the hotspots identified through PETIS imaging represent the instantaneous distribution of carbon metabolites secreted from the roots. Considering that the dynamics of 137Cs in the rhizosphere are influenced cumulatively over time by root activity, further investigation is needed to understand the temporal changes in Cs behavior in the rhizosphere soil. |
