Some indica rice varieties are potential phytoextractors for paddy fields polluted with Cd because of their high biomass and because they can accumulate Cd to moderate levels in their shoots. To establish a practical phytoextraction system, phytoextraction using two indica rice cultivars (MORETSU and IR-8) was carried out in a paddy field polluted with moderate Cd levels (2.91 and 2.52 mg kg -1 , respectively). The Cd concentration and Cd uptake of MORETSU increased when irrigation water was drained at the maximum tillering stage, and the paddy soil was under oxidative conditions until harvesting. The Cd uptake of MORETSU and IR-8 increased and reached 516 and 657 g ha -1 , respectively, at the beginning of October. After phytoextraction using these high Cd accumulating rice varieties for 2 years, the Cd concentration in the paddy field decreased by 18% compared with the initial Cd concentration. The Cd concentration in the rice grains of a japonica ordinary rice variety (HINOHIKARI) subsequently grown on the field after the phytoextraction was lower than the concentration in rice grown on a nonphytoextracted field. These results suggest that phytoextraction using high Cd accumulating rice varieties with early drainage of irrigation water is a practical remediation system for moderate Cd polluted paddy fields in southwest Japan.
Soybeans (Glycine max (L) Merr.) are the major summer crop grown in Japanese upland fields (characterized by aerobic soil) that have been converted from paddies. To evaluate the effect of phytoextraction by rice on the seed cadmium (Cd) content of soybeans grown subsequently, we grew Milyang 23, a high-Cd-accumulating rice cultivar, and then grew soybeans in three paddy soils contaminated with moderate Cd concentrations (2.50-4.27 mg Cd kg(-1)). The rice accumulated 7-14% of the total soil Cd in its shoots. The soybean seed Cd contents were 24-46% less than those grown on control soils. Phytoextraction by Milyang 23 rice is thus a promising remediation method for reducing seed Cd contents of soybeans grown on paddy soils under aerobic soil conditions.
To develop a new method for the extraction of plant‐available cadmium (Cd), the correlation between the Cd content of polluted soil extracted with several extractants and the Cd content of wheat plants (variety “A”; Triticum aestivum L.) was examined. Among the HCI concentrations of soil extractants tested, the content of 0.025 mol L−1 HCI (HCl0.025)‐extractable Cd of soil showed a significant correlation with the Cd content of wheat grain and Cd uptake by shoot. The correlation between the soil Cd content extracted with 1 mol L−1 NH4Cl and the Cd content of wheat grain was nearly the same as that in the case of HCI0.025. In contrast, other reagents such as MgCl2, diethylenetriaminepentaacetate (DTPA) and tetra‐sodium pyrophosphate (Na4P2O7) could not give a good correlation between the extractable Cd content of soil and the Cd content of wheat grain. Therefore, it was considered that HCl0.025‐extractable Cd is a suitable indicator of the content of plant‐available Cd of soil to wheat. This extraction method can be applied to weakly acid Grey Lowland soil. Using the equation for the relation between the content of HCl0.025‐extractable Cd of soil and the Cd content of grain of the wheat variety “A”, the Cd content of grain of other 11 wheat varieties was evaluated. The wheat variety “A” showed the lowest absorption of Cd. The Cd content of variety “B” grain was located on the regression line of that of variety “A”, suggesting that both varieties exhibited the lowest ability to absorb Cd among the varieties tested. Grain yields of both “A” and “B” varieties were reasonably high. On the other hand, other varieties showed a substantially high ability to absorb soil Cd compared with the varieties “A” and “B”. The new method proposed for the determination of the content of plantavailable Cd proved to be suitable for the evaluation of plant‐available Cd of soil and also for the screening of wheat varieties with a high or low Cd accumulation capacity in grain.
Phytoextraction using indica rice plants (Indian Rice Oryza ssp.) is a promising technique for remediating cadmium (Cd)-polluted paddy fields. Because this technique has only been established for paddy fields, we decided to examine phytoextraction in upland fields that have been converted from paddy fields. Although "CHOKOUKOKU" shows a shattering habit and lodging, its Cd uptake was significantly higher than that of other indica rice varieties. On the other hand, "IR8", which was able to accumulate only moderate levels of Cd in its shoots, has a lodging tolerance, making it an optimal variety for southwest Japan, which experiences several typhoons each year. Therefore, both "CHOKOUKOKU" and "IR8" were useful in estimating practical phytoextraction in upland fields. A practical phytoextraction examination showed that the total Cd uptake of "CHOKOUKOKU" and "IR8" was 822 and 545 g ha , respectively, after a 4-year period. After phytoextraction by planting high Cd-accumulating rice plants, the Cd concentration of the plot soil decreased by approximately 35%, compared to the initial Cd concentration. To evaluate phytoextraction efficiency in the upland field, wheat (Triticum aestivum) was subsequently grown in the remediation field. The Cd concentration in the grains of "SHIROGANEKOMUGI" grown in the phytoextraction plot was lower than that grown in a non-phytoextraction plot; regrettably, it exceeded the Codex Alimentarius Commission standard, whereas the grain Cd concentrations of "CHUGOKU165" planted in phytoextraction plots complied with the Codex standard for wheat grain. These results suggest that phytoextraction using high Cdaccumulating rice varieties is a practical remediation system for low Cd-polluted upland fields. Moreover, we determined the end point of the phytoextraction process, which occurs when the soil Cd concentration of the phytoextraction fields is less than 0.6 mg kg −1 if "CHUGOKU165" is cultivated in this area.
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