Phytoextraction by high-Cd-accumulating rice to reduce Cd in wheat grains grown in Cd-polluted fields

Abstract

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−1, 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 Cd-accumulating 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.