Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/27/2003
Publication Date: 11/1/2003
Citation: Simmons, R.W., Pongsakul, P., Chaney, R.L., Saiyasitpanich, D., Klinphoklap, S., Nobuntou, W. 2003. The comparative exclusion of zinc and iron from rice grain in relation to rice grain cadmium: implications for human health. Plant and Soil Journal. 257(1):163-170. Interpretive Summary: Excessive crop Cd comprises a health risk to subsistence rice consumers, but not other crops. Study of animal absorption of Cd has indicated that the low Fe, Zn, and Ca levels in rice grain cause higher intestinal absorption of Cd, and rice has been found to induce Fe and Zn deficiency in subsistence consumers. The present study was conducted to test whether rice exclusion of Zn from grain might also contribute to the greater potential of rice to cause human Cd disease. Samples of soil, rice grain and leaf, and soybean grain were collected from farm fields in an area contaminated by use of irrigation water rich in Cd and Zn from upstream Zn mine and ore deposits. The fields were somewhat to highly contaminated with Cd and Zn which came from the same geogenic source. Rice grain Cd but not Zn was strongly increased in mature rice grown on the fields, while both Cd and Zn was increased in the soybean. These results illustrate a second component of why subsistence rice production on Zn-mine contaminated soils causes human Cd disease, but not other crops: Exclusion of Zn and Fe from grain by physiological and soil-chemical processes in flooded soils causes grain Cd to have higher bioavailability than Cd in other crops grown on such soils.
Technical Abstract: Rice grain samples collected from within an isolated and geo-physically unique Zinc/Cadmium (Zn/Cd) co-contaminated area in Thailand suggests that the rice plant effectively controls grain Zn and Iron (Fe) uptake even though concurrent aqua regia digested soil Zn and Fe concentrations ranged from 254-8040 mg Zn kg-1 and 4610-24,950 mg Fe kg-1, respectively. However, the rice plant is unable to control the accumulation of Cd to grain resulting in very high Cd:Zn and Cd:Fe ratios. Further, in contrast to Zn and Fe, rice grain Cd concentrations were significantly correlated (R2 =0.543) with aqua regia digested soil Cd concentrations. In comparison, soybean Zn concentrations of samples collected within the study area were positively correlated with aqua regia digested soil Zn. However, soybean effectively controls seed Cd and Fe accumulation. These results indicate that in the Zn/Cd co-contaminated paddy soils investigated, the rice plant effectively controls grain Zn and Fe uptake to levels indicative of human dietary Fe and Zn deficiencies without a reciprocal control on Cd.