Plant and Soil Factors That Influence Bioavailability of Heavy Metals in Crops
Crop Systems & Global Change
Project Number: 8042-42000-015-00
Start Date: Feb 23, 2011
End Date: Feb 22, 2016
Objective 1: Characterize the influence of zinc and iron concentrations in edible crop tissues and crop species on the bioavailability of crop cadmium to animals (C1; PS 1.F).
Objective 2: Characterize the potential transfer of soil lead, arsenic, and other trace elements by vegetable crops grown on element enriched urban and orchard soils and develop methods to prevent this transfer. (C1; PS 1.F).
Objective 3: Characterize genetic resources and inheritance of grain Cd to reduce cadmium in durum wheat, flax and soybean. (C1; PS 1.F).
Objective 4: Evaluate information about the risk from Cd in foods to support public decisions for foods of both plant and animal origin. (C1; PS 1.F).
The ultimate goals of this Project Plan are to improve the science about risk of heavy metals in soils and crops in order to obtain improved regulatory limits for Cd in crops under Codex, and the information needed to provide improved advice about the risk of Pb in urban garden soils and crops. Essentially all human Cd disease from soil Cd has resulted from paddy rice grown on fields contaminated by mining or smelter emissions. Garden vegetables and other grains have not been found to induce Cd disease in highly exposed populations who grow crops on highly Cd+Zn contaminated soils. Some Europeans want to set crop Cd limits based on “attainable” levels rather than on the basis of potential risk to consumers. Such non-risk based standards will harm U.S. growers of durum wheat, sunflower kernels, flax, soybean, and some other crops. If crops must contain lower levels of Cd to win importation by other nations, both soil treatments, selection of soil series which produce lower Cd crops, and improved cultivars which accumulate lower amounts of Cd will contribute to growers needs. Providing a clear technical basis for the bioavailability of Cd in different crops appears to be the central issue which could change the demand for lower limits for crop Cd, and protect growers from unnecessary costs to produce lower Cd crops which may have no benefit. Only animal feeding tests can provide valid information about the bioavailability of crop Cd to animals, and the concentrations fed must represent levels in foods rather than toxic levels fed in most previous research. Because Zn is usually greatly increased in crops (except rice) when Cd is increased, the presence of Zn may substantially reduce the bioavailability of crop Cd, alleviating presumed risk of crop Cd. Because commercial carrots were found with higher than normal Pb concentrations when grown on historic orchard soils, U.S.-FDA requested that ARS examine the basis for carrot Pb accumulation and to learn if agricultural amendments can reduce carrot Pb when they are grown on high Pb soils. Certain root crops have xylem elements growing through the edible storage root, so if Pb is trapped within the xylem during normal growth of the crop, it will be in the edible root. But such Pb might have much lower bioavailability than the Pb-acetate used to establish diet Pb risk; 2-10% of food Pb is absorbed, while 60-80% of soluble Pb in water is absorbed by human volunteers.
Characterize the influence of zinc and iron concentrations in edible crop tissues and crop species on the bioavailability of crop cadmium to animals. Zinc incorporated in lettuce reduces weanling rat absorption of lettuce Cd. Using controlled chelator-buffered nutrient solutions (similar to Kukier and Chaney, 2002), Romaine lettuce will be grown to contain basal and sub-phytotoxic concentrations of foliar Zn (25 and 450 mg kg-1 DW), and the Codex Cd limit (4 mg Cd kg-1 DW); the high Zn represents maximum Zn levels normally present if lettuce is grown in acidic soils with geogenic Cd+Zn contamination at the beginning of yield reduction due to Zn phytotoxicity. Fe concentration in lettuce is tightly controlled genetically, but lettuce can supply higher bioavailable Fe than the marginal AIN diet, and thus plant Fe can reduce Cd bioavailability. The lettuce will be fed to weanling rats following the protocol of Reeves and Chaney (2004) in which American Institute of Nutrition (AIN) purified diets with marginal or adequate Fe-Zn-Ca were fed. The basal and high Zn lettuce will be mixed with both the marginal and adequate diets and fed for 28 days without radioisotope labeling used previously. At autopsy, the liver, kidney and duodenum will be removed for analysis. Tissues and blood will be tested to establish Fe status of the test animals. The tissues will be digested in HNO3, and Cd, Zn and Fe levels measured by ICP-Atomic Emission Spectrometry or ICP-Mass Spectrometry. Eight replicate rats will be fed each diet to accommodate the natural variation in such feeding tests (Reeves and Chaney, 2001). If the lettuce experiment shows a strong effect of crop Zn on crop Cd bioavailability, other crops may be studied using a similar approach (durum wheat; soybean).