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ARS Home » Northeast Area » Beltsville, Maryland (BARC) » Beltsville Agricultural Research Center » Adaptive Cropping Systems Laboratory » Research » Research Project #430993

Research Project: Environmental and Plant Factors That Influence Trace Element Bioavailability in Food Crops

Location: Adaptive Cropping Systems Laboratory

2017 Annual Report

1a. Objectives (from AD-416):
Objective 1: Characterize effects of flood and soil management on As (inorganic and total) and Cd accumulation in rice grain. 1A: Does alternate wetting and drying (AWD) irrigation management reliably lower inorganic As (iAs) in rice grain without causing excessive grain Cd in rice genotypes? 1B: Develop extraction and analysis methods to rapidly measure the inorganic As (iAs) in rice grain samples. Objective 2: Characterize competition of other cations with Cd accumulation by rice and vegetables. 2A: Clarify whether increased Zn2+ activity reduces Cd2+ accumulation by spinach, lettuce and rice and whether plant Zn deficiency causes up-regulation of Zn transporters which increases uptake of Zn and Cd compared to Zn-sufficient crops. 2B: Clarify whether increased manganese (Mn2+) is more important than Zn2+ in inhibiting uptake of Cd2+ by rice and vegetables from nutrient solutions with environmentally relevant activity of Cd2+. Objective 3: Characterize effects of soil amendments on Pb, Cd and As accumulation by garden crops to improve advice to urban gardeners regarding risk reduction for contaminated urban soils. 3A: Characterize the effects of soil amendments and mulch on Pb and Cd accumulation by garden crops from contaminated urban garden soils. 3B: Characterize the effects of soil amendments on Pb and Cd accumulation by crops grown in contaminated urban gardens. Objective 4: Continue evaluation of the effects of crop species and crop Zn on bioavailability of crop Cd to animals. Test whether crop Zn concentration and crop species affect the bioavailability of crop Cd to monogastric animals in order to clarify if crop species and crop Zn should be included in technically valid limits for Cd in crops.

1b. Approach (from AD-416):
Because rice accumulates higher levels of inorganic As (iAs) than other crops, regulations are being imposed on iAs in rice grain which are lower than achieved by some growers. More aerobic soil management both lowers grain iAs and increases grain Cd. In order to identify methods to produce rice with lower levels of iAs, samples of rice grain from cooperators’ field tests of the effect of irrigation management, genetic variation, and soil amendment application will be analyzed for iAs and Cd (which may be increased by aerobic soil management to lower grain iAs). Previous methods for analysis of iAs in rice were so expensive that few samples could be evaluated to identify the needed genetic variation and irrigation management variation needed to lower grain iAs. An improved method for simple analysis of iAs using hydride generation from only iAs in samples was developed and will be validated by an international inter-laboratory project. Simpler and faster methods to extract iAs and Cd from rice flour will be evaluated so that market demanded analyses might be made available to the industry. Chelator-buffered nutrient solution methods will be developed to test the competition among elements for absorption of Cd by rice and spinach using levels of Cd, Mn and Zn chemical activity in the solution similar to soil solution. Recent studies indicated that Cd is absorbed on a Mn transporter in rice, but a Zn transporter in other species. Flooded soil production increases dissolved Mn but reduces dissolved Zn in rice soils until fields are drained; after drainage, much higher Cd absorption can occur. Cooperators field tests of irrigation management which limits both As and Cd in grain will be investigated. In other soils, crop Cd accumulation may exceed allowable limits in spinach and lettuce. High natural soil Cd and low Zn in spinach production fields in California allows excessive Cd accumulation in spinach and lettuce. Characterizing the direct competition among these trace elements in representative crop species will clarify needed management changes to limit Cd accumulation by both crop groups. Besides testing direct inhibition of Cd uptake by Zn and Mn in both species, the effect of Mn and Zn deficiency on up-regulation of the uptake rate of Cd will be evaluated. The potential utility of soil amendments on uptake of Pb, Cd and As by garden crops, and bioaccessibility of soil Pb to mammals will be evaluated both in the greenhouse, and when improved methods have been identified, will be tested in cooperating urban gardens with high soil Pb, Cd or As. Because food Cd limits presently do not consider variation in Cd bioavailability among crops, or the effect of crop Zn on Cd bioavailability, the effect of usual co-contaminating Zn on accumulation of diet Cd by weanling pigs will be tested with diets containing 20% Romaine lettuce or spinach. Bioavailability will be evaluated by analyzing duodenum, liver and kidney from test animals. These studies should provide evidence that the effect of Zn on Cd absorption, and effect of spinach oxalate on Cd absorption should be included in international regulation of Cd in crops.

3. Progress Report:
Objective 1A. Concentration of inorganic Arsenic (iAs) in rice grain was analyzed using methods developed in this lab to support the breeding and other research of ARS and university cooperators in Arkansas and California. Alternating wetting and drying (AWD) flood management was very effective in limiting inorganic As accumulation in grain compared with traditional continuous flood management, with a similar effect of alternate wetting and drying on inorganic arsenic accumulation by different cultivars and experimental genotypes of rice. Objective 1B. A rapid, inexpensive, and convenient method to analyze iAs in rice was developed and tested in an inter-laboratory evaluation project. The method used analyzed inorganic arsenic directly rather than conducting speciation of all arsenic chemicals in the grain. Used U.S. Food and Drug Administration method to extract arsenic from powdered rice grain, and then used relatively inexpensive analytical instrumentation for hydride generation to analyze for inorganic As. This method was then subjected to inter-laboratory evaluation with internationally qualified labs. The method was shown to be reliable in all but one lab where equipment problems limited success. Report on project submitted to sponsor, USA Rice. Manuscript is in preparation. Objective 2A. Various nutrient solutions were tested in order to develop a method that would be the same for growing both rice and spinach in order to evaluate competition between Cd and zinc in Cd uptake by rice and spinach. Because rice uses a different Fe uptake method than dicot plants such as spinach, this was a complex goal. Initial experiments were conducted to find the chemical activity of Cd and Zn which provided a way to test the whole spectrum of conditions, from deficient to adequate to phytotoxic for rice and spinach. A solution was developed to allow for future experiments. Objective 2B. Initial calibration experiments using chelator-buffering were conducted to find the chemical activity of Cd, Zn and Mn at deficient, adequate and phytotoxic levels for rice and spinach. This system will be used to explore the interaction between these 3 elements and the resultant impact on Cd uptake. It is believed that rice and spinach use very different methods in regulating Cd uptake. Progress was achieved in developing the solutions that will allow for testing of the interaction of these 3 metals. Objective 3A. ARS researcher, in collaboration with scientist from Johns Hopkins University, identified urban farmers and home gardeners within the city of Baltimore. A survey was conducted to determine the types of vegetables each urban farmer and home gardener anticipate planting. Over 500 soil samples were collected before crops were planted to determine lead, cadmium and zinc concentrations. Soil samples are being processed for analysis. Root and leafy vegetable crops will be collected during the growing period from each farm and garden for lead, cadmium and zinc determination. Objective 3B. Cadmium accumulation by spinach and lettuce from Cd-mineralized soils in Salinas Valley, CA, limits grower’s use of these soils for production of leafy vegetables. Our previous research indicated that if large amounts of zinc were added and the soils made calcareous, crop Cd could be reduced to meet Cd limits and protect growers and food processors from crop recalls. In addition, our previous research showed that the sorbent materials in biosolids composts (organic matter, Fe oxides) and Zn applied by such composts persistently limited Cd accumulation by leafy vegetables. We conducted a test of using Los Angeles biosolids compost, Zn, Mn, and limestone amendments to Lockwood loam to determine which combinations were most effective in limiting Cd accumulation by spinach and lettuce. The compost reduced Cd uptake despite lowering pH of the amended soil, while Mn salt addition acidified the soil and caused a substantial increase in crop Cd. Combinations of limestone with Zn, compost, or Zn and compost strongly limited Cd accumulation by both spinach and lettuce, allowing production of compliant crops on these soils. This greenhouse study provides additional information for future work with soil amendment use in urban garden situations. Objective 4. Previous studies indicated that if a crop contained high levels of Zn, the Cd in the crop had lower bioavailability to animals than if the Zn was low. Because nearly all Cd contamination of cropland is accompanied by 100-fold higher Zn, Cd in crops grown on such soil might comprise no risk from the somewhat higher crop levels of Cd. We tested this hypothesis by feeding weanling pigs diets with tolerable crop Cd (4 mg/kg) and either normal low levels of Zn, or sub-phytotoxic levels of Zn which would occur if soils with Cd with 100-fold higher Zn were acidified. The pigs were fed until all test feeds were consumed, and tissues were collected for analysis. Tissues were prepared by freeze-drying and are presently undergoing analysis.

4. Accomplishments
1. Use of canopy reflectance to monitor arsenic in rice plants. Rice production in the United States is over a 2 billion dollar industry. Due to the growing concern of elevated levels of arsenic in rice grown under flooded conditions, early dictions of arsenic in the tissue will allow a rice farmer to identify potential arsenic contaminated fields. ARS researcher in Beltsville, Maryland, in collaboration with scientist from the University of Maryland, conducted research that evaluates the feasibility of using reflectance spectroscopy to monitor arsenic in rice plants. Results demonstrated that leaf reflectance characteristics of rice plants were changed in plants having high concentrations of arsenic. This finding will provide valuable information to rice producers in identifying areas within rice fields with a high concentration of arsenic before harvest. This will allow farmers to separate the arsenic contaminated grain from non-contaminated grain during harvest, reducing the potential of exceeding the allowable arsenic levels in rice.

2. Research continues to find ways to measure and reduce inorganic arsenic (iAs) in rice. The level of iAs in rice is of major importance both in the U.S. and internationally. Rice is the second leading food crop in the world and was worth $2.38 billion to the U.S. economy in 2016. Meeting limits on iAs in rice is important to producers who need information in order to assess contamination and management practices. Research done by ARS researchers in Beltsville, Maryland, resulted in the development of a simple, rapid method for analysis of iAs. In addition, cooperative work between ARS researchers in Stuttgart, Arkansas and Beltsville, Maryland, has helped to provide important information on both production practices for rice and important rice cultivars that will provide for decreased levels of iAs in rice. This finding will provide valuable information to the rice industries, U.S. Food and Drug Administration and farmers.

Review Publications
Lahue, G.T., Chaney, R.L., Adviento-Borbe, M.A., Linquist, B.A. 2016. Alternate wetting and drying in high yielding direct-seeded rice systems accomplishes multiple environmental and agronomic objectives. Agriculture, Ecosystems and Environment. 229:30-39.
Xiao, Z., Codling, E.E., Luo, Y., Nou, X., Lester, G.E., Wang, Q. 2016. Microgreens of brassicaceae: mineral composition and content of 30 varieties. Journal of Food Composition and Analysis. 49:87-93.
Bandaru, V., Daughtry, C.S., Codling, E.E., Hansen, D., White-Hansen, S., Green, C.E. 2016. Evaluating leaf and canopy reflectance of stressed rice plants to monitor arsenic contamination. International Journal of Environmental Research and Public Health. 13:606. doi:10.3390/ijerph13060606.
Green, C.E., Chaney, R.L., Bouwkamp, J. 2016. Lack of Zn inhibition of Cd accumulation by rice (Oryza sativa L.) supports non-Zn transporter uptake of Cd. Journal of Plant Nutrition. 40:869-877.