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United States Department of Agriculture

Agricultural Research Service

Research Project: PLANT VARIATION IN CD, PB, ZN AND AS ACCUMULATION AND BIOAVAILABILITY AND METHODS TO LIMIT RISK
2011 Annual Report


1a.Objectives (from AD-416)
Characterize the influence of zinc and iron concentrations in edible crop tissues on bioavailability of crop cadmium to animals; characterize potential transfer of soil lead, arsenic, and copper by vegetable crps grown on long-term orchard soils and other contaminated agricultural soils and methods to prevent this transfer; characterize genetic resources and inheritance of grain Cd to reduce cadmium in durum wheat, flax, and nonoilseed sunflower and release improved lower Cd germplasm; develop methods to identify levels of heavy metals, such as lead, arsenic and cadmium that might be food safety/security risks to give us the tools to prevent contamination of food of both plant and animal origin.


1b.Approach (from AD-416)
1) conduct animal feeding studies on the effect of dietary iron, zinc and calcium supply, and crop Zn level, on absorption of Cd in lettuce, polished rice and other crops for which Cd is important in understanding of human Cd risks from foods (durum wheat; bread wheat; etc.).
2)Grow commercial and garden carrot varieties with a wide range of properties on contaminated orchard soils rich in Pb and As; include tests of soil amendments expected to reduce uptake of Pb or As. Measure in vitro bioaccessibility and if needed bioavailability of crop Pb or As to animals. Examine metal residues in peel layer vs. internal storage root tissue..
3)Complete testing of inheritance of grain Cd concentration in sunflower hybrids, flax genotypes shown to differ in grain Cd accumulation, and durum wheat breeding lines; assist plant breeders develop germplasm releases with lower Cd than present commercial types. Examine physiology of genetic differences in Cd accumulation in relation to soil properties where crops are grown..
4)Test methods for rapid direct analysis, or preparation or extraction of Cd, As, and/or Pb in foods of plant or animal origin for spectrometric analysis at lower cost than present usual methods of analysis and verify the application of the methods developed for commercial food samples.


3.Progress Report
The project covered reducing the risks from trace elements in soils and crops and better understanding food-chain transfer and bioavailability of trace elements in foods. Important progress was made in understanding bioavailability of crop cadmium in relation to bioavailable levels of zinc, iron and calcium in crops. Although crops vary widely in iron concentration, only genetic changes can alter bioavailable plant iron levels. Present studies showed that phytoavailable zinc was critical in limiting lettuce cadmium accumulation, probably because both are transported on the zinc transporter in epidermal cells of roots. Using this information, a method was developed to strongly reduce cadmium in lettuce grown on Salinas Valley cadmium-mineralized soil by adding both limestone and zinc fertilizer. The increase in lettuce zinc should also reduce bioavailability of crop cadmium. In the lead and arsenic studies, an explanation for accumulation of lead in peeled carrots was achieved. By measurement of localization using synchrotron X-Ray spectroscopy, lead in peeled carrots was shown to be nearly entirely within the xylem which grows through carrot storage roots. High phosphate additions to lead rich soils reduced lead uptake into carrot roots and shoots, but mobilized arsenate which could leach deeper into soil profiles. Rice uptake of arsenic on aerobic absorption compared with flooded soils, showed much higher accumulation of arsenic in shoots of rice grown under flooded conditions. Lastly, no further progress was made in characterizing bioavailability of cadmium in crops. The previous cooperator at Grand Forks, ND, died and no one at that location will continue the cooperation. A new animal scientist cooperator was identified during development of a new Project Plan.


4.Accomplishments
1. Analysis of cadmium in durum wheat from breeders’ field tests, allowed evaluation of genetic variation in grain cadmium levels in several growing regions. Export of durum wheat and several other crops may be limited by grain cadmium concentration. Both soil variation and crop genetic variation can strongly affect grain cadmium levels. To retain markets, several U.S. durum breeders have been conducting research to breed cultivars with lower cadmium levels using a gene identified by Canadian researchers. The project analyzed grain from genetic trials in MT, CA, and AZ to support breeder’s progress. Significant variation has been found due to both genetics and to the use of irrigation with low chloride water supplies. Breeders will continue to work to improve overall durum wheat traits while incorporating the lower cadmium trait.

2. Localization of lead in xylem elements of root crops. Previous studies in this project identified higher than normal levels of lead in carrots grown on old orchard soils where lead-arsenate insecticide had been used before 1950. Peeled carrots were shown to have higher carrot lead, showing that the contamination pathway was not due to soil adherence to the roots. Thus both manual separation of carrot into peel, pulp, and xylem portions, and use of synchrotron X-Ray Spectroscopy were used to examine the localization of lead in carrots grown on high lead soils. Both approaches showed that lead had accumulated in the xylem portion of the root with very little lead in the rest of the storage root. Additional crops which have expanded hypocotyls (beet, turnip, radish) were similarly tested and lead accumulation was observed but considerably lower than that found in carrot. This appears to result from the long xylem through the carrot compared to the wider diameter of the other root crops. Potato had very low lead when grown on the same soils, showing that phloem-fed tissues such as tubers, fruits and grains accumulate very low levels of lead even on high lead soils. Overall, the findings support the Food and Drug Administration’s goal of understanding how carrots can be enriched in lead, and the industry’s need to limit production of crops with high lead levels.


Last Modified: 9/10/2014
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