Limiting Risk From Trace Elements in Crops By Breeding, Agronomy, and Understanding Bioavailability.

Some crops accumulate levels of cadmium (Cd), arsenic (As), lead (Pb), selenium (Se) and sulfur (S) which comprise risk to humans or livestock. Starting in 1970, Cd poisoning of subsistence rice farmers in Japan raised concern about dietary Cd. By the 1980s, levels of Cd in durum wheat, confectionery sunflower kernels and flax started to be limited by European importers. Limits were established and US exports had to meet EU or national limits of the importing nation. US exports of sunflower kernels were threatened by natural soil Cd in several states. Investigations identified that poorly drained soils rich in chloride (but not Cd contamination) promoted Cd accumulation enough to interfere with exports, so processors arranged to contract with growers who had soil series which could produce low Cd kernels. Although the North Dakota State University-Agricultural Research Service-National Sunflower Association cooperative studies characterized genetic variation and even bred low Cd inbred and restorer lines so that lower Cd hybrids were available, these were not adopted to date. Exports of durum wheat have the complication that the grain needs to be shipped as whole grain to prevent spoilage, but if milled much of the Cd is in the bran such that durum flour meets EU standards for Cd even when the whole grain exceeds import limits. Again, high soil chloride is a key factor in higher grain Cd although low soil Zn may promote Cd uptake. Within the US, spinach and lettuce grown on certain California soils with geogenic Cd can greatly exceed CODEX limits, but there are no Cd limits on US crops at this time because no disease from dietary Cd has been identified in the US. Because these soils have geogenic Cd enrichment without higher Zn, liming may actually cause higher crop Cd concentration unless Zn fertilizer is added. Research in pots has shown that the combination of high Zn fertilizer application plus limestone to make the soil calcareous can limit spinach and lettuce Cd to CODEX limits. Several US exports are threatened by possible EU intent to lower allowable Cd in many crops based on a European Food Safety Agency risk assessment which suggested a much lower allowable daily Cd intake (2.5 ìg/kgBW/week or ~43% of JECFA) than the international WHO/JECFA risk assessment (25 ìg/kgBW/month). Many epidemiologic studies in Japan where large numbers of subsistence farm families suffered Cd disease (renal proximal tubular dysfunction) and a few suffered the itai-itai bone disease do not support the EU risk assessment. Low rice grain bioavailable Fe and Zn levels strongly increase the bioavailability of rice Cd compared to other grains for subsistence rice consumers (up to 10-fold).

In the case of arsenic, extensive disease in Asia resulted from well waters from mineralized sediments which were used for drinking and cooking as well as irrigating rice. Drinking water is the dominant exposure, while use of contaminated water in cooking with 6 times more water than rice can actually considerably increase As in the cooked rice. Further, As phytotoxicity to rice may occur when soil As accumulates over time. Flood produces anaerobic conditions which generate arsenite; arsenite is absorbed on the silicate transporter of rice, and part moves to grain. Aerobic production can drastically reduce grain As, but strongly reduces grain yield and increases grain Cd. Genetic solutions may be available to limit Cd accumulation in rice, but no strong genetic As reduction has been identified, nor has a method to obtain normal yields with aerobic soil production. Added Fe and silicate may reduce rice As but may not be cost effective. Because rice bran is 10-times higher in As than polished grain, it seems likely that rice bran products will be discouraged or banned in the near future.

The Pb case shows how plant accumulation can occur in unexpected ways. Leafy and root crops can be enriched in Pb when grown on Pb rich soils, especially if the soil is low in pH, OM, and phosphate. High Pb carrots were discovered by US-FDA which asked ARS to identify how this occurred. To our surprise, Codling et al. found that Pb was accumulated in the xylem part of the expanded hypocotyl edible carrot. Other expanded hypocotyl vegetables also accumulate Pb in their xylem, but phloem fed potatoes do not. Low growing leafy vegetables and herbs may also comprise Pb risk due to soil splash and Pb uptake on high Pb soils. But soil ingestion is a higher risk than crop consumption for most Pb contaminated soils. Pb in crops has quite low bioavailability to humans compared to Pb in drinking water.

Thus crop genetics, fertilizers, soil properties and management may all affect risk from crop trace elements. Markets may require change in genetics and crop production practices.