The Occurrence of Rare Earth Elements in Edible Tissues of Selected Vegetable Crops.

Rare earth elements (REE) are of interest as: (1) materials of strategic and economic value, especially in high-tech applications such as specialized batteries and magnets, (2) substances that may stimulate plant growth, and (3) potential analogues for transuranic elements in radioecological studies.  In order to better understand the bioavailability of REE in the human diet, and the cycling of REE in terrestrial soil-plant systems, we examined edible plant tissues (n= 27), including leaves and fruits, harvested from field trials of vegetable crops grown at a test plot in southern Maryland. These samples were analyzed for major elements, trace elements, and REE, determined on nitric acid/peroxide digests using ICP-MS.  Multiple regression analysis showed REE concentrations (La to Gd, excluding Eu) to be positively correlated (r > 0.75) with Ca content. A similar pattern was seen for Sr, with such correlations extending to the heavier REE. As a proxy for REE uptake, we examined the total oxalate concentration in the plant tissue by extraction with 2N HCl and determination by enzymatic assay.  Rationale for this proxy is based on: (1) La has been used as physiological analog to Ca in many biochemical studies, (2) Ca storage in plant tissues has been linked to the formation of Ca oxalate crystals, and (3) Ca oxalate precipitation represents an established method of scavenging REE in analytical chemistry procedures. A plot of Ca content vs oxalate content showed a positive correlation (r = 0.71, p < 0.001). X-ray diffraction analysis of low temperature ash from selected high Ca content tissues confirmed the presence of the Ca oxalate monohydrate mineral whewellite as a dominant phase. The possible role of biomineralization in the retention of REE in plant tissue will be discussed, with particular reference to historical studies from the biogeochemical literature.