Perchlorate, nitrate, and iodine uptake and distribution in lettuce (Lactuca sativa L.) and potential impact on background levels in humans

Much focus has been placed on the impact of exposure to perchlorate (ClO₄^-) from drinking water. Recently, it has become more apparent that a significant percentage of the total ClO₄^- exposure may be due to ingestion of food. Most studies have only evaluated the uptake and distribution of ClO₄^- by plants without considering the potential for uptake of iodine (I) by the plant and the subsequent impacts on ClO₄^- uptake and distribution on human health. The objectives of this research effort were to evaluate the relative uptake of ClO₄^- and I supplied as either KI or KIO₃, the two major environmental forms of I in a standard hydroponic nutrient solution using butter head lettuce. No interaction of ClO ₄^- uptake and distribution was found in the presence of I^- or IO₃^- relative to previous studies evaluating ClO₄^- alone. Bioconcentration factors for ClO₄^- and total I in butter head lettuce when coexposed to both anions were similar for outer (292 ± 17 and 294 ± 12 L kg^-1 of dry weight, respectively) and inner (76 ± 18 and 60 ± 8 L kg^-1 of dry weight, respectively) leaves but not for roots (23 ± 3.7 and 359 ± 1.7 L kg^-1 of dry weight, respectively) when the iodine was supplied as I^-. The uptake of iodine was lower (BCF = 47 ± 3.8, 19 ± 0.6, and 189 ± 16, L kg^-1 of dry weight for the outer and inner leaves and roots, respectively) for all tissues when iodine was supplied as IO₃ ^-, with the greatest accumulation by the roots. These results suggest that if lettuce is grown using fertilizers containing both ClO₄ ^- and I^-, then the final ratio of I_T/ClO ₄ in the leaves will be essentially equal to the ratio in the fertilizer but lower if the I is supplied as IO₃^-. Therefore, the impact of the consumption of lettuce containing ClO ₄^- may be mitigated if the lettuce is grown using fertilizer with an appropriate amount of I to maintain the existing ratio of serum I to total goitrogen load (TGL). Nevertheless, the TGL in lettuce appeared to be almost completely dominated by NO₃^- with only a minor contribution of ClO₄^-, even for the highest exposure to ClO₄^-.