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Title: Perchlorate content of plant foliage reflects a wide range of species-dependent accumulation but not ozone-induced biosynthesis

item GRANTZ, DAVID - University Of California
item Burkey, Kent
item JACKSON, ANDREW - Texas Tech University
item VU, HAI-BANG - University Of California
item HARVEY, GREGORY - United States Air Force

Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/28/2013
Publication Date: 1/1/2014
Citation: Grantz, D., Burkey, K.O., Jackson, A., Vu, H., Harvey, G. 2014. Perchlorate content of plant foliage reflects a wide range of species-dependent accumulation but not ozone-induced biosynthesis. Environmental Pollution. 184: 690-696.

Interpretive Summary: Perchlorate salts are detected throughout the environment including surface waters such as the Colorado River, and ground water such as the Ogalalla Aquifer in the Midwestern U.S. Perchlorate ions at high levels are considered toxic to mammals, including humans, because perchlorate disrupts thyroid metabolism by interfering with the uptake of iodide. Natural sources of perchlorate include atmospheric oxidation of chloride mediated by lightning, ultraviolet radiation, or tropospheric ozone. In addition to natural atmospheric sources, a number of anthropogenic point sources have been identified that include industrial and military applications, consumer products such as fireworks and highway flares, and in Chilean nitrate fertilizer which contains high natural concentrations of perchlorate. The current ambient distribution of perchlorate does not correspond with the abundance of known sources, suggesting that additional sources may remain to be identified. We tested the hypothesis that plants exposed to ozone may synthesize perchlorate from chloride present in plant tissues. A wide range of vegetable (spinach, lettuce, broccoli, bush bean) and crop (soybean, Pima cotton, sorghum, sugarcane, and maize) plants were exposed in greenhouse exposure chambers to ozone concentrations representative of present day ambient air pollution conditions. In the absence of ozone treatment, plant species varied in the amount of perchlorate accumulated from fertilizer with the highest perchlorate levels observed in spinach. In ozone treated plants, no evidence was found to suggest that plant metabolism is capable of perchlorate biosynthesis.

Technical Abstract: Perchlorate interferes with uptake of iodide in humans. Emission inventories do not explain observed distributions. Ozone is implicated in the natural origin of perchlorate, and has increased since pre-industrial times. Ozone produces perchlorate in vitro from chloride, and plant tissues contain chloride and redox reactions. We hypothesize that ozone exposure may induce plant synthesis of perchlorate. We exposed contrasting crop species to environmentally relevant ozone concentrations. In the absence of ozone exposure, species exhibited a large range of perchlorate accumulation with fertilizer serving as a known source, but there was no relationship between leaf or soil perchlorate content and ozone treatment, whether expressed as exposure or flux. Older, senescing leaves accumulated more perchlorate than younger leaves. Ozone exposed vegetation is not a source of environmental perchlorate.