Abstract
Lead (Pb) ranks among the most problematic environmental pollutants. Background contamination of soils is nearly ubiquitous, yet plant Pb accumulation is barely understood. In a survey covering 165 European populations of the metallophyte Arabidopsis halleri, several field samples had indicated Pb hyperaccumulation, offering a chance to dissect plant Pb accumulation. Accumulation of Pb was analysed in A.halleri individuals from contrasting habitats under controlled conditions to rule out aerial deposition as a source of apparent Pb accumulation. Several elemental imaging techniques were employed to study the spatial distribution and ligand environment of Pb. Regardless of genetic background, A.halleri individuals showed higher shoot Pb accumulation than A.thaliana. However, dose-response curves revealed indicator rather than hyperaccumulator behaviour. Xylem sap data and elemental imaging unequivocally demonstrated the in planta mobility of Pb. Highest Pb concentrations were found in epidermal and vascular tissues. Distribution of Pb was distinct from that of the hyperaccumulated metal zinc. Most Pb was bound by oxygen ligands in bidentate coordination. A.halleri accumulates Pb whenever soil conditions render Pb phytoavailable. Considerable Pb accumulation under such circumstances, even in leaves of A.thaliana, strongly suggests that Pb can enter food webs and may pose a food safety risk.
Highlights
Lead (Pb) is one of the most problematic environmental pollutants, ranking, for example, second on the Agency for Toxic Substances and Disease Registry 2017 substance priority list
In a large-scale field study, several A. halleri individuals had been identified as putative Pb hyperaccumulators (Stein et al, 2017)
All A. halleri accessions contained more Pb than the reference plant A. thaliana after growth in the same contaminated soil in the same experiment
Summary
Lead (Pb) is one of the most problematic environmental pollutants, ranking, for example, second on the Agency for Toxic Substances and Disease Registry 2017 substance priority list. Despite the ban on tetraethyl Pb in gasoline and on the use of Pb in many consumer products since the 1970s, Pb production remains at almost 5 million tons per year according to data compiled by the US Geological Survey (https://minerals.usgs.gov/minerals/pubs/com modity/lead/). Widespread Pb pollution results in continuous low-level exposure of organisms including humans to Pb. For Cd and As it is well documented that they are taken up from the soil as Cd2+ and arsenate or arsenite, respectively, into plants and enter food webs in this way (Adriano, 2001; McLaughlin et al, 2011). Concentrations of ionic Pb in soil solutions are very low. This is predominantly explained by the strong sorption of Pb to soil particles and dissolved organic matter (Degryse et al, 2007). When Pb was a major target of early phytoremediation research, chelators were frequently added to the soil in order to improve Pb bioavailability (Huang et al, 1997; Wang et al, 2007; Saifullah et al, 2009)
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