Selenium phytoremediation potential of Stanleya pinnata

Disposal of saline irrigation wastewater in hydrologically closed sinks in the semi-arid western U.S. has concentrated selenium-rich salts to hazardous levels and phytoextraction, along with plant-enhanced volatilization of methyl-selenides, is an active area of research. Here, we provide an overview of our ongoing studies of Stanleya pinnata (Brassicaceae), a previously unstudied candidate that is a primary accumulator (hyperaccumulator) of Se that is widespread and broadly adapted in the western U.S. When grown in sand culture under uniform greenhouse conditions, 16 populations representing S. pinnataʹs broad biogeographical range differed in shoot Se concentration by 1.4- to 3.6-fold, and the shoot concentrations were positively correlated with the indigenous soil Se levels at the collection sites. Thus, S. pinnata exhibits significant ecotypic variation in Se accumulation. All populations accumulated SeO4 2- preferentially over SO4 2- consistent with S. pinnataʹs classification as a primary Se accumulator, while hydroponically-grown Brassica juncea consistently accumulated sulfate preferentially over selenate. The Se in S. pinnata shoots was predominately in the soluble amino-acid pool, which may serve as direct precursor to volatile forms such as dimethyldiselenide; inorganic forms (e.g. selenate) dominated in B. juncea. Preliminary results suggest that S. pinnata may volatilize unusually large quantities of Se when grown at high sulfate concentrations, an unexpected result not heretofore reported in any species. In a sand–culture experiment, S. pinnata exhibited excellent tolerance of excess boron, but only moderate tolerance of salinity, and superior genotypes will likely be needed for phytoremediation of highly salinized soils and sediments. Stanleya pinnata is a perennial that responded favorably to repeated cuffing in the greenhouse, a trait that could prove valuable in field-scale phytoremediation. Environmental concerns about Se are common in the western USA, and S. pinnata is a potentially useful species for phytoremediation due to its broad adaptation to western soils and environments, and its uptake, metabolism and volatilization of Se.