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- Materials and methods
- Results and Discussion
• Ecotypic variation in selenium (Se) hyperaccumulation in plants is reported here among populations of Stanleya pinnata (Brassicaceae), which has a broad biogeographical range in the western USA.
• In a glasshouse study, Se and sulfur accumulation were examined in 16 populations of S. pinnata. Plants grown from seed (collected from sites representing the species range) were subjected to five treatments differing in selenate (SeO42−) and sulfate (SO42−) concentrations.
• The populations differed in shoot Se concentration by 1.4- to 3.6-fold, depending on the treatment, and these concentrations were positively correlated with the indigenous soil Se levels at the collection sites. Shoot S concentrations varied by less than two-fold, and did not correlate with the shoot Se levels. All populations accumulated SeO42− preferentially over SO42−. By contrast, Brassica juncea seedlings grown in a similar solution series consistently accumulated SO42− preferentially over SeO42−. Biomass production differed up to three-fold between populations.
• S. pinnata is a primary Se accumulator, but populations exhibit significant ecotypic differences in Se accumulation. Environmental concerns about Se are common, and the broad adaptation of S. pinnata makes it an attractive candidate for phytoremediation.
- Top of page
- Materials and methods
- Results and Discussion
In 1831 at Ft. Randal, South Dakota, USA, it was reported that livestock grazing in certain areas would contract ‘alkali’ disease. In the 1920s, high selenium (Se) in forage was found to be the causative agent of alkali disease, thus identifying Se as an ecotoxicological hazard, and spurring numerous investigations into the geobotany of Se. This led to the identification of unique plants, mostly endemic to western USA, capable of (hyper)accumulating Se to thousands (primary accumulators) or hundreds (secondary accumulators) of mg kg−1 (Rosenfeld & Beath, 1964; Mayland et al., 1989).
More recently, disposal of saline irrigation wastewater in hydrologically closed sinks in semiarid western USA has concentrated salts, Se, and other trace elements to levels sufficient to harm wildlife (Presser et al., 1994). A striking example of the ecotoxicology of Se occurred in the mid 1980s at Kesterson Reservoir (Merced Co., CA, USA), where high Se in agricultural wastewater was linked to deformities and deaths of waterfowl embryos (Ohlendorf et al., 1986). Due to the threat it poses to wildlife, methods for removing Se from both contaminated soils and sediments are being sought.
Phytoremediation has been proposed as one solution to this problem wherein plants remove Se, predominantly as selenate (SeO42−), from the soil and accumulate it in their shoots (Banuelos et al., 1990, 1997a; Parker et al., 1991). Seleniferous plant material could be harvested and landfilled, used as a supplement to low-Se animal diets, or incorporated back into the soil to promote microbial Se volatilization (Parker et al., 1991). Plant-enhanced volatilization of methyl-selenide compounds from leaves and/or the rhizosphere has also been proposed to augment removal in harvested shoots (Terry et al., 1992; Zayed & Terry, 1994). The search for superior germplasm is ongoing (Banuelos et al., 1997c), and the ideal candidate plant for Se phytoremediation should be tolerant of the wide range of aerial and edaphic conditions found in western USA, including heat, drought, salinity, and high boron (Parker et al., 1991). Such plants should also have a rapid growth rate and large biomass production, coupled with the ability to accumulate high concentrations of Se in shoot tissue even when the soil is high in soluble SO42−, as is often the case (Bell et al., 1992).
To date, the only primary accumulators evaluated specifically for use in phytoremediation have been species of Astragalus, which have proven difficult to grow, and tend to produce small biomass (Parker et al., 1991; Bell et al., 1992; Duckart et al., 1992; Retana et al., 1993). Other studies have focused on Brassica species (mostly B. juncea and B.napus), which seem to be secondary Se accumulators, probably because they are avid accumulators of S (Banuelos et al., 1997a). Stanleya pinnata, a member of the Brassicaceae, is a previously unstudied candidate that is attractive because it is a putative primary Se accumulator, with a reported maximum of 2380 mg kg−1 Se (Inhat, 1989), it is widespread and broadly adapted in western USA, and it has a perennial habit and potentially large biomass. With respect to Se metabolism, S. pinnata has been shown to be similar to the Se-accumulating Astragalus species, producing the characteristic Se-methylselenocysteine and selenocystathionine (Shrift, 1969). Additionally, S. pinnata produces a seleno-wax unknown in any other species (McColloch et al., 1963).
Bell et al. (1992) described the unique ability of primary Se accumulators to accumulate SeO42−preferentially over SO42−. Sulphur and Se share similar chemistries so that SeO42− and SO42− enter plants via the same carrier and compete strongly for plant uptake and assimilation (Legget & Epstein, 1956). Bell et al. (1992) proposed a method for classifying plants as primary, secondary, or nonSe accumulators based on a discrimination coefficient (DC) derived from the ratio of [plant Se/S] to [nutrient solution Se/S]. Bell et al. (1992) found that the nonaccumulator Medicago sativa (alfalfa) had a DC of <1, indicating discrimination against SeO42−. The primary accumulator Astragalus bisulcatus had a mean DC of 5.43, which indicates significant preferential accumulation of SeO42− over SO42−. The Brassicaceae are well-known accumulators of S (Mengel & Kirkby, 1997), so high shoot Se concentrations reported for S. pinnata may thus be a function of avid S uptake coupled with indiscriminant Se uptake. Alternatively, S. pinnata might be a true primary accumulator that exhibits preferential uptake of SeO42− over SO42−.
Plant populations are classified as ecotypes when they have evolved heritable, phenotypic differences that directly correlate to environmental factors (Turresson, 1922). Baker (1987) and Macnair (1993) have reviewed plant adaptation to high-metal soils, and report that ecotypic variation with regard to tolerance and accumulation of metals is a common phenomenon. Recently, differential cadmium accumulation in populations of Thlaspi caerulescens has been investigated in the context of identifying superior germplasm for phytoremediation (Lombi et al., 2000). To date, there have been no reports of true ecotypic variation in Se accumulation. Due to its extensive range in western USA, S. pinnata offers an opportunity to investigate ecotypic differentiation with respect to soil Se.
The first objective of this research was to determine whether populations of S. pinnata differ ecotypically with respect to Se accumulation, and, thus, to identify populations with greater promise for phytoremediation of Se-laden soils. Secondly, we sought to clarify whether S. pinnata behaves more as a primary or secondary accumulator with respect to preferential uptake of SeO42− vs SO42−. For comparison, we also examined Se accumulating tendencies in a single genotype of B. juncea, a species that has been extensively studied for phytoremediation of Se (Banuelos et al., 1997b,c; de Souza et al., 1998; Pilon-Smits et al. 1999) and other trace metals (Kumar et al., 1995; Salt et al., 1995).