The question of whether or not selenium is a micronutrient for plants is still considered unresolved (3). Selenium has not been classified as an essential element for plants, but its role as a beneficial element in plants that are able to accumulate large amounts of it has been considered (11). Uptake and accumulation of selenium by plants is determined by the form and concentration of selenium, the presence and identity of competing ions, and affinity of a plant species to absorb and metabolize selenium (10). Variation in selenium contents of plants seems to exceed that of nearly every other element (12). Nonconcentrator or nonaccumulator plant species will accumulate >25 mg Se kg-1 dry weight. Most crops species such as grains, grasses, fruits, vegetables, and many weed species are considered nonconcentrators (8,13). Secondary absorbers normally grow in areas with low to medium soil-selenium concentrations and can accumulate from 25 to 100 mg Se kg-1 dry weight. They belong to a number of different genera, including Aster, Atriplex, Castelleja, Grindelia, Gutierrezia, Machaeranthera, and Mentzelia. The primary indicator or selenium-accumulator species can accumulate from 100 to 10,000 mg Se kg-1 dry weight. This group includes species of Astragalus, Machaeranthera, Haplopappus, and Stanleya (14). These plant species are suspects for causing acute selenosis, or selenium toxicity, of range animals that consume the plants as forages (10,15). Selenium-accumulator plants can contain 100 times more selenium than nonaccumulator plants when grown on the same soil (16). Surveys of selenium concentrations in crops reveal that areas producing low-selenium crops (<0.1 mg Se kg-1) are more common than those producing crops with toxic selenium levels (>2 mg Se kg-1) (16).
Many studies have demonstrated an antagonistic relationship for uptake between SeO42- and SO42- (10,19,22-25). When SeO42- is present in high concentrations, it can competitively inhibit SO42- uptake. Adding SeO42- lowered SO42- absorption and transport in excised barley (Hordeum vulgare L.) roots. Conversely, adding SO42- lowered SeO42- absorption and transport (19,26). These studies involved an SeO42-/SO42- ratio of 1:1. In a preliminary solution culture experiment, an SeO42-/SO42- ratio of 1:3 resulted in the death of onion (Allium cepa L.) plant within 6 weeks (D.A. Kopsell and W.M. Randle, University of Georgia, unpublished results, 1994). When the SeO42-/SO42- ratio was lowered to 1:500 or 1:125 in solution culture, Kopsell and Randle (27) reported significant increases in SO42- uptake by whole onion plants. Increasing SO4-2 levels from 0.25 to 10 mM in solution culture inhibited SeO42- uptake of broccoli (Brassica oleracea var. botrytis L.), Indian mustard (Brassica juncea Czern.), sugarbeet (Beta vulgaris L.), and rice (Oryza sativa L.) by 90% (22). Applications of gypsum (CaSO42H2O) at the rates of 5.6 to 16.8 t ha-1 reduced selenium uptake in alfalfa (Medicago sativa L.) and oats (Avena sativa L.) grown on flyash landfill soils (28).
Although phosphate (H2PO4-) is not expected to affect SeO42- uptake because of the chemical dissimilarities of the two radicals, the relationship between phosphate additions and selenium levels in plants has been inconsistent (9,10,29). Hopper and Parker (29) reported that a 10-fold increase (up to 200 µM) in phosphate solution culture decreased the selenium content of ryegrass (Lolium perenne L.) shoots and roots by 30 to 50% if selenium was supplied as SeO3. In contrast, Carter et al. (30) reported that applying up to 160 kg P ha-1 either as H3PO4 or concentrated superphosphate to Gooding sandy loam increased selenium concentrations in alfalfa.
Selenate can accumulate in plants to concentrations much greater than that of selenium in the surrounding medium. In contrast, SeO32- did not accumulate to levels surpassing the selenium levels of the external environment (31). When broccoli, Indian mustard, and rice were grown in the presence of SeO42-, SeO32-, or selenomethionine (Se-Met), plants accumulated the greatest amount of shoot selenium when selenium was supplied as SeO42-, followed by those provided with Se-Met (22). In the same study, sugarbeet (Beta vulgaris L.) accumulated the most shoot-Se when treated with Se-Met (22). Broccoli, swiss chard (Beta vulgaris var. cicla L.), collards (Brassica oleracea var. acephala D.C.), and cabbage (Brassica oleracea var. capitata L.) grown in soil treated with 4.5 mg SeO32- kg-1 or 4.5mg SeO42- kg-1 had a tissue concentration of Se in the range from 0.013 to 1.382 g Se kg-1 dry weight and absorbed 10 times the amount of selenium if treated with SeO42- than with SeO32- (32). When roots of bean (Phaseolus vulgaris L.) were incubated in 5mmol m-3 Na2SeO3 or 5mmol m-3 Na2SeO4 for 3 h, there was no significant difference in selenium accumulation, but distribution within the plant was different (33). In contrast, time-dependent kinetic studies showed that Indian mustard absorbed SeO42- up to 2-fold faster than SeO32- (34).
Increasing levels of selenium in plants may act to suppress the tissue concentrations of nitrogen, phosphorus, and sulfur. It can also inhibit the absorption of several heavy metals, especially manganese, zinc, copper, iron, and cadmium (35). This detoxifying effect of selenium has been demonstrated as reducing cadmium effects on garlic (Allium sativum L.) cell division (36). In contrast, the application of nitrogen, phosphorus, or sulfur is known to detoxify selenium. This effect may be due to either lowering of selenium uptake by the roots or to establishment of a safe ratio of selenium to other nutrient elements (35).
Selenomethionine was readily taken up by wheat (Triticum aestivum L.) seedlings, and the uptake followed a linear pattern in response to increasing selenium solution concentrations up to 1.0 µM (37). Western wheatgrass (Pascopyrum smithii L�ve) also showed linear selenium uptake with Se-Met solution concentrations up to 0.6 mg Se L-1 (38). Results from Ba�uelos et al. (39) showed that alfalfa accumulated selenium in plant tissues when selenium-laden mustard plant tissue was added to the soil. These studies provide evidence that organic selenium compounds in the soils may become available sources of selenium (40).
Genetic differences for selenium uptake and accumulation within species have also been reported. In 1939, Trelease and Trelease reported that cream milkvetch (cream locoweed, Astragalus racemosus Pursh.), a selenium-accumulator, produced 3.81 g dry weight in solution culture with 9 mg Na2SeO3 L-1, whereas ground plum (A. crassicarpus Nutt.), a nonaccumulator, produced only 0.20 g dry weight (41). Shoots of different land races of Indian mustard grown hydroponically in the presence of 2.0 mg Na2SeO4 L-1 ranged from 501 to 1092 mg Se kg-1 dry matter, whereas shoots grown in soil culture at 2.0 mg Na2SeO4 kg-1 concentration ranged from 407 to 769 mg Se kg-1 dry matter (42). Total accumulation of selenium in onion bulb tissue ranged from 60 to 113 µg Se g-1 dry weight among 16 different cultivars responding to 2.0 mg Na2SeO4 L-1 nutrient solution (43).
Volatilization of selenium is also influenced by the chemical form of selenium in the growing medium. The rate of selenium volatilization of a hybrid poplar (Populus tremula x alba) was 230- fold higher in sand culture if 20 µM Se was supplied as Se-Met than as SeO32-, and volatilization from SeO32- was 1.5-fold that from SeO42- (49). Selenium volatilization by shoots of broccoli, Indian mustard, sugarbeet, or rice supplied with Se-Met was also many folds higher than that from plants supplied with SeO32- (22). In Indian mustard, Se-volatilization rates were doubled or tripled in sand culture amended with 20 µM SeO32- relative to rates with 20 µM SeO42- (34). These data indicate that selenium volatilization from SeO42- is limited by the rate of SeO42- reduction as well as by the form of selenium available (22,34).
An increasing problem with irrigation agriculture in arid and semi-arid regions is the appearance of selenium in soils, ground water, and drainage effluents (12,55,56). The greatest concerns for selenium contamination come in areas where water systems drain seleniferous soils. One area of the United States that has come under close investigation because of elevated levels of selenium in the water is the San Joaquin Valley in California (57,58). Selenium enters the groundwater as soluble selenites and selenates and as suspended particles of sparingly soluble and organic forms of the element (8). The mobility of selenium in groundwater is related to its speciation in the aqueous solution, sorption properties of the substrate, and solubility of the solid phases (59). The ability of certain plants to take up, accumulate, and volatilize selenium has an important application in phytoremediation of selenium from the environment (3). Phytoremediation of selenium from contaminated soils is more practical and economical than its physical removal (60). Bioaccumulation of selenium in wetland habitats is also a problem and results in selenium toxicity to wildlife (61). There is a danger of selenium re-entering the local ecosystem if plant tissues that have accumulated selenium are consumed by wildlife or allowed to degrade (62).
The search for germplasm with the potential for effective phytoremediation has begun (63). The most ideal plant species for selenium phytoremediation should have the ability for rapid establishment and growth, ability to accumulate or volatilize large amounts of selenium, tolerate salinity and elevated soil boron, and develop large amounts of biomass on high-selenium soils (3,62–64). Indian mustard was more efficient at accumulating selenium than milkvetch (Astragalus incanus L.), Australian saltbush (Atriplex semibaccata R. Br.), old man saltbush (Atriplex nummularia Lindl.), or tall fescue (Festuca arundinacea Schreb.) when grown in potting soil amended with 3.5 mg Se6> kg-1 or 3.5 mg Se4> kg-1 as selenate or selenite (60).
Two of the options available once selenium is phytoextracted from contaminated soils are volatilization of methylated Se forms or harvest and removal of selenium-enriched plant biomass. Plant species with a high affinity for phytovolatilization could remove selenium from the environment by releasing it into the atmosphere, where it is dispersed and diluted by air currents (3,11,62). Most of the selenium in the air comes from windblown dusts, volcanic activity, and discharges from human activities such as the combustion of fossil fuels, smelting and refining of nonferrous metals, and the manufacturing of glass and ceramics (8). The large particulate and aerosol forms of selenium generally are not readily available for intake by plants or animals. When 15 crop species were grown in solution culture with 20µM SeO42-, rice, broccoli, or cabbage volatized 200 to 350µg Se m-2 leaf area day-1, whereas sugar beet, bean, lettuce (Lactuca sativa L.), or onion volatized less than 15µg Se m-2 leaf area day-1 (52). One of the proposed disposal schemes for selenized plants from phytoremediation is as a source of forage for selenium-deficient livestock (3,60) Accurate determination of selenium levels as well as other trace elements in plant tissues and the use of other forages in a blended mixture would be needed to ensure proper dietary selenium levels in animal feeds (60,62).
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