Contrasting Selenate‐Sulfate Interactions in Selenium‐Accumulating and Nonaccumulating Plant Species

Abstract The shoots of primary Se‐accumulating plant species can accrue Se to several thousand micrograms per gram of dry weight, even when growing in gypsiferous soils, yet no detailed studies of the mutual antagonism between SeO 4 and SO 4 during plant uptake have been conducted with these species. In a comparative study, we grew a nonaccumulator (alfalfa, Medicago sativa L.) and a primary Se accumulator Astragalus bisulcatus (Hook.) A. Gray in identical nutrient solutions with varied SeO 4 (2–80 µ M ) and SO 4 (0.5–15.5 m M ) concentrations for 21 (alfalfa) or 32 to 35 d ( A. bisulcatus ). Shoot S concentrations in alfalfa were increased by increases in solution SeO 4 , but only when shoot Se was above ≈20 µg g −1 , suggesting that SeO 4 ‐induced stimulation of S uptake may be a result of incipient Se toxicity. Similar stimulations of S uptake were less apparent in A. bisulcatus . Alfalfa shoot Se concentrations ranged from 4 to 154 µg g −1 , while the same treatments resulted in shoot levels of 175 to 1200 µg g −1 in A. bisulcatus . Uptake of SeO 4 by alfalfa was profoundly inhibited by increases in solution SO 4 , while Se uptake by A. bisulcatus was much less, although still significantly, affected. Comparison of molar Se/S ratios in plants and nutrient solutions indicated discrimination against Se by alfalfa, but preferential accumulation of Se by A. bisulcatus . A reevaluation of previously published results was in general agreement with our findings, and suggested that primary Se accumulators have a unique ability to accumulate SeO 4 in the face of competition from SO 4 . These species could thus prove useful in efforts to remediate Se‐contaminated soils or sediments that are also enriched in SO 4 . Overall plant discrimination between SeO 4 and SO 4 may be related to (i) discrimination between the two analogues during initial absorption, or (ii) differential retranslocation of these elements to the root with subsequent afflux to the external solution.