Effect of salinity and its composition on the accumulation of selenium by alfalfa

Alfalfa (Medicago sativa L.) was grown in greenhouse sand culture to examine the effect of salinity composition and concentration on Se accumulation by plants. In a 2×2×4 factorial experiment, salinity was added as either C1⁻ or SO ₄ ²⁻ salts to the irrigating solution to achieve an electrical conductivity of 0.5, 1.5–3.0, or 6.0 dS m⁻¹. Selenium was added to the nutrient solution at a concentration of 0.25 or 1.0 mg Se(VI)I⁻¹. Following the third cutting, the roots were washed and all plant material analyzed for dry weight and Se. Plant biomass production decreased with additions of either Se or salinity, regardless of composition. In the presence of Se, the yield reduction was greater with Cl⁻ salinity than with SO ₄ ²⁻ salinity. Plant Se accumulation was reduced from 948 mg Se kg⁻¹ to 6 mg Se kg⁻¹ in the presence of SO ₄ ²⁻ salts (0.5 mmol SO ₄ ²⁻ l⁻¹ vs. 40 mmol SO ₄ ²⁻ l⁻¹) due to an apparent Se(VI) −SO ₄ ²⁻ antagonism. This Se−SO ₄ ²⁻ antagonism prevented accumulation of Se and reduced Se-induced toxicity. A lesser antagonistic effect on Se accumulation was observed between Cl⁻, and Se. A synergistic interaction between SO ₄ ²⁻ and Se(VI) increased plant S concentrations in the presence of the relatively low basal SO ₄ ²⁻ concentrations but not at the higher solution SO ₄ ²⁻ concentrations. In many areas, soil and water containing high Se concentrations also contain large amounts of SO ₄ ²⁻ . The occurrence of SO ₄ ²⁻ with Se reduces plant accumulation of Se(VI) and may lower the risk of Se overexposure to animals feeding on forage material grown in high Se−SO ₄ ²⁻ regions.