Influence of Binary Interactions of Arsenate, Molybdate, and Selenate on Yield and Composition of Alfalfa

Arsenic (As), selenium (Se), and molybdenum (Mo) are known to coexist in the soil solution as anions and thus may exert antagonistic or synergistic effects on their absorption by and translocation within plants. To evaluate the effects of binary interactions [As(V) × Se(VI), Se(VI) × Mo(VI) and As(V) × Mo(VI)] on growth and composition of alfalfa (Medicago sativa L.), three glasshouse sand culture experiments were conducted. Treatments applied in the half-strength Hoagland's solution alone and in all possible factorial combinations for a given interaction consisted of 0, 0.25, 0.5, and 1.0 mg As(V) L⁻¹ added as Na₂HAsO₄; 0 , 0.5, and 1.0 mg Se(VI) L⁻¹ added as Na₂SeO₄; and 0.01, 1, 3, and 5 mg Mo(VI) L⁻¹ added as Na₂MoO₄. Shoots were harvested three times and roots at the termination of the experiment. Plant tissues were dried, weighed, digested and analyzed for total As, Se, and Mo. Added Mo(VI) concentrations in the nutrient solutions had no significant effect on the tissue yield of alfalfa. Arsenic showed some adverse effect on shoot and root yields. The highest Se(VI) treatment decreased alfalfa shoot yield by 13.7 to 44.2% and root yield by 33% to 56.1%. The tissue As, Se, and Mo concentrations increased correspondingly with the increasing solution concentrations. At equal levels in the solution, alfalfa accumulated Se > Mo ≫ As. Increasing As(V) in solution caused significant increases in shoot Se concentrations and uptake, whereas solution Se(VI) showed a strong antagonistic effect on As uptake. The Se treatments significantly (P ~ 0.05) increased concentrations and uptake by shoot and root at all levels of solution Mo, except 0.01 mg L⁻¹ where 1.0 mg Se L⁻¹ increased Mo concentration, but had no significant effect on Mo uptake. Increasing Mo levels in the nutrient solution showed no significant effect on Se concentration and uptake by root. However, at 1.0 mg Se(VI) L⁻¹, shoot Se concentration increased significantly at 3.0 and 5.0 mg Mo L⁻¹. The solution As(V) showed no significant effect on tissue Mo at 0.01 and 1.0 mg Mo(VI) L⁻¹ but increased Mo concentrations in shoots significantly (P ≤ 0.05) at 3.0 and 5.0 mg Mo(VI) L⁻¹. These interactions may help to understand the complex interactions in soilplant systems. Based on the observed elevated tissue concentrations of As, Se, and Mo, these results might have important environmental implications for phytotoxicity to plants and foraging animals. Contribution from the Dep. of Soil and Environ. Sciences, Univ. of California, Riverside, CA 92521.