Antagonistic Relationship between Selenate and Sulfate Uptake in Onion (Allium cepa):  Implications for the Production of Organosulfur and Organoselenium Compounds in Plants

The relationship between S and Se uptake in onion plants was investigated in solution culture. In the absence of Se, plant S concentrations ranged from 0.6 to 1.45% at S solution culture levels of 0.8 mol m-3 (standard nutrient solution) and 14.8 mol m-3 SO4 (gypsum saturated), respectively. Increasing levels from 1 to 8 g m-3 SeO42--Se did not increase Se content beyond 158 ± 19 μg/plant, although plant concentrations ranged from 200 to over 1000 μg of Se/g of dry matter. At a constant Se level, increasing S in solution culture reduced plant dry matter Se concentration by 80%. When the antagonistic relationship between these two elements is expressed as a molar ratio, S/Se in plant dry matter is nearly identical to S/Se in solution culture. These data suggest that S must be supplied at relatively low levels to produce high concentrations of plant Se.