Selenate Concentration Affects Selenium and Sulfur Uptake and Accumulation by `Granex 33' Onions

Selenium and sulfur have similar chemical structures. This allows Se to be absorbed and incorporated in the same assimilation pathways as S. Onions (Allium cepa L.) are a crop with unique S metabolism, responsible for growth and flavor intensity. Because of the antagonistic behavior of the two ions, the effects of Se on S and Se nutrient depletion and tissue accumulation were investigated. `Granex 33' onions were grown in nutrient solutions with one concentration of S and increasing Se concentrations. Selenium was applied as sodium selenate (Na₂SeO₄) at concentrations of 0, 0.5, 1.0, 1.5, and 2.0 mg·L^-1. Selenium depletion from the nutrient solution increased linearly with increasing Na₂SeO₄ treatment concentrations. Sulfur depletion increased and then decreased with increasing Na₂SeO₄ treatment concentrations. Selenium and S accumulation were highest in leaf tissues, less in root tissues, and lowest in bulb tissues at plant maturity. Selenium accumulation increased linearly with increasing Na₂SeO₄ for all tissues analyzed. Sulfur accumulation in leaf and bulb tissues was quadratic in response to increasing SeO₄^-2, while S in root tissues decreased linearly with increasing Na₂SeO₄. Low concentrations of Na₂SeO₄ in our study enhanced S uptake and accumulation. Previously, Se was thought to competitively inhibit S uptake and metabolism.