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.
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