Regulation of sulfate uptake by excised barley roots in the presence of selenate

Active transport of SO₄²⁻ and SeO₄²⁻ has been evaluated during 60-hour contact of barley roots with nutrient solutions containing either ³⁵SO₄²⁻ or ⁷⁵SeO₄²⁻, or both ions, at 0.1 milli-equivalent per liter. In the SO₄²⁻ solution the time course of active transport follows a straight line; if SeO₄²⁻ is also present transport is strongly inhibited after 20 to 30 hours for both ions. The S-Se uptake ratio remains 1.4 during the 60 hours; S-Se ratio shifts from 3.0 to 3.3 in proteins and falls to 0.6 in free amino acids. S-Se discrimination is mainly operating at the level of amino acid incorporation into proteins. The presence of Se-amino acids blocks this incorporation and brings about an accumulation of free amino acids; at the same time carrier activity is inhibited. The addition of methionine or Se-methionine causes a 60 to 80% inhibition of the active transport.