For sulfate uptake by barley roots, competition studies reveal that uptake and phase transitions are caused by interaction of ions with separate sites on or in the plasmalemma. Uptake is competitively, and unequally, inhibited by sulfate analogues but not by other divalent anions. In contrast, divalent phosphate and di‐ and trivalent pyrophosphate are equally effective in causing transitions. Phosphate is taken up mainly or entirely as H2PO4− by a similar but separate multiphasic mechanism. At pH 8, sulfate uptake is mediated by fewer phases than at low and intermediate pH.
Uptake of sulfate by excised barley roots increases upon their washing in aerated water or dilute CaCl2 solutions. Washing increases the values for Vmax and the sulfate concentrations required for transition between the lower phases, but the KM‐values remain essentially constant. At low sulfate concentrations, phase transitions do not occur in the absence of calcium or other divalent cations. These ions are about equally effective in enhancing short‐term sulfate uptake. Phase transitions were not principally altered by sulfhydryl or protein reagents. These concentration‐dependent transitions appear unrelated to temperature‐dependent phase transitions as evidenced by similar multiphasic patterns at low and high temperature.
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