Sulfate Uptake and Its Regulation in Lemna paucicostata Hegelm. 6746

The results of studies of SO(4) (2-) uptake by Lemna paucicostata are most simply interpreted by the hypothesis that at least two components are involved, one saturating and one linear, ;nonsaturating.' The saturating component has a low K(m) and high specificity for SO(4) (2-). Uptake by the nonsaturating component is less affected by pH and temperature than is that of the saturating system. SO(4) (2-) efflux is not quantitatively important in Lemna under standard conditions (20 micromolar SO(4) (2-)) (Datko AH, SH Mudd 1980 Plant Physiol 65: 906-912). 55% of newly taken up (35)SO(4) (2-) enters a slowly turning over compartment (vacuole?); 45% remains in a compartment (cytoplasm?) in which it is rapidly metabolized to organic compounds.Growth in increased concentrations of SO(4) (2-) or cystine, but not methionine, down-regulates the saturating, but not the nonsaturating, system. Growth in limiting SO(4) (2-) up-regulates the saturating system. Overall, a 500-fold change was observed. Reciprocal inhibition experiments demonstrated that molybdate and SO(4) (2-) are taken up by a common mechanism, but growth in molybdate failed to up-regulate SO(4) (2-) uptake. Regulation by growth in SO(4) (2-) or cystine did not markedly affect uptake of phosphate or of several organic compounds.The saturating system contributes 99% of SO(4) (2-) uptake under standard conditions, providing sufficient SO(4) (2-) so it is not limiting. In nature the same system likely contributes at least 65 to 70%.