Sulfate uptake and its regulation in Lemna paucicostata Hegelm. 6746

The results of studies of SO4 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 Km and high specificity for SO4 2-. Uptake by the nonsaturating component is less affected by pH and temperature than is that of the saturating system. SO4 2- efflux is not quantitatively important in Lemna under standard conditions (20 micromolar SO4 2-) (Datko AH, SH Mudd 1980 Plant Physiol 65: 906-912). 55% of newly taken up 35SO4 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 SO4 2- or cystine, but not methionine, down-regulates the saturating, but not the nonsaturating, system. Growth in limiting SO4 2- up-regulates the saturating system. Overall, a 500-fold change was observed. Reciprocal inhibition experiments demonstrated that molybdate and SO4 2- are taken up by a common mechanism, but growth in molybdate failed to up-regulate SO4 2- uptake. Regulation by growth in SO4 2- or cystine did not markedly affect uptake of phosphate or of several organic compounds. The saturating system contributes 99% of SO4 2- uptake under standard conditions, providing sufficient SO4 2- so it is not limiting. In nature the same system likely contributes at least 65 to 70%.