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Articles

Characteristics of Sulfate Transport Across Plasmalemma and Tonoplast of Carrot Root Cells ¹

School of Biological Sciences (A12), University of Sydney, New South Wales 2006, Australia

Compartmental analysis of ³⁵SO₄^2– exchange kinetics is used to obtain SO₄^2– fluxes and compartment contents in carrot (Daucus carota L.) storage root cells, where 2 to 5% of the SO₄^2– taken up is reduced to organic form. The necessary curve fitting is verified by (a) consistency between `content versus time' and `rate versus time' plots of washout data; (b) agreement between loading and washout kinetics; and (c) correct identification of the fastest exchange phase as being from extracellular spaces.

Sulfate is actively transported up an electrochemical potential gradient at both plasmalemma and tonoplast. The plasmalemma influx is from 2 to 10 times higher than the tonoplast influx, is much greater than the SO₄^2– reduction rate, and would not limit the rate of either. This is consistent with the finding that the plasmalemma influx is not regulated by internal SO₄^2– or cysteine (Cram 1982 Plant Sci Lett, in press).

Both SO₄^2– influxes rise with only limited saturation as the external SO₄^2– concentration increases up to 50 millimolarity. Both effluxes appear to be passive, with extensive recycling in the plasmalemma influx pump. SO₄^2– permeability is about 10^–11 meter per second at both membranes.

The high, nonlimiting fluxes of SO₄^2– at the plasmalemma relative to the tonoplast (found also in Lemna; Thoiron, Thoiron, Demarty, Thellier 1981 Biochim Biophys Acta 644: 24-35) contrasts with SO₄^2– fluxes in bacteria and with Cl^– fluxes in plant cells. Their implications for work on characteristics and regulation of SO₄^2– uptake in roots and tissue cultures are discussed.