Sulfate Transport in Cultured Tobacco Cells : EFFECTS OF CALCIUM AND SULFATE CONCENTRATION

Sulfate transport by tobacco cells (var. Xanthi) cultured in liquid medium was investigated. Monophasic uptake was observed over a sulfate concentration range from 0.01 to 10 millimolar, and the K_m was 20 micromolar. A time-dependent stimulation of transport was observed when cells were incubated in medium containing 0.5 millimolar Ca²⁺. Calcium stimulation was dependent on the culture cycle and was maximal during the early exponential phase. It was not observed in sulfur-deficient cells with high transport rates and was relatively small in sulfate-loaded cells with low transport rates. A kinetic analysis showed that Ca²⁺ increased the maximum rate of transport without affecting the K_m.