Localization of sulfate uptake and pH changes at sulfur-deprived roots of intact Brassica pekinensis seedlings by using H⁺-selective microelectrodes

Proton-selective microelectrodes were used to determine sulfate uptake by roots of intact plant seedlings. The response of H+ fluxes to sulfate addition showed to be a good proxy for sulfate uptake by the sulfate/H+ co-transport system. H+ influx and increase in root surface pH was much higher in sulfate-deprived seedlings than in seedlings grown with sufficient sulfate. The opposite was true for the response of H+ fluxes to nitrate addition. By using this method sulfate uptake could be mapped along the root axis, which revealed higher uptake rates in mature regions. Sulfate deprived roots showed a lower root surface pH, which correlated strongly with the response to sulfate addition. A possible contribution of this component to a higher sulfate uptake capacity under sulfur deficiency was further tested by using the fungal toxin fusicoccin, which permanently activates the plasma membrane H+-pumping ATPase. Application of fusicoccin lowered the pH of sufficient roots to the level of deficient roots, indicating a more activated state of the ATPase under sulfur deficiency rather than a higher abundance.