Membrane potential and cytoplasmic pH in leaf cells of Olea europaea L. during urea treatments. A proposed, chemiosmotic model for Urea/H+ cotransport

Membrane potential (Em) and cytoplasmic pH (pHc) were monitored in leaf cells of Olea europaea L. during urea treatments. At 100 mM of urea, reversible membrane depolarisation of about 13 mV and transient cytoplasmic acidification were observed. When pH of the ureic solutions was buffered at 7.0 and 8.0, membrane depolarisation was, respectively, lower or almost absent. At 5°C or in presence of the uncoupler CCCP, depolarisation was completely inhibited. Instead with the photosynthesis inhibitor DCMU, depolarisation by urea was less than half. All these results are consistent to support a metabolic and pH dependent transport for urea. A chemiosmotic model for urea/H+ cotransport is proposed.