Vacuoles from Sugarcane Suspension Cultures : III. PROTONMOTIVE POTENTIAL DIFFERENCE.

The electrochemical proton gradient across the tonoplast of isolated (Saccharum sp.) vacuoles and vacuoles in situ was measured. The isolated vacuoles show no significant protonmotive potential difference, the pH gradient of 0.8 (inside acid) was balanced by a membrane potential of about -80 mv (inside negative). From pH and uncoupler insensitivity and K(+) sensitivity, it was concluded that the experimentally caused K(+) gradient created the electric potential.Qualitatively different results were obtained on vacuoles in situ: the pH gradient is greater (1.3), the membrane potential positive inside. Uncoupler sensitivity is evidence for an enzyme system transducing protons inward as a cause for the considerable protonmotive force at the tonoplast in situ.