Bi-phasic composition of trans-root electrical potential in roots of Plantago species: involvement of spatially separated electrogenic pumps.

The effect of oxygen on the trans-root potential (TRP) of excised roots in Plantago media L. and P. maritima L. was investigated. Two distinct reactions were found. In some experiments (type A roots) the reaction of TRP to anoxia was bi-phasic, and this reaction fits well into a model, assuming the presence of two spatially separated proton pump sites in the roots: one at the plasmalemma of epidermal and cortical cells and the other at the symplast/xylem interface. The two pumps work in opposite directions. In other experiments (type B roots) no hyperpolarization as a response to anoxia at the inner symplast membrane was observed. There is evidence that the inner pump is also present in these roots, but only in an inactive or electroneutral state. It is concluded that O2-deficiency prevails more often in the central part of the root than in epidermal and cortical cells, when roots are brought gradually under anoxia. This causes the pump located at the symplast/xylem interface to be inhibited more quickly than the other at decreasing O2-concentrations in the bathing solution.