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

Abstract

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.

Trans‐root potential, xylem pressure, and root cortical membrane potential of ‘low‐salt’ maize plants as influenced by nitrate and ammonium

“…Assuming that a radial electrical gradient did not exist in the symplast (Dunlop & Bowling 1971a), the TRP was considered to be the difference between the membrane potentials of cortical cells and xylem parenchyma cells (both membranes being opposite in polarity with respect to the overall potential difference; Dunlop & Bowling 1971a,b). Similar conclusions were obtained when the effect of sugars on cortical membrane potentials and on the TRP were studied (Kennedy & Steward 1982) as well as when roots were treated with anoxia (De Boer, Prins & Zanstra 1983). These data have been interpreted by analogue electrical circuits of the root (Ginsburg 1972;Cortes 1993;Due 1993).…”

Simultaneous recording of xylem pressure and trans‐root potential in roots of intact glycophytes using a novel xylem pressure probe technique

“…For example, K + -ion gradients in the root tissue as revealed with the electronprobe microanalysis technique Dunlop & Bowling 1971a) could not be detected by successive radial insertion of a K + -selective microelectrode into the root tissue. Furthermore, measurements of transroot potentials lead to conflicting data concerning the role of stelar proton pumps (Dunlop 1973;Dunlop & Bowling 1971b;De Boer et al 1983).In this communication we demonstrate that the problems of the microelectrode technique mentioned above can be overcome by incorporation of a potential-recording Ag/AgCl wire into the xylem pressure probe (Balling & Zimmermann 1990; Schneider et al 1997a,b). Such a xylem pressure-potential probe has the advantages (1) that a proper probing of a vessel is indicated by the pressure signal, (2) that clogging of the microcapillary tip can easily be removed by the displacement of the metal rod within the pressure probe and (3) that the integrated electrode in the probe can be used to determine separately the radial electrical resistance of the root (which is an indicator for a short-circuit of the tissue).…”

“…For example, K + -ion gradients in the root tissue as revealed with the electronprobe microanalysis technique Dunlop & Bowling 1971a) could not be detected by successive radial insertion of a K + -selective microelectrode into the root tissue. Furthermore, measurements of transroot potentials lead to conflicting data concerning the role of stelar proton pumps (Dunlop 1973;Dunlop & Bowling 1971b;De Boer et al 1983).…”

“…Some authors (Bowling & Spanswick 1964;Shone 1968Shone , 1969Davis & Higinbotham 1969;Dunlop & Bowling 1971a; De Boer, Prins & Zanstra 1983;Kennedy 1977;Vuletic & Vucinic 1996) determined this parameter on excised roots by measuring the voltage drop between the exudate and the bathing solution. Other authors (Dunlop & Bowling 1971a,b;Bowling 1972;Dunlop 1973Dunlop , 1982 recorded the potential profile between the 'xylem' and the external medium of intact plants by insertion of a microelectrode into the root tissue.…”

Control of Vascular Sap pH by the Vessel-Associated Cells in Woody Species (Physiological and Immunological Studies)

“…The results may also be compared with data from perfusion experiments in which young roots of herbaceous species were used. Here the opposite effects of anoxia and FC on the pH of the perfusion liquid flowing through the vessels of primary xylem indicated that the vascular sap pH depends on the activity of the plasma membrane H + -ATPase of cells probably located in the stele or surrounding the stele (De Boer et al, 1983;Clarkson and Hanson, 1986). Immunocytolocalization of this enzyme or in situ expression in sections of young roots indicated that the H + -ATPase was highly concentrated in the endodermis (Parets-Soler et al, 1990) or pericycle (Samuels et al, 1992), which is adjacent to the primary vessels.…”