The pressure-dependence of the hydraulic conductivity, the membrane resistance and membrane potential during turgor pressure regulation inValonia utricularis

Abstract

The pressure-dependence of the hydraulic conductivity and of electrical membrane parameters in Valonia utricularis are determined over a turgor pressure range of 4 to 4.5 arm by means of a direct measurement of the hydrostatic pressure inside the cell. The dependence of the hydraulic conductivity on pressure is calculated from the volume flows and subsequent changes in the turgot induced by both osmotic and hydrostatic gradients. L v is independent of pressure above 1 atm and equal under osmotic and hydrostatic conditions. Polarity of water movement is not observed. At pressures below 1 atm, Lv increases up to 10 times on approaching the point of plasmolysis. This increase is discussed in terms of membrane folding and of the stretching of pores within the membrane. In contrast to this finding the membrane resistance (normally 300 to 500 f~ cm z) increases markedly in response to higher pressures and reaches a maximum value of 1500 f~ cm 2 at about 2 atm. Further increase of the hydrostatic pressure reduces the membrane resistance again until the original value is reached at 3 to 4 atm. In the range of the maximum resistance the membrane potential (0 to + 15 mV at 1 arm) drops by 10 to 40 mV. At low pressures the membrane potential drops in the dark, while at high pressures and reduced membrane potential an increase of the p.d. is observed. From these results it is suggested that the electrogenic potassium pump postulated by Gutknecht is pressure-dependent and causes the observed changes in membrane resistance. According to this hypothesis, with increasing pressure the potassium influx would be reduced and the potassium efflux accelerated. This means that the electrogenic pump reverses its direction at high pressures.A characteristic feature of many algae is their capacity to regulate the internal water and ionic content and the cell turgor pressure in response to changes in the osmolarity and ionic composition of the environment. Three mechanisms of regulation are found in nature: regulation via water transport [10], via synthesis of osmotically active substances [4,16,21] and via ion transport [18,24,29]. The last mechanism has been observed in cells of Chaetomorpha linum, a littoral alga, which can adapt to large variations of the external osmotic pressure. As outlined in a previous paper [29], we 22, 332 U. Zimmermann and E. Steudle demonstrated that the regulation is controlled phenomenologically by the external potassium concentration and by the osmotic pressure of the external medium. To get more insight into the molecular mechanism of the turgor regulation which is coupled with the active transport of ions, we must know the dependence of several transport and membrane parameters, i.e. of the hydraulic conductivity Lp, of the membrane potential Go, of the membrane resistance Po, and of the ion fluxes on the hydrostatic pressure in the cell, P. A method of direct and continuous measurement of changes in the hydrostatic pressure inside the cells has been described for Valonia utricularis by us elsewhere [23,2...

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

“…However, it can be argued that these changes did not reflect the occurrence of streaming potentials, but rather changes in the membrane potentials of root cells generated by osmotically induced changes of the turgor pressure of these cells. Turgor pressure-dependent transport processes in the plasmalemma membrane of algae and higher plants are well known (Zimmermann & Steudle 1974;Zimmermann & Beckers 1977;Wendler, Zimmermann & Bentrup 1983;Stahlberg & Cosgrove 1997;Lew 1996).…”

Electrophysiology of Turgor Regulation in Marine Siphonous Green Algae

“…What does electrophysiology tell us about these processes? A particularly powerful tool is the simultaneous measurement of turgor and electrophysiological parameters in a single cell during the course of regulation (Zimmermann & Steudle, 1974;Beilby & Bisson, 1999;Bisson & Beilby, 2002;Heidecker et al, 2003b).…”

“…When turgor decreases, either through hyperosmotic stress or controlled by the turgor probe (Zimmermann & Steudle, 1974;Beilby & Bisson, 1999;Bisson & Beilby, 2002;Heidecker et al, 2003b), the membrane PD becomes more positive in most cells. The cell conductance also increases (Bisson & Beilby, 2002).…”

Turgor Pressure Sensing in Plant Cell Membranes

“…Although this has been the subject of many studies (1,2,12,14,15,17,19,25,27,29,31), the mechanisms by which turgor pressure information is sensed and utilized in osmoregulation and growth remain unknown.…”

“…For a membrane or membrane segment, which need not necessarily be structurally homogeneous, the mechanical restoring force (Pm), per unit area, is given by Pm = Ym(te") et ,'=Lo' (1) The regulation of turgor pressure in plant cells plays a vital role in both cell growth and homeostasis. Although this has been the subject of many studies (1,2,12,14,15,17,19,25,27,29,31), the mechanisms by which turgor pressure information is sensed and utilized in osmoregulation and growth remain unknown.…”