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

Wegner, Lars H.; Sattelmacher, B.; Läuchli, André; Zimmermann, U.

doi:10.1046/j.1365-3040.1999.00506.x

Cited by 22 publications

(21 citation statements)

References 45 publications

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“…As shown in this study, bicarbonate addition to maize roots is always associated with a hyperpolarization of the TRP. A rapid response of the TRP most likely reflects a change in the membrane potential of cortex cells, as demonstrated previously for roots supplied with nitrate and ammonium (Wegner et al, 1999). Thus, a hyperpolarization would be in agreement with an electrogenic mechanism of bicarbonate uptake into root cortex cells.…”

Section: Discussionsupporting

confidence: 76%

Bicarbonate-Induced Alkalinization of the Xylem Sap in Intact Maize Seedlings as Measured in Situ with a Novel Xylem pH Probe

Wegner¹,

Zimmermann²

2004

Plant Physiology

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In higher plants the pH of the xylem sap plays an important role in drought signaling, growth regulation, and plant nutrition. However, the interpretation of the data is very controversial. The main reason for this is that the xylem pH in intact plants was not directly accessible hitherto. We present here a novel, minimally-invasive probe based on the xylem pressure-potential probe (used for measuring directly xylem pressure and the electrical potential between root xylem sap and medium). Singletipped, double-barreled capillaries were used, one barrel served as H 1 -selective electrode, whereas pressure and electrical potential were recorded by the other one. Upon insertion of the probe into the root xylem of maize (Zea mays) seedlings, pH values ranging between about 4.2 and 4.9 were monitored when the roots were immersed in standard nutrient solution. The pH did not respond to changes in light irradiation (up to 300 mmol m 22 s 21 ), but increased upon exposure of the root to 5 or 20 mM bicarbonate in the bath solution. Changes in pH could also be recorded in transpiring plants when the root was cut below the insertion point of the probe and placed in media with different pH. The data support the hypothesis of Mengel ([1994] Plant Soil 165: 275-283) that upon external supply with bicarbonate Fe is immobilized in the leaf apoplast via changes in xylem pH.

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Section: Discussionsupporting

confidence: 76%

Bicarbonate-Induced Alkalinization of the Xylem Sap in Intact Maize Seedlings as Measured in Situ with a Novel Xylem pH Probe

Wegner¹,

Zimmermann²

2004

Plant Physiology

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“…Despite the interesting features of this model, little effort has been spent in exploring electric‐hydraulic coupling phenomena in tall trees. Recent integration of a microelectrode into a xylem pressure probe and application of this xylem pressure‐potential probe to ‘laboratory‐sized’ plants has provided some insight into this subject matter (Wegner and Zimmermann 1998, Wegner et al. 1999).…”

Section: Involvement Of Electric Driving Forces In Water Risementioning

confidence: 99%

“…Interestingly, the response of the trans ‐root potential was always faster than the response of the xylem pressure upon illumination (Wegner and Zimmermann 1998). Maize plants that were temporarily deprived of any N‐source additionally exhibited a close electrical coupling between the xylem and the accessory cells (Wegner et al. 1999).…”

Section: Involvement Of Electric Driving Forces In Water Risementioning

confidence: 99%

What are the driving forces for water lifting in the xylem conduit?

Zimmermann

Schneider

Wegner

et al. 2002

Physiologia Plantarum

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After Renner had shown convincingly in 1925 that the transpirational water loss generates tensions larger than 0.1 MPa (i.e. negative pressures) in the xylem of cut leafy twigs the Cohesion Theory proposed by Böhm, Askenasy, Dixon and Joly at the end of the 19th century was immediately accepted by plant physiologists. Introduction of the pressure chamber technique by Scholander et al. in 1965 enforced the general belief that tension is the only driving force for water lifting although substantial criticism regarding the technique and/or the Cohesion Theory was published by several authors. As typical for scientific disciplines, the advent of minimal- and non-invasive techniques in the last decade as well as the development of a new, reliable method for xylem sap sampling have challenged this view. Today, xylem pressure gradients, potentials, ion concentrations and volume flows as well as cell turgor pressure gradients can be monitored online in intact transpiring higher plants, and within a given physiological context by using the pressure probe technique and high-resolution NMR imaging techniques, respectively. Application of the pressure probe technique to transpiring plants has shown that negative absolute pressures (down to - 0.6 MPa) and pressure gradients can exist temporarily in the xylem conduit, but that the magnitude and (occasionally) direction of gradients contrasts frequently the belief that tension is the only driving force. This seems to be particularly the case for plants faced with problems of height, drought, freezing and salinity as well as with cavitation of the tensile water. Reviewing the current data base shows that other forces come into operation when exclusively tension fails to lift water against gravity due to environmental conditions. Possible candidates are longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels as well as gel- and gas bubble-supported interfacial gradients. The multiforce theory overcomes the problem of the Cohesion Theory that life on earth depends on water being in a highly metastable state.

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“…if a hydraulic continuum exists throughout the leaf tissue and xylem), measurements of the balancing pressure value of non-transpiring plants are a direct measure of µ w,h=0 in the xylem and, in turn, of xylem pressure when the osmotic pressure is very low and gel-like substances are absent. This had been demonstrated for well-hydrated and non-transpiring maize and sugarcane plants and tobacco leaves by simultaneous application of the presfrom on-line measurements in the xylem of intact plants by using xylem pressure probes equipped with potential measuring and ion-selective microelectrodes (Wegner and Zimmermann 1998;Wegner et al 1999;Schneider et al 2000a). Of equal importance for the evaluation of the contribution of osmotic pressure to xylem water lifting is the "compression-decompression" method introduced by Schill et al (1996).…”

Section: Tension Versus Water Activitymentioning

confidence: 99%

Implications of mucilage on pressure bomb measurements and water lifting in trees rooting in high-salinity water

Zimmermann

Heidecker

et al. 2001

Trees

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Various studies on mangroves and other tall trees rooting in high-salinity water have given compelling evidence that tension is not the only factor in water lifting as thought by plant physiologists. A characteristic feature of these trees is that the tissue cells, the apoplastic space and, in particular, the lumen and the inner walls of many xylem vessels of the roots, the trunk and the branches (up to the apex) contain mucilage. Data on single marine giant algal cells are presented that show that mucilage reduces the chemical activity of water. Longitudinal gradients in the chemical activity of water and interfacial forces are presumably the dominant forces for water lifting. In order to save water on its tortuous pathway to the uppermost foliage trees apparently use different strategies (as revealed by 1 H-NMR imaging), e.g. reduction of the conducting xylem area in the branches at intermediate height by mucilage or interruption of the xylem water columns by gas-filled segments and water lifting through mucilage networks and surface films. Pressure bomb experiments over the entire height of the trees revealed clearly that balancing pressure values cannot be taken as a measure for xylem tension. Such values can be used generally for an estimation of the chemical potential of water in the xylem of leafy twigs under atmospheric pressure, µ w,h=0 , provided that a species-specific "threshold pressure" (depending on wood density, elastic forces of the tissue, hydraulic coupling between xylem and tissue cells, intercellular spaces, cellular osmotic pressure etc.) is subtracted from the balancing pressure values. Transpiration increases the "threshold pressure" considerably and in an unpredictable way. Thus, as shown here, predawn balancing pressure data taken at various heights can yield information about the height dependence of µ w (measured at h=0) under field conditions, particularly when the water content of the xylem is simultaneously determined in a reliable manner (e.g. by the compression/decompression method in combination with centrifugation).

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

Cited by 22 publications

References 45 publications

Bicarbonate-Induced Alkalinization of the Xylem Sap in Intact Maize Seedlings as Measured in Situ with a Novel Xylem pH Probe

Bicarbonate-Induced Alkalinization of the Xylem Sap in Intact Maize Seedlings as Measured in Situ with a Novel Xylem pH Probe

What are the driving forces for water lifting in the xylem conduit?

Implications of mucilage on pressure bomb measurements and water lifting in trees rooting in high-salinity water

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