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

Abstract

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.

OsALMT7 Maintains Panicle Size and Grain Yield in Rice by Mediating Malate Transport

“…As most anion channels mediate anion transport depending on the membrane potential and anion concentration on both sides of the plasma membrane, and the membrane potential of most plant cells is negative (hyperpolarized potential) under normal physiological conditions, anion channels generally favor the function of anion efflux from the cytosol. However, some cells such as xylem cells show depolarized membrane potential in plants (Wegner and Zimmermann, 2004;Wegner et al, 2011). Given that OsALMT7 mediates large inward/outward currents in oocytes ( Figures 6C to 6E, 7A and 7B), we speculate that OsALMT7 may mediate influx or efflux in spikelet cells on the panicle, depending on the membrane potential and cytosolic and extracellular malate concentration.…”

Flooding Impairs Fe Uptake and Distribution in Citrus Due to the Strong Down-Regulation of Genes Involved in Strategy I Responses to Fe Deficiency in Roots

“…It is here where bicarbonate production is enhanced as long-term waterlogging leads to oxygen depletion and CO 2 accumulation due to a hindered gas exchange from soil solution to ambient air [ 33 ]. Thus it is well-known that a high bicarbonate level induces Fe deficiency in plants through buffering soil solution to inhibit FC-R induction, which enhances the alkalinisation of xylem sap and the cell apoplast, and prevents Fe uptake and transport from roots to shoots [ 14 , 38 , 73 ]. However, our experiment was performed in the absence of HCO 3 - , which indicates that inhibition of Fe uptake by flooding under field conditions is not due only to the presence of this ion.…”

Transport and Use of Bicarbonate in Plants: Current Knowledge and Challenges Ahead

“…Enhanced Cl − supply under stress causes alkalization of the root apoplast due to the symport of 2 H + per 1 Cl − [ 127 ]. Increasing the external pH of the root bathing solution also increases pH of both A. thaliana root cell walls [ 128 ] and xylem sap [ 129 ]. This favors HCO 3 − over CO 2 formation.…”