Aluminum Toxicity in Roots : Correlation among Ionic Currents, Ion Fluxes, and Root Elongation in Aluminum-Sensitive and Aluminum-Tolerant Wheat Cultivars

The inhibition of root growth by aluminum (Al) is well established, yet a unifying mechanism for Al toxicity remains unclear. The association between cell growth and endogenously generated ionic currents measured in many different systems, including plant roots, suggests that these currents may be directing growth. A vibrating voltage microelectrode system was used to measure the net ionic currents at the apex of wheat (Triticum aestivum L.) roots from Al-tolerant and Al-sensitive cultivars. We examined the relationship between these currents and Al-induced inhibition of root growth. In the Al-sensitive cultivar, Scout 66, 10 micromolar Al (pH 4.5) began to inhibit the net current and root elongation within 1 to 3 hours. These changes occurred concurrently in 75% of experiments. A significant correlation was found between current magnitude and the rate of root growth when data were pooled. No changes in either current magnitude or growth rate were observed in similar experiments using the Al-tolerant cultivar Atlas 66. Measurements with ion-selective microelectrodes suggested that H⁺ influx was responsible for most of the current at the apex, with smaller contributions from Ca²⁺ and Cl⁻ fluxes. In 50% of experiments, Al began to inhibit the net H⁺ influx in Scott 66 roots at the same time that growth was affected. However, in more than 25% of cases, Al-induced inhibition of growth rate occurred before any sustained decrease in the current or H⁺ flux. Although showing a correlation between growth and current or H⁺ fluxes, these data do not suggest a mechanistic association between these processes. We conclude that the inhibition of root growth by Al is not caused by the reduction in current or H⁺ influx at the root apex.     

Sodium and Potassium Ion Effluxes from Squid Axons under Voltage Clamp Conditions

Squid giant axons loaded with Na²⁴ were subjected to short duration (0.5 msec.) clamped depolarizations of about 100 mv at frequencies of 20/sec. and 60/sec. while in choline sea water. Under such conditions the early outward current was just about maximal at the time of termination of the clamping pulse. An integration of the early current versus time record gave 1.2 μcoulomb/cm² pulse, while a measurement of the extra Na²⁴ efflux resulting from repetitive pulsing gave a charge transfer of 1.4 μcoulomb/cm² pulse. In sodium-containing sea water and with pulses 50-75 mv more positive than E_Na the Na²⁴ efflux is about 3 times the measured charge transfer. The efflux of K⁴² from a previously loaded axon into normal sea water is only 50 per cent of the measured charge transfer when the membrane is held for about 5 msec. at a potential such that there is no early current, and such pulses are at 10-20/sec. The experiments appear to confirm the suggestion that the early current during bioelectric activity is sodium but provide unsatisfactory support for the identification of the delayed but sustained current solely with potassium ions. Resting Na⁺ efflux is 0.6 pmole/cm² sec. mmole [Na]₁, while the apparent K⁺ efflux is about 250 pmole/cm² sec. and is little affected by hyperpolarization.     

Chloride Currents in Skeletal Muscles of Bufo arenarum

Cl⁻ currents (I _Cl) were measured in short fibers (1–2 mm) from the lumbricalis muscle of toads (Bufo arenarum) with two microelectrodes (15°C). Initially the fibers were equilibrated in a high K⁺-containing solution: (mm) K₂SO₄ 68; Na₂SO₄ 20; KCl 60; CaSO₄ 8; MgSO₄ 1; HEPES 2.5. Constant pulses were applied when all the external K⁺ was replaced by Cs⁺: Cs₂SO₄ 68; Na₂SO₄ 20; CsCl 60; CaSO₄ 8; HEPES 2.5 (pH 7.5). Under these conditions about 80–90% of the current is carried by Cl⁻. The current-voltage relation is almost linear implying constant conductance and hence voltage-independent permeability. The voltage dependence of the net Cl⁻ current could be fitted by constant field equation with a P _Cl of 3.3 × 10⁻⁶ cm/sec. In a separate group of experiments a two-pulse technique was used to estimate the availability and the inactivation of the initial I _Cl during a test pulse. After returning the potential to the holding potential for various times, test pulses of the same amplitude and duration of the prepulses were applied. The initial current during the test pulse was 70% of the initial current during the prepulse and the recovery was complete in less than 300 msec with a linear relationship between the current during the test pulse and the amplitude of the preceding prepulse. When the test pulses were preceded by a positive prepulse, the initial current for any given test pulse was larger than with a negative prepulse. If we assumed that the initial current during the test pulse is a measure of the number of channels open at the end of the prepulse, these results suggest that hyperpolarizing pulses inactivate and depolarizing prepulses activate the I _Cl. Received: 31 March 1995/Revised: 27 October 1995     

K+/H+ antiporter in alkaliphilic Bacillus sp. no. 66 (JCM 9763)

A K⁺/H⁺ antiport system was detected for the first time in right-side-out membrane vesicles prepared from alkaliphilic Bacillus sp. no. 66 (JCM 9763). An outwardly directed K⁺ gradient (intravesicular K⁺ concentration, K_in, 100 mM; extravesicular K⁺ concentration, K_out, 0.25 mM) stimulated uphill H⁺ influx into right-side-out vesicles and created the inside-acidic pH gradient (ΔpH). This H⁺ influx was pH-dependent and increased as the pH increased from 6.8 to 8.4. Addition of 100 μM quinine inhibited the H⁺ influx by 75%. This exchange process was electroneutral, and the H⁺ influx was not stimulated by the imposition of the membrane potential (interior negative). Addition of K⁺ at the point of maximum ΔpH caused a rapid K⁺-dependent H⁺ eflux consistent with the inward exchange of external K⁺ for internal H⁺ by a K⁺/H⁺ antiporter. Rb⁺ and Cs⁺ could replace K⁺ but Na⁺ and Li⁺ could not. The H⁺ efflux rate was a hyperbolic function of K⁺ and increased with increasing extravesicular pH (pH_out) from 7.5 to 8.5. These findings were consistent with the presence of K⁺/H⁺ antiport activity in these membrane vesicles. Received: March 20, 1997 / Accepted: May 22, 1997     

Effects of ethanol,octanoic and decanoic acids on fermentation and the passive influx of protons through the plasma membrane of Saccharomyces cerevisiae

Ethanol, octanoic and decanoic acids are known toxic products of alcoholic fermentation and inhibit yeast functions such as growth and fermentation. pH-stat measurements showed that, in a concentration range up to 20 mg/l, octanoic and decanoic acids increase the rate of passive H⁺ influx across the plasma membrane of Saccharomyces cerevisiae IGC 3507. Decanoic acid was more active than octanoic acid, which agrees with its higher liposolubility. The fatty acids probably act as H⁺ carriers, since the magnitude of the effect depended on pH and correlated with the concentration of protonated fatty acids. Esterification of the fatty acids partially abolished the enhancing effect on passive H⁺ influx. Passive H⁺ influx showed saturation kinetics with half-maximal activity at 6.6 M H⁺ (pH 5.2). Contrary to previous findings, ethanol inhibited H⁺ influx exponentially up to a concentration of 8% (v/v). At higher concentrations, ethanol reactivated H⁺ influx; the original rate of H⁺ uptake was reached at 14% (v/v) ethanol. In the same concentration ranges that affected passive H⁺ influx, ethanol, octanoic and decanoic acids inhibited the fermentation rate. This inhibitory effect of the fatty acids on fermentation rate depended on liposolubility, pH, and esterification in the same way as that found for their effect on passive H⁺ influx. Inhibition of fermentation by octanoic and decanoic acids could therefore result from their effect on the rate of passive H⁺ influx. Correspondence to: S. Stevens     

Role of calcium and other ions in directing root hair tip growth in Limnobium stoloniferum

The magnitude and spatial localization of Ca²⁺, K⁺ and H⁺ fluxes in growing and non-growing Limnobium stoloniferum root hairs was determined using non-invasive, ion-selective vibrating microelectrodes. Both the spatial pattern and magnitude of the ionic flux was dependent on the particular ion in question. Both H⁺ and Ca²⁺ influx was localized almost exclusively to the tips of growing root hairs, suggesting that these fluxes may be involved in directing growth. Influx of K⁺ showed no distinct localization and uptake appeared uniform along the length of the root hair. Competitive inhibition of Ca²⁺ influx using a range of Mg⁺ concentrations indicated that the magnitude of the Ca²⁺ flux entering the root hair tip did not determine growth rate; however, the presence of Ca²⁺ on the external face of the membrane was implicit for root hair integrity. Aluminum proved to be a potent inhibitor of root hair growth. At an exogenous Al concentration of 20 M a complete blockage of Ca²⁺ influx into root hair tips was observed, suggesting that Al blockage of Ca²⁺ influx could be involved in Al toxicity. However, at a lower Al concentration (2 M), Ca²⁺ fluxes were unaffected while inhibition of growth was still observed along with a distinct swelling of the root hair tip. The swelling at the root hair tips was identical in appearance to that seen in the presence of microtubule inhibitors, suggesting that Al could influence a number of different sites at the plasma-membrane surface and within the cell. The possible role(s) of Ca²⁺ and H⁺ fluxes in directing tip growth are discussed.     

Boric acid stimulates the plasma membrane H+-ATPase of ungerminated lily pollen grains

The stimulation of the plasma membrane (PM) H⁺-ATPase by boric acid was studied on a microsomal fraction (MF) obtained from ungerminated, boron-dependent pollen grains of Lilium longiflorum Thunb. which usually need boron for germination and tube growth. ATP hydrolysis and H+ transport activity increased by 14 and 18%, respectively, after addition of 2-4 mM boric acid. The optimum of boron stimulation was at pH 6.5-8.5 for ATP hydrolysis and at pH 6.5-7.5 for H⁺ transport. No boron stimulation was detected when vanadate was added to the MF, whereas an increase of 10-20% in ATP hydrolysis and H⁺ transport was still measured in the presence of inhibitors specific for V -type ATPase (nitrate and bafilomycin) and F-type ATPase (azide), respectively. A vanadate-sensitive increase in ATP hydrolysis activity was also observed in partially permeabilized vesicles (0.001%[w/v] Triton X-100) suggesting a direct interaction between borate and the PM H⁺-ATPase rather than a weak acid-induced stimulation. Additionally, we measured the effect of boron on membrane voltage (V_m) of ungerminated pollen grains and observed small hyperpolarizations in 48% of all experiments. Exposing pollen grains to a more acidic pH of 4 caused a depolarization, followed in some experiments by a repolarization (21%). In the presence of 2 mM boron such hyperpolarizations, perhaps caused by an enhanced activity of the H⁺-ATPase, were measured in 58% of all tested pollen grains. The effects of boron on V_m may be reduced by additional stimulation of a K⁺ inward current of opposite direction to the H⁺-ATPase. All experiments indicate that boron stimulates an electrogenic transport system in the plasma membrane which is sensitive to vanadate and has a pH optimum around 7, i.e. the plasma membrane H⁺-ATPase. A boron-increased PM H⁺-ATPase activity in turn may stimulate germination and growth of pollen tubes.     

Sodium ATPase and Sodium/proton Antiporter Are Not Obligatory for Sodium Homeostasis of Enterococcus hirae at Acid pH

Enterococcus hirae grows in a broad pH range from 5 to 11. An E. hirae mutant 7683 lacking the activities of two sodium pumps, Na⁺-ATPase and Na⁺/H⁺ antiporter, does not grow in high Na⁺ medium at pH above 7.5. We found that 7683 grew normally in high Na⁺ medium at pH 5.5. Although an energy-dependent sodium extrusion at pH 5.5 was missing, the intracellular levels of Na⁺ and K⁺ were normal in this mutant. The Na⁺ influx rates of 7683 and two other strains at pH 5.5 were much slower than those at pH 7.5. These results suggest that Na⁺ elimination of this bacterium at acid pH is achieved by a decrease in Na⁺ entry and a normal K⁺ uptake.     

Acid-base regulation during nitrate assimilation in Hydrodictyon africanum

Abstract The acid-base balance during NO₃^? assimilation in Hydrodictyon africanum has been investigated during growth from (1) an analysis of the elemental composition of the cells, (2) the alkalinity of the ash and (3) the net H⁺ changes in the medium during growth. These investigations agree in showing that some 0.25 excess organic negative charges are generated per N assimilation from No₃^? as N-source and C02 as C-source; the excess OH^? (0.75 OH^? per NO₃^? assimilated) appears in the medium. Approximately half of the excess organic negative charge is attributable to cell wall uronates; the remainder is intracellular. All of the excess OH^? appearing in the medium must have crossed the plasmalemma (as net downhill H⁺ influx or OH^? efflux). Previous work has shown that the value of ψco is more negative than ψ_K⁺ during NO₃^? assimilation, suggesting that the active electrogenic H⁺ extrusion pump is still operative despite the net downhill H⁺ influx. The interpretation of this in terms of H⁺?NO₃^? symport which causes the entry of more H⁺ than is consumed in NO₃^? metabolism, with extrusion of the excess H⁺via the active, electrogenic H⁺ pump, was tested by measuring short-term H⁺ influx upon addition of NO^?₃. A net H⁺ influx occurs before NOa assimilation (as indicated by additional O₂ evolution in the light) has commenced, suggesting a mechanistic relation of H⁺ and NO₃^? influxes. This is consistent with the interpretation suggested above. Determinations of cytoplasmic pH showed no significant effect of NO₃^? assimilation, suggesting that cytoplasmic pH changes sufficient to change the ‘pH-regulating’ H⁺ fluxes are smaller than the errors in the determination of cytoplasmic pH.     

Activation of Na+,H+-Exchange in Erythrocytes of the River Lamprey Lampetra fluviatilis

To study H⁺ transport, the lamprey red blood cells were acidified to pH 6.0 by a pretreatment with an ionophore, nigericin. Incubation of the acidified cells in NaCl-medium at pH 8.0 was accompanied by a rapid H⁺ efflux from the erythrocytes. There was a tenfold decrease of the H⁺ efflux rate on addition to NaCl-medium of dimethylamiloride or on replacing Na⁺ in the medium (KCl-medium, pH 8.0). A high rate of Na+ influx into the acidified erythrocytes occurred only in the presence of H⁺ gradient (pH medium 8.0), but not in its absence (pH medium 6.0). The Na⁺-dependent H⁺ efflux from the cells and H⁺-dependent Na⁺ influx into the cells were quantitatively similar (about 700 mmol/l cells/h). A rapid elevation of the intracellular Na⁺ concentration as measured by flame photometry was also observed during incubation of the acidified cells in NaCl-medium (pH 8.0). The H⁺-dependent Na⁺ influx and an increase of the Na⁺ content in the acidified cells were significantly inhibited by amiloride. The data obtained for the first time prove with certainty the presence of the Na⁺/H⁺ exchanger in erythrocytes of the river lamprey.     

31P NMR Measurements of Cytoplasmic pH Changes in Maize Root Tips

³¹P nuclear magnetic resonance (NMR) spectroscopy has been usedto measure the changes caused by sodium azide and anaerobiosisin the cytoplasmic pH of maize (Zea mays L.) root tips. Measurementsof H⁺ influx, lactate accumulation and ATP hydrolysis, togetherwith estimates of the cytoplasmic volume and the cytoplasmicbuffering capacity, were used to calculate the expected pH changesunder the same conditions. In the case of azide inhibition,where there was a large H⁺ influx, there was good agreementbetween the calculated and measured pH changes. In contrastduring anoxia, where the accumulation of lactate was the mostimportant source of protons, the predicted cytoplasmic pH changewas greater than the observed change. It was concluded thatin the absence of a compensating H⁺ efflux, the excess H⁺ productionwas offset by the production of H⁺-consuming metabolites suchas -aminobutyrate. Key words: Cytoplasmic pH, ³¹P NMR, Membrane H⁺ fluxes.     

H Cotransports in Corn Roots as Related to the Surface pH Shift Induced by Active H Excretion

The surface pH shift induced by active H⁺ excretion in corn (Zea mays L.) roots was estimated using acetic acid influx as a pH probe (H Sentenac, C Grignon 1987 Plant Physiol 84: 1367-1372). At constant bulk pH, buffering the medium strongly reduced the magnitude of the surface pH shift. This was used to study the effect of surface pH shift on H⁺ cotransports. In the absence of buffers, the surface pH shift increased with the bulk pH. Buffers decreased ³²Pi influx and this effect was stronger at pH 7.2 than at pH 5.8, and stronger in the absence than in the presence of an inhibitor of the proton pump (vanadate). Buffers exerted a similar depressive and pH-dependent effect on net NO₃⁻ uptake. They hyperpolarized the cell membrane, and stimulated ⁸⁶Rb⁺ influx, K⁺:H⁺ net exchange, and malate accumulation. These results are consistent with the hypothesis that H⁺ accumulation at the cell surface is effective in driving H⁺ reentry. We concluded that the surface pH shift due to proton pump activity is involved in the energetic coupling of H⁺ cotransports.     

Ion currents and the nitrogen status of roots of Hordeum vulgare and non-nodulated Trifolium repens

Abstract. Profiles of self-generated ion currents associated with the growing primary root tips of intact Hordeum vulgare L. and Trifolium repens L. (nonnodulated) seedlings were measured using a highly sensitive vibrating electrode in media containing NH⁺₄ or NO^-₃, and compared to control roots growing in nitrogen free media. Under these three nutrient regimes, positive current entered the root at regions corresponding to the meristematic tissues and main elongation zones of root tips and left from the mature root tissues. Mapping the surface of the roots with a pH-sensitive microelectrode revealed regions of external alkalinity where positive electrical current entered the root, and external acidity where positive current exited. The correlation between pH-profile and the pattern of ion current generation in these experiments suggests that H⁺ ions were responsible for carrying the bulk of the root-generated current. Assimilation of NHJ results in net H⁺ extrusion while assimilation of NO^-₃, results in net OH^-₃ efflux. Growth on NH⁺₄, as compared to growth on NO^-₃, stimulated the magnitude of the electrical current but did not affect significantly the growth rate of the roots. However, despite the differing stresses on internal pH regulation that arise due to growth on the two exogenous forms of combined nitrogen, the current profiles were qualitatively similar under the different conditions that were examined. The role of the circulating proton current is not yet known; however, the constancy of the current profile under different nutrient regimes sustains the hypothesis that the current may have a role in the regulation of root polarity.     

K+ Uptake by Fermenting Escherichia coli Cells: pH Dependent Mode of the TrkA System Operating

Escherichia coli accumulates K⁺ by means of multiple transportsystems, of which TrkA is the most prominent at neutral and alkalinepH while Kup is major at acidic pH. In the present study, K⁺ uptakewas observed with cells grown under fermentative conditions at an initialpH of 9.0 and 7.3 (the medium pH decreased to 8.4 and 6.8, respectively,during the mid-logarithmic growth phase), washed with distilled water andresuspended in a K⁺ containing medium at pH 7.5 in the presence ofglucose. The kinetics for this K⁺ uptake and the amount of K⁺accumulated by the wild type and mutants having a functional TrkA orKup could confirm that K⁺ uptake by E. coli grown either at pH 9.0or pH 7.3 occurs mainly through TrkA. The following results distinguishpH dependent mode of TrkA operating: (1) K⁺ uptake was inhibited byDCCD in cells grown either at pH 9.0 or pH 7.3, although the stoichiometryof K⁺ influx to DCCD-inhibited H⁺ efflux for bacteria grownat pH 9.0 varied with external K⁺ concentration, but remained constantfor cells grown at pH 7.3; (2) K⁺ uptake was observed with an atpDmutant grown at pH 9.0 but not at pH 7.3; (3) The DCCD-inhibited H⁺efflux was increased 8-fold less by 5 mM K⁺ added into a K⁺ freemedium for bacteria grown at pH 9.0 than that for cells grown at pH 7.3;(4) the DCCD-inhibited ATPase activity of membrane vesicles from bacteriagrown at pH 9.0 was reduced a little in the presence of 100 mM K⁺,but stimulated more than 2.4-fold at pH 7.3.     

Sucrose and proton cotransport in Ricinus cotyledons

In Ricinus cotyledons, evidence for proton extrusion came from observation of direct acidification of the medium in the presence of potassium salts. Increasing K⁺ influx with increasing pH suggested a link between K⁺ influx and H⁺ efflux by an H⁺ pump. The kinetics of K⁺ influx and H⁺ efflux were consistent with a 1:1 stoichiometry K⁺:H⁺, which may indicate either electrical coupling or carrier mediated exchange. The results were consistent with an H⁺ pump setting up an electrochemical potential gradient which provides the driving force for an H⁺-sucrose cotransport and the movement of K⁺. With reference to this, a model for phloem loading is suggested.     

Acid-base regulation during ammonium assimilation in Hydrodictyon africanum

Abstract The acid-base balance during ammonium (used to mean NH ₄⁺ and/or NH₃) assimilation in Hydrodictyon africanum has been measured on cells growing with about 1 mol m^?3 ammonium at an external pH of about 6.5. Measurements made included (1) ash alkalinity (corrected for intracellular ammonium) which yields net organic negative charge, (2) the accumulation of organic N in the cells and (3) the change in extracellular H⁺ (from the pH change and the buffer capacity). These measurements showed that some 0.25 excess organic negative charge (half in the cell wall, half inside the plasmalemma) accumulates per organic N synthesized, while some 1.25H⁺ accumulate in the medium per organic N synthesized. Granted a permeability (PNH₃) of some 10^?3 cm s^?1, and a finite [NH₃] in the cytoplasm of these N-assimilating cells it is likely that most of the ammonium entering these growing cells is as NH ₄⁺. This means that most of the H ⁺ appearing in the medium must have originated from inside the cell and have been subjected to active efflux at the plasmalemma: H⁺ accumulates in the medium equivalent to any NH₃ entry by requilibration from exogenous NH ₄⁺. The cell composition (net organic negative charge, organic N content) is very similar in these ammonium-grown cells to that of NO₃⁺grown cells, suggesting that there is no action of a ‘biochemical pH stat’ during longterm assimilation of NO₃⁺in H. africanum. Short-term experiments were carried out at an external pH of 7.2 in which ammonium at various concentrations were supplied to NO₃⁺-grown cells. There was in all cases a rapid influx followed by a slower uptake; at least at the lower concentrations (less than 100 μmol dm^?3) the net influx was all attributable to NH₄⁺influx via a uniporter, probably partly short-circuited by a passive NH₃ efflux due to intrinsic membrane permeability to NH₃. The net ammonium influx was in all cases associated with H⁺ accumulation in the medium. (1.3-1.7 H ⁺ per ammonium taken up); as in the growth experiments, most of the ammonium taken up was assimilated. Determinations of cytoplasmic pH showed either no effect on, or a slight decrease in, pH during ammonium assimilation; the changes that occurred were in the direction expected for actuating a ‘pH-regulating’ change in H⁺ fluxes.     

Transcellular ion currents and extension ofNeurospora crassa hyphae

Summary Hyphae ofNeurospora crassa, like many other tipgrowing organisms, drive endogenous electric currents through themselves such that positive charges flow into the apical region and exit from the trunk. In order to identify the ions that carry the current, the complete growth medium was replaced by media lacking various constituents. Omission of K⁺ or of phosphate diminished the zone of inward current, effectively shifting the current pattern towards the apex. Omission of glucose markedly reduced both inward and outward currents; addition of sodium azide virtually abolished the flow of electric current. Growing hyphae also generate a longitudinal pH gradient: the medium surrounding the apex is slightly more alkaline than the bulk phase, while medium adjacent to the trunk turns acid. The results suggest thatNeurospora hyphae generate a proton current; protons are expelled distally by the H⁺-ATPase and return into the apical region by a number of pathways, including the symport of protons with phosphate and potassium ions. Calcium influx may also contribute to the electric current that enters the apical region. There seems to be no simple obligatory linkage between the intensity of the transcellular electric current and the rate of hyphal extension. Calcium ions, however, are required in micromolar concentrations for extensions and morphogenesis of hyphal tips.     

Aluminum‐dependent dynamics of ion transport in Arabidopsis: specificity of low pH and aluminum responses

Low‐pH and Al³⁺ stresses are the major causes of poor plant growth in acidic soils. However, there is still a poor understanding of plant responses to low‐pH and Al³⁺ toxicity. Low‐pH or combined low‐pH and Al³⁺ stress was imposed in order to measure rhizosphere pH, ion fluxes, plasma membrane potential and intracellular H⁺ concentration in distal elongation and mature zones (MZs) along the longitudinal axis of Arabidopsis thaliana roots. Low‐pH stress facilitated H⁺ influx into root tissues and caused cytoplasmic acidification; by contrast, combined low‐pH/Al³⁺ treatment either decreased H⁺ influx in the distal elongation zone (DEZ) or induced H⁺ efflux in the MZ, leading to cytoplasmic alkalinization in both zones. Low‐pH stress induced an increase in rhizosphere pH in the DEZ, whereas combined low‐pH/Al³⁺ stress resulted in lower rhizosphere pH in both root zones compared with the low‐pH treatment alone. Low‐pH stress facilitated K⁺ efflux; the presence of Al³⁺ diminished K⁺ efflux or favored K⁺ influx into root tissues. In both zones, low‐pH treatment induced plasma membrane (PM) depolarization, which was significantly diminished (P≤ 0.05) when combined stresses (low‐pH/100 µM Al³⁺) were imposed. After 60 min of exposure, low pH caused PM depolarization, whereas low pH/100 µM Al³⁺ caused PM hyperpolarization. Thus, low pH and Al³⁺ toxicity differentially affect root tissues and, consequently, the rhizosphere, which might underpin the differential mechanisms of plant adaptation to these abiotic stresses.     

H and k electrogenic exchanges in corn roots

The membrane potential difference, the net H⁺ exchange rate, the K⁺ net flux, and the K⁺ (⁸⁶Rb⁺) influx were measured in excised corn roots as functions of the K⁺ concentration in the medium at various pH values, in the presence of poorly permeant anions. The roots behaved as a K⁺/H⁺ exchange system. By comparing the results in normal or hypoxic conditions, or in the presence of vanadate, it was possible to distinguish the active components of membrane potential and transports from the passive ones. The magnitude of the electrogenic potential was not related to the active H⁺ extrusion rate. At pH 6, the variations of the electrogenic potential resulted from variations of the stoichiometry of the active H⁺/K⁺ exchange. The same relationship between this stoichiometry and the K⁺ concentration was observed in conditions ensuring different membrane polarizations (pH 6, pH 4, or pH 6 with fusicoccin). Both metabolic and Mg-ATPase specific inhibitors stopped the active H⁺ transport and the net K⁺ influx. Nevertheless, the tracer influx in the presence of vanadate remained higher than the passive influx calculated from the permeability coefficient determined in hypoxia. It is proposed that vanadate uncouples the K⁺ moiety of the H⁺/K⁺ antiport and allows it to mediate isotopic exchanges.     

Extent of intracellular pH changes during H+ extrusion by maize root-tip cells

³¹P-Nuclear-magnetic-resonance spectra of maize (Zea mays L.) root tips, that had been induced to extrude large amounts of H⁺ in response to fusicoccin (FC) in the presence of potassium salts, indicate that the cytoplasmic pH does not become higher than that of controls. In fact, the cytoplasmic pH may become slightly (approx. 0.1 pH unit) lower in cells extruding H⁺. Estimations of the buffer capacity of the cells show that without active intracellular pH regulation, H⁺ extrusion caused by FC would cause the intracellular pH to rise by at least 0.6 pH unit h^-1. Our results indicate that intracellular pH is tightly regulated even during extreme rates of acid extrusion, and that a rise in cytoplasmic pH is not the signal linking H⁺ extrusion with enhanced organic-acid synthesis or other intracellular responses to H⁺ pumping.Abbreviations FC fusicoccin - P_i inorganic phosphate - NMR nuclear magnetic resonance - chemical shift - MDP methylene diphosphonic acid