Proton Fluxes and the Activity of a Stelar Proton Pump in Onion Roots

The xylem vessels of excised adventitious roots of onion, Alliumcepa, were perfused with unbuffered nutrient solution adjustedinitially to either pH 9·3 or 3·9; the pH of thesolution after passage through the xylem, at rates not lessthan 2 xylem volume changes min^–1, was close to pH 6·5in both instances. The flux of H⁺ across the xylem/symplastboundary into mildly alkaline, phosphate-buffered solutionsperfusing the vessels could be increased greatly with increasingbuffer strength, up to a maximum value between 0·5–1·0pmol H⁺ mm^–2 s^–1. The apparent neutralization ofacidic malic acid buffers had a slightly lower maximum capacity,equivalent to –0·3 to –0·5 pmol H⁺mm^–2 s^–1. The addition of 5·0 pmol m^–3fusicoccin (FC) to the xylem perfusion solution stimulated theentry of H⁺ into the xylem; in unbuffered perfusion solutionsthe pH fell to pH 3·6 after a lag of 25–35 min.FC additions to phosphate-buffered solutions also stimulatedthe H⁺ flux to an extent similar to that in unbuffered solution,viz. 0·2–0·4 pmol mm^–2 s^–1. The release of K⁺ (³⁶Rb-labelled) into xylem sap transientlyincreased as the [K⁺] in weakly buffered perfusion solutionswas raised stepwise; a very marked increase being seen whenthe concentration was raised to 100 mol m^–3 from 40 molm^–3. The addition of 5·0 mmol m^–3 FC to theperfusing solution containing 100 mol m^–3 K⁺ rapidly decreasedthe K⁺ flux to the xylem as the H⁺ flux increased. Fusicoccinalso inhibited the flux of K⁺ into unbuffered perfusion solutionsbut the effect appeared reversible. Addition of 10 mmol m^–3abscisic acid (ABA) to the perfusion solution quickly producedtransient increases in both K⁺ and H⁺ fluxes into the xylem.In this and other experiments using weakly phosphate-bufferedperfusing solutions, H⁺ fluxes were comparable in size to thoseof K⁺ The results are consistent with the idea that the stele of onionroots contains a proton trarislocating ATPase whose activityresponds to the pH of the xylem sap. It is evident that theactivity of the proton secreting and proton neutralizing mechanismsin the xylem parenchyma control the movement of other ions acrossthe xylem/symplast boundary. Key words: Xylem perfusion, fusicoccin, abscisic acid, pH gradient     

Effects of Two Plasmalemma ATPase Inhibitors on H+ Extrusion and Intracellular pH in Elodea densa Leaves

Beffagna, N. and Romani, G. 1988. Effects of two plasmalemmaATPase inhibitors on H⁺ extrusion and intracellular pH in Elodeadensa leaves.—J. exp. Bot. 39: 1033–1043. Elodea leaves in the dark show very little exchange of H⁺ withthe medium in the external pH range between 5.0 and 6.0. Thepresence of fusicoccin and potassium in the medium markedlystimulates H⁺ extrusion. Fusicoccin- and K⁺ -induced H⁺ extrusionis inhibited by either erythrosin B (EB) or Na-orthovanadate,two inhibitors of H⁺ transporting plasma membrane ATPase. EBcompletely inhibits it from the first 30 min of treatment, whensupplied at pH 5.5 at a concentration of 30 mmol m^–3.Vanadate also inhibits H⁺ extrusion, this effect becoming evidentonly after 45 min of treatment. After this time inhibition iscomplete with 250 mmol m^–3 vanadate but only partial forlower concentrations. In the presence of either inhibitor the intracellular pH, measuredas cell sap pH, is significantly lowered. When the intracellularpH changes are determined on vacuole and, separately, on cytoplasmby the weak acid and base distribution method, acidificationof both compartments is found to accompany the blocking of H⁺extrusion by either of the inhibitors. Key words: Intracellular pH, vanadate, erythrosin B, H⁺pumping     

Phosphate uptake and transport in Agrostis capillaris L Effects of non-toxic levels of aluminium and the significance of P and Al speciation

Investigations on the effects of low levels of Al on P adsorption,uptake and translocation in seedlings of the indigenous grassAgrostis capillaris were undertaken. Apparent uptake and transportof H₂ ³²PO₄ from nutrient solutions containing 10 or 100mmolm^–3 phosphate were characterized as functions of timeand concentration. Experiments on ³²P uptake and transport insolutions containing no Al (control) or Al ranging from 3.7to 185 mmol m^–3 at pH ranging from 4.3 to 4.6, showedthat in 10 mmol m^–3 P, effects of Al at 3.7 and 37 mmolm^–3 on the size of the initial uptake shoulder were small,but some increase in subsequent P uptake to the roots was observed,though transport to the shoots was suppressed. With 37 mmolm^–3 Al in nutrient solution containing 100 mmol m^–3P, the uptake shoulder was much increased above the control.Subsequent root uptake was stimulated but transport was unaffected.Lack of toxicity of the Al concentrations used was indicatedby a lack of significant effect on plant fresh weight. AbsorbedAl was almost totally retained in the root in all treatments.Speciation calculations showed that the major species in Alamended nutrient solution at pH 4.4 were H₂PO₄^–, AI³⁺and AIHPO₄⁺, together with substantial amounts of AISO₄⁺ andsoluble aluminium hydroxy complexes (AIOH²⁺, AI(OH)₂⁺), dependingon the relative concentrations of P and Al. The effects of Al,with 10 mmol m^–3 P, on adsorption of complexed P werepartly accounted for in terms of preferential cell wall adsorptionof Al complexes not containing P. Conclusions were drawn aboutthe P-economy of A. capillaris plants growing on soils withlow levels of P and Al. Key words: Phosphorus, aluminium, speciation, Agrostis capillahs L     

Flexible Coupling of Phosphate Uptake in Dunaliella acidophila at Extremely Low pH Values

The acidophilic alga Dunaliella acidophila exhibits optimalgrowth at pH 1. We have investigated the regulation of phosphateuptake by this alga using tracer techniques and by performingintracellular phosphate measurements under different growthconditions including phosphate limitation. In batch culturewith 2·2 mol m^–3 phosphate in the medium the uptakeof phosphate at micromolar phosphate concentrations followeda linear time dependence in the range of minutes and rates werein the range of 1 µmol phosphate mg^–1 chl h^–1,only. However, under discontinuous phosphate-limited growthconditions, tracer influx revealed a biphasic pattern at micromolarphosphate concentrations: An initial burst phase resulted ina 10⁴-fold internal phosphate accumulation and levelled offafter about 10 s. A double reciprocal plot of the initial influxrates obtained for phosphate-limited and unlimited algae exhibitedMichaelis-Menten kinetics. Phosphate limitation caused a significantactivation of the maximum velocity of uptake, yielding V_maxup to 1 mmol mg^–1 chl h^–1 as compared to valuesin the order of 50 µmol phosphate mg^–1 chl h^–1for the second phase (this magnitude is also representativefor non-limited batch cultures). Concomitantly the Michaelisconstant was altered from 4 mmol m^–3 to 0·7 mmolm^–3. The rapid uptake of phosphate was inhibited by arsenateand FCCP and was not stimulated by Na⁺. The pH dependence oftracer accumulation and measurements of the intracellular phosphatepool under different growth conditions indicate that at lowpH and low external phosphate concentrations the high protongradient present under these conditions is utilized for a H₃PO₄uptake or a H⁺/H₂PO^–4 cotransport. However, when the externalphosphate concentration was increased to levels sufficientlyhigh for transport to be driven by the positive membrane potential(10 mol m^–3 phosphate), the pH dependence of phosphateuptake was more complex, but could be explained by the uptakeof H₃PO₄ or a H⁺/H₂PO^–₄-cotransport at low pH and a differenttype H₂PO^–4-transport (with unknown type of ion coupling)at high pH-values. It is suggested that this flexible couplingof phosphate transport is of essential importance for the acidresistance of Dunaliella acidophila. Key words: Acid resistance, Dunaliella acidophila, phosphate cotransport, phosphate limitation, plasma membrane, sodium     

Photoperiodically Induced Changes in Seed Growth Rate of Soybean as Related to Endogenous Concentrations of ABA and Sucrose in Seed Tissues

Short-day photoperiods can increase the partitioning of assimilatesto filling seeds of soybean (Glycine max L. Merr.), resultingin higher seed growth rates. The plant growth substance ABAhas been implicated in the regulation of assimilate transferwithin filling soybean seeds. Thus, we hypothesized that anincreased concentration of endogenous ABA in seeds may enhancesucrose accumulation and seed growth rate of soybeans exposedto short-day photoperiods. Plants of cv. Hood 75 were grownin a greenhouse under an 8-h short-day photoperiod (SD) until11 d after anthesis (DAA) of the first flower, when half ofthe plants were transferred to a night-interruption (NI) treatment(3 h of low-intensity light inserted into the middle of thedark period). Plants remaining in SD throughout seed developmenthad seed growth rates 43% higher than that of plants shiftedto NI (7·6 mg seed^–1 d^–1 vs. 5·3 mgseed^–1 d^–1). On a tissue-water basis, the concentrationof ABA in SD seeds increased rapidly from 7.6 µmol l^–1at 11 DAA to 65·2 µmol l^–1 at 18 DAA, butthen declined to 6·6 µmol l^–1 by 39 DAA.In contrast, the concentration of ABA increased more slowlyin NI seeds, reaching only 47·4 µmol l^–1by 18 DAA, peaking at 57·0 µmol l^–1 on 25DAA, and declining to 10·2 µmol l^–1 by 39DAA. The concentration of sucrose in SD embryos peaked at 73·5mmol l^–1 on 25 DAA and remained relatively constant forthe remainder of the seed-filling period. In NI, the concentrationof sucrose reached only 38·3 mmol 1^–1 by 25 DAA,and peaked at 61·5 µmol l^–1 on 32 DAA. Thusin both SD and NI, sucrose accumulated in embryos only afterthe peak in ABA concentration, suggesting that ABA may havestimulated sucrose movement to the seeds. The earlier accumulationof ABA and sucrose in SD suggests that ABA may have increasedassimilate availability during the critical cell-division period,thus regulating cotyledon cell number and subsequent seed growthrate for the remainder of the seed-filling period. Glycine max L. Merr. cv. Hood 75, soybean, assimilate partitioning, abscisic acid, photoperiod, source-sink     

H+ Transport across the Plasmalemma and Tonoplast of Nitellopsis obtusa during Excitation

Measurements of intra- and extracellular pH during the excitationof Nitellopsis obtusa were carried out with antimony microelectrodesat conditions of dark adaptation and of continuous illumination.The pH of the vacuolar sap of both dark-adapted and illuminatedcells increased during cell excitation by 0·1–0·15units. H⁺ ions which had entered the cytoplasm during excitationin dark-adapted cells were extruded back into the vacuole acrossthe tonoplast. After cell excitation in the light H⁺ ions wereextruded from the cytoplasm also into the external medium probablyacross the light-stimulated active H⁺-channels. Protoplasmicstreaming ceased during excitation in the dark for 1–3min, and during excitation in the light—for 5–20s.     

H+ Efflux and Trans-Root Potential Measured while Increasing the Temperature of Solutions Bathing Excised Roots of Zea mays

Kennedy, C. D. and Gonsalves, F. A. N. 1988. H⁺ efflux and trans-rootpotential measured while increasing the temperature of solutionsbathing excised roots of Zea mays.—J. exp. Bot. 39: 37–49. Novel temperature-ramp procedures have been used to measureH⁺ efflux and trans-root potential of excised roots of Zea mays(var. Fronica). Two types of experiment were performed: (1),increasing temperature from 17°C, and (2), pre-cooling theroots to 1°C before starting the temperature ramp. The ratesof increase of temperature for H⁺ efflux and trans-root potentialexperiments were 0·5 and 2·1°C min^–1respectively The H⁺ scans revealed strong sharp maxima at 30°C and 32°C,for non-pre-cooled and pre-cooled roots respectively, the latterbeing significantly smaller. The trans-root potential scansfor the pre-cooled roots showed a corresponding maximum at 30°C,which was inhibited by KCN (1-0 mmol dm^–3) with or withoutSHAM (10 mmol dm^–3), or Hg²⁺ (1, 10, 100 µmol dm^–3)in the bathing solutions. Some of the evidence suggests thatthese maxima are associated with electrogenic H⁺ pumping, mediatedby a plasma membrane-bound ATPase. However, no correspondingmaximum was observed in the trans-root potential scans for non-pre-cooledroots, the potential remaining at about — 75 m V from20°C to 35°C. As there is a 7-fold increase in H⁺ effluxbetween 20°C and 30°C, the relationship between netH+ efflux and electrogenic proton pumping in these roots isby no means clear. Some possibilities are considered here. Pre-cooled and non-pre-cooled roots show clear maxima in thetrans-root potential scans at about 46°C, at which temperaturethere is a slight net H⁺ influx. This, and other less prominentfeatures observed, are briefly discussed. Key words: H⁺ efflux, trans-root potential, temperature-ramp procedure, Zea mays, roots     

Comparison of the Effects of Ammonia and Methylamine on Chloride Transport and Intracellular pH in Chara corallina

Ammonium and methylammonium ions greatly increase the rate ofCl^– transport in Chara corallian. This effect is dependenton the pH of the bathing solution. The amine-stimulated Cl^–influx is small at pH 5·5, increases to a maximum atpH 6·5–7·5, and decreases again as the pHis raised to 8·5. Increased Cl^– influx is accompaniedby an increase in cytoplasmic pH, as calculated from the distributionof DMO. When the external pH lies between 5·5 and 7·3,cytoplasmic pH in the absence of amine is 7·65–7·70,with an increase of 0·15–0·25 in the presenceof amine. As external pH is increased above 7·3, cytoplasmicpH also increases, with progessively less effect of amine. Although the relationship between Cl^– influx and cytoplasmicpH is not simple, the results provide evidence in accord withthe hypothesis that Cl^– transport in Chara involves H⁺—Cl^–symport, or the equivalent OH^–—Cl^– antiport.The possible role of cytoplasmic pH as a factor involved inthe regulation of membrane transport in Chara is discussed.     

Calcium Inhibits Ion-Stimulated Stomatal Opening in Epidermal Strips of Commelina communis L.

Inoue, H. and Katoh, Y. 1987. Calcium inhibitsion-stimulatedstomatal opening in epidermal strips of Commelina communis L.—J.exp. Bot. 38: 142–149. Ca²⁺ suppressed both the ion-stimulated stomatal opening andH⁺ extrusion of pre-illuminated epidermal strips isolated fromCommelina communis L. In the absence of Ca²⁺, the rate of H⁺release was 18 nmol H⁺ cm^–2 h^–1 per epidermal stripunit area in 150 mol m^–3 KCL at pH 7?4. Half-maximum inhibitionof stomatal opening was observed with 220 mmol m^–3 ofCa²⁺. The hexavalent dye, ruthenium red, showed concentration-dependentprevention of the inhibition by Ca²⁺ of the ion-stimulated stomatalopening. The effect of ruthenium red was non-competitive, andthe K₁ for the calcium inhibition was found to be 3?6 mmol m^–3.The calcium inhibition of H⁺ extrusion was also prevented byruthenium red. These results suggest that Ca²⁺ inhibits theactivity of electrogenic H⁺ translocating ATPase of the guardcell plasma membrane and leads to the suppression of stomatalopening. Key words: Calcium, Commelina communis, ruthenium red, stomata     

Inhibition of Malate Synthesis by Vanadate: Implications for the Cytosolic Alkalinization Induced by Vanadate Concentrations Partially Inhibiting the Plasmalemma H+ Pump

The effects of Na-orthovanadate, at concentrations only partiallyinhibiting net H⁺ extrusion, were determined on vacuolar andcytosolic pH by the weak base and weak acid distribution atequilibrium. Treatment with vanadate induces in Elodea densaleaves and in Arabidopsis thaliana seedlings a moderate acidificationof both cell sap and vacuole. Conversely, it induces an alkalinizationof cytosol, this effect being in apparent contrast with a conditionof reduced activity of the H⁺-transporting plasmalemma ATPase,which should be associated with a cytosolic acidification. InArabidopsis seedlings treated with vanadate, the increase inpH of both cytosol and external medium is associated with adecrease in cell sap buffer capacity, more evident for highervanadate concentrations, and particularly marked in the pH rangebetween 3·5 and 5·5. In these conditions, themalate content is strongly reduced, its decrease almost completelyaccounting for the decrease in cell sap buffer capacity. Anin vitro analysis of the vanadate effect on phosphoenolpyruvatecarboxylase indicates that the decrease in malate content seemssubstantially due to an inhibiting effect of vanadate on thisenzyme. These results stress that the in vivo use of vanadateas an inhibitor of the plasmalemma H⁺-ATPase must be taken withcaution; in particular, for studying the correlations betweenchanges in net H⁺ extrusion and changes in cytosolic pH andrelated processes. Key words: Vanadate, malate, cytosolic pH, Elodea densa, Arabidopsis thaliana     

Chloride Transport in Chara corallina and the Electrochemical Potential Difference for Hydrogen Ions

The pH of the cytoplasm of Chara corallina cells has been measuredwith the weak acid 5,5-dimethyloxazolidine-2,4-dione (DM0).Over an external pH range 4·5–9·5 the resultsfit the regression equation pH_cytoplasm=6·28+0·22pH_out. Using measured values of the electric potential difference acrossthe plasmalemma we have calculated the electrochemical potentialdifference across this membrane for H⁺ and Cl^–. Thesedata are used to test the hypothesis that the inward transportof Cl^– is coupled to the inthix of H⁺ or, which comesto the same thing, efflux of OH^–. One-for-one couplingwill not give net Cl^– uptake from solutions with pH greaterthan about 7·2, unless the cytoplasmic Cl^– concentrationis lower than 10 mM, or the pH just outside the membrane islower than that in the bulk solution. It is shown that net Cl^–uptake proceeds from solutions with pH up to 9. The alternative possibility is that Cl^– transport is broughtabout by co-transport of two H⁺ for each Cl^–; this isnot ruled out by the results reported. Such a mechanism mightbe detectable by its electrogenic effect: although such effectshave not been detected, it is shown that they would be smallunder most conditions. Other possible mechanisms are discussed.     

Simulating the Mineral Environment of Aluminium Toxic Soils in Plant Cell Culture

The development of a medium for studying aluminium toxicityin plant cell cultures is described. To prevent the precipitationof Al added to the standard cell culture medium, it was necessaryto lower the phosphate concentration from 1250 mmol m^–3to10 mmol m^–3, and the pH from 5.8 to 4-0. Two additionalmodifications were the use of unchelated iron and a reductionin the calcium concentration from 3.0 mol m^–3 to 0.1 molm^–3. Since the gelling properties of agar are inhibitedat pH 4.0, cells were cultured on filter paper supported bypolyurethane foam sturated with liquid medium. The only limitationto the growth of plated Nicotiana plumbaginifolia Viv. cellson the modified medium was the reduced phosphate concentration.This was partly overcome by ‘preloading’ the cellswith phosphate prior to each experiment. In addition, the filterpaper with adhering cells was transferred to fresh medium everysecond day to replenish phosphate, and to re-establish the initialpH of4.0 (which otherwise drifts upward). With the modifiedmedium, Al toxicity was observed in plated N. plumbaginifoliacells at both 200 mmol m^–3 and 400 mmol m^–3 Al.There was no toxicity at these Al concentrations when the normalphosphate concentration or pH were restored to the modifiedmedium. Partial alleviation of Al toxicity occurred with restorationof the normal calcium concentration or chelated iron. Chelationof Al with citrate or EDTA also mitigated Al toxicity. In additonto Al toxicity, the modified medium should also prove usefulfor studying other metal toxicities in plant cell culture. Key words: Al toxicity, Cell culture, Nicotiana plumbaginifolia     

Effects of Aluminium on Honeylocust (Gleditsia triacanthos L.) Seedlings in Solution Culture

Honeylocust (Gleditsia triacanthos L.) seedlings were grownin solution culture at pH 4.0) with 50, 150, 600 and 1500 mmolm^–3 aluminium. All levels of aiuminium reduced the sizeand weight of roots, shoots and leaves with the exception ofroot elongation at 50 mmol m^–3 Al. Aluminium content ofroots was 50 to 100 times that of shoots. With increasing concentrationof aluminium, aluminium content of leaves and roots increasedexponentially while a linear increase was observed for stems.The nutrient content of seedlings was improved in 50 mmol m^–3where increases in shoot calcium, magnesium, and phosphorusconcentrations were observed. Aluminium concentrations greaterthan 50 mmol m^–3 reduced shoot nutrient content. Presenceof aluminium increased the root phosphorus and calcium levelsbut had no effect on potassium and magnesium concentrations.Results show that honeylocust is an aluminium sensitive treespecies whose growth may be reduced by high soil Al levels. Key words: Aluminium toxicity, Gleditsia triacanthos, nutrient solution     

Modifying Effects of Non-toxic Levels of Aluminium on the Uptake and Transport of Phosphate in Ryegrass

Apparent uptake and transport of H₂³²PO₄^– from nutrientsolutions containing 100 mmol m^–3 phosphate were characterizedasfunctions of time, concentration and pH in ryegrass seedlings.On a log/log plot, concentration versus uptake to the root resolvedintotwo linear phases, suggesting a change in uptake mechanism orefflux at the break. These results were compared with thosefor ³²P uptake and transport in solutions containing Al rangingfrom 0–185 mmol m^–3. Al addition depressed pH, butbecauseuptake of P was unaffected by pH below 5–0, noadjustments were attempted. Uptake time-courses revealed clearlythe usualinitial adsorption shoulder in the uptake curve, increasingwith Al concentration up to 37 mmol m^–3. Beyond about2 h, P uptaketo the root became linear, at rates increasingwith external Al concentration up to 37 mmol m^–3. Concentrationsof Al muchabove 100 mmol m^–3 were toxic. Al treatmentsdid not affect P transport to the shoot and absorbed Al wasconfined to the root.The quantities of P and Al taken up intothe root indicated storage in cortex cell vacuoles, lockingup significant amounts of P.Experiments with tillering plantsshowed similar characteristics to those with seedlings. Sequesteringof P with Al within the rootcortex cells was evident, particularlyin plants which had been grown in nutrient containing Al fromsoon after germination. Aland P solution chemistry is discussedin the context of this work and the consequences of effectson P uptake for the economy ofphosphate poor upland soils wereconsidered. Key words: Phosphate, aluminium, adsorption, uptake, Lolium perenne L     

Changes of the Apoplastic pH and K+ Concentration in the Phaseolus Pulvinus in situ in Relation to Rhythmic Leaf Movements

The apoplastic pH and K⁺ concentration of the extensor of thePhaseolus primary-leaf pulvinus in relation to rhythmic leafmovements have been investigated with double-barrelled ion-sensitivemicro-electrodes. Simultaneous measurements of leaf movementand ion activities in a fine hole of the extensor in situ showedco-existence of ultradian and circadian leaf movements as wellas of ultradian and circadian pH changes in the Water Free Space(WFS) of the extensor apoplast in situ. During circadian leafmovement the H⁺ and K⁺ activities in the WFS of the extensorchange in an antagonistic manner. When extensor cells swell(upward movement of the lamina) the H⁺ activity increases fromapproximately pH 6.7 to 5.9 and the K⁺ concentration decreasesfrom approximately 50 to 10 mol m^–3 and vice versa whenextensor cells shrink. These changes in the ionic activitiesin the WFS must be correlated with large changes in the ioncontent of the DFS and thus support the hypothesis that thecell walls of pulvinar cells serve as reservoirs for K⁺ andH⁺. Key words: Phaseolus pulvinus, apoplastic ionic activities, rhythmic leaf movements, ion-sensitive micro-electrodes (double-barrelled)     

Rates of Hydrogen Ion Efflux by Nodulated Legumes Grown in Flowing Solution Culture with Continuous pH Monitoring and Adjustment

The net efflux of H⁺ from lucerne (Medicago saliva L.), redclover (Trifolium pratense L.) and white clover (Trifolium repensL.) growing in flowing solution culture and dependent upon symbioticfixation of atmospheric N, was measured over a 75 d experimentalperiod. Considerable and rapid increases in acidity of the nutrientsolution of up to 1.45 pH units were recorded when the pH wasriot held constant over a 30 h period. There was little differencein H⁺ efflux when solution pH was held constant at 4.75, 5.75or 6.75, but there was an immediate cessation when it was adjustedto 3.75. Differences in the daily net efflux of H⁺ closely followedthe pattern of daily differences in incoming radiation, andthere was also evidence of a diurnal pattern of H⁺ efflux. Althoughthere were initially distinct differences between the speciesin the calculated rate of net H⁺ efflux (µg H⁺ g^–1dry shoot d^–), by day 75 these had diminished. In allspecies, however, the maximum rate of efflux per unit of shootsoccurred during the earlier rapid phases of growth. The measuredefflux of H⁺ was well equated with the plant content of excesscations (as measured by ash alkalinity) and, on average, theratio of acidity produced to N assimilated (expressed as anequivalent) was 0-24. Medicago sativa L., Trifolium pratense L., Trifolium repens L., lucerne, red clover, white clover, acidification, cation/anion balance, flowing solution culture, H⁺ efflux, nitrogen fixation     

H+ tolerance of Chara Internodal Cells and Apparent Net Influx of H+ in Weakly Acidic Solutions: Implication of the Net Flux of H+ as a Minor Component among the Total Net Flux of Ions across the Intact Cell Membrane of Chara

Precise measurements of the net flux of protons in Chara internodalcells were made with a recently designed high-resolution pH-meter.Survival of intact Chara internodal cells in artificial pondwater (APW) that contained HC1 at various concentrations wasalso examined. The apparent net flux of H⁺ was inward and muchsmaller than that reported so far. In APW at pH 4.005, a valuehigher than the extracellular pH expected from the values ofH⁺ efflux reported to date, all of the intact Chara internodalcells died within a day. With reference to the data on the circadianflow of ions in the pulvinus of Phaseolus [Kiyosawa (1979) PlantCell Physiol. 20: 1621–1634, Hosokawa and Kiyosawa (1983)Plant Cell Physiol. 24: 1065–1072] and ionic regulationin Chara L-cells [Kiyosawa and Okihara (1988) Plant Cell Physiol.29: 9–19], a discussion is presented of the prossiblyminor contribution of the net flux of H⁺ in the generation ofthe electrical membrane potential. Regulation of the net fluxof H⁺ in weakly acidic APW is also discussed. (Received September 4, 1989; Accepted January 25, 1990)     

Effects of Cations on the Cytoplasmic pH of Chara corallina

Smith, F. A. and Gibson, J.–L. 1985. Effects of cationson the cytoplasmic pH of Chara corallina.—J.exp. Bot.36: 1331–1340 Removal of external Ca²⁺ from cells of Chara corallina lowersthe cytoplasmic pH, as determined by the intracellular distributionof the weak acid 5,5–dimethyloxazolidine2–,4–dione(DM0), when the external pH is below about 60. This effect isreversed, at least partially, by addition of the following cationsto Ca²⁺-free solutions: tetraethylammonium (TEA⁺) and Na⁺ at5 or 10 mol m^-3, Li⁺ and Cs⁺ (10 mol m^-3), or Mg²⁺, Mn²⁺ andLa³⁺ (02 or 05 mol m^-3). Under the same conditions, increasesin pH sometimes, but not always, occur in the presence of 10mol m^-3 K⁺ or Rb⁺ The results are discussed in relation to the major transportprocesses that determine pH and the electric potential differenceacross the plasma membrane, namely fluxes of H⁺ and of K⁺. Thesimplest explanation of the effects of the various cations testedin this study is that they primarily affect pHi_c via changesin influx of H⁺ but direct effects on the H⁺ pump or on K⁺ fluxesmay also be involved Key words: Chara corallina, cytoplasmic pH, cations, H⁺transport     

Plant Growth in Relation to the Supply and Uptake of NO3-: A Comparison Between Relative Addition Rate and External Concentration as Driving Variables

Two approaches to quantifying relationships between nutrientsupply and plant growth were compared with respect to growth,partitioning, uptake and assimilation of NO₃^– by non-nodulatedpea (Pisum sativum L. cv. Marma). Plants grown in flowing solutionculture were supplied with NO₃^– at relative addition rates(RAR) of 0·03, 0·06, 0·12, and 0·18d^–1, or constant external concentrations ([NO₃^–)of 3, 10, 20, and 100 mmol m^–3 over 19 d. Following acclimation,relative growth rates (RGR)approached the corresponding RARbetween 0·03–0.12 d^-1, although growth was notlimited by N supply at RAR =0.18 d^-1. Growth rates showed littlechange with [NO₃–] between 10–100 mmol m^–3(RGR=0·15 –0·16 d^-1). The absence of growthlimitation over this range was suggested by high unit absorptionrates of NO₃^–, accumulation of NO₃^– in tissues andprogressive increases in shoot: root ratio. Rates of net uptakeof NO₃^– from 1 mol m^–3 solutions were assessed relativeto the growth-related requirement for NO₃^–, showing thatthe relative uptake capacity increased with RGR between 0·03–0·06d^–1 , but decreased thereafter to a theoretical minimumvalue at RGR     

Aluminium Effects on ATPase Activity and Lipid Composition of Plasma Membranes in Sugar Beet Roots

The effect of aluminium (Al) in vivo and in vitro on root plasmamembranes has been studied in two sugar beet (Beta vulgarisL.) cultivars, Monohill (Al-sensitive) and Regina (relativelyAl-tolerant). Although Al in vitro inhibited the MgATPase inan uncompetitive way for both cultivars raised in the absenceof Al, the specific K⁺-activation of the MgATPase was only inhibitedby Al in cv. Monohill. Arrhenius analysis of the MgATPase activity showed that theeffect of Al in vitro depended on whether or not the plantswere exposed to Al in vivo. Al treatment in vitro of the MgATPasefrom control plants cultivated at a low pH (5·4) causedan increase in the phase transition temperature from 17 to 22°C. Only at a higher pH range (pH 6·1) could a secondtransition temperature be induced (at 9 °C). By additionof Al in vitro to plants cultivated with Al at pH 5·4,the slopes of the activity plots did not change. Aluminium changedthe K_m of the ATPase for MgATP in an opposite way by treatmentin vivo and in vitro. Lipid analyses of the plasma membranes showed that the acylcomposition differed little following Al treatment in vivo,but that the ratio of phosphatidylcholine: phosphatidylethanolamineincreased. The changes correlated with the observed change inthe K_m for the MgATPase. We conclude that the main effect ofAl on the MgATPase is not due to the formation of an Al-ATPcomplex. Instead, Al may bind to the membrane-bound enzyme(s)and/or modify the lipid environment. Key words: Aluminium, ATPase, Beta vulgaris, lipids