Shoot-Dependent Regulation of Sodium and Potassium Fluxes in Roots of Whole Barley Seedlings

Unidirectional fluxes and the cytoplasmic and vacuolar contentsof potassium and sodium in root cells of intact barley seedlings(Hordeum vulgare L., cv. Villa) were determined by use of compartmentalanalysis. In addition, the net vacuolar accumulation J_cv andthe xylem transport ø_cx of K⁺ and Na⁺ were measured.Both of these data were needed for the evaluation of the effluxdata. Fluxes and compartmental contents of K⁺ and Na⁺ were comparableto data obtained with excised roots. The effect of the shoot-to-rootratio—as varied by partial excision of the seedlings seminalroots—on the fluxes and contents was investigated. Highershoot-to-root ratios induced an increase in xylem transport,in plasmalemma influx, and also in the cytoplasmic content ofK⁺ and Na⁺. With potassium the plasmalemma efflux was almostunaltered while the tonoplast fluxes and vacuolar content weredecreased (in presence of Na⁺). With sodium, on the other hand,the plasmalemma efflux and the tonoplast fluxes were also increasedin the plants having one root and a high shoot-to-root ratio.These changes occurred even under conditions of low humidity,when transpiration was low and guttation occurred. The latterwas also increased at the high shoot-to-root ratio. The observedchanges could be due to a relieved feedback control of ion fluxesby the shoot and mediated in part by a relatively higher supplyof photosynthates in the plants having one root In addition,hormonal signals were suggested to participate. In particulara possibly decreased level of cytokinins in the plants havingonly one root could contribute to the signal. The observed changesappear to be responses of the plant to an alteration that canoccur under natural conditions when the root system is damaged.     

Effects of Growth and Assay Temperatures on Unidirectional K+ Fluxes in Roots of Rye (Secale cereale)

The effects of growth and assay temperature on unidirectionalK⁺ fluxes in excised roots of rye (Secale cereale cv. Rheidol)were studied using ⁸⁶Rb⁺ as a tracer. Both K⁺ influx to thevacuole, estimated as K⁺ uptake between 3 and 12 h after transferof unlabelled roots to radioactive solution, and movement ofK⁺ to the xylem were determined directly. Other fluxes weredetermined on excised roots of plants, which had been labelledwith ⁸⁶Rb⁺ since germination, by conventional triple exponentialefflux analysis. When assayed at 20°C, roots of plants previously grown at20°C(WG roots) had lower rates of net K⁺ uptake than rootsof low temperature-acclimated plants, grown with a temperaturediferential between roots (87°C) and shoots (20°C) eithersince germination (DG roots) or for 3 d prior to experiments(DT roots). This resulted from a greater unidirectional K⁺ effluxacross the plasma membrane and a reduced K⁺ flux to the xylemin WG roots, compared to DG or DT roots, rather than a decreasein unidirectional K⁺ influx or a decrease in the net K⁺ fluxto the vacuole. Indeed, although WG roots had lower rates ofK⁺ influx and K⁺ efflux across the tonoplast at 20°C thanDG or DT roots, roots of plants from all growth temperaturetreatments showed an equivalent net K⁺ flux to the vacuole. Although all unidirectional K⁺ fluxes in roots from plants grownunder all temperature regimes were reduced by lowering the temperatureof the root, these fluxes were differentially affected in rootsof plants from contrasting growth temperature treatments. Rapidcooling to 8°C of WG roots resulted in a lower rate of K+influx and a transient increase in K⁺ efflux across both theplasma membrane and tonoplast, compared to DG and DT roots.Furthermore, since the K⁺ flux to the xylem was lower in WGroots, the net K⁺ uptake at 8°C into WG roots was considerablyreduced compared to DG and DT roots. These results suggest thatlow temperature-acclimation of K⁺ fluxes in rye roots may involvea reduction in the temperature sensitivity of K⁺ influx anda curtailment of K⁺ efflux across both the plasma membrane andtonoplast at low temperatures. Key words: K⁺influx, K⁺ efflux, low temperature, potassium, rye (Secale cereale cv. Rheidol)     

Vacuolar Na/K Exchange, its Occurrence in Root Cells of Hordeum, Atriplex and Zea and its Significance for K/Na Discrimination in Roots

Using excised low-salt roots of barley and Atriplex hortenslsthe transport of endogenous potassium through the xylem vesselswas studied It was enhanced by nitrate and additionally by sodiumions which apparently replaced vacuolar potassium which wasthen available in the symplasm of root cells for transport tothe shoot Vacuolar Na/K exchange also has been investigatedby measurements of longitudinal ion profiles in single rootsof both species. In Atriplex roots a change in the externalsolution from K⁺ to Na⁺ induced an exchange of vacuolar K⁺ forNa⁺, in particular in the subapical root tissues and led toincreased K⁺ transport and loss of K⁺ from the cortex. In inverseexperiments a change from Na⁺ to K⁺ did not induce an exchangeof vacuolar Na⁺; merely in meristematic tissues Na⁺—apparentlyfrom the cytoplasm—was extruded in exchange for K⁺. Inroots of barley seedlings without caryopsis, as in excised roots,a massive exchange of K⁺ for Na⁺ was observed in the continuouspresence of external 1.0 mM Na and 0.2 mM K. This exchange alsowas attributed to the vacuole and was most pronounced in theyoung subapical tissues. It did not occur, however, in the correspondingtissues in roots of fully intact barley seedlings. In these,the young tissues retained a relatively high K/Na ratio alsoin their vacuoles. Similarly, contrasting results were obtainedwith intact and excised roots of Zea mays L. Based on theseresults a scheme of the events that lead to selective cationuptake in intact barley roots is proposed. In this scheme acrucial factor of selectivity is sufficient phloem recirculationof K⁺ by the aid of which K⁺ rich cortical cells are formednear the root tip. When matured these cells are suggested tomaintain a high cytoplasmic K/Na ratio due to K⁺ dependent sodiumextrusion at the plasmalemma and due to recovery of vacuolarK⁺ by Na/K exchange across the tonoplast. Key words: Potassium/Sodium selectivity, Vacuolar exchange, Xylem transport, Hordeum, Zea, Atriplex     

Coupling of Proton Fluxes in the Polar Leaves of Potamogeton lucens L

An attempt has been made to quantify the light-induced H⁺ effluxand influx observed in polar leaves of Potamogeton lucens.Theseproton fluxes are spatially separated. The H⁺ efflux, mediatedby a plasmalemma bound H⁺ –ATPase, occurs across theplasmamembrane at the morphological lower epidermis and is accompaniedby an H⁺ influx (or OH^– efflux) at the upper side oftheleaf. As a result, these leaves exhibit a remarkable pH–polarityin the light. The pH near the lower epidermis may drop to avalueas low as 3.5, while a pH of about 10.5 can be observed at theupper epidermis. Obviously this phenomenon requires theco–ordinationof transport processes in the different cell layers of the leaftissue. These observations led to quantitative studies oftherelation between the H⁺ fluxes at either plasmalemma. Thesefluxes were calculated from the pH values recorded at twodistancesfrom the leaf surface. Although the H⁺ influx always exceededthe efflux, a coupling between the transport processesacrosseither plasma membrane became evident from the time–coursesof the two fluxes. Key words: Potamogeton lucens, proton flux, flux coupling, pH–;polarity     

Potassium Fluxes in Excised Barley Roots

The method of the modified compartmental analysis for excisedroots has been adopted for measuring K⁺-fluxes and compartmentationin barley (Hordeum distichon) roots. Efflux of ⁴²K and ⁸⁶Rbindicated that more than two intracellular compartments wereinvolved in the tracer exchange; the ⁴²K data clearly showedthe components. On the basis of the efflux behaviour of theapical and more basal tissues of the roots, the three componentsof efflux were attributed to the cytoplasm of differentiated(fast) and meristematic tissues (intermediate) and to the vacuoles(slow exchange) of the roots. A model is proposed on the basisof which, the fluxes corresponding to the meristematic and differentiatedtissues of the root can be estimated. Additionally, fluxes ofthe differentiated root tissues were determined by using effluxdata obtained with root segments without apical tissues. Thedata obtained in both ways compare reasonably well and agreeto independent chemical measurements. Comparison of the ⁴²K and ⁸⁶Rb efflux data show strong discriminationof K^– in favour of Rb⁺ and indicate that ⁸⁶Rb is not suitableas a tracer for K⁺ in efflux measurements, at least with barleyroots.     

Ionic Relations of Garden Orache, A triplex hortensis L.: Growth and Ion Distribution at Moderate Salinity and the Function of Bladder Hairs

The growth of garden orache, A triplex hortensis was studiedunder conditions of mild NaCl or Na₂SO₄ salinity. Growth, drymatter production and leaf size were substantially stimulatedat 10 mM and 50 mM Na⁺ salts. Increased growth, however, appearedto be due to a K⁺-sparing effect of Na⁺ rather than to salinityper se. The distribution of K⁺ and Na⁺ in the plant revealeda remarkable preference for K+ in the roots and the hypocotyl.In the shoot the K/Na ratio decreased strongly with leaf age.However, the inverse changes in K⁺ and Na⁺ content with leafage were dependent on the presence of bladder hairs, which removedalmost all of the Na⁺ from the young leaf lamina. Measurementsof net fluxes of K⁺ and Na⁺ into roots and shoots of growingAtriplex plants showed a higher K/Na selectivity of the netion flux to the root compared to the shoot. With increasingsalinity the selectivity ratio S_{K, Na}^* of net ion fluxes tothe roots and to the shoots was increased. The data suggestthat recirculation of K⁺ from leaves to roots is an importantlink in establishing the K/Na selectivity in A. hortensis plants.The importance of K⁺ recirculation and phloem transport forsalt tolerance is discussed. Key words: Atriplex hortensis, Salinity, Potassium, Sodium, K⁺ retranslocation, Bladder hairs, Growth stimulation     

Unidirectional Ca2+ Fluxes in Roots of Rye (Secale cereale L). A Comparison of Excised Roots with Roots of Intact Plants

The unidirectional Ca²⁺ fluxes across the plasma membrane andtonoplast were determined in both excised roots and roots ofintact seedlings of rye (Secale cereale L. cv. Rheidol). Theunidirectional Ca²⁺ fluxes across the plasma membrane and tonoplastmeasured in excised roots were of a similar order of magnitudeto those determined in roots of intact plants. Influx and effluxof Ca²⁺ across the root plasma membrane were similar (estimatedto be between 0·7 and 3·4 µmol g     

Fluxes of Sodium and Potassium in Acetabularia mediterranea

Sodium efflux in Acetabularia mediterranea occurs against agradient of electrochemical potential and is a light-stimulated,temperature-sensitive process; it is not sensitive to the uncouplerCCCP. Sodium influx is stimulated in CCCP and at low temperature.Potassium influx is temperature- and uncoupler-sensitive, butis not light-stimulated. Tracer K efflux shows complex kinetics,which cannot be explained by any arrangement of intracellularcompartments; it appears to be stimulated at low temperatureand is insensitive to light and uncouplers. There is no evidencefor any chemical linkage between fluxes of Na⁺, K⁺, or Cl^–.It is concluded that Na^– efflux at the plasmalemma isan active process, but no consistent explanation can be advancedto account for the results of K⁺ flux measurements.     

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     

Compartmental analysis of 35SO2-4exchange kinetics in roots and leaves of a tropical legume Macroptilium atropurpureum cv. Siratro

Compartmental analysis of ³⁵SO^2-₄ tracer exchange kinetics hasbeen used to estimate unidirectional fluxes and compartmentcontents in excised root and leaf tissue of the tropical legume,Macroptilium atropurpureum. In excised root tissue only 5% ofthe sulphate taken up across the plasmalemma was reduced toorganic forms whereas in excised leaf tissue approximately 20%was reduced. It was necessary, therefore, to incorporate themetabolism of sulphate during the course of the experiment intothe compartmental models. In root tissue, wash-out data was fitted by three exponentials,assumed to correspond to exchange in the extracellular spaces,cytoplasm and vacuole, but in leaf tissue two large, slowlyexchanging compartments have been postulated in order to achievea fit to the data. It is likely that differences in leaf cellpopulations cause the ‘anomalous’ tracer exchangekinetics and the justification of this assumption is discussed. The fluxes of sulphate at the plasmalemma were greater thanthe corresponding fluxes at the tonoplast in both roots andleaves. The flux of SO^2-₄ from the cytoplasm to the externalsolution did not appear to limit the loss of SO^2-₄ from thevacuole. At an external SO^2-₄concentration of 0.25 mol m^-3 therate constants for exchange in the vacuole were two orders ofmagnitude greater in roots than in the slowest exchanging leafcell population. It is possible, therefore, that the slow lossof SO^2-₄ from leaf cell vacuoles may limit the redistributionof sulphate during S-stress. Key words: Compartmental analysis, sulphate, deficiency, Macroptilium atropurpureum     

Compartmental analysis of efflux of K+(86Rb+) and Rb+(86Rb+) from roots of intact plants of barley (Hordeum vulgare) of high and low K+ status, and after excision of the shoot

The classic compartment analysis of ion efflux from roots is often applied with the assumption that there is a system of 3 compartments in series. However, complex ion transport across the root tissues, as well as influences from the shoot, may complicate the picture. The present experiments were performed to study the immediate effects that excision of the shoot before the experiment exerts on the efflux of Rb⁺(⁸⁶Rb⁺) and of K⁺(⁸⁶Rb⁺) from 9-day-old roots of plants of barley (Hordeum vulgare L. cv. Salve). The efflux from high K⁺ and low K⁺ roots of intact and detopped plants were compared. After excision of the shoot of high K⁺ plants, a marked increase in efflux was observed after 2.5 h with a maximum at about 7 h. The increase in efflux was seen as a peak in plots of efflux versus time. Excision of the shoot from low K⁺ roots did not give rise to a consistent increase in efflux. Regular K⁺ ion efflux curves were observed from roots of intact plants of high or low K⁺ status. Furthermore, after a pulse treatment of 9-day-old roots of intact plants of high or low K⁺ status with a solution containing Rb⁺(⁸⁶Rb⁺), the Rb⁺(⁸⁶Rb⁺) transport to the shoots was not reduced during the following 3 h in unlabelled solution. It is suggested that both the peak appearing in the efflux plots and the maintained tracer transport to the shoots after transfer of the roots to an unlabelled solution indicate the existence of a K⁺/Rb⁺ transport system in the symplasm of the roots that has only a slow exchange with the bulk cytoplasm and vacuoles.     

Sodium Accumulation in Pappophorum I. Uptake, Transport and Recirculation

The uptake, transport and accumulation of sodium were comparedin two grasses: Pappophorum pappifervm (Lam.) O. Kuntze, a glycophyteand P. philippianum L. R. Parodi, a facultative halophyte. Atlow salinity levels, (50 mM NaCl) shoots of salt-treated P.pappiferum accumulated lower Na⁺ concentrations than the otherspecies. This difference does not seem to be related to Na⁺uptake, as in short-time experiments (< I h), whole plantsof both species showed similar rates of Na⁺ uptake and transport Sodium recirculation was assessed in split-root experiments.It was similar in control (previously non-salinized) plantsof both species, but in salt-treated plants it was more significantin P. pappiferum. This mechanism, along with increased lossof recently acquired Na⁺, could contribute to keep Na⁺ levelslower in shoots of P. pappiferum than in P. philippianum. Pappophorum, Gramineae, sodium recirculation, salinity     

K+-dependent net Na+ efflux in roots of barley plants

Summary The influence of K⁺ ions on the net Na⁺ fluxes in cells of excised barley roots (Hordeum distichon L.) and roots of whole barley plants was investigated. The fluxes were determined by flame photometry in the external solution. In both cases a transient net Na⁺ efflux against the external Na⁺ concentration was observed upon addition of K⁺. The results stress the effectiveness of the K⁺-dependent Na⁺ efflux mechanism residing at the plasmalemma, and its involvement in K–Na-selectivity in whole barley plants.     

Effect of Potassium on Photosynthate Unloading from Seed Coats of Phaseolus vulgaris L. Specificity and Membrane Location

Rates of ¹⁴C-photosynthate unloading from excised seed-coathalves of Phaseolus vulgaris L. plants were stimulated by externalKCI concentrations in excess of 10 mM with an optimal responseat 100–150 mM KCI. The cellular pattern of ¹⁴C-photosynthatemetabolism was not altered by KCI but the treatment preferentiallystimulated the release of sucrose from the seed-coats. Photosynthateunloading was insensitive to Cl^– and was stimulated bya range of membrane-permeable cations (Na⁺, Mg²⁺ and tetraphenylphosphoniumion) in addition to K⁺. The K⁺ ionophore, valinomycin, abolishedthe K⁺ stimulation of ¹⁴C-photosynthate unloading. A switchto a wash solution containing K⁺ elicited a rapid burst of ¹⁴C-photosynthateunloading; the rate constant for the final phase of ¹⁴C-efflux(probably across the tonoplast) was unaffected by K⁺. The KCItreatment did not change the passive permeability of eitherthe plasmalemma or tonoplast. While sucrose influx across theplasmalemma was insensitive to K⁺, sucrose transfer to the vacuolewas slowed. The results obtained support the postulate thatK⁺ (and other membrane permeable cations) preferentially stimulatesucrose efflux across the plasmalemma of the unloading cellsby serving to carry positive charge in the opposite direction. Phaseolus vulgaris, bean, photosynthate unloading, potassium stimulation, seed-coat     

Cellular Cobalt Fluxes in Roots and Transport to the Shoots of Wheat Seedlings

Macklon, A. E. S. and Sim, A. 1987. Cellular cobalt fluxes inroots and transport to the shoots of wheat seedlings.—J.exp. Bot. 38: 1663–1677. From compartmental analysis of radioisotope elution measurements,concentrations and fluxes of cobalt were estimated for corticalcells of wheat (Triticum aestivum L. cv. Fenman) seedling roots,relative to a complete nutrient solution containing 20 mmolm³ Co^{2 +} . The water free space, Donnan free space and cytoplasmcontained little Co relative to the high levels accumulatedin the vacuole, resulting in time courses of Co uptake whichwere essentially linear in continuous light. In light/dark cyclesthere was almost complete inhibition of uptake during the darkperiods. Comparison of estimated concentrations of Co in cytoplasmand vacuole with values to be expected, if passive diffusionalone was operative, indicated that active transport was occurringin an outward direction at the plasmalemma and into the vacuoleat the tonoplast. Transport to the shoot, which was unaffectedby transpirational water flux through the plant, reached about10% of root absorbed Co, most of which was stored in the corticalcell vacuoles and removed from the transport pathway. Between0·02 and 20 mmol m ³ Co in external solution, uptakewas linearly related to concentration on a log/log plot, andthe proportion of transported Co was constant. The implicationsof these findings are discussed. Key words: Cobalt, transmembrane fluxes, transport.     

Inhibition of Ion Transport in Excised Barley Roots by Abscisic Acid; Relation to Water Permeability of the Roots

Previous papers have shown that abscisic acid can inhibit transportof ions across the root to the xylem vessels, resulting in reducedexudation from excised roots or inhibiting guttation from intactplants. However, it has not been established whether the inhibitionwas due to a reduction in salt transport (J_s) or in permeabilityof the roots to water (L_p). This paper investigates the effectof ABA on L_p and J_s separately. It is shown that L_p increasedin ABA and then fell, but was about the same as in control rootswhen transport was inhibited. The effect of ABA on exudationtherefore appeared to be mainly due to reduction in J_s. Inhibitionof J_s was also present in intact, transpiring plants and sowas not due to reduced water flow. The inhibition of ion releaseto the xylem affected Na⁺, Mg²⁺, Ca²⁺, and phosphate as wellas the major ion in the exudate, K⁺. It is concluded that ABAinhibits salt transport to the shoot by acting on ion transportinto the xylem, and not by reducing water flow coupled withsalt transport.     

A Relationship between Phosphate Uptake and Membrane Electrical Potential in Excised Roots of Helianthus annuus

Phosphate uptake by excised roots of sunflower (Helianthus annuus)was determined by the disappearance of phosphate from the externalsolution and by the accumulation of phosphate labelled with³²P. Over a 24 h period it was observed that net phosphate uptakedeclined to zero whilst uptake of ³²P continued unabated. Theelectrical PD of the cortical cell membranes declined in parallelwith net phosphate uptake and it was found that both could berestored by creating a pH gradient across the plasmalemma. Itwas concluded that net phosphate uptake was responsible fora component of the membrane PD of the root cortical cells. Key words: Roots, Phosphate, Membranes     

K+-Na+ Exchange and Selectivity in Barley Root Cells: Effect of Na+ on the Na+ Fluxes

Using the compartmental analysis the unidirectional Na⁺ fluxesin cortical cells of barley roots, the cytoplasmic and vacuolarNa⁺ contents Q_c and Q_v, and the trans-root Na⁺ transport R'have been studied as a function of the external Na⁺ concentration.Using the re-elution technique the effect of low K⁺ concentrationson the plasmalemma efflux _co of Na⁺ (K+-Na+ exchange) and onR' was investigated at different Na+ concentrations and correspondinglydifferent values of the cytoplasmic sodium content Qc. The relationof the K+-dependent Na+ efflux coK+-dep to Qc or to the cytoplasmicNa+ concentration obeyed Michaelis-Menten kinetics. This isconsistent with a linkage of co, K+-dep to K+ influx by a K⁺-Na⁺exchange system. The apparent Km corresponded to a cytoplasmicNa⁺ concentration of 28 mM at 0·2 mM K⁺ and about 0·2mM Na⁺ in the external solution. 0·2 mM K⁺ stimulatedthe plasma-lemma efflux of Na⁺ and inhibited Na⁺ transport selectivelyeven in the presence of 10 mM Na⁺ in the external medium showingthe high efficiency of the K⁺-Na⁺ exchange system. However,_co, K⁺-dep was inhibited at 10 mM Na¹ compared to lower Na¹concentrations suggesting some competition of Na¹ with K¹ atthe external site of the exchange system. The effect of theNa+ concentration on Na¹ influx _oc is discussed with respectto kinetic models of uuptake.     

Wirkung von K+ auf die Fluxe und den Transport von Na+ in Gerstenwurzeln,K+-stimulierter Na+-Efflux in der Wurzelrinde

Summary Barley roots grown on a nutrient solution containing 1 mM Na⁺ but no K⁺ are capable of a considerable Na⁺ transport via the symplasm of the root and the xylem vessels. K⁺ added to the medium surrounding the root cortex severely inhibits this transport after a lag period at a high rate constant (Fig. 3).It is likely that the fluxes of Na⁺ are changed drastically during this transition from low to high K⁺ status. Although originally limited to steady state fluxes, the extended method of efflux analysis for excised roots (Pitman, 1971) has been applied to the non-steady fluxes which occur upon the addition of K⁺ to the roots. It is shown that besides other changes the efflux of ²²Na⁺ through the cortex of barley roots is stimulated instantaneously (Fig. 5) by the addition of K⁺ and presumably by an influx of K⁺ ions. From this a transient, K⁺-stimulated Na⁺ efflux at the plasmalemma of the cortical cells can be estimated. It amounts to 10.9 moles/g fw · h compared to the control efflux of 3.3 moles/g fw · h without K⁺.The stimulated efflux is attributed to a Na⁺ efflux pump at the plasmalemma and is thus related to the K-Na-selectivity of barley plants. The inhibition of the Na⁺ transport by K⁺ is probably a consequence of this increased efflux of Na⁺ from the symplasm through the root cortex.     

Acidification of Deionized Water by Roots of Intact Rice Seedlings

This investigation was designed to examine whether or not deionizedwater could be acidified by roots of intact rice seedlings.Roots of intact rice seedlings caused significant acidificationof the deionized water in which they were immersed and thisacidification could be repeated after replacement of acidifiedwater with fresh deionized water. The addition of K⁺, Na⁺, andMg²⁺ to the deionized water significantly increased the rateand extent of acidification. However, no such increase was foundwhen Ca²⁺ was present in the water. The inhibition of acidificationby vanadate and its promotion by fusicoccin indicated that theacidification of water by roots of intact rice seedlings originatedfrom an ATP-driven proton pump located in the plasmalemma. Ferricyanide was effectively reduced by the roots of intactrice seedlings. This reduction was associated with the acidificationof the bathing solution. 8-Hydroxyquinoline and p-nitrophenyl-acetateinhibited both the reduction of ferricyanide and ferricyanide-inducedacidification. Vanadate, although it slightly inhibited thereduction of ferricyanide, did not inhibit the ferricyanide-stimulateddecrease in pH. It seems that the involvement of redox activityassociated with the plasmalemma in the acidification of deionizedwater cannot be excluded. (Received August 30, 1989; Accepted April 5, 1990)