Influx and efflux of organic acids across the soil-root interface of Zea mays L. and its implications in rhizosphere C flow

The influx and efflux of organic acids across the root-soil interface were investigated in intact, sterile maize (Zea mays L.) roots under a variety of experimental conditions. Under nutrient-sufficient conditions the efflux of organic acids was shown to constitute < 1% of the total C lost across the root-soil interface. Under severe nutrient stress, however, the rates of malate and citrate efflux from the root increased 33 and 12 fold respectively. Influx experiments indicated that roots could not directly reabsorb citrate-Fe³⁺ or other metal complexes from solution. Influx of citrate was observed only at high external citrate concentration ( 1 mM) or from solutions with low ionic strengths. It was postulated that citrate influx is of little importance in a soil environment.     

Screening sorghum for tolerance to excess manganese in solution culture

This study was conducted to define traits to screen sorghum (Sorghum bicolor L. Moench) genotypes for tolerance to excess Mn. Visual Mn toxicity symptoms, net and total root lengths, shoot and root dry matter yields, and shoot and root Mn concentrations were determined for plants grown in nutrient solutions (pH 4.5) at different levels of Mn (0, 3, 6, 9 and 12 mM above the initial 18 M) to assess plant responses to excess Mn. Dry matter yields showed greatest variability among genotypes, and was an effective trait to evaluate sorghum for tolerance to excess Mn. Reductions in dry matter yields did not occur until Mn levels were above 3 mM. Levels of Mn between 3 and 6 mM could effectively be used to screen sorghum for genotypic differences to excess Mn. Manganese levels above 6 mM were too severe to allow good genotypic differentiation. Of genotypes tested, NB9040 and Wheatland showed good tolerance and SC283 and ICA-Nataima were sensitive to excess Mn.     

Ferric chelate reduction by suspension culture cells and roots of soybean: A kinetic comparison

The abilities of suspension cultures and intact roots of soybean (Glycine max L. cv. Hawkeye) to reduce ferric chelate were compared. Ferric chelate was supplied as ferric hydroxyethylethylenediaminetriacetic acid (FeHEDTA) and reduction was measured spectrophotometrically using bathophenan-throlinedisulfonic acid (BPDS) as the ferrous scavenger. Ferric chelate reduction by cell suspension cultures showed typical saturation kinetics; however, no difference was observed between cells that had been continuously grown with Fe (+Fe) and those that had been grown for four days without added Fe (–Fe). Values for K_m and V_max, determined from a Lineweaver-Burk plot, were 57 M and nmoles mg^-1 dry weight for the +Fe cells and 50 M and 22 nmoles mg^-1 dry weight for the -Fe cells, respectively. Ferric chelate reduction by Fe-deficient roots also exhibited saturation kinetics, while roots grown with adequate Fe did not reduce ferric chelate. The K_m and V_max values for Fe-deficient roots were 45 M and 20 nmoles mg^-1 dry weight, respectively, and did not differ from values obtained for cells in culture. This study offers strong evidence that the mechanism responsible for the reduction of ferric chelate is the same for cultured cells and roots and that the process is controlled at the cellular level. We propose that suspension cultures can be used as an alternative to intact roots in the study of ferric chelate reduction.     

The action of divalen Zn,Cd, Hg,Cu and Pb ions on the ATPase activity of a plasma membrane fraction isolated from roots ofZea mays

The effects of the divalent metal ions Zn, Cd, Hg, Cu and Pb on the ATPase activity of a plasma membrane fraction isolated from roots ofZea mays have been investigated. When Mg-ions (3 mM), with or without K-ions (50mM) are included in the reaction medium, inhibition of ATPase activity was found in all cases, the relative order of the inhibitors over the concentration range 10 to 100M, being Hg>>CuCd>ZnPb. Below 1.0M only Hg caused substantial inhibition. In the absence of Mg ions, Zn and to a lesser extent Cd, activated the enzyme up to a concentration of 1 mM, activity being further stimulated in the presence of K-ions (50mM). No activation of ATPase activity was found for Hg, Cu or Pb. It was concluded that Zn-ATP and Cd-ATP are both alternative substrates for the enzyme. Further experiments showed that both K_m and V_max for the substrates Zn-ATP and Cd-ATP are very much lower than for the usual substrate Mg-ATP.These present results are discussed in relation to the known actions of these divalent cations on the trans-root potential and H-ion efflux in excised maize roots (Kennedy and Gonsalves, 1987).     

Sulphate influx in wheat and barley roots becomes more sensitive to specific protein-binding reagents when plants are sulphate-deficient

When young wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) plants were deprived of an external sulphate supply (-S plants), the capacity of their roots to absorb sulphate, but not phosphate or potassium, increased rapidly (derepression) so that after 3–5 d it was more than tenfold that of sulphate-sufficient plants (+S plants). This increased capacity was lost rapidly (repression) over a 24-h period when the sulphate supply was restored. There was little effect on the uptake of L-methionine during de-repression of the sulphate-transport system, but S input from methionine during a 24-h pretreatment repressed sulphate influx in both+S and-S plants.Sulphate influx of both+S and-S plants was inhibited by pretreating roots for 1 h with 4,4-diisothiocyanatostilbene-2,2-disulphonic acid (DIDS) at concentrations > 0.1 mol · m^-3. This inhibition was substantially reversed by washing for 1 h in DIDS-free medium before measuring influx. Longer-term pretreatment of roots with 0.1 mol·m^-3 DIDS delayed de-repression of the sulphatetransport system in-S plants but had no influence on+S plants in 3 d.The sulphydryl-binding reagent, n-ethylmaleimide, was a very potent inhibitor of sulphate influx in-S roots, but was much less inhibitory in +S roots. Its effects were essentially irreversible and were proportionately the same at all sulphate concentrations within the range of operation of the high-affinity sulphate-transport system. Inhibition of influx was 85–96% by 300 s pretreatment by 0.3 mol·m^-3 n-ethylmaleimide. No protection of the transport system could be observed by including up to 50 mol·m^-3 sulphate in the n-ethylmaleimide pre-treatment solution. A similar differential sensitivity of-S and+S plants was seen with p-chloromercuriphenyl sulphonic acid.The arginyl-binding reagent, phenylglyoxal, supplied to roots at 0.25 or 1 mol·m^-3 strongly inhibited influx in-S wheat plants (by up to 95%) but reduced influx by only one-half in+S plants. The inhibition of sulphate influx in-S plants was much greater than that of phosphate influx and could not be prevented by relatively high (100 mol·m^-3 sulphate concentrations accompanying phenylglyoxal treatment. Effects of phenylglyoxal pretreatment were unchanged for at least 30 min after its removal from the solution but thereafter the capacity for sulphate influx was restored. The amount of new carrier appearing in-S roots was far greater than in+S roots over a 24-h period.The results indicate that, in the de-repressed state, the sulphate transporter is more sensitive to reagents binding sulphydryl and arginyl residues. This suggests a number of strategies for identifying the proteins involved in sulphate transport.Abbreviations DIDS 4,4-diisothiocyanatostilbene-2,2-disulphonic acid - NEM n-ethylmaleimide - PCMBS p-chloromercuriphenyl sulphonic acid     

Effects of External Potassium and Strophanthidin on Sodium Fluxes in Frog Striated Muscle

Unidirectional Na fluxes in isolated fibers from the frog''s semitendinosus muscle were measured in the presence of strophanthidin and increased external potassium ion concentrations. Strophanthidin at a concentration of 10^-5 M inhibited about 80 per cent of the resting Na efflux without having any detectable effect on the resting Na influx. From this it is concluded that the major portion of the resting Na efflux is caused by active transport processes. External potassium concentrations from 2.5 to 7.5 mM had little effect on resting Na efflux. Above 7.5 mM and up to 15 mM external K, the Na efflux was markedly stimulated; with 15 mM K the Na influx was 250 to 300 per cent greater than normal. On the other hand, Na influx was unchanged with 15 mM K. The stimulated Na efflux with the higher concentrations was not appreciably reduced when choline or Li was substituted for external Na, but was completely inhibited by 10^-5 M strophanthidin. From these findings it is concluded that the active transport of Na is stimulated by the higher concentrations of K. It is postulated that this effect on the Na "pump" is produced as a result of the depolarization of the muscle membranes and is related to the increased metabolism and heat production found under conditions of high external K.     

Influence of boron and calcium on the tolerance to salinity of nitrogen-fixing pea plants

The effects of different levels of B (from 9.3 to 93 M B) and Ca (from 0.68 to 5.44 mM Ca) on plant development, nitrogen fixation, and mineral composition of pea (Pisum sativum L. cv. Argona) grown in symbiosis with Rhizobium leguminosarum bv. viciae 3841 and under salt stress, were analysed. The addition of extra B and extra Ca to the nutrient solution prevented the reduction caused by 75 mM NaCl of plant growth and the inhibition of nodulation and nitrogen fixation. The number of nodules recovered by the increase of Ca concentration at any B level, but only nodules developed at high B and high Ca concentrations could fix nitrogen. Addition of extra B and Ca during plant growth restored nodule organogenesis and structure, which was absolutely damaged by high salt. The increase in salt tolerance of symbiotic plants mediated by B and Ca can be co-related with the recovery of the contents of some nutrients. Salinity produced a decrease of B and Ca contents both in shoots and in nodulated roots, being increased by the supplement of both elements in the nutrient solution. Salinity also reduced the content in plants of other nutrients important for plant development and particularly for symbiotic nitrogen fixation, as K and Fe. A balanced nutrition of B and Ca (55.8 M B, 2.72 mM Ca) was able to counter-act the deficiency of these nutrients in salt-stressed plants, leading to a huge increase in salinity tolerance of symbiotic pea plants. The necessity of nutritional studies to successfully cultivate legumes in saline soils is discussed and proposed.     

Phosphorus nutrition of barley, buckwheat and rape seedlings. II. Influx and efflux of phosphorous by intact roots of different P status

Seedlings of barley (Hordeum vulgare L. cvs Salka and Zita), buckwheat (Fagopyrum esculentum Moench) and rape (Brassica napus L. ssp. napus cv. Line) were raised at 8 or 10 different extenral P concentrations in the range 0–2000 μM. Apart from P, the nutrient solutions were complete. Phosphate influx in roots of different P status was determined by use of a nutrient solution containing 0.1 mM³²P-labelled phosphate. A double labelling technique was used for simultaneous determination of influx (³³P) and efflux (³²P) of phosphorus by roots of barley and rape with three selected P levels. Flux determinations were also done in presence of a metabolic uncoupler (2,4-dinitrophenol) and a protein synthesis inhibitor (cycloheximide). Influx of phosphate was maximal at a certin optimal P level of the roots and decreased at both lower and higher P levels. Maximum phosphate influex [μmol (g root)-^?1 h^?1] were: rape 4,4, buckwheat 2.2, barley cv. Salka 1.6, barley cv. Zita 1.5. Both Hill plots and plots of the untransformed decreasing phosphate influx vs root P concentrations above the optimal were linear and had high correlation coefficients. The Hill coefficient varied between -3.1 and -4.2. The decrease of phosphate influx from the maximum to the lowest value at the highest P concentration of the root was 60–70%. Hence, phosphate influex appeared to be regulated through negative feedback by the internal level of phosphorous in the roots. The regulation mechanism seems bascially similar for the three species and may be of an allosteric type. P efflux from roots of low and optimal (with regard to P influx) P status was 15–20% of the simultaneous P influx. Contary to P influx, P efflux increased at high P status and almost eliminated (barley) or halved (rape) net P uptake. 2,4-Dinitrophenol reduced both P influx and P efflux by low P roots and gave linearly increasing P efflux with increasing root P status. This indicates that P efflux partly occurred by counter transport and ion exchange at the uptake sites, partly by passive P efflux along an electrochemical potential gradient. Phosphate influx was not affected by inhibition of barley root growth with cycloheximide, but P efflux increased considerably.     

Choline permeability in cardiac muscle cells of the cat

Permeability of the cardiac cell membrane to choline ions was estimated by measuring radioactive choline influx and efflux in cat ventricular muscle. Maximum values for choline influx in 3.5 and 137 mM choline were respectively 0.56 and 9 pmoles/cm²·sec. In 3.5 mM choline the intracellular choline concentration was raised more than five times above the extracellular concentration after 2 hr of incubation. In 137 mM choline, choline influx corresponded to the combined loss of intracellular Na and K ions. Paper chromatography of muscle extracts indicated that choline was not metabolized to any important degree. The accumulation of intracellular choline rules out the existence of an efficient active pumping mechanism. By measuring simultaneously choline and sucrose exchange, choline efflux was analyzed in an extracellular phase, followed by two intracellular phases: a rapid and a slow one. Efflux corresponding to the rapid phase was estimated at 16–45 pmoles/cm²·sec in 137 mM choline and at 1.3–3.5 pmoles/cm²·sec in 3.5 mM choline; efflux in 3.5 mM choline was proportional to the intracellular choline concentration. The absolute figures for unidirectional efflux were much larger than the net influx values. The data are compared to Na and Li exchange in heart cells. Possible mechanisms for explaining the choline behavior in heart muscle are discussed.     

Influence of Excision and Aging upon K+ Influx into Barley Roots: Recovery or Enhancement? 1

The influx of K⁺ from ⁸⁶Rb-labeled solutions in the concentration range 0.008 to 0.2 mm into roots of intact plants and excised roots of barley plants (Hordeum vulgare [L.]) previously grown in 5 mm CaSO₄ (low K⁺ roots) or 0.5 mm CaSO₄ plus 5 mm KCl (high K⁺ roots) was measured. A consistent observation of these experiments was a substantial reduction of influx (usually by about 50%) following excision. The possible leakage of K⁺ into the medium and subsequent dilution of specific activity of labeled solutions was eliminated as an explanation for influx reduction in excised low K⁺ roots. Reduction of transpirational rates was also without effect upon influx into low K⁺ roots. Excision followed by 2 hours aging in 0.5 mm CaSO₄ solution revealed that influx values recovered within the 2 hours to the values obtained in intact roots. It is concluded that much of the literature which describes the enhancement of ion uptake following excision actually describes excision damage followed by recovery.     

Zinc-binding peptides as a function of Zn and S in soybeans

Soybean (Glycine max [L.] Merr. cv. Ransom) plants were grown for two weeks in nutrient solutions at 0.8 and 40 M Zn and 0.02 and 20 mM S. Zinc toxicity appeared to induce synthesis of low molecular weight Zn-binding peptides in soybean roots and leaves. This binding of Zn-ions seemed to increase in leaves when the S concentration was high in the nutrient solution.     

Macronutrient concentrations of soybean infected with soybean cyst nematode

Soybean cultivars (Glycine max(L.) Merr.) infected with soybean cyst nematode (SCN; Heterodera glycinesIchinohe) often show symptoms similar to K deficiency. The objectives of this experiment were to determine if SCN infection affected macronutrient concentrations in soybean seedling vegetative tissues, determine whether increased K fertility can overcome these possible effects, and to determine if these possible effects are localized at the site of infection or expressed systemically throughout the root system. Soybean plants were grown with root systems split into two halves. This allowed differential K (0.2, 2.4 and 6.0 mM K nutrient solutions) and SCN (0 and 15 000 eggs/plant) treatments to be applied to opposite root-halves of the same plant. Thirty days after plants were inoculated with SCN, macronutrient concentrations of shoot and root tissues were determined. Potassium concentration in leaf blades was not affected; but K concentrations in leaf-petiole and stem tissues were increased with SCN infection. Roots infected with SCN contained lower K concentrations than uninfected roots, but only for the 2.4 mM K treatment. Thus, at the medium level of K fertility, SCN reduced K concentration in soybean roots, and increasing K fertility to the high level overcame the effect. Because K concentrations in the shoot tissues were not reduced by SCN infection, above ground portions of the plant may be able to overcome limitations that occur in roots during the first 30 days of infection. Increasing K fertility level in soybean fields may not benefit vegetative growth of soybean infected with SCN.     

Sodium Movements in Perfused Squid Giant Axons : Passive fluxes

Sodium movements in internally perfused giant axons from the squid Dosidicus gigas were studied with varying internal sodium concentrations and with fluoride as the internal anion. It was found that as the internal concentration of sodium was increased from 2 to 200 mM the resting sodium efflux increased from 0.09 to 34.0 pmoles/cm²sec and the average resting sodium influx increased from 42.9 to 64.5 pmoles/cm²sec but this last change was not statistically significant. When perfusing with a mixture of 500 mM K glutamate and 100 mM Na glutamate the resting efflux was 10 ± 3 pmoles/cm²sec and 41 ± 10 pmoles/cm²sec for sodium influx. Increasing the internal sodium concentration also increased both the extra influx and the extra efflux of sodium due to impulse propagation. At any given internal sodium concentration the net extra influx was about 5 pmoles/cm²impulse. This finding supports the notion that the inward current generated in a propagated action potential can be completely accounted for by movements of sodium.     

Nitrogen-13 Studies of Nitrate Fluxes in Barley Roots: I. COMPARTMENTAL ANALYSIS FROM MEASUREMENTS OF 13N EFFLUX

The short-lived radio-isotope nitrogen-13 (half-life 10 min)was used as a tracer in studying fluxes of N in the roots ofintact barley plants. After supplying the plants with ¹³N-nitratefor 30 min, efflux of ¹³N into an unlabelled (wash) solutionwas followed under steady-state conditions for a further 10min. Tests with ion exchange resins suggested that all of the¹³N released during this period was in the form of nitrate. In addition to nitrate from a surface film of solution and fromthe free space of the roots, efflux from another compartmentwas detected, tentatively identified as the cytoplasmic nitratepool. In plants grown with nitrate as the only external N-source,efflux from this compartment decreased with a rate constantabout 0·17 min^–1 (half-time 4 min). Adding ammoniumsulphate to the wash solution alone did not significantly affecteither the initial rate, or the rate constant, of efflux of¹³N from these roots. However, ¹³N efflux decreased more rapidly(rate constant about 0·32 min^–1, half-time 2·2min) in roots grown in, and subsequently washed with, solutioncontaining ammonium nitrate. In barley plants grown with 1·5 mol m^–3 nitrate,the cytoplasmic nitrate pool was estimated to contain about2% of the total nitrate in the roots, corresponding to a cytoplasmicnitrate concentration 26 mol m^–3. Nitrate efflux was equivalentto almost 40% of nitrate influx in the roots of these plants. Key words: Ion transport, nitrate, ammonium, efflux analysis, compartmentation     

Effects of Sodium Azide on Sodium Fluxes in Frog Striated Muscle

Unidirectional Na fluxes from frog''s striated muscle were measured in the presence of 0 to 5 mM sodium azide. With azide concentrations of 2 and 5 mM the Na efflux was markedly stimulated; the Na efflux with 5 mM azide was about 300 per cent greater than normal. A similar increase was present when all but the 5.0 mM sodium added with azide was replaced by choline. 10^-5 M strophanthidin abolished the azide effect on Na²⁴ efflux. Concentrations of azide of 1.0 mM or less had no effect on Na efflux. The Na influx, on the other hand, was only increased by 41 per cent in the presence of 5 mM NaN₃. From these findings it is concluded that the active transport of Na is stimulated by the higher concentrations of azide. The hypothesis is advanced that the active transport of Na is controlled by the transmembrane potential and that the stimulation of Na efflux is produced as a consequence of the membrane depolarization caused by the azide.     

Do cluster roots of Hakea actities (Proteaceae) acquire complex organic nitrogen?

Although there has been speculation that cluster roots play a role in plant nitrogen (N) acquisition there have been few experimental studies, demonstrating this. We grew the subtropical wet heathland plant Hakea actities in sand and in axenic vermiculite-sand cultures to investigate its use of different N sources. Seedlings produced greater quantities of cluster roots when grown with NO₃ ^–, glutathione or protein as N sources than with NH₄ ⁺, glutamine or without N addition, suggesting that N source influences cluster root initiation and development. Axenically grown seedlings acquired more N when grown with 4.5 mM than with 0.5 mM glutathione or protein as N sources suggesting that seedlings are able to assimilate complex organic N. However, control seedlings that did not receive N other than seed storage N (approximately 0.2 mg N) acquired 2 mg N from an unknown source, so that data are not unequivocal. Peptidase activity in extracts derived from non-axenic cluster root tissue changed with age of cluster roots. Developing and mature cluster roots had higher peptidase activity than young and senescing cluster roots. To determine whether peptidases are associated with the outer surface of cluster roots, entire cluster roots were gently centrifuged and the resulting solution analysed for peptidase activity using gelatine-containing PAGE. Different peptidases were active at different times of cluster root development, suggesting that plant or microbially derived peptidases could play a role in organic N acquisition by cluster roots. Roots and cluster roots expressed a putative amino acid transporter (HaAAT4-1) and peptide transporter (HaPepT1) as determined by northern blot analysis. Expression of HaPepT1 in cluster roots increased throughout cluster root development although further studies need to confirm this.     

Relative importance of Na+, Cl−, and abscisic acid in NaCl induced inhibition of root growth of rice seedlings

The relative importance of endogenous abscisic acid (ABA), as well as Na⁺ and Cl^– in NaCl-induced responses related to growth in roots of rice seedlings were investigated. The increase in ammonium, proline and H₂O₂ levels, and cell wall peroxidase (POD) activity has been shown to be related to NaCl-inhibited root growth of rice seedlings. Increasing concentrations of NaCl from 50 to 150 mM progressively decreased root growth and increased both Na⁺ and Cl^–. Treatment with NaCl in the presence of 4,4-diisothiocyano-2,2-disulfonic acid (DIDS, a nonpermeating amino-reactive disulfonic acid known to inhibit the uptake of Cl^–) had less Cl^– level in roots than that in the absence of DIDS, but did not affect the levels of Na⁺, and responses related to growth in roots. Treatment with 50 mM Na-gluconate (the anion of which is not permeable to membrane) had similar Na⁺ level in roots as that with 100 mM NaCl. It was found that treatment with 50 mM Na-gluconate effected growth reduction and growth-related responses in roots in the same way as 100 mM NaCl. All these results suggest that Cl^– is not required for NaCl-induced responses in root of rice seedlings. Endogenous ABA level showed no increase in roots of rice seedlings exposed to 150 mM NaCl. It is unlikely that ABA is associated with NaCl-inhibited root growth of rice seedlings.     

The physiological basis of increased biomass partitioning to roots upon nitrogen deprivation in young clonal tea (Camellia sinensis (L.) O. Kuntz)

Deprivation of nitrogen (N) increases assimilate partitioning towards roots at the expense of that to shoots. This study was done to determine the physiological basis of increased root growth of tea (sCammellia sinensis L.) under N shortage. Nine-month-old clonal tea (clone TRI2025) was grown in quartz sand under naturally lit glasshouse conditions. Three levels of N (0, 3.75 and 7.5 mM N) were incorporated in to the nutrient solution and applied daily. Plant growth, photosynthesis, root respiration and plant N contents were measured at 10-day intervals over a 45-day period. Root dry weight showed a sharp increase during the first 15 days after the plants were transferred to 0 mM N, whereas no such increase was shown in plants transferred to 7.5 mM N. In contrast, shoot dry weight increased at 7.5 mM N and was significantly greater than at 0 mM N, where no increase was observed. Due to the above changes, root weight ratio increased and leaf weight ratio decreased during the first 15 days of N deprivation. Leaf photosynthetic rates did not vary between N levels during the initial 15-day period. Thereafter, photosynthetic rates were greater at 7.5 mM and 3.75 mM N than at 0 mM N. Root respiration rate decreased at 0 mM N, whereas it increased at 3.75 and 7.5 mM N, probably because of the greater respiratory cost for nitrate uptake. Root respiratory costs associated with maintenance (R _m) and nitrate uptake (R _u) were calculated to investigate whether the sharp increase of root growth observed upon nitrogen deprivation was solely due to the reduced respiratory costs for nitrate uptake. The estimated values for R _m and R _u were 3.241 × 10^–4 mol CO₂ g^–1 (root dry matter) s^–1 and 0.64 mol CO₂ (mol N)^–1, respectively. Calculations showed that decreased respiratory costs for nitrate uptake could not solely account for the significant increase of root biomass upon N deprivation. Therefore, it is concluded that a significant shift in assimilate partitioning towards roots occurs immediately following N deprivation in tea.     

Transport of glycine-betaine by Listeria monocytogenes

Uptake of [¹⁴C]glycine-betaine by Listeria monocytogenes was stimulated by NaCl with optimal stimulation at 0.4–0.5 M. The glycine-betaine transport system had a K _m of 22 M and a V _max of 11.7 nmol^-1 min^-1 mg^-1 protein when grown in the absence of NaCl. When grown in the presence of 0.8 M NaCl the V _max increased to 27.0 nmol^-1 min^-1 mg^-1 protein in 0.8 M NaCl. At NaCl concentrations above 0.5 M the uptake rate of glycine-betaine was reduced. Measurement of intracellular K⁺ concentrations and fluorescent dye quenching indicated that higher NaCl concentrations also led to a decrease in the electrochemical potential difference across the cytoplasmic membrane. Uptake of glycine was also observed, but this was not stimulated by NaCl.     

Pump-leak sodium fluxes in low salt corn roots

Summary The influx and efflux of sodium from 4-hr washed, low salt corn roots (Zea mays L.) has been studied for characterization of passive and active components. Initial Na⁺ content of the roots is very low, 2.25±0.4 mol/g fresh weight. Na⁺ influx in the presence of 0.2mm Ca²⁺ and 0.002 to 20mm K⁺ is passive (a leak) based upon Goldman-type models, being determined by Na⁺ and cell potential (). Na⁺ was not transported by the K⁺ carrier and influx was unaffected by 50 m dicyclohexylcarbodiimide (DCCD). Permeability of the cells to Na⁺ was of the same order asP _k.Efflux of Na⁺ was by an efficient and rapid active transport system (a pump), thus accounting for the failure of these roots to accumulate high levels of Na⁺. In short-term loading and efflux experiments, internal Na⁺ turnover had a half-time of about 5 min. Sodium efflux was unaffected by DCCD. Net H⁺ flux was zero in the presence of DCCD regardless of sodium efflux, indicating absence of Na⁺/H⁺ antiport. Efflux of Na⁺ was equally rapid into medium containing no Na⁺ and only 0.002mm K⁺. K⁺ influx accounted for less than 4% of Na⁺ efflux, prompting the hypothesis that the Na⁺ (or cation?) efflux pump is the second electrogenic system previously defined based upon electrophysiological measurements.