Effect of ATPase inhibitors on cell potential and k influx in corn roots

Experiments were performed to determine the effect of plasmalemma ATPase inhibitors on cell potentials (Ψ) and K⁺ (⁸⁶Rb) influx of corn root tissue over a wide range of K⁺ activity. N,N′Dicyclohexylcarbodiimide (DCCD), oligomycin, and diethylstilbestrol (DES) pretreatment greatly reduced active K⁺ influx and depolarized Ψ at low, but not at high, K⁺ activity (K°). More comprehensive studies with DCCD and anoxia showed nearly complete inhibition of the active component of K⁺ influx over a wide range of K°, with no effect on the apparent permeability constant. DCCD had no effect on the electrogenic component of the cell potential (Ψ_p) above 0.2 millimolar K°. Net proton efflux was rapidly reduced 80 to 90% by DCCD. Since tissue ATP content and respiration were only slightly affected by the DCCD-pretreatment, the inhibitions of active K⁺ influx and Ψ_p at low K° can be attributed to inhibition of the plasmalemma ATPase.     

Energy-linked Potassium Influx as Related to Cell Potential in Corn Roots

Cell potentials and K⁺ (⁸⁶Rb) influx were determined for corn roots over a wide range of external K⁺ activity (K°) under control, anoxic, and uncoupled conditions. The data were analyzed using Goldman theory for the contribution of passive influx to total influx. For anoxic and uncoupled roots the K⁺ influx shows the functional relationship with K° predicted with constant passive permeability, although K⁺ permeability in uncoupled roots is about twice that of anoxic roots. In control roots the equation fails to describe K⁺ influx at low K°, but does so at high K°, with a gradual transition over the region where the electrical potential becomes equal to the equilibrium potential for K⁺ (ψ = E_K). In the low K° range, where net K⁺ influx is energetically uphill, participation of an energy-linked K⁺ carrier is indicated. In the high K° range, K⁺ influx becomes passive down the electrical gradient established by the cell potential. Since the cell potential includes a substantial electrogenic component, anoxia or uncoupling reduces passive influx.     

Material Studies of Lipid Vesicles in the Lα and Lα-Gel Coexistence Regimes

In this work, we utilize micropipette aspiration and fluorescence imaging to examine the material properties of lipid vesicles made from mixtures of palmitoyloleoylphosphocholine (POPC) and dipalmitoylphosphatidylcholine (DPPC). At elevated temperatures/low DPPC fractions, these lipids are in a miscible liquid crystalline (L_α) state, whereas at lower temperatures/higher DPPC fractions they phase-separate into L_α and gel phases. We show that the elastic modulus, K, and critical tension, τ_c, of L_α vesicles are independent of DPPC fraction. However, as the sample temperature is increased from 15°C to 45°C, we measure decreases in both K and τ_c of 20% and 50%, respectively. The elasticity change is likely driven by a change in interfacial tension. We describe the reduction in critical tension using a simple model of thermally activated membrane pores. Vesicles with two-phase coexistence exhibit material properties that differ from L_α vesicles including critical tensions that are 20–40% lower. Fluorescence imaging of phase coexistent POPC/DPPC vesicles shows that the DPPC-rich domains exist in an extended network structure that exhibits characteristics of a solid. This gel network explains many of the unusual material properties of two-phase membranes.     

Modulation of β-Cell Ouabain-Sensitive 86Rb+ Influx (Na+/K+ Pump) by D-Glucose,Glibenclamide or Diazoxide

The activity of the β-cell Na⁺/K⁺ pump was studied by using ouabain-sensitive (lmM ouabain) ⁸⁶Rb⁺ influx in β-cell-rich islets of Umeå-ob/ob mice as an indicator of the pump function. The present results show that the stimulatory effect of glucose on ouabain-sensitive ⁸⁶Rb⁺ influx reached its approximate maximum at 5mM glucose. Pre-treatment of the islets with 20mM glucose for 60 min strongly reduced the glucose-induced stimulation of the Na⁺/K⁺ pump. Pre-treatment (60 or 180 min) of islets at 0mM glucose, on the other hand, did not affect the magnitude of the glucose-induced stimulation of ⁸⁶Rb⁺ influx dunng the subsequent 5-min incubation. Glibenclamide stimulated the ouabain-sensitive ⁸⁶Rb⁺ uptake in the same manner as glucose. The stimulatory effect, showed its apparent maximum at 0.5μM. Pre-treatment (60 min) of islets with 1μM glibenclamide did not reduce the subsequent stimulation of the ouabain-sensitive ⁸⁶Rb⁺ influx. The stimulatory effect of glibenclamide and D-glucose were not .additive, suggesting that they may have the same mechanism of action. No direct effect of glibenclamide (0.01-1μM) was observed on the Na⁺/K⁺ ATPase activity in homogenates of islets. Diazoxide (0.4mM) inhibited the Na⁺/K⁺ pump. This effect was sustained even after 60 min of pre-treatment of islets with 0.4mM diazoxide. The stimulatory effect of glibenclamide and D-glucose were abolished by diazoxide. It is concluded that nutrient as well as non-nutrient insulin secretagogues activate the Na⁺/K⁺ pump, probably as part of the membrane repolarisation process.     

Time-resolved x-ray diffraction and calorimetric studies at low scan rates: II. On the fine structure of the phase transitions in hydrated dipalmitoylphosphatidylethanolamine

The phase transitions of dipalmitoylphosphatidylethanolamine (DPPE) in excess water have been examined by low-angle time-resolved x-ray diffraction and calorimetry at low scan rates. The lamellar subgel/lamellar liquid-crystalline (L_c → L_α), lamellar gel/lamellar liquid-crystalline (L_β → L_α), and lamellar liquid-crystalline/lamellar gel (L_α → L_β) phase transitions proceed via coexistence of the initial and final phases with no detectable intermediates at scan rates 0.1 and 0.5°C/min. At constant temperature within the region of the L_β → L_α transition the ratio of the two coexisting phases was found to be stable for over 30 min. The state of stable phase coexistence was preceded by a 150-s relaxation taking place at constant temperature after termination of the heating scan in the transition region. While no intermediate structures were present in the coexistence region, a well reproducible multipeak pattern, with at least four prominent heat capacity peaks separated in temperature by 0.4-0.5°C, has been observed in the cooling transition (L_α → L_β) by calorimetry. The multipeak pattern became distinct with an increase of incubation time in the liquid-crystalline phase. It was also clearly resolved in the x-ray diffraction intensity versus temperature plots recorded at slow cooling rates. These data suggest that the equilibrium state of the L_α phase of hydrated DPPE is represented by a mixture of domains that differ in thermal behavior, but cannot be distinguished structurally by x-ray scattering.     

Propagation Speed in Myelinated Nerve: I. Experimental Dependence on External Na+ and on Temperature

Conduction speed (θ) in single myelinated Rana pipiens sciatic nerve fibers has been precisely measured using intracellular recording and on-line digital computer techniques. The dependence of relative speed on external Na concentration at 15°C has been found to be ln(θ₁/θ₂) = 0.524 (±0.018) ln ([Na⁺]₁/[Na⁺]₂) + 0.003. Thus θ has very close to a square root dependence on [Na⁺]₀ for these fibers. This experimental finding is not in complete agreement with a theoretical prediction based on a solution of the Hodgkin-Huxley (H.H.) equations. The effect of small temperature variations around 15°C on θ has also been measured for Rana fibers in Ringer's solution. θ has close to an exponential dependence on T and a Q₁₀ of 2.95 has been estimated.     

Diffusion coefficient of fluorescent phosphatidylinositol 4,5-bisphosphate in the plasma membrane of cells

Phosphatidylinositol 4,5-bisphosphate (PIP₂) controls a surprisingly large number of processes in cells. Thus, many investigators have suggested that there might be different pools of PIP₂ on the inner leaflet of the plasma membrane. If a significant fraction of PIP₂ is bound electrostatically to unstructured clusters of basic residues on membrane proteins, the PIP₂ diffusion constant, D, should be reduced. We microinjected micelles of Bodipy TMR-PIP₂ into cells, and we measured D on the inner leaflet of fibroblasts and epithelial cells by using fluorescence correlation spectroscopy. The average ± SD value from all cell types was D = 0.8 ± 0.2 μm²/s (n = 218; 25°C). This is threefold lower than the D in blebs formed on Rat1 cells, D = 2.5 ± 0.8 μm²/s (n = 26). It is also significantly lower than the D in the outer leaflet or in giant unilamellar vesicles and the diffusion coefficient for other lipids on the inner leaflet of these cell membranes. The simplest interpretation is that approximately two thirds of the PIP₂ on inner leaflet of these plasma membranes is bound reversibly.     

Membrane Permeation Induced by Aggregates of Human Islet Amyloid Polypeptides

Several neurodegenerative diseases such as Alzheimer’s and Parkinson’s diseases as well as nonneuropathic diseases such as type II diabetes and atrial amyloidosis are associated with aggregation of amyloid polypeptides into fibrillar structures, or plaques. In this study, we use molecular dynamics simulations to test the stability and orientation of membrane-embedded aggregates of the human islet amyloid polypeptide (hIAPP) implicated in type II diabetes. We find that in both monolayers and bilayers of dipalmitoylphosphatidylglycerol (DPPG) hIAPP trimers and tetramers remain inside the membranes and preserve their β-sheet secondary structure. Lipid bilayer-inserted hIAPP trimers and tetramers orient inside DPPG at 60° relative to the membrane/water interface and lead to water permeation and Na⁺ intrusion, consistent with ion-toxicity in islet β-cells. In particular, hIAPP trimers form a water-filled β-sandwich that induce water permeability comparable with channel-forming proteins, such as aquaporins and gramicidin-A. The predicted disruptive orientation is consistent with the amphiphilic properties of the hIAPP aggregates and could be probed by chiral sum frequency generation (SFG) spectroscopy, as predicted by the simulated SFG spectra.     

Structural and dielectric properties of synthetic glycolipids in mixtures with water

Three synthetically produced glycolipids, N-(β-D-glucopyranosyl)-N-octadecyl-stearoylamide (OSGA), N-(β-D-glucopyranosyl-N-octadecyl-oleoylamide (OOGA), N-(β-D-galactopyranosyl)-N-octadecyl-lauroylamide (OLGA) have been studied in different mixtures with water by x-ray diffraction and dielectric measurements with microwaves at 9.4 GHz. The measurements were performed in the temperature range -50-70°C. X-Ray diffraction revealed a direct L_β' → H transition at 20°C, 60°C, and 45°C depending on the glycolipid species but nearly not on the water content. The hexagonal phases are saturated at a water content of ≈20 wt%. The lamellar phase absorbs even less water (< 10 wt%). The dielectric data show that in the H phase the binding of water is stronger than in the L_β' phase. In the temperature range below 0°C, OSGA and OOGA show a “subzero transition” due to the freeze-out of water in a separate ice phase. This transition can be seen in an abrupt decrease of the dielectric function because the dielectric response of ice is much smaller at microwave frequencies. OLGA does not show the subzero transition but an additional transition, hexagonal → distorted hexagonal at 60°C.     

Coarsening Dynamics of Domains in Lipid Membranes

We investigate isothermal diffusion and growth of micron-scale liquid domains within membranes of free-floating giant unilamellar vesicles with diameters between 80 and 250 μm. Domains appear after a rapid temperature quench, when the membrane is cooled through a miscibility phase transition such that coexisting liquid phases form. In membranes quenched far from a miscibility critical point, circular domains nucleate and then progress within seconds to late stage coarsening in which domains grow via two mechanisms 1), collision and coalescence of liquid domains, and 2), Ostwald ripening. Both mechanisms are expected to yield the same growth exponent, α = 1/3, where domain radius grows as time^α. We measure α = 0.28 ± 0.05, in excellent agreement. In membranes close to a miscibility critical point, the two liquid phases in the membrane are bicontinuous. A quench near the critical composition results in rapid changes in morphology of elongated domains. In this case, we measure α = 0.50 ± 0.16, consistent with theory and simulation.     

Alkylguanidines as inhibitors of k transport in isolated barley roots

It has been shown that plants can accumulate K⁺ through an energy-dependent process. The effect of alkylguanidines, in particular octylguanidine on the uptake of ⁸⁶Rb⁺ by excised barley roots (Hordeum vulgare var. Apizaco LV-72), has been studied. ⁸⁶Rb⁺ was used as tracer of K⁺. The uptake of ⁸⁶Rb⁺ which is linear with time and shows saturation kinetics is inhibited by octylguanidine. Half-maximal inhibition of ⁸⁶Rb⁺ uptake is attained at 50 μM octylguanidine. Octylguanidine induces a decrease in the V_max of the process and increases the Km of the system for Rb⁺. When the effects of various alkylguanidines were studied, the following order of effectiveness was encountered; octylguanidine = hexilguanidine > butylguanidine > ethylguanidine > guanidine. This suggests that guanidines inhibit Rb⁺ uptake by interacting through its positively charged guanidinium group with a Rb⁺ carrier while the alkyl chain interacts with the hydrophobic milieu of the membrane.     

Calcium Triggered Lα-H2 Phase Transition Monitored by Combined Rapid Mixing and Time-Resolved Synchrotron SAXS

Background

Awad et al. [1] reported on the Ca²⁺-induced transitions of dioleoyl-phosphatidylglycerol (DOPG)/monoolein (MO) vesicles to bicontinuous cubic phases at equilibrium conditions. In the present study, the combination of rapid mixing and time-resolved synchrotron small-angle X-ray scattering (SAXS) was applied for the in-situ investigations of fast structural transitions of diluted DOPG/MO vesicles into well-ordered nanostructures by the addition of low concentrated Ca²⁺ solutions.

Methodology/Principal Findings

Under static conditions and the in absence of the divalent cations, the DOPG/MO system forms large vesicles composed of weakly correlated bilayers with a d-spacing of ∼140 Å (L_α-phase). The utilization of a stopped-flow apparatus allowed mixing these DOPG/MO vesicles with a solution of Ca²⁺ ions within 10 milliseconds (ms). In such a way the dynamics of negatively charged PG to divalent cation interactions, and the kinetics of the induced structural transitions were studied. Ca²⁺ ions have a very strong impact on the lipidic nanostructures. Intriguingly, already at low salt concentrations (DOPG/Ca²⁺>2), Ca²⁺ ions trigger the transformation from bilayers to monolayer nanotubes (inverted hexagonal phase, H₂). Our results reveal that a binding ratio of 1 Ca²⁺ per 8 DOPG is sufficient for the formation of the H₂ phase. At 50°C a direct transition from the vesicles to the H₂ phase was observed, whereas at ambient temperature (20°C) a short lived intermediate phase (possibly the cubic Pn3m phase) coexisting with the H₂ phase was detected.

Conclusions/Significance

The strong binding of the divalent cations to the negatively charged DOPG molecules enhances the negative spontaneous curvature of the monolayers and causes a rapid collapsing of the vesicles. The rapid loss of the bilayer stability and the reorganization of the lipid molecules within ms support the argument that the transition mechanism is based on a leaky fusion of the vesicles.     

Delta-endotoxin-induced leakage of 86Rb+-K+ and H2O from phospholipid vesicles is catalyzed by reconstituted midgut membrane

Brush border membrane from Heliothis virescens catalyzed delta-endotoxin-induced leakage of ⁸⁶Rb⁺-K⁺ and H₂O from phospholipid vesicles. Activated delta-endotoxin [CrylA(c)-55 kDa] from Bacillus thuringiensis kurstaki strain EG2244 producing a single CrylA(c) toxin, when incorporated into phospholipid vesicles, made these vesicles more leaky to ⁸⁶Rb⁺-K⁺ than phospholipid vesicles without toxin. This effect was assayed by following the movement of ⁸⁶Rb⁺ into the vesicles in response to a KCl gradient. When toxin was added to the outside of phospholipid vesicles, ⁸⁶Rb⁺ uptake was impeded. Vesicles prepared with H. virescens brush border membrane catalyzed the association of the toxin with the vesicle, and stimulated KCl gradient-induced ⁸⁶Rb⁺ uptake. Toxin did not catalyze the leakage of ³⁶Cl⁻, suggesting that the toxin created a cation-selective leak. Toxin enhanced the permeability of phospholipid vesicles to H₂O, demonstrated by the enhanced rate of vesicle shrinking under increased osmotic pressure. This was analyzed spectrophotometrically by following the rate of vesicle shrinking in response to a 10 mM KCl gradient. In the presence of concentrated phosphatidylcholine vesicles, toxin spontaneously associated with the vesicles so as to enhance the rate of vesicle shrinking in an osmotic gradient. The rate of vesicle shrinking the presence of KCl and toxin was catalyzed by the presence of brush border reconstituted into the vesicles, reducing the effective toxin concentration 1000-fold.     

GUVs Melt Like LUVs: The Large Heat Capacity of MLVs Is Not Due to Large Size or Small Curvature

The excess heat capacity functions (ΔC_p) associated with the main phase transition of large unilamellar vesicles (LUVs) and multilamellar vesicles (MLVs) are very different. Two explanations are possible. First, the difference in vesicle size (curvature) results in different gel-fluid interactions in the membrane; those interactions have a large effect on the cooperativity of the phase transition. Second, there is communication between the bilayers in an MLV when they undergo the gel-fluid transition; this communication results in thermodynamic coupling of the phase transitions of the bilayers in the MLV and, consequently, in an apparent increase in the cooperativity of the transition. To test these hypotheses, differential scanning calorimetry was performed on giant unilamellar vesicles (GUVs) of pure dipalmitoylphosphatidylcholine. The ΔC_p curve of GUVs was found to resemble that of the much smaller LUVs. The transition in GUVs and LUVs is much broader (half-width ∼1.5°C) than in MLVs (∼0.1°C). This similarity in GUVs and LUVs indicates that their size has little effect on gel-fluid interactions in the phase transition. The result suggests that coupling between the transitions in the bilayers of an MLV is responsible for their apparent higher cooperativity in melting.     

Geometrical Membrane Curvature as an Allosteric Regulator of Membrane Protein Structure and Function

Transmembrane proteins are embedded in cellular membranes of varied lipid composition and geometrical curvature. Here, we studied for the first time the allosteric effect of geometrical membrane curvature on transmembrane protein structure and function. We used single-channel optical analysis of the prototypic transmembrane β-barrel α-hemolysin (α-HL) reconstituted on immobilized single small unilamellar liposomes of different diameter and therefore curvature. Our data demonstrate that physiologically abundant geometrical membrane curvatures can enforce a dramatic allosteric regulation (1000-fold inhibition) of α-HL permeability. High membrane curvatures (1/diameter ∼1/40 nm⁻¹) compressed the effective pore diameter of α-HL from 14.2 ± 0.8 Å to 11.4 ± 0.6 Å. This reduction in effective pore area (∼40%) when combined with the area compressibility of α-HL revealed an effective membrane tension of ∼50 mN/m and a curvature-imposed protein deformation energy of ∼7 k_BT. Such substantial energies have been shown to conformationally activate, or unfold, β-barrel and α-helical transmembrane proteins, suggesting that membrane curvature could likely regulate allosterically the structure and function of transmembrane proteins in general.     

Electrodiffusion,Barrier, and Gating Analysis of DIDS-insensitive Chloride Conductance in Human Red Blood Cells Treated with Valinomycin or Gramicidin

Current-voltage curves for DIDS-insensitive Cl⁻ conductance have been determined in human red blood cells from five donors. Currents were estimated from the rate of cell shrinkage using flow cytometry and differential laser light scattering. Membrane potentials were estimated from the extracellular pH of unbuffered suspensions using the proton ionophore FCCP. The width of the Gaussian distribution of cell volumes remained invariant during cell shrinkage, indicating a homogeneous Cl⁻ conductance among the cells. After pretreatment for 30 min with DIDS, net effluxes of K⁺ and Cl⁻ were induced by valinomycin and were measured in the continued presence of DIDS; inhibition was maximal at ∼65% above 1 μM DIDS at both 25°C and 37°C. The nonlinear current-voltage curves for DIDS-insensitive net Cl⁻ effluxes, induced by valinomycin or gramicidin at varied [K⁺]_o, were compared with predictions based on (1) the theory of electrodiffusion, (2) a single barrier model, (3) single occupancy, multiple barrier models, and (4) a voltage-gated mechanism. Electrodiffusion precisely describes the relationship between the measured transmembrane voltage and [K⁺]_o. Under our experimental conditions (pH 7.5, 23°C, 1–3 μM valinomycin or 60 ng/ml gramicidin, 1.2% hematocrit), the constant field permeability ratio P_K/P_Cl is 74 ± 9 with 10 μM DIDS, corresponding to 73% inhibition of P_Cl. Fitting the constant field current-voltage equation to the measured Cl⁻ currents yields P _Cl = 0.13 h⁻¹ with DIDS, compared to 0.49 h⁻¹ without DIDS, in good agreement with most previous studies. The inward rectifying DIDS-insensitive Cl⁻ current, however, is inconsistent with electrodiffusion and with certain single-occupancy multiple barrier models. The data are well described either by a single barrier located near the center of the transmembrane electric field, or, alternatively, by a voltage-gated channel mechanism according to which the maximal conductance is 0.055 ± 0.005 S/g Hb, half the channels are open at −27 ± 2 mV, and the equivalent gating charge is −1.2 ± 0.3.     

Conformation and Orientation of Gramicidin a in Oriented Phospholipid Bilayers Measured by Solid State Carbon-13 NMR

Three analogues of the helical ionophore gramicidin A have been synthesized with ¹³C-labeled carbonyls (¹³C=O) incorporated at either Gly², Ala³, or Val⁷. A fourth compound incorporated ¹³C at both the carbonyl and α-carbon of Gly² within the same molecule. These labels were studied using solid-state, proton-enhanced, ¹³C nuclear magnetic resonance (NMR) in hydrated dispersions of dimyristoylphosphatidylcholine (DMPC)-gramicidin A. The dispersions were aligned on glass coverslips whose orientation to the magnetic field could be varied through 180°. The orientation dependence of the NMR spectrum was used to obtain an accurate measurement of the ¹³C chemical shift anisotropy (CSA), and in the case of the fourth compound, the ¹³C—¹³C dipolar coupling constant. From the measured CSA and estimates of the orientation of the ¹³C shielding tensor, we are able to determine the direction of the ¹³C=O bonds and to compare these with the predictions of the various reported models for the configuration of gramicidin A in phospholipid bilayers. Our results are consistent with the left-handed ππ^6.3_LD single-stranded helix (Urry, D. W., J. T. Walker, and T. L. Trapane. 1982. J. Membr. Biol. 69:225-231). The right-handed ππ^6.3_LD single-stranded helix observed for gramicidin A in sodium dodecyl sulfate micelles (Arseniev, A. S., I. L. Barsukov, V. F. Bystrov, A. L. Loize, and Yu A. Ovchinnikov. 1985. FEBS (Fed. Eur. Biochem. Soc.) Lett. 186:168-174) yields a poorer fit to the data. However, the width of the carbonyl resonances suggests a distribution of molecular geometries possibly resulting from a spread in the helix pitch and handedness. Double-stranded helices and β sheet structures are excluded. In dispersions in which the lipid is in the L_α phase, the gramicidin A undergoes rapid reorientation about an axis which is centered on the normal to the plane of the coverslips. When the supporting lipid is in the L_β′ phase the helices are rigid on the timescale of ¹³C-NMR. The configuration of gramicidin A is unaltered by L_α-L_β′ phase transition of the bilayer lipid.     

Permeability properties of unilamellar vesicles containing choline plasmalogens and comparison with other choline glycerophospholipid species

The rates of non-electrolyte and ion diffusion across bilayer membranes consisting of choline plasmologens or of their alkyl and acyl analogs were studied. The influx of [¹⁴C]glucose, ⁸⁶Rb⁺ and ³⁶Cl^? into small unilamellar vesicles made from a semisynthetic choline plasmalogen and from synthetic diacyl, alkylacyl and dialkyl analogs with comparable side chain compositions were measured. Rates of glucose and Rb⁺ diffusion are about equal in alkenylacyl- and diacyl-glycerophosphocholine (GPC) bilayers, but are reduced in dialkyl-GPC membranes; the permeability coefficients correlate with the packing densities of the respective choline glycerophospholipids in monolayers at the air water interface. Rates of chloride diffusion are consistently higher in membranes formed from phospholipids containing alkenyl or alkyl other bonds as compared to the diacyl analogs. Highest rates of Cl^? diffusion are observed with choline plasmalogen vesicles. The phospholipid side chain composition has little influence on Cl^? permeation, but glucose and Rb⁺ diffusion are markedly affected. Incorporation of cholesterol (30 mol%) into choline plasmalogen membranes reduces their solute permeability by approximately 70%. A similar effect is found with the other choline phospholipid analogs. Thus, the choline phospholipid—cholesterol interaction, as far as it is reflected in reduced bilayer permeability, is not influenced by the presence of the alkenylether bond of plasmalogens.     

Temperature-dependent water and ion transport properties of barley and sorghum roots : I. Relationship to leaf growth

Root temperature strongly affects shoot growth, possibly via “nonhydraulic messengers” from root to shoot. In short-term studies with barley (Hordeum vulgare L.) and sorghum (Sorghum bicolor L.) seedlings, the optimum root temperatures for leaf expansion were 25° and 35°C, respectively. Hydraulic conductance (L_p) of both intact plants and detached exuding roots of barley increased with increasing root temperature to a high value at 25°C, remaining high with further warming. In sorghum, the L_p of intact plants and of detached roots peaked at 35°C. In both species, root temperature did not affect water potentials of the expanded leaf blade or the growing region despite marked changes in L_p. Extreme temperatures greatly decreased ion flux, particularly K⁺ and NO₃⁻, to the xylem of detached roots of both species. Removing external K⁺ did not alter short-term K⁺ flux to the xylem in sorghum but strongly inhibited flux at high temperature in barley, indicating differences in the sites of temperature effects. Leaf growth responses to root temperature, although apparently “uncoupled” from water transport properties, were correlated with ion fluxes. Studies of putative root messengers must take into account the possible role of ions.