Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis II. Axial potential difference

Vital staining with pH indicator dyes made it possible to identifythe xylem with the specific channel A, reported previously,through which hydrogen ions flow generating a resting potentialdifference along the germ axis as their diffusion potential.The distribution of K+ concentration within this channel showedno similarity to electric potential distribution, in contrastto the distribution of H+. The axial P.D. between both ends of a segment cut from a hypocotylresponded reversibly to the change in O₂ tension of the surroundinggas phase. After air had been quickly replaced by N₂, a lagperiod appeared before the sudden potential drop took place.The lag period () was largely dependent on temperature. Apparentactivation energy of the process characterized by 1/ was 18Kcal/mole between 14–30?C, approximately equal to thatof the O₂-uptake within the same temperature range. The relationbetween O₂ concentration and the maximum rate of recovery ofP.D. from anoxia was of the Michaelis-Menten type; the apparentKm was calculated as 2.1 ? 10^–5M O₂ being of the sameorder as that of cytochrome oxidase in higher plants. The O₂-uptakerate "per unit of hypocotyl length" showed a distinct maximumin the elongating region where the axial distribution of bothelectric potential and pH within channel A had their minimums. (Received July 21, 1972; )     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis I. Distribution of overall ion concentration and the role of hydrogen ion in generation of potential difference

Overall concentration of free inorganic ions distributes inthe hypocotyl of a bean seedling {Vigna sesquipedalis) at aconstant level (H⁺) or decreases monotonously from the cotyledonarynode towards the base (K⁺, Na⁺, Ca⁺⁺ and Mg⁺⁺, phosphate, NO₃^–).According to our theory, this is inconsistent with the distributionof electric potential having a definite minimum in the elongatingregion. The discrepancy can not be explained by regional variancein radial potential difference or histological differentiationin passive ionic permeability of the cell membrane. Short circuitcurrent observed through a hypocotyl segment corresponded toa net flux in ions of 10–24 pEq/cm².sec. It is questionable,however, whether this is due to active ion transport, whichcan be the source of electric potential difference, or is apassive flow due to histological heterogeneity in ion concentration. In order to investigate the latter possibility, pH of sap exudingfrom stumps made at various intervals along the hypocotyl axiswas measured, since H⁺ is the ion electro-osmotically most effective.pH Values of acropetal exudates distributed along the axis closelycorresponding to the distribution of electric potential. Thissuggests that potential distribution is determined by a passiveflow of H⁺ through a specific channel in the vascular system.The fact that H⁺ production and the uptake of ions and waterare most active at the elongating zone of hypocotyl is discussedfrom a physiological point of view. (Received December 3, 1969; )     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis V. Electrogenic activity of the parenchyma cells in hypocotyl segments

Intracellular potential of parenchyma cells (V_ps) in the hypocotylsegment of Vigna sesquipedalis was initially low after excision,then gradually increased to a more negative level. ThereafterV_ps could be remarkably reduced under anoxia then recoveredwith reaeration, accompanied by several cycles of damped oscillations.Both the rapidity of the decrease of V_ps caused by anoxia andits temperature dependency suggest an electrogenic mechanism.No marked spatial differences along the germ axis and the radiusof hypocotyl were observed in the V_ps level and its electrogeniccomponent when the reference electrode was placed on the surfaceof the hypocotyl segment. V_ps also decreased rapidly in an atmospherecontaining 80% CO+20% O₂ in the dark, then was recovered immediatelyin the light or spontaneously but very slowly in the dark. Theextent of the decrease of V_ps caused by CO depended on the growthactivity of the cell and the time lapse after excision. Theseresults suggest the possible conversion or replacement of theterminal oxidase. Sometimes phenomena resembling those of actionpotentials occurred spontaneously or during the reduction ofV_ps due to inhibition of the energy metabolism. V_ps in the elongatingregion varied transiently with the change in illumination. Interrelationshipbetween V_ps and the surface resting potential is discussed. (Received July 20, 1977; )     

The Movement of Ions to the Xylem Exudate of Maize Roots: III. THE LOCATION OF THE ELECTRICAL AND ELECTROCHEMICAL POTENTIAL DIFFERENCES BETWEEN THE EXUDATE AND THE MEDIUM

Trans-root and membrane potentials have been measured simultaneouslyin the same maize root by using microelctrodes inserted in theexuding sap, external bathing solution, and a vacuole of anepidermal cell. On rapidly increasing the KCl concentrationof the external solution, the membrane and trans-root potantialsfell simultaneously. This initial rapid phase of depolarizationwas complete within 20s of changing the external solution whenthe membrane potential had reached a new stable value. However,the trans-root potential continued to fall slowly and this phaseof depolarization lasted for about 25 min. Then followed a riseto a stable value at 1.5–2.0 h. This secondary rise wasrelatively small compared with the initial fall. The major part(approx. 80 per cent) of the depolarization of the trans-rootpotential occured during the initial rapid phase. These results indicate that the major component of the trans-rootpotential resides at the plasmalemma of the epidermal cellswith a smaller contribution from the cells underlying the epidermis.The rise in the trans-root potential after 25 min suggestedthat this back potential was associated with the plasmalemmaof the xylem parenchyma. From knowledge of the elelctrical propertiesof these cells this back potential could be calculated and trans-rootpotentials accurately predicted from values of the membranepotentials of root cells. It is concluded that in maize roots, ion movement to the xylemvessels is mainly symplasmic, that the outer boundery of thesymplasm is the plasmalemma of the epidermal cells and thatthe inner boundary is the plasmalemma of the xylem parenchyma.This hypothesis has enabled trans-root electrochemical potentialdifferences to be predicted accurately from vacuolar values.     

Histochemical Localization of a Chimeric Gene (rolC-GUS) Expression in Zygotic Embryos of Transgenic Tobacco Plants

Histochemical localization of the expression pattern of a chimericgene (rolC-GUS) in zygotic embryo development in tobacco plantswas analysed. The results indicate that strong expression waslocalized mainly in the vascular cylinders of the cotyledonsand central axis of the hypocotyl. Quantitative analysis indicatedan increase of gene expression in embryos up to 20 d after pollination(DAP), but decreased at 30 DAP. Continuous increase of GUS activitywas recorded up to 12 d after imbibition (DAI) in germinatingseeds. The xylem cells were visualized following phloem differentiationin the cotyledons at 3 DAI.Copyright 1994, 1999 Academic Press Tobacco (Nicotiana tabacum cv. Samsun), transgenic plants, rolC promoter-GUS chimeric gene, germinating seeds, transition region, zygotic embryos     

Observations on the Germ Aleurone of Barley. Phenol Oxidase and Peroxidase Activity

Strips of tissue containing the germ aleurone layer were removedfrom dry, harvest-ripe grains of barley (Hordeum vulgare L.)and incubated in buffered solutions of phenolic compounds, withand without the addition of hydrogen peroxide. Peroxidase ando-diphenol oxidase activity were found in the material releasedinto the incubation medium, and in the cytoplasm of the germaleurone cells. Peroxidase activity was located in the cellwalls and appeared to be high in the region where the germ aleuronecovering the embryonic axis merges into that which adheres tothe scutellum i.e. the region in which a row of germ aleuronecells becomes lignified following germination. Monophenol oxidaseactivity was not detected in the released enzymes or in theintact tissue. Although hydroquinone was oxidized in the cytoplasmof the germ aleurone tissue, unequivocal evidence of the presenceof laccase was not obtained. The oxidation of endogenous phenolicsubstances by phenol oxidases and peroxidases is discussed inrelation to anti-microbial defence mechanisms which appear tooperate in the germ aleurone during germination.Copyright 1994,1999 Academic Press Barley, Hordeum vulgare L., germ aleurone, catechol oxidase, laccase, peroxidase, defence mechanisms, germination     

Simultaneous recording of xylem pressure and trans-root potential in roots of intact glycophytes using a novel xylem pressure probe technique

The radial electrical potential difference between the root xylem and the bathing solution, i.e. the so-called trans-root potential, was measured in intact maize and wheat plants using a xylem pressure probe into which an Ag/AgCl electrode was incorporated. Besides other advantages (e.g. detection and removal of tip clogging; determination of the radial root resistance), the novel probe allowed placement of the electrode precisely in a single xylem vessel as indicated by the reading of sub-atmospheric or negative pressure values upon penetration. The trans-root potentials were of the order of 0 to – 70 mV and + 40 to – 20 mV for 2- to 3-week-old maize and wheat plants, respectively. Osmotic experiments performed on maize demonstrated that addition of 100 mM mannitol to the solution resulted in a decrease of xylem pressure associated with a slow, but continuous depolarization. The depolarization was reversible upon removal of the mannitol. For wheat plants it could be shown that the oscillations of the xylem pressure described recently by Schneider et al. (1997, Plant, Cell and Environment 20, 221–229) were accompanied by (rectangular, saw-tooth and/or U-shaped) oscillations in the trans-root potential (but not by corresponding changes of the membrane potential of the cortical cells measured simultaneously with conventional microelectrodes). Increase of the light intensity (up to 550 μmol m^–2 s^–1) resulted in a drop of the xylem pressure in wheat, whereas the trans-root potential showed a biphasic response: first hyperpolarization (by about 10 mV) was observed, followed by depolarization (by up to about + 40 mV). Similar light-induced biphasic (but often less pronounced) changes in the trans-root potential were also recorded for maize plants. Most interestingly, the response of the trans-root potential was always faster (by about 1–3 min) than the response of the xylem pressure upon illumination, suggesting that changes in the transpiration rate are reflected very quickly in the electrical properties of the root tissue. The impact of this and other findings on long-distance transport of solutes and water as well as on long-distance signalling is discussed.     

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     

Concentrations of abscisic acid and other solutes in xylem sap from root systems of tomato and castor-oil plants are distorted by wounding and variable sap flow rates

We investigated if concentrations of abscisic acid (ABA) andother solutes measured in the first few droplets of xylem sapfrom detopped root systems, are good estimates of those in thetranspiration stream as it enters the shoot-base of whole plants.Xylem sap from root systems of pot-grown tomato plants (Lycopersiconesculentum Mill., cv. Ailsa Craig), at the seven-leaf stage,was obtained by placing root systems in chambers pressurizedto 0.3 MPa with air. The first sample was taken from the cut-surfaceof the hypo-cotyl stump within 30 s of removing the shoot. ABA,sucrose and other osmolytes were more concentrated in the initial100–200 mm³ of xylem sap than in subsequent samples. Thissuggested the sap was contaminated and not unchanged transpirationfluid. The effect was reproduced on the same plant, severaltimes, by recutting the hypocotyl prior to reassembling thesap collecting set-up and repressurizing. Similar results werefound with castor-oil plants (Ricinus communis L., cv. Gibsonii).However, neither release of ABA from the cut surface of thehypocotyl stump, nor the effects of pressure to the roots causedthe contamination. Instead, small radial pressures exerted bya rubber sleeve attached to the hypocotyl stump, for collectingthe sap, were responsible. The effect was reproduced by lightlysqueezing the hypocotyl by hand. The possibility was examined that reliable estimates of ABAconcentrations in transpiration stream fluid may be obtainedfrom sap samples taken immediately after rejecting the initial,contaminated 200 mm³. However, ABA concentrations in these latersamples were also unsatisfactory since they changed with rateof sap flow. The problem may be overcome by analysing sap inducedto flow through detached root systems at rates close to thoseof whole-plant transpiration. Key words: Tomato, Lycopersicon esculentum Mill., Castor-oil plant, Ricinus communis L., roots, root to shoot communication, xylem sap, abscisic acid, sucrose, transpiration stream     

Structural Changes in the Embryonic Axis of Theobroma cacao L., during Germination and Early Seedling Establishment

The changes occurring in the axis of the developing seedlingof Theobroma cacao L. were observed over a 96-h period. It wasfound that during the first 24-h period, greatest change hadoccurred in the hypocotyl; the epicotyl and the root meristemhad apparently remained dormant. In the hypocotyl, vascularizationwhich had begun in the mature embryo continued: starch grainshad disappeared from the cells of the cortex and pith, the latterof which were seen to be binucleate. During the second 24-hperiod, greatest change was seen to have taken place in theroot meristem, resulting in the production of a root which consistedof a central pith, a poly-arch stele, and cortical tissue. Betweenthe root and the hypocotyl an apparently undifferentiated regionwas observed. This region was seen to have differentiated duringthe third 24-h period, leading to the production of a completering of xylem, external to which were groups of phloem, theformer being separated from the latter by parenchymatous tissue.This region is interpreted as being transitional between rootand shoot. During the fourth 24-h period, the adventitious andlateral root primordia initiated earlier, were seen to havedifferentiated to the extent that the disposition of their tissueswas evident.     

Glucose Induced H+ Influx and Transient Currents in Excised Roots: particularly those of Zea mays

The application of D-glucose to solutions bathing excised maize,wheat, pea and bean roots causes a rapid depolarization of theelectrical potentials between the cut tops of the roots andthe bathing solutions. Similar effects are observed for theplasma membrane potentials of maize lateral roots. A flow cell apparatus was used to demonstrate qualitative andquantitative relations between glucose induced H⁺ influx andthe transient decrease in current through the root. The currentchanges appear to be due entirely to H⁺ fluxes. Current andH⁺ fluxes are strongly influenced by external pH, the optimumpH for glucose induced current change being about 4.0. A similarpH optimum was found for 3-O-methyl-D-glucopyranoside but 1-O-methyl--D-glucopyranosidedid not significantly affect the trans-root potential at anypH, suggesting a significant role for the anomeric hydroxylgroup of glucose. Compounds which depolarize the trans-root potential also inhibitthe glucose induced depolarization. Surface -SH groups are probablynot involved in the glucose/H⁺ cotransport. Eadie-Hofstee plots relating the depolarization of trans-rootpotential to the concentrations of D-glucose or 3-O-methyl-D-glucopyranosidehave shown that K_m values increase with increasing monosaccharideconcentration and are very similar to reported values of 3-O-methyl-D-glucopyranosideuptake in maize root segments. K_m values for a similar rangeof D-glucose concentrations do not vary significantly with pHor with membrane depolarization due to a 10-fold increase ofKCl concentration. However, V_max is lowered by an increase inexternal pH or a decrease in trans-root potential. It appearsthat both proton and electrical gradients can affect glucoseinduced H⁺ influx. The auxin herbicide, 2, 4-dichlorophenoxyethanoic acid (0.01mM) stimulates the glucose induced depolarizations in a mannerconsistent with an increase in cytoplasmic pH. This is discussedin relation to the reported action of indole-3-acetic acid andfusicoccin on maize root tissue.     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis IV. The effects of carbon monoxide and cyanide on the axial potential difference of hypocotyl segments

The effects of carbon monoxide and cyanide on the electric potentialdifference between both cut ends of a hypocotyl segment excisedfrom a bean embryo were investigated. Carbon monoxide definitely diminished the PD. This inhibitionwas photoreversible. The inhibition ratio was quantitativelyrelated to the [CO]/[O₂] ratio of the gas mixture. Prolongedexposure to a CO gas mixture induced spontaneous recovery ofthe PD, and this CO-resistant electrogenic activity was sensitiveto CN and also disappeared under anoxia. Cyanide also diminished the PD and often induced oscillationof the potential. The type of oscillation and the inhibitionratio varied with the concentration of CN. Sometimes CN pretreatmentcaused changes in the anoxia response of the potential. The time course of the spontaneous change in PD after cuttingof the hypocotylsegment also was investigated. (Received August 8, 1977; )     

Exposure to alcohol vapours reduces chilling-induced injury of excised cucumber cotyledons, but not of seedlings or excised hypocotyl segments

Cucumber (Cucumis sativus L. cv. Poinsett 76) seedlings withfully expanded cotyledons, and excised cotyledons, first trueleaves, hypocotyl segments and fruit mesocarp discs were exposedto vapours from a series of aqueous alcohol solutions of 0 to320 mM methanol, ethanol, n-propanol, n-butanol, and n-pentanolduring chilling at 2.5C for 5 d. Certain concentrations ofeach alcohol reduced subsequent chilling-induced ion leakagefrom the cotyledons and leaves. Exposure of cotyledons to certainmethanol or ethanol solutions also reduced chilling-inducedethylene production, but not carbon dioxide production. In contrast,exposing cucumber seedlings with fully expanded cotyledons tothe same series of alcohol concentrations that resulted in reducedchilling-induced ion leakage and ethylene production of excisedcotyledons actually increased chilling injury of the seedlings.The hypocotyl region directly below the cotyledons was the siteof chilling-induced injury and contained the most chilling-sensitivehypocotyl tissue. Exposing hypocotyl segments excised from thissensitive region to alcohol solutions did not significantlyreduce chilling-inducedion leakage. Exposing excised cucumbercotyledons or hypocotyl segments to equivalent osmotic nonvolatilesolutions of mannitol and glycerol at 2.5C or to alcohol solutionsat 12.5C had no significant effect on the rate of ion leakage.For the series of alcohols used, the relationship between thelog of the alcohol concentration that minimized chilling-inducedion leakage from cucumber cotyledon discs held at 2.5C for5 d and the log of the partition coefficient of the alcoholinto olive oil or the log of the molecular weight of the alcoholswas highly significant. The same concentrations of alcoholsthat reduced chilling-inducedion leakage also reduced stomatalaperture as measured as decreased porosity of excised cotyledons.The correlation between reduced chilling injury and stomatalconductance of cotyledons exposed to a series of ethanol solutionswas highly significant. It appears that alcohols may reducechilling injury of cucumber cotyledons by inducing stomata closure.Sufficient endogenously synthesized ethanol accumulated in discsheld in N2 at 10C for 1 d to confer tolerance to chilling at2.5C for 5 d. Key words: Anaerobic, Cucumis sativus, ethanol, ion leakage, stomatal conductance     

Seedling Growth in Phaseolus vulgaris L. The Early Growth of Cultivars Selected for Seedling Cold Tolerance

Silbernagel has described a test for selecting cultivars ofPhaseolus vulgaris L. which exhibit rapid seedling emergenceat low temperatures, and using this test has identified threecold-tolerant cultivars. We have compared the growth of thesecultivars with that of three control cultivars. Both at 25/20°C and at 20/15 °C day/night temperatures, Silbernagel'scultivars emerged more rapidly than the controls. This was dueto more rapid hypocotyl elongation, not to earlier germination.Rapid hypocotyl elongation was associated with high relativegrowth rates of the seedling axis and rapid loss in weight ofthe cotyledons. After emergence, cotyledons of all cultivarscontinued to lose weight at a constant exponential rate. Relativegrowth rates of the axes were not constant but declined withtime. There was no evidence that genotypic differences in growthrates before emergence were reflected in growth rates afteremergence. A simple quantitative analysis suggested that thedecline in axis relative growth rate after emergence was dueto a declining contribution from cotyledonary reserves. There were significant differences between cultivars in theinitial weight of the seedling axis. Axis weight (A) was notlinearly proportional to seed weight (S), but the curvilinearallometric relationship A = 0.0773 S^0.697 satisfactorily accountedfor most of the variation in initial axis weight between cultivars.In all cultivars, axis weights at emergence were smaller inthe cool regime than in the warm, because low temperatures depressedaxis relative growth rates relative to the rate of emergence.The biggest difference between the Q₁₀ of relative growth rate,and of emergence rate, and hence the biggest effect of low temperatureon axis weight at emergence, occurred in the cold-susceptiblecultivar Seafarer. However, genotypic cold tolerance duringthe period when growth is dependent on reserves did not appearto guarantee cold tolerance during the main period of growth. Phaseolus vulgaris, L., bean, seedling growth, temperature, cold tolerance     

Distribution of electric potential and ion transport in the hypocotyl of Vigna sesquipedalis VI. The dual structure of radial electrogenic activity

The electrophysiological structure in bean hypocotyl was investigatedby the intracellular electrode method in combination with surfaceelectric potential (V_s) measurement and respiratory inhibitionby anoxia, with special reference to the membrane transportof ions and the formation of an absorption centre in the elongating(E) zone. The radial potential difference (V_sx: electric potentialdifference between the organ surface and a xylem vessel), onwhich axial distribution of V_a was dependent, comprised twocomponents; V_ax=V_px–V_ps. |V_px| (the potential differencebetween the inside of a parenchyma symplast and a xylem vessel)was at a maximum in the E-zone, while |V_ps| (the intracellularelectric potential with respect to the organ surface) was largestin the G-zone (mature zone), resulting in the characteristicdistribution pattern of V_s with a minimum in the E-zone. Therewere two independent electromotive forces which were both partiallydependent on respiration; one corresponding to V_ps located atthe surface of the parenchyma symplast (P) and the other toV_px located between P and the xylem (X). The electrogenic componentof Vpx was relatively small both in the hook (H) zone and theG-zone, but maximal in the E-zone of the hypocotyl. This resultwas consistent with the emergence of a maximum pH differencebetween P and X in the E-zone, where accumulation of K⁺ andwater were at a maximum, suggesting maximum activity of an H⁺-pumpextruding protons from P into X in exchange for K⁺. (Received July 17, 1978; )     

Trans-root potential, xylem pressure, and root cortical membrane potential of 'low-salt' maize plants as influenced by nitrate and ammonium

Upon addition of nitrate and ammonium, respectively, to the bath of intact ‘low salt’ maize plants, the cortical membrane potential and the trans-root potential changed in a similar and synchronous way as revealed by applying conventional microelectrode techniques and the xylem pressure-potential probe ( Wegner & Zimmermann 1998). Upon addition of nitrate, a hyperpolarization response was observed which was frequently preceded by a short depolarization phase. In contrast, addition of ammonium resulted in an overall depolarization response both of the cortical membrane potential and the trans-root potential. The nitrate-induced hyperpolarization response and the depolarization following the addition of ammonium were concentration-dependent. The data suggest that a tight electrical coupling exists between the cellular and tissue level in the root of the intact plant and that the resistance of the cellular (symplastic) space is much less than the resistance of the apoplast.     

Homeostatic Regulation of Membrane Potential by an Electrogenic Ion Pump against Change in the K+ Concentration of the Extra- and Intra-Organ Perfusion Solutions

The dependence of membrane potentials on changes in the extra-cellularK⁺ concentration [K⁺]_e was investigated in potato tuber sliceswith dripping perfusion, and in growing Vigna hypocotyl segmentswith pressurized intra-organ perfusion methods. Only under anoxiawere the membrane potential of potato tuber slices and the electricpotential difference between the parenchyma symplast and xylem(V_px) of Vigna hypocotyl segments depolarized markedly (46 mVand 42 mV/log[K⁺]_e unit, respectively) with increasing [K⁺]_eabove the critical values. The electric potential differencebetween the parenchyma symplast and organ surface (V_ps of thehypocotyl segments remained nearly unchanged up to 30 mEq [K⁺]_e.Under highly aerobic conditions the membrane potentials wererelatively independent of [K⁺]_e except at very high K⁺ concentrations.V_ps showed even hyperpolarization with the increasing KCl concentrationin the perfusion solution that is not in direct contact withthe surface membrane of the parenchyma symplast. The respiration-dependentelectrogenic components of the membrane potentials regularlyincreased with the increasing [K⁺]_e. A voltage-dependent homeostaticcontrol of membrane potential is discussed. (Received August 13, 1984; Accepted December 21, 1984)     

Free radical involvement in the generation of trans‐root potential

The identity of the naturally occurring compounds that accept electrons from plasma membrane-bound redox systems in vivo is obscure. We analysed the effect of ascorbate, oxygen, iron, as well as their free radical forms, and also the free radical-generating and -quenching systems on the trans-root electrical potential, which had previously been shown to be coupled to plasma membrane-bound redox systems. The material was the primary root of 8-day-old maize (Zea mays L.) seedlings. Trans-root electrical potential difference was measured across excised roots. Different ascorbate (ascorbate, dehydroascorbate and ascorbate free radical) and oxygen redox forms (superoxide and hydroxide radicals and hydrogen peroxide), as well as scavenging agents of oxygen species (superoxide dismutase, catalase, mannitol), and ferric and ferrous ions were added to the solution flowing around the root. Ascorbate free radical induced the greatest depolarization of the trans-root potential when compared to other ascorbate redox forms, which is consistent with its suggested role as a natural electron acceptor. Addition of xanthine oxidase, with or without xanthine, also produced depolarizing effects. The presence of SOD magnified this effect both with ascorbate free radical and xanthine oxidase. When ferric or ferrous chloride and ferric EDTA were applied to the bathing medium, only free ferric ion produced a very pronounced depolarization. The magnitude and kinetics of trans-root potential depolarization, induced by the ascorbate redox forms and systems for the generation and scavenging of oxygen species, argue in favour of the mutually competing electron transfer role of ascorbate free radicals and superoxide radicals in the extracellular space of the root. These results provide evidence that at least a part of the electrical potential difference occurring across plant roots arises from current flow from the symplast, via the plasma membrane-bound redox systems, to naturally occurring compounds in the apoplast, and that this transfer is achieved through the mediation of their free radical forms.     

Longitudinal Gradients of Indole-3-Acetic Acid and Abscisic Acid in the Hypocotyl of Etiolated Bean Seedlings

The longitudinal distribution of abscisic acid (ABA) and indole-3-aceticacid (IAA) along the hypocotyl of 5-d-old etiolated Phaseolusvulgaris L. cv. Limburg seedlings was measured. IAA was analysedby the L-methyl-indole--pryone assay (2-MIP) and ABA by electroncapture gas chromatography (ECD-GC). Length and width of theinner parenchyma cells, growth rate and protein content werealso measured. Cell expansion occurred predominantly in a region20 mm below the centre of the hook where elongation rate wasmaximal and where protein concentration decreased rapidly withdistance from the hook. The ratio between ABA and IAA was constant along the lengthof the hypocotyl. On a fresh weight basis the concentrationof both growth substances was maximal in the upper (youngest)part, decreased in slightly older sections where cell expansionwas proceeding and was smallest in the basal regions where cellexpansion was complete. However, when expressed on a proteinbasis the concentration gradient of the hormones was the reverseof that described on a fresh weight basis. Key words: IAA, ABA, hypocotyl, etiolated, bean     

Longitudinal component of trans-root electrical potential difference: evidence from application of metabolic inhibitors to the cut end of excised maize roots

Changes in trans-root electrical potential induced by application of metabolic inhibitors (carbonyl cyanide m-chlorophenylhydrazone, vanadate, diethylstilbestrol, N-ethyl maleimide, p-chloromercuribenzene sulfonic acid, KCN, salicylhydroxamic acid) and electron acceptors (hexachloroiridate IV and hexacyanoferrate III) to the cut end of excised roots of maize demonstrated existence of a longitudinal component of trans-root electrical potential. It was probably associated with redox plasma membrane bound system(s) and coupled to the cyanide sensitive and alternative respiration pathways. This revised version was published online in July 2006 with corrections to the Cover Date.