Control of Sodium Transport in Sunflower Roots

Electrochemical potential differences (driving forces) for sodiumdistributed between the outside solution and the exuding sapof water-culture-grown sunflower plants (Helianthus annuius)have been determined. The results indicated that sodium wasmoving from the outside solution to the xylem against the electrochemicalpotential gradient at external concentrations below approximately0.30 mM Na. At higher external concentrations sodium appearedto be actively excluded from the xylem. An electrical potential difference between the exuding sap andthe external solution of approximately 30 mV was observed. Itwas unaffected by the external sodium concentration. Use ofa short-circuiting technique indicated that the trans-root potentialresides at the plasmalemma of the cortical cells. Driving forces on sodium distributed between the external solutionand the root and between the xylem sap and the root were calculated.They indicated that the root is able to accumulate sodium activelyboth from the external solution and the xylem sap. It is concludedthat sodium transport to the xylem in this species is controlledby the balance of these two opposing forces.     

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.     

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.     

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.     

Sequential depolarization of root cortical and stelar cells induced by an acute salt shock - implications for Na(+) and K(+) transport into xylem vessels

Early events in NaCl-induced root ion and water transport were investigated in maize (Zea mays L) roots using a range of microelectrode and imaging techniques. Addition of 100 mm NaCl to the bath resulted in an exponential drop in root xylem pressure, rapid depolarization of trans-root potential and a transient drop in xylem K(+) activity (A(K+) ) within ～1 min after stress onset. At this time, no detectable amounts of Na(+) were released into the xylem vessels. The observed drop in A(K+) was unexpected, given the fact that application of the physiologically relevant concentrations of Na(+) to isolated stele has caused rapid plasma membrane depolarization and a subsequent K(+) efflux from the stelar tissues. This controversy was explained by the difference in kinetics of NaCl-induced depolarization between cortical and stelar cells. As root cortical cells are first to be depolarized and lose K(+) to the environment, this is associated with some K(+) shift from the stelar symplast to the cortex, resulting in K(+) being transiently removed from the xylem. Once Na(+) is loaded into the xylem (between 1 and 5 min of root exposure to NaCl), stelar cells become more depolarized, and a gradual recovery in A(K+) occurs.     

The Effect of D-Glucose and Other Sugars on the Trans-root Potential of Zea Mays

The addition of D-glucose and certain other sugars to the bathingsolutions of young excised maize roots (Zea mays L., var. Pioneer)gives rise to rapid changes in the electrical potential differencebetween the bathing solution and the xylem fluid. The resultssuggest the presence of a fairly non-specific sugar transportsystem in the plasma membranes of the root epidermal cells inwhich the hydroxyl group in the I-carbon position of n-glucoseis cntically involved. D-Mannose, 2-deoxy-D-glucose, and 2-deoxy-D-galactose have adelayed but more profound effect on this potential, reducingit to a much smaller value. The subsequent addition of adequateamounts of D-glucose restores this potential to about its formervalue, suggesting that these three compounds interfere withthe supply of endogenous glucose or glucose-derived productswithin the root.     

On-line measurements of K+ activity in the tensile water of the xylem conduit of higher plants

In higher plants the xylem is the main pathway for anti-gravitational, long-distance transport of nutrients and water from the root through the shoot to the upper leaves. In the xylem conduit water is in a metastable state if tension larger than 0.1 MPa (i.e. negative pressure) is developed. While diurnal changes in negative pressure of individual xylem vessels can quite accurately be recorded by the minimal-invasive xylem pressure probe technique and water flow by non-invasive NMR techniques, the problem of continuous monitoring of solute flow remains a hitherto unresolved challenge. As shown here, integration of a K+ selective and a potential measuring microelectrode into the xylem pressure probe allowed on-line measurements of the K+ activity in individual xylem vessels of maize roots together with pressure and trans-root potential, the potential difference between the xylem and the external medium (i.e. the overall driving force of ions through the root tissue). When light irradiation was increased from 10 micro mol m(-2) s(-1) to 300 micro mol m(-2) s(-1) and negative pressure developed in the vessel, xylem K+ activity dropped from 3.6 +/- 2.6 mm to 0.9 +/- 0.7 mm (n = 16), whereas the trans-root potential depolarized from -2 +/- 11 mV to + 12 +/- 11 mV (n = 11), i.e. by + 14 +/- 7 mV. The effect of light on all three parameters was reversible. Exposure of the root to various K+ activities in the bath ranging from 0.1 to 43 mm revealed that the K+ activity of the xylem sap was shielded against short-term fluctuations in K+ supply to a large extent. In contrast, control experiments in which the root was cut 1 cm below the probe insertion point, allowing direct entry of external K+ into the xylem vessels, demonstrated that the xylem equilibrated rapidly with external K+. This was taken simultaneously as a proof for the correct reading of the probe.     

Membrane Potentials in the Xylem in Roots of Intact Plants

The membrane potential differences (PDs) of root cells of intact,illuminated Trifolium repens L. and Lolium perenne L. have beenmeasured. In T. repens the PDs were the same for all cell typesexcept for the xylem vessels, which were more positive, andfor some cells immediately adjacent to the xylem vessels whichwere 10 mV more negative. The mean PD for all cells was emdash164.6 ± 0.6 mV and the mean for cells adjacent to thexylem vessels with elevated PDs was 178.4 ± 2.4 mV. Whenthe electrode tip was in a xylem vessel a low but stable PD(mean = emdash 89.9 mV) was recorded. The results for L. perennewere similar except that there were no cells with elevated PDsadjacent to the xylem vessels. An inhibitor of ion transport from the root to the shoot, p-fluorophenylalanine(p-FPA), caused a depolarization of 10 mV in the cell PDs butin the xylem vessels the depolarization was 50 mV. The possibility that the elevated PDs of cells adjacent to thexylem vessels are related to the transport of ions into thevessels is discussed.     

The haustorium of the root parasite Triphysaria (Scrophulariaceae), with special reference to xylem bridge ultrastructure

Haustoria of Triphysaria pusilla and T. versicolor subsp. faucibarbata from a natural habitat were analyzed by light and electron microscopy. Secretory trichomes (root hairs) participate in securing the haustorium to the surface of the host root. The keel-shaped intrusive part of the secondary haustorium penetrates to the depth of the vascular tissue of the host. Some of the epidermal interface cells differentiate into xylem elements. A significant number of haustoria do not differentiate further, but in most haustoria one to five of the epidermal xylem elements terminate a similar number of xylem strands. The strands mostly consist of vessel members and they connect host xylem or occasionally host parenchyma to the plate xylem adjacent to the stele of the parasite root. Each strand of this xylem bridge is accompanied by highly protoplasmic parenchyma cells with supposed transfer cell function. Increased surface area of the plasmalemma occurs in these cells as it does in interface parenchyma cells. Graniferous tracheary elements are restricted to the haustorium and occur most frequently in the plate xylem. The plate xylem is also accompanied by highly protoplasmic parenchyma cells. Hyphae of mycorrhizal fungi of the host root occasionally penetrate into the distal part of the xylem bridge. We combine structural observations and physiological facts into a hypothesis for translocation of water and nutrients between host and parasite. Some evolutionary aspects related to endogeny/exogeny of haustoria are discussed, and it is argued that the Triphysaria haustorium represents a greatly advanced and/or reduced condition within Scrophulariaceae.     

Ion Channels in the Xylem Parenchyma of Barley Roots (A Procedure to Isolate Protoplasts from This Tissue and a Patch-Clamp Exploration of Salt Passageways into Xylem Vessels

To identify mechanisms for the simultaneous release of anions and cations into the xylem sap in roots, we investigated voltage-dependent ion conductances in the plasmalemma of xylem parenchyma cells. We applied the patch-clamp technique to protoplasts isolated from the xylem parenchyma by differential enzymic digestion of steles of barley roots (Hordeum vulgare L. cv Apex). In the whole-cell configuration, three types of cation-selective rectifiers could be identified: (a) one activated at membrane potentials above about -50 mV; (b) a second type of outward current appeared at membrane potentials above +20 to +40 mV; (c) below a membrane potential of approximately -110 mV, an inward rectifier could be distinguished. In addition, an anion-specific conductance manifested itself in single-channel activity in a voltage range extending from about -100 to +30 mV, with remarkably slow gating. In excised patches, K+ channels activated at hyperpolarization as well as at depolarization. We suggest that salt is released from the xylem parenchyma into the xylem apoplast by simultaneous flow of cations and anions through channels, following electrochemical gradients set up by the ion uptake processes in the cortex and, possibly, the release and reabsorption of ions on their way to the xylem.     

Transfer Cells in Roots of Phaseolus coccineus: Ultrastructure and Possible Function in Exclusion of Sodium from the Shoot

A study was made of ultrastructural aspects and ion distributionin roots of Phaseolus coccineus as affected by NaCl and Na₂SO₄salinity. In the proximal region of the root, xylem parenchymacells are differentiated as transfer cells with well developedwall protuberances adjacent to the half-bordered pits of thevessels. The cytoplasm of these transfer cells contains cisternaeof rough endoplasmic reticulum, the number of which was increasedgreatly when the plants were grown in the presence of NaCl orNa₂SO₄. The cisternae of the endoplasmic reticulum are oftenassociated closely with the plasmalemma and interconnected withit by fibrillar bridges. Wall protuberances occur also in the exodermis and epidermisof the more apical region of the root. Their function is stillunknown. P. coccineus excludes Na, but not Cl, from the leaves by retainingit particularly in the proximal region of the root. X-ray microanalysisof unfixed, frozen, hydrated specimens revealed that the transfercell-type xylem parenchyma cells in salt-treated roots accumulatedNa relative to both the adjoining xylem vessels and the corticalcells and showed very high Na/K and Na/Cl ratios. It is suggestedthat the xylem parenchyma cells can reabsorb Na from the vessels,probably in exchange for K, and that Na exclusion from the shootis at least partly mediated by this process. The implicationof this for regulation of salt transport in salt sensitive glycophytesis discussed.     

CHANGES IN THE ULTRASTRUCTURE OF XYLEM PARENCHYMA CELLS OF PEACH (PRUNUS PERSICA) AND RED OAK (QUERCUS RUBRA) IN RESPONSE TO A FREEZING STRESS

An ultrastructural investigation was conducted of xylem parenchyma cells of peach (Prunus persica [L.] Batsch.) cv. Harbrite and red oak (Quercus rubra L.) in response to a freezing stress. Freezing curves of xylem tissues, as determined by differential thermal analysis, were used to predict temperatures at which both living and dead cells would be observed. Tissues were exposed to low temperatures (-15 to -35 C) and fixed in a frozen state at -10C and at thawing. Current models of the freezing behavior of supercooled plant cells suggest that xylem parenchyma cells behave as individual water droplets. This implies that cells are unresponsive to the presence of low temperature and extracellular ice until internal nucleation triggers lethal, intracellular freezing. For these reasons, deep supercooling has been described as an avoidance mechanism. Results of this study confirmed earlier reports that xylem parenchyma cells freeze as individuals or in small groups. Individual cells, however, did not exhibit a neutral response. Instead, a range of responses was observed that included internal and external vesiculation, deep invaginations of the plasma membrane, and the formation of electron-dense deposits external to the plasmalemma. In general, our observations suggested that the cells responded to a dehydrative stress. Results are discussed in context of the biophysical data associated with deep supercooling phenomena and compared to responses of cells that exhibit extracellular freezing.     

The Action of Divalent Zinc, Cadmium, Mercury, Copper and Lead on the Trans-Root Potential and H+, Efflux of Excised Roots

Kennedy, C. D. and Gonsalves, F. A. N. 1987. The action of divalentzinc, cadmium, mercury, copper and lead on the trans-root potentialand H⁺ efflux of excised roots.—J. exp. Bot. 38: 800–817. The action of Zn²⁺, Cd²⁺, Hg²⁺, Cu²⁺ and Pb²⁺ ions on the trans-rootpotential and H⁺ efflux of young excised maize roots has beenstudied. Micro-electrode implantations into root epidermal cellsconfirmed the root outer membranes as the major contributorin the trans-root potential changes. The effects of these ionson H⁺ efflux were studied over a period of time in a continuousflow cell apparatus, adequate controls allowing for transientinterference due to divalent cations at the pH probe. The addition of Zn²⁺, 5 to 100 µmol dm^–3, to thesolution bathing the roots reduces H⁺ efflux and depolarizesthe trans-root potential. However, in the presence of Mg²⁺,0?1 or 1?0 mmol dm^–3, not only is this depolarizationinhibited, but hyperpolarization is observed instead. Cd²⁺ affectstrans-root potential and H⁺ efflux at a much slower rate thanZn²⁺, suggesting a lower membrane permeability. Without Mg²⁺,Cd²⁺ hyperpolarizes the trans-root potential, but this is bettersustained in its presence. Hyperpolarization did not occur withHg²⁺, Cu²⁺ or Pb²⁺ whether or not Mg²⁺ was present Hg²⁺ andto a lesser extent Cu²⁺ are potent depolarizers of the trans-rootpotential and strongly inhibit H⁺ efflux. The maximum rates of depolarization observed in the absenceof Mg²⁺ increase in the order Cd PCMBS <<.lt; Zn Cu< Hg. This is similar to the relative maximum rates of H⁺inhibition, Pb Cd <<.lt; Zn < Cu < Hg, suggestingconsiderable differences in mode of action and/or membrane permeability.The lower membrane permeability of the sulphydryl reagent PCMBSapparently prevents ready access to the site(s) of action availableto Hg²⁺. The reductions in trans-root potential and H⁺ gradients inducedby this range of cations would be detrimental to the acquisitionof nutrients using these gradients as an energy source. In contrast,Zn^2+, , in the presence of adequate Mg²⁺, could be beneficialto nutrient uptake by maintaining a higher membrane potentialthan would occur in its absence. Possible modes of action for the observed effects are discussed. Key words: Trans-root potentials, H⁺ efflux, heavy metal ions     

Rapid alterations in growth rate and electrical potentials upon stem excision in pea seedlings

Excision of the epicotyl base of pea (Pisum sativum L.) seedlings in air results in a fast drop in the growth rate and rapid transient membrane depolarization of the surface cells near the cut. Subsequent immersion of the cut end into solution leads to a rapid, transient rise in the epicotyl growth rate and an acropetally propagating depolarization with an amplitude of about 35 mV and a speed of approx. 1 mm · s^–1. The same result can be achieved directly by excision of the pea epicotyl under water. Shape, amplitude and velocity of the depolarization characterize it as a slow-wave potential. These results indicate that the propagating depolarization is caused by a surge in water uptake. Neither a second surge in water uptake (measured as a rapid increase in growth rate when the cut end was placed in air and then back into solution) nor another cut can produce the depolarization a second time. Cyanide suppresses the electrical signal at the treated position without inhibiting its transmission through this area and its development in untreated parts of the epicotyl. The large depolarization and repolarization which occur in the epidermal and subepidermal cells are not associated with changes in cell input resistance. Both results indicate that it is a transient shut-down of the plasma-membrane proton pump rather than large ion fluxes which is causing the depolarization. We conclude that the slow wave potential is spread in the stem via a hydraulic surge occurring upon relief of the negative xylem pressure after the hydraulic resistance of the root has been removed by excision.Abbreviations and Symbols GR growth rate - P_x xylem pressure - R_in cell input resistance - SWP slow wave potential - V_m membrane potential - V_s surface potential This work was supported by grants to D.J.C. from the National Science Foundation and the U.S. Department of Energy.     

Membrane Depolarization by Hexacyanoferrate (III), Hexabromoiridate (IV) and Hexachloroiridate (IV)

Three artificial electron acceptors of different E_o and charge,hexacyanoferrate (III) (K₃Fe(CN)₆), hexachloroiridate (IV) (K₂IrCl₆),and hexabromoiridate (IV) (K₂IrBr₆), were compared with respectto their rate of reduction by roots of Zea mays L., the concomitantproton secretion, and to the effect on plasmalemma depolarization. It has been shown that these plasma membrane impermeable electronacceptors were reduced by a plasmalemma reductase activity.At low concentrations proton secretion was slightly inhibited,at higher concentrations, however, the rate of proton secretionwas stimulated. The root cell plasmalemma showed a transientdepolarization after addition of all three electron acceptors.The depolarization was concentration-dependent for the iridatecomplexes but not for hexacyanoferrate (III). For both iridatecomplexes maximum depolarization was reached at 50 µmoldm^–3. A hypothetical model as an explanation of the redox dependentproton secretion will be given. Key words: Hexachloroiridate (IV), hexabromoiridate (IV), hexacyanoferrate (III), plasmalemma redox, membrane potential, Zea mays     

A possible stress physiological role of abscisic acid conjugates in root-to-shoot signalling

Abscisic acid (ABA) conjugates, predominantly their glucose esters, have recently been shown to occur in the xylem sap of different plants. Under stress conditions, their concentration can rise substantially to levels that are higher than the concentration of free ABA. External ABA conjugates cannot penetrate apoplastic barriers in the root. They have to be hydrolysed by apoplastic enzymes in the root cortex. Liberated free ABA can then be redistributed to the root symplast and dragged directly across the endodermis to the stele. Endogenous ABA conjugates are formed in the cytosol of root cells, transported symplastically to the xylem parenchyma cells and released to the xylem vessels. The mechanism of release is unknown; it may include the action of ABC-transporters. Because of its extremely hydrophilic properties, ABA-GE is translocated in the xylem of the stem without any loss to the surrounding parenchyma. After arrival in the leaf apoplast, transporters for ABA-GE in the plasmalemma have to be postulated to redistribute the conjugates to the mesophyll cells. Additionally, apoplastic esterases can cleave the conjugate and release free ABA to the target cells and tissues. The activity of these esterases is increased when barley plants are subjected to salt stress.     

Plasma membrane H+‐ATPase in the root apex: Evidence for strong expression in xylem parenchyma and asymmetric localization within cortical and epidermal cells

The cell and subcellular localization of plasma membrane P‐type H⁺‐ATPase in root apices from Zea mays L. (maize) seedlings was investigated by immunofluorescence microscopy. H⁺‐ATPase was highly abundant in cells of epidermal and endodermal tissues as well as in phloem companion cells. Strong immunodecoration was also observed in a subset of xylem parenchyma cells forming a connection between the endodermis and metaxylem. Evidence that these cells are equipped for active membrane transport raises the potential that they play a special role in xylem loading. Significant amounts of H⁺‐ATPase were also observed in outer cortical cells. Progressively less H⁺‐ATPase was seen in cortical cells further away from the root‐soil interface. The H⁺‐ATPase was asymmetrically localized within both epidermal and outer cortical cells, with higher levels detected on cell surfaces closest to the root‐soil interface. This asymmetric localization of H⁺‐ATPase is consistent with the hypothesis that transport systems for uptake of nutrients from the soil are selectively targeted to cell surfaces most exposed to nutrients.     

Water relations, xylem embolism and histological features of Pinus thunbergii inoculated with virulent or avirulent pine wood nematode, Bursaphelenchus xylophilus

The development process of pine wilt disease caused by Bursaphelenchusxylophilus (Steiner and Buhrer) Nickle, pine wood nematode,was studied ecophysiologically and histologically in relationto pathogenicity of B. xylophilus. Judging from the predawnxylem pressure potential of needles, the heat pulse velocity,and the soil water potential, the control Pinus thunbergii Parl.,used for the study, was not water-stressed. Virulent B. xylophilusisolate can kill non-waterstressed pines. In virulent B. xylophilusisolateinoculated pines, the predawn xylem pressure potentialof needles abruptly decreased when the colour of 1-year-oldneedles changed to brown and then the water conducting functionof the xylem was lost completely. Avirulent B. xylophilus isolatedoes not affect the needle colour and the xylem pressure potentialof pines. Avirulent B. xylophilus isolate-inoculated pines,however, responded to nematode invasion by decreasing hydraulicconductance of stem and root xylems. In addition, oleoresinexudation slightly decreased. The decreased hydraulic conductanceresults from embolism of tracheids caused by cavitation in thecentral part of the xylem. From histological observation, allof the parenchyma cells in virulent B. xylophilus isolateinoculatedpines died. In contrast, the parenchyma cells, degenerated inavirulent B. xylophilus isolateinoculated pines, were limitedin the embolized region of the xylem. The difference betweenthe response of pine to the virulent B. xylophilus isolate invasionand that to avirulent B. xylophilus isolate invasion indicatesthat nematode-induced death of pine relates to the death ofparenchyma cells, as well as the decrease in xylem hydraulicconductance. Key words: Embolism, hydraulic conductance, parenchyma cells, pathogenicity of pine wood nematode, pine wilt disease.     

Effects of External KCl on Electrical Properties and K$(86Rb) Transport in the Elongating Region of Intact Phaseolus mungo Roots

Two simultaneous measurements, extracellular potential V andK^$(⁸⁶Rb) transport, and the intracellular potential of corticalcell E and potential V, were used to study the effects of externalKCl on two-day-old bean roots. High, external KCl concentrations(>10 mM) markedly enhanced K^$ loss from tissues in the elongatingregion to the external solution and induced depolarization ofthe membrane potential difference (PD=V–E). When Phaseolus roots were returned to a solution with a lowerconcentration of K^$, the K^$ loss and the potential difference,PD, were restored to their previous values. K^$ transport fromother parts of the root to the elongating region, however, didnot recover, and the potential, E, increased. These resultsclearly demonstrate that treatment of Phaseolus roots with ahigh external K^$ concentration inhibits K^$ translocation throughthe stele to the elongating cortical cells and is dependenton depolarization of the intracellular potential. (Received October 14, 1983; Accepted January 20, 1984)     

The Effect of Water and Salt Fluxes on the Trans-root Potential in Helianthus annuus

The effect of an artificially imposed water flux on the trans-rootelectrical potential difference has been studied in excisedsunflower roots. It was found that the potential of the xylemsap became more negative with respect to the external mediumas the rate of water flow was increased. This change appearedto be related to an accompanying increase in the flux of ions. The effect of increasing the water flux on the vacuolar potentialdifference of the epidermal cells was also investigated. Italso became more negative with increasing water flux. Both potentialswere measured simultaneously in the same root. On increasingthe water flux it was found that the trans-root potential beganto rise immediately but there was a time lag of approximately2 min before the vacuolar potential began to change. The relevance of these potential changes to the mechanism andpathway of ion transport across the root is discussed.