The Effect of Hydrosatic Pressure Gradients on the Movement of Sodium and Calcium across the Root Cortex

The hydrostatic pressure gradient across the root cortex inducedby transpiration was simulated by raising the pressure on themedium surrounding the roots of detopped tomato (Lycopersicumesculentum) and castoroil plants (Ricinus communis).Applyinga pressure of 2 atm. resulted in a doubling of the sodium fluxfrom medium to xylern (table I). This flux depended on a maintainedsuply of Na in the medium (fig.1) indicating that there wasa small but readily available storage and exchange capacityfor Na in the root tissues. In the presence of metabolic inhaibitorsthe Na flux was considerablyu reduced and equalled the fluxof mannitol (talble II) believed to be a passive mass flow.The Na flux under a pressure gradient appeared therefore tobe metabollism-facilitated. Metabolic inhibition after the rootshad been allowed to take up Na led to a large flux of sodiuminto the xylem (Fig.2). This was considered to come from a largeunavailable store in the roots. Metabolism-facilitated processes also appeared to predominatein the flux of calcium to the xylem of Ricinus, buit not inthe flux through tomato roots. The location of the centripetal metabolism-facilitated transferand the available and unavailable stores is discussed in relationto the effects of pressure gradients.     

Silicon deposition in the root reduces sodium uptake in rice (Oryza sativa L.) seedlings by reducing bypass flow

Sodium chloride reduces the growth of rice seedlings, which accumulate excessive concentrations of sodium and chloride ions in their leaves. In this paper, we describe how silicon decreases transpirational bypass flow and ion concentrations in the xylem sap in rice (Oryza sativa L.) seedlings growing under NaCl stress. Salt (50 mM NaCl) reduced the growth of shoots and roots: adding silicate (3 mM) to the saline culture solution improved the growth of the shoots, but not roots. The improvement of shoot growth in the presence of silicate was correlated with reduced sodium concentration in the shoot. The net transport rate of Na from the root to shoot (expressed per unit of root mass) was also decreased by added silicate. There was, however, no effect of silicate on the net transport of potassium. Furthermore, in salt-stressed plants, silicate did not decrease the transpiration, and even increased it in seedlings pre-treated with silicate for 7 d prior to salt treatment, indicating that the reduction of sodium uptake by silicate was not simply through a reduction in volume flow from root to shoot. Experiments using trisodium-8-hydroxy-1,3,6-pyrenetrisulphonic acid (PTS), an apoplastic tracer, showed that silicate dramatically decreased transpirational bypass flow in rice (from about 4.2 to 0.8%), while the apparent sodium concentration in the xylem, which was estimated indirectly from the flux data, decreased from 6.2 to 2.8 mM. Direct measurements of the concentration of sodium in xylem sap sampled using Philaenus spumarius confirmed that the apparent reduction was not a consequence of sodium recycling. X-ray microanalysis showed that silicon was deposited in the outer part of the root and in the endodermis, being more obvious in the latter than in the former. The results suggest that silicon deposition in the exodermis and endodermis reduced sodium uptake in rice (Oryza sativa L.) seedlings under NaCl stress through a reduction in apoplastic transport across the root.     

Apoplastic transport across young maize roots: effect of the exodermis

The uptake of water and of the fluorescent apoplastic dye PTS (trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate) by root systems of young maize (Zea mays L.) seedlings (age: 11–21 d) has been studied with plants which either developed an exodermis (Casparian band in the hypodermis) or were lacking it. Steady-state techniques were used to measure water uptake across excised roots. Either hydrostatic or osmotic pressure gradients were applied to induce water flows. Roots without an exodermis were obtained from plants grown in hydroponic culture. Roots which developed an exodermis were obtained using an aeroponic (=mist) cultivation method. When the osmotic concentration of the medium was varied, the hydraulic conductivity of the root (Lp _r in m³ · m⁻² · MPa⁻¹ · s⁻¹) depended on the osmotic pressure gradient applied between root xylem and medium. Increasing the gradient (i.e. decreasing the osmotic concentration of the medium; range: zero to 40 mM of mannitol), increased the osmotic Lp _r. In the presence of hydrostatic pressure gradients applied by a pressure chamber, root Lp _r was constant over the entire range of pressures (0–0.4 MPa). The presence of an exodermis reduced root Lp _r in hydrostatic experiments by a factor of 3.6. When the osmotic pressure of the medium was low (i.e. in the presence of a strong osmotic gradient between xylem sap and medium), the presence of an exodermis caused the same reduction of root Lp _r in osmotic experiments as in hydrostatic ones. However, when the osmotic concentration of the medium was increased (i.e. the presence of low gradients of osmotic pressure), no marked effect of growth conditions on osmotic root Lp _r was found. Under these conditions, the absolute value of osmotic root Lp _r was lower by factors of 22 (hydroponic culture) and 9.7 (aeroponic culture) than in the corresponding experiments at low osmotic concentration. Apoplastic flow of PTS was low. In hydrostatic experiments, xylem exudate contained only 0.3% of the PTS concentration of the bathing medium. In the presence of osmotic pressure gradients, the apoplastic flow of PTS was further reduced by one order of magnitude. In both types of experiments, the development of an exodermis did not affect PTS flow. In osmotic experiments, the effect of the absolute value of the driving force cannot be explained in terms of a simple dilution effect (Fiscus model). The results indicate that the radial apoplastic flows of water and PTS across the root were affected differently by apoplastic barriers (Casparian bands) in the exodermis. It is concluded that, unlike water, the apoplastic flow of PTS is rate-limited at the endodermis rather than at the exodermis. The use of PTS as a tracer for apoplastic water should be abandoned. Received: 9 October 1997 / Accepted: 5 February 1998     

The concentration of xylem sap constituents in root exudate, and in sap from intact, transpiring castor bean plants (Ricinus communis L.)

Root exudates were sampled from detopped root systems of castor bean (Ricinus communis). Different volume flux rates were imposed by changing the pneumatic pressure around the root system using a Passioura-type pressure chamber. The concentrations of cations, anions, amino acids, organic acids and abscisic acid decreased hyperbolically when flux rates increased from pure root exudation up to values typical for transpiring plants. Concentrations at low and high fluxes differed by up to 40 times (phosphate) and the ratio of substances changed by factors of up to 10. During the subsequent reduction of flux produced by lowering the pneumatic pressure in the root pressure chamber, the concentrations and ratios of substances deviated (at a given flux rate) from those found when flux was increased. The flux dependence of exudate composition cannot therefore be explained by a simple dilution mechanism. Xylem sap samples from intact, transpiring plants were collected using a Passioura-type root pressure chamber. The concentrations of the xylem sap changed diurnally. Substances could be separated into three groups: (1) calcium, magnesium and amino acid concentrations correlated well with the values expected from their concentration-flux relationships, whereas (2) the concentrations of sulphate and phosphate deviated from the expected relationships during the light phase, and (3) nitrate and potassium concentrations in intact plants varied in completely the opposite manner from those in isolated root systems. Abscisic acid concentrations in the root exudate were dependent on the extent of water use and showed strong diurnal variations in the xylem sap of intact plants even in droughtstressed plants. Calculations using root exudates overestimated export from the root system in intact plants, with the largest deviation found for proton flux (a factor of 10). We conclude that root exudate studies cannot be used as the sole basis for estimating fluxes of substances in the xylem of intact plants. Consequences for studying and modelling xylem transport in whole plants are discussed.     

The Effect of Hydrostatic Pressure Gradients on the Movement of Potassium across the Root Cortex

Transpiration reduces the hydrostatic pressure in the xylemand water moves into the root and across the root cortex inresponse to the pressure gradient so produced. This effect wassimulated by raising the pressure on the external medium surroundinga detopped root system. The flux of sap from the stem stumpand the concentration of potassium in the sap were measuredand the flux of potassium into xylem obtained as (sap flux Xconcentration of potassium). The application of a pressure of 2 atm. caused an approximatelyfourfold increase in potassium flux. This increase was independentof the presence of potassium in the external medium and wasdue, therefore, to an efflux of ions already stored in the roottissues. The effect was specifically due to hydrostatic pressureand not to the D.P.D. difference across the cortex, and wascaused, in part at least, by an increase in the permeabilityof the tissues to iona. This is evident since the increasedpotassium flux occurred in response to pressure, even when theconcentration of potassium in the xylem increased at the sametime, thus precluding any ‘dilution effects‘. It was confirmed that metabolic inhibitors and low temperaturereduce the sap flux but leave the concentration of the sap unchanged.Under a pressure gradient, however, there was an even greaterporportionate reduction of sap flux by equivalent concentrationsof inhibitor or low temperature and the concentration of potassiumin the sap gradually increased. Adding calcium or magnesium ions to the medium caused an increaseof potassium concentration in the sap, demonstrating exchangeprocesses in the transpiration pathway. The difficulty of framing a hypothesis to cover all the factsis discussed and it is tentatively suggested that movement acrossthe cortex to the xylem is a catenary process in which soluteand water move independently at one stage and together as asolution at another stage.     

Analysis of Growth and Water Relations of Tomato Fruits in Relation to Air Vapor Pressure Deficit and Plant Fruit Load

The influence of air vapor pressure deficit (VPD) and plant fruit load on the expansion and water relations of young tomato fruits grown in a glasshouse were evaluated under summer Mediterranean conditions. The contributions of phloem, xylem and transpiration fluxes to the fruit volume increase were estimated at an hourly scale from the growth curves of intact, heat-girdled and detached fruits, measured using displacement transducers. High VPD conditions reduced the xylem influx and increased the fruit transpiration, but hardly affected the phloem influx. Net water accumulation and growth rate were reduced, and a xylem efflux even occurred during the warmest and driest hours of the day. Changes in xylem flux could be explained by variations in the gradient of water potential between stem and fruit, due to changes in stem water potential. Misting reduced air VPD and alleviated the reduction in fruit volume increase through an increase in xylem influx and a decrease in fruit transpiration. Under low fruit load, the competition for assimilates being likely reduced, the phloem flux to fruits increased, similarly to the xylem and transpiration fluxes, without any changes in the fruit water potential. However, different diurnal dynamics among treatments assume variable contributions of turgor and osmotic pressure in F3 and F6 fruits, and hypothetical short-term variations in the water potential gradient between stem and fruit, preventing xylem efflux in F3 fruits.     

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     

Calcium in Xylem Sap and the Regulation of its Delivery to the Shoot

Amounts of total and free calcium in root and shoot xylem sapwere quantified for a number of species grown in comparableenvironments and in a rooting medium not deficient in calcium.The potential for the shoot to sequester calcium was also examined,along with the ability for ABA to regulate calcium flux to theleaf. Xylem sap calcium showed considerable interspecific and diurnalvariation, even though the plants were grown with similar rhizosphericcalcium concentrations. The potential for the shoot to sequesterxylem sap calcium was also highly variable between species andimplied an ability, at least in some species, to regulate thecalcium reaching the shoot in the transpiration stream. Long distance transport of calcium in the xylem was not primarilyby mass flow, because neither calcium uptake nor distributionwere closely related to water uptake or transpiration. The diurnalchanges in xylem sap total ion concentration appeared to benegatively correlated with transpiration while, in contrast,the calcium ion concentration showed two peaks, one occurringin the dark and the other in the light period. The application of ABA to roots caused an increase in the rateof exudation from the xylem of detopped well-watered plants.These experiments suggest that changes in root water relationsdriven by ionic fluxes were the likely cause for enhanced sapexudation from ABA-treated roots. The steady-state concentrationof calcium in the xylem sap was unaffected by ABA when exudationrate increased and, consequently, the flux of calcium must alsohave increased. Key words: Abscisic acid, calcium, xylem sap, ionic fluxes     

Export of Abscisic Acid, 1-Aminocyclopropane-1-Carboxylic Acid,Phosphate, and Nitrate from Roots to Shoots of Flooded Tomato Plants (Accounting for Effects of Xylem Sap Flow Rate on Concentration and Delivery)

We determined whether root stress alters the output of physiologically active messages passing from roots to shoots in the transpiration stream. Concentrations were not good measures of output. This was because changes in volume flow of xylem sap caused either by sampling procedures or by effects of root stress on rates of whole-plant transpiration modified concentrations simply by dilution. Thus, delivery rate (concentration x sap flow rate) was preferred to concentration as a measure of solute output from roots. To demonstrate these points, 1-aminocyclopropane-1-carboxylic acid (ACC), abscisic acid, phosphate, nitrate, and pH were measured in xylem sap of flooded and well-drained tomato (Lycopersicon esculentum Mill., cv Ailsa Craig) plants expressed at various rates from pressurized detopped roots. Concentrations decreased as sap flow rates were increased. However, dilution of solutes was often less than proportional to flow, especially in flooded plants. Thus, sap flowing through detopped roots at whole-plant transpiration rates was used to estimate solute delivery rates in intact plants. On this basis, delivery of ACC from roots to shoots was 3.1-fold greater in plants flooded for 24 h than in well-drained plants, and delivery of phosphate was 2.3-fold greater. Delivery rates of abscisic acid and nitrate in flooded plants were only 11 and 7%, respectively, of those in well-drained plants.     

Uptake of EDTA-complexed Pb, Cd and Fe by solution- and sand-cultured Brassica juncea

Direct plant uptake of metals bound to chelating agents has important implications for metal uptake and the free-ion activity model. Uptake of hydrophilic solutes such as metal–EDTA complexes is believed to occur via bypass apoplastic flow, but many questions remain about the relative importance and selectivity of this pathway. In this study, Brassica juncea (Indian mustard) plants grown in solution- and sand-culture conditions were exposed to metal–EDTA complexes and to PTS, a hydrophilic fluorescent dye previously used as a tracer of apoplastic flow. The results suggest that there are two general phases of solute uptake. Under normal conditions, xylem sap solute concentrations are relatively low (i.e., <0.5% of concentration in solution) and there is a high degree of selectivity among different solutes, while under conditions of stress, xylem sap concentrations are significantly higher (i.e., >3% of concentration in solution) and the selectivity among solutes is less. In healthy plants, xylem sap metal–EDTA concentrations were generally an order of magnitude higher than those of PTS and differences among complexes were observed, with CdEDTA²⁻ exhibiting slightly higher xylem sap concentrations than PbEDTA²⁻ or FeEDTA⁻. Metal–EDTA complexes were found to dominate xylem sap metal speciation and the fraction of metal in xylem sap present as metal–EDTA was greater for non-nutrient metals (Pb, Cd) than for the nutrient metal Fe. Despite differences in root morphology between plants grown under solution- and sand-culture conditions, uptake of solutes was similar under both sets of growth conditions.     

Water Flow Through Cotton Roots in Relation to Xylem Anatomy

The effect of root anatomy on water flow was studied in 7-d-oldcotton (Gossypium hirsutum L.) seedlings grown in solution culture.The total water flux of the intact root system was measuredusing a pressure chamber. Then successive terminal root sectionswere removed at 2,6,10 and 12 cm behind the root tip and theflux was remeasured after each successive cut was made. Xylemdevelopment at different distances behind the root apex wasstudied with a microscope using sections cut free-hand and stainedwith toluidine blue. Water flux increased with the removal ofsuccessive terminal root sections and this coincided with thedegree of basipetal primary xylem development. The large increasein water flux at 10 to 12 cm was associated with secondary xylemdevelopment and increased xylem vessel number. A comparison of water flow and xylem anatomy between roots withtetrarch (Stoneville 506 and Deltapine 41) and pentarch (T25strain) vascular bundle arrangements showed no significant differencesin the measured values of water flux for the primary root. Waterflux, estimated using Poiseuille's equation and measured xylemdimensions, was greater for the tetrarch roots, primarily becauseof the larger diameter of individual vessel elements. The increasednumber of vessel elements in the pentarch primary root of T25did not result in any apparent decrease in axial resistanceto water flow. Key words: Gossypium hirsutum L., roots, vascular bundle, xylem water flux, xylem     

Xylem Transport of 1-Aminocyclopropane-1-carboxylic Acid, an Ethylene Precursor, in Waterlogged Tomato Plants

Waterlogging is known to cause an increase in ethylene synthesis in the shoot which results in petiole epinasty. Evidence has suggested that a signal is synthesized in the anaerobic roots and transported to the shoot where it stimulates ethylene synthesis. Experimental data are presented showing that 1-aminocyclopropane-1-carboxylic acid (ACC), the immediate precursor of ethylene, serves as the signal. Xylem sap was collected from detopped tomato plants (Lycopersicon esculentum Mill. cv. VFN8). ACC in the sap was quantitated by a sensitive and specific assay, and its tentative chemical identity verified by paper chromatography. ACC levels in both roots and xylem sap increased markedly in response to waterlogging or root anaerobiosis. The appearance of ACC in the xylem sap of flooded plants preceded both the increase in ethylene production and epinastic growth, which were closely correlated. Plants flooded and then drained showed a rapid, simultaneous drop in ACC flux and ethylene synthesis rate. ACC supplied through the cut stem of tomato shoots at concentrations comparable to those found in xylem sap caused epinasty and increased ethylene production. These data indicate that ACC is synthesized in the anaerobic root and transported to the shoot where it is readily converted to ethylene.     

Study of root uptake and xylem translocation of cinmethylin and related compounds in detopped soybean roots using a pressure chamber technique

A pressure chamber technique was used to study the root uptake and xylem translocation of nonradiolabeled cinmethylin and its analogs in detopped soybean (Glycine max) roots. Quantifications of compounds were achieved by gas chromatography analysis using a mass spectrometry detector under selected ion monitoring. The compounds tested, with octanol-water partition coefficients (log Kow values) ranging from 0.96 to 5.3, were all nonionizable under the experimental conditions. Root efflux curves of all compounds exhibited a steady-state kinetic profile. The time required to achieve the steady state efflux concentration in the xylem sap correlated with log Kow values in a manner very similar to the root binding profile reported previously by GG Briggs et al. ([1982] Pestic Sci 13: 495-504). After reaching the steady state efflux, the concentration ratio of each compound in the xylem sap to the final concentration in the pressure chamber was taken as the transpiration stream concentration factor (TSCF). A nonlinear relationship was observed between TSCF and log Kow values. The highest TSCF value was between 0.6 to 0.8 for compounds with log Kow between 2.5 to 3.5. The range of optimal log Kow values was slightly higher than that reported earlier by Briggs et al. ([1982] Pestic Sci 13: 495-504). After taking into account the binding of the compound to soil, the apparent optimal Kow value for best root-to-shoot translocation is lowered to around 1. The relationship of root-to-shoot and phloem translocation was also discussed to promote a better understanding at the whole plant level of the uptake and translocation of a soil-applied xenobiotic.     

Pressure-Induced Water and Solute Flow Through Plant Roots

Water and salt flows through detopped sunflower, tomato andred kidney bean roots under applied pressure were studied usinga pressure chamber. Values of J_v for these root systems weremeasured applying variable pressure on the root medium, andL_p calculated. The K, Na and Cl fluxes under applied pressure were comparedwith those in intact plants at the same water flow rates. Tento 100 times higher Na and Cl fluxes were observed through detoppedroots under pressure as compared to those in the unpressurized,intact plants. It is suggested that the roots under pressureare not completely analogous to intact plant roots, and thatpressure-induced flow may not be a reliable method of determiningcharacteristics of ion flow in roots in relation to water flow. Key words: Volume flow, Hydraulic conductivity, K selectivity     

Diurnal variation in the radial reflection coefficient of intact maize roots determined with the xylem pressure probe

Xylem pressure and its relative response to the imposition ofan external osmotic stress (the so-called radial reflectioncoefficient) were recorded in roots of intact maize plants usingthe xylem pressure probe technique. Consecutive insertion oftwo probes into the same xylem vessel or into adjacent vesselsof intact roots of plants exposed to high light intensity andsalt stress under laboratory conditions showed that the xylemtension was not changed by vessel probing. It was also shownby using the double probe approach that the plants were capableof overcoming artificially induced leakages. This and otherevidence reported in the literature convincingly demonstratedthat the probe accurately reads xylem pressure and xylem pressureresponses to osmotic stress. Additional experiments were performedon plants grown in a greenhouse at a subtropical latitude. Underthese conditions the plants were exposed to strong diurnal fluctuationsin light intensity, relative humidity and temperature. The resultsshowed that the absolute xylem pressure in the roots of untreatedplants decreased with increasing transpiration rate from positivevalues in the early morning to negative values around noon (averagevalue –0.15 MPa; maximum negative value –0.57 MPa).As the day progressed and the transpiration rate decreased,xylem pressure increased again to positive values. Correspondingly,the radial reflection coefficient for NaCI increased from aboutzero in the early morning to about unity at noon when transpirationreached its highest value and decreased again to very low valuestowards the evening. The data raise questions concerning conclusionsabout the mechanism of water transport in intact roots drawnfrom the low radial reflection coefficients measured on excisedroots using the root pressure probe. Key words: Xylem pressure probe, osmotic stress, reflection coefficient, transpiration, diurnal changes     

Ion supply capacity of roots in relation to rejuvenation of primary leaves in vivo

Removal of the shoot above the primary node (detopping) of 3-week-old bean plants ( Phaseolus vulgaris L. cv. Contender) altered the metabolism and development of the remaining leaves. An increase in levels of chlorophyll, protein, stomatal opening, photosynthesis and growth, i.e. rejuvenation of primary leaves, was established within 7 days of detopping. These levels were maintained while the primary leaves of equivalent intact plants senesced.
The flux of xylem solution (mineral ions, cytokinins and water) into leaves is related to the leaf area to be supplied and root supply capacity; it has been suggested that detopping leads to an increased availability of root-supplied solutes and hence rejuvenation of the remaining leaves. This assumes however that root output of solutes is not decreased by the defoliation treatment.
We found that root output of ions (electrical conductivity of passive xylem exudate) in detopped plants was 30% lower than in intact plants after 24 h and 60% lower after 7 days. The output of Ca²⁺, Mg²⁺ and K⁺ were similarly reduced 7 and 14 days after detopping as were fresh and dry weights of roots. Furthermore, neither the calculated xylem flux of ions nor directly measured levels of Ca²⁺, Mg²⁺ and K⁺ were significantly increased in leaves of detopped plants during their rejuvenation. We therefore conclude that root output is tightly coupled to shoot demand and that the apparent rejuvenation of primary leaves caused by detopping bean plants is not a consequence of increased xylem flux of mineral ions into the leaves.     

Effect of calcium on transport of cations to the xylem exudate of tobacco

Summary Roots of detopped tobacco plants (Nicotiana tabacum var. Virginia Gold) were exposed to Na, K, and Ca salts or to water, and cation transfer to xylem vessels was measured. In some cases plants had been exposed to Na in addition to regular nutrient solutions before detopping. Calcium in the external medium greatly depressed the transport of Na from the external medium to the xylem vessels and it often stimulated the transfer of K from the external medium to the xylem vessels. The K/Na ratio in the exudate thus was dependent upon the Ca content of the external medium under these conditions. In contrast, externally applied Ca or Ca deficiency had very little effect on the transfer of preaccumulated K and Na from compartments within roots to the xylem vessels. The K/Na ratio in the exudate under these conditions was not related to Ca levels nor to mild Ca deficiency. The ratios decreased with time after detopping regardless of Ca level. Intact plants accumulated more Na than did root systems of detopped plants in a 6-day period.Riverside University of CaliforniaSoil Science and Agricultural Engineering     

The Contribution of an Apoplastic Pathway to Sodium Uptake by Rice Roots in Saline Conditions

An apoplastic pathway, the so-called bypass-flow, across riceroots to the xylem has been investigated and approximately quantifiedusing the apoplastic tracer dye 8-hydroxy-l,3,6-pyrenetrisulphonicacid (PTS); former nomenclature 3-hydroxy-5,8,10-pyrenetrisulphonicacid. It was confirmed that PTS was confined to a compartmentno greater than the apparent free space in living rice roots.Experimental handling did not contribute to bypass-flow. Riceroots recovered rapidly from severe damage: following root pruning,sodium and calcium uptake returned to steady values in about6 h. Apoplastic flow declined after damage as a first-orderkinetic with a half time of 75 min. Analysis of the pattern of elution of PTS from preloaded roots(intact, excised and heat-killed), and from cellulose, was followedto compare PTS movement in the extracellular compartment withthat of water and small hydrated ions. Consideration is givento the factor by which the bypass-flow estimated with the dyewould need to be corrected to reflect the proportion of thetranspiration stream flowing in the apoplastic pathway. Thedata suggest that this factor would be at least 10 for transpiringrice plants. There was large individual variation in the transport both ofsodium and of the apoplastic tracer PTS to the shoot. Plantswith high sodium transport also had high PTS transport and itis concluded that some proportion of the sodium reaching thexylem in rice does so by a pathway which is also available toPTS, presumably direct apoplastic contact across the endodermis.A median value for the bypass-flow of water (corrected fromPTS) would be 0.5 to 1.0 percent of the transpirational volumeflow, but individuals with the highest sodium transport wouldhave bypass-flow values of several percent. Evidence is discussedwhich suggests that apoplastic transport may increase in stressconditions, and it is argued that bypass-flow is a major contributionto sodium uptake in rice in saline conditions. Key words: Oryza saliva, salinity, roots, radial ion transport, apoplast, bypass-flow     

Cd accumulation in roots and shoots of durum wheat: the roles of transpiration rate and apoplastic bypass

Cadmium is readily taken up from soils by plants, depending on soil chemistry, and variably among species and cultivars; altered transpiration and xylem transport and/or translocation in the phloem could cause this variation in Cd accumulation, some degree of which is heritable. Using Triticum turgidum var. durum cvs Kyle and Arcola (high and low grain Cd accumulating, respectively), the objectives of this study were to determine if low-concentration Cd exposure alters transpiration, to relate transpiration to accumulation of Cd in roots and shoots at several life stages, and to evaluate the role of apoplastic bypass in the accumulation of Cd in shoots. The low abundance isotope (106)Cd was used to probe Cd translocation in plants which had been exposed to elemental Cd or were Cd-na?ve; apoplastic bypass was monitored using the fluorescent dye PTS (8-hydroxy-1,3,6-pyrenetrisulphonate). Differential accumulation of Cd by 'Kyle' and 'Arcola' could be partially attributed to the effect of Cd on transpiration, as exposure to low concentrations of Cd increased mass flow and concomitant Cd accumulation in 'Kyle'. Distinct from this, exposure of 'Arcola' to low concentrations of Cd reduced translocation of Cd from roots to shoots relative to root accumulation of Cd. It is possible, but not tested here, that sequestration mechanisms (such as phytochelatin production, as demonstrated by others) are the genetically controlled difference between these two cultivars that results in differential Cd accumulation. These results also suggest that apoplastic bypass was not a major pathway of Cd transport from the root to the shoot in these plants, and that most of the shoot Cd resulted from uptake into the stele of the root via the symplastic pathway.     

Hydraulic properties of living late metaxylem and interactions between transpiration and xylem pressure in maize

A new approach to study dynamic interactions between transpiration and xylem pressure in intact plants is presented. Pressure probe measurements were preformed in living (immature) late metaxylem of maize roots rather than in adjacent mature xylem. This eliminated technical limitations related to the measurement of negative pressures. Water relations of single cells showed that turgor and volumetric elastic modulus were significantly larger in living metaxylem than in cortical cells; hydraulic conductivity was similar in both types of root cells. Increasing transpiration induced an immediate decrease of xylem pressure, and vice versa. Turgor in the living metaxylem could be continuously recorded for more than 1 h. The relationship between xylem pressure and transpiration yielded a root hydraulic resistance of 1.3 x 10⁹ MPa s m^-3. Control experiments indicated that the response of living xylem in the positive pressure range essentially paralleled that of mature root xylem in the negative range. In mature xylem, pressures as low as -0.55 MPa were recorded for short periods (several minutes). Several tests verified that the pressure probe was in contact with mature xylem during the measurements of tensions. The results demonstrate convincingly that transpiration generates an effective driving force for water uptake in roots, a central feature of the cohesion theory.Key words: Hydraulic conductivity, negative pressure, root development, turgor, water transport, Zea mays.