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.     

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.     

盐胁迫对大豆根系木质部压力和Na+吸收的影响

取栽培大豆的水培幼苗为材料,用木质部压力探针和原子吸收分光光度计测定了盐胁迫条件下其根木质部压力和伤流液中Na~+含量的变化,以分析大豆抗盐吸水的机制.结果表明:在25～150 mmol/L NaCl的浓度范围内,随着盐胁迫强度的增加,大豆根木质部负压力的绝对值逐渐增大,但相对负压力和根的径向反射系数则逐渐减小;木质部伤流液中Na~+含量逐渐增加,但Na~+的相对含量则逐渐降低.同时,虽然根系吸水所需的木质部负压力(压力势)及根木质部伤流液的渗透势随着盐胁迫强度的增加都有所下降,但两者共同作用使木质部水势下降的幅度远远小于根外溶液水势(渗透势)下降的幅度,即随着根外溶液盐浓度的升高,根木质部溶液的总水势逐渐高出根外溶液的水势.上述结果说明,在盐胁迫下大豆可以利用相对小的木质部负压力逆水势梯度吸水,且通过避免对Na~+的过量吸收来适应盐胁迫环境.     

Xylem perfusion of tap root segments of Plantago maritima: the physiological significance of electrogenic xylem pumps

Abstract A method is described for perfusing xylem vessels in tap root segments of the halophyte P. maritima. Use of excised segments allowed recording of the trans-root potential (TRP) at both ends of a segment. It was shown that there can be a spatial variation of electrogenic ion pump activity along the xylem in one root segment. The pH of perfusion solutions, differing in buffering capacity, was adjusted by the root segment to pH 5.1–5.6 during How through the xylem. This pH range was similar to that of sap produced by root pressure. The K⁺ activity in the outflow solution (K⁺_out) was rather constant at 12–13 mol m^?l3 despite input K⁺ activities ranging from 8 to 20 mol m^?l3. Addition of fusicoccin (10^?l2 mol m^?l3) to the perfusion solution induced a strong acidification of the xylem sap, a decrease in K⁺_out and an increase in Na⁺_out. Inhibition of aerobic respiration through anoxia inhibited electrogenic proton pumping into the xylem and led to an increase in K⁺_out and a decrease in Na⁺_out. It is suggested that transport of K⁺ and Na⁺ to the shoot of the halophyte P. maritima is regulated in the tap root by means of ion exchange between xylem vessels and xylem parenchyma and that this exchange is energized by proton translocating ATPases.     

Electrical signalling and cytokinins mediate effects of light and root cutting on ion uptake in intact plants

Nutrient acquisition in the mature root zone is under systemic control by the shoot and the root tip. In maize, exposure of the shoot to light induces short-term (within 1–2 min) effects on net K⁺ and H⁺ transport at the root surface. H⁺ efflux decreased (from −18 to −12 nmol m⁻² s⁻¹) and K⁺ uptake (∼2 nmol m⁻² s⁻¹) reverted to efflux (∼−3 nmol m⁻² s⁻¹). Xylem probing revealed that the trans-root (electrical) potential drop between xylem vessels and an external electrode responded within seconds to a stepwise increase in light intensity; xylem pressure started to decrease after a ∼3 min delay, favouring electrical as opposed to hydraulic signalling. Cutting of maize and barley roots at the base reduced H⁺ efflux and stopped K⁺ influx in low-salt medium; xylem pressure rapidly increased to atmospheric levels. With 100 m m NaCl added to the bath, the pressure jump upon cutting was more dramatic, but fluxes remained unaffected, providing further evidence against hydraulic regulation of ion uptake. Following excision of the apical part of barley roots, influx changed to large efflux (−50 nmol m⁻² s⁻¹). Kinetin (2–4  µ m ), a synthetic cytokinin, reversed this effect. Regulation of ion transport by root-tip-synthesized cytokinins is discussed.     

Xylem pressure response in maize roots subjected to osmotic stress: determination of radial reflection coefficients by use of the xylem pressure probe

The absolute pressure in conducting xylem vessels of roots of 2-week-old, slowly transpiring intact maize plants (bathed in nutrition medium) was determined to be +0·024 ± 0·044 MPa using the xylem pressure probe. When the roots were subjected to osmotic stress (NaCI, KCI or sucrose), the xylem pressure decreased immediately and became more negative. However, the response of xylem pressure to osmotic stress was considerably attenuated, indicating that the radial reflection coefficients, σ₁₃ of the maize root for these solutes were rather low (between 0·2 and 0·4 depending on the concentration of the osmoticum). The low values of a, may be caused (partly) by unstirred layer effects. In repeated osmoticum/nutrition regimes a complex pattern of changes in xylem pressure was observed which was apparently linked to the interplay between transpiration and (passive and/or active) solute loading of the xylem. These processes were not observed when the roots were subjected to osmotic stress after excision. In this case, a biphasic response was observed comparable to that found for excised roots using the root pressure probe.     

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.     

Na^＋ and Water Uptake in Relation to the Radial Reflection Coefficient of Root in Arrowleaf Saltbush Under Salt Stress

The response of halophyte arrowleaf saltbush (Atriplex triangularis Willd) plants to a gradient of salt stress were investigated with hydroponically cultured seedlings. Under salt stress, both the Na+ uptake into root xylem and negative pressures in xylem vessels increased with the elevation of salinity (up to 500 mol/m3) in the root environment. However, the increment in negative pressures in root xylem far from matches the decrease in the osmotic potential of the root bathing solutions, even when the osmotic potential of xylem sap is taken into consideration. The total water potential of xylem sap in arrowleaf saltbush roots was close to the osmotic potential of root bathing solutions when the salt stress was low, but a progressively increased gap between the water potential of xylem sap and the osmotic potential of root bathing solutions was observed when the salinity in the root environment was enhanced. The maximum gap was 1.4 MPa at a salinity level of 500 mol/m3 without apparent dehydration of the tested plants. This discrepancy could not be explained with the current theories in plant physiology. The radial reflection coefficient of root in arrowleaf saltbush decreased with the enhanced salt stress was and accompanied by an increase in the Na+ uptake into xylem sap. However, the relative Na+ in xylem exudates based on the corresponding NaCl concentration in the root bathing solutions showed a tendency of decrease. The results showed that the reduction in the radial reflection coefficient of roots in the arrowleaf saltbush did not lead to a mass influx of NaCl into xylem when the radial reflection coefficient of the root was considerably small; and that arrowleaf saltbush could use small xylem pressures to counterbalance the salt stresses, either with the uptake of large amounts of salt, or with the development of xylem pressures dangerously negative. This strategy could be one of the mechanisms behind the high resistance of arrowleaf saltbush plants to salt stress.     

The mechanism of water-stress-induced embolism in two species of chaparral shrubs

The mechanism of water-stress-induced embolism of xylem was investigated in Malosma laurina and Heteromeles arbutifolia, two chaparral shrub species of southern California. We tested the hypothesis that the primary cause of xylem dysfunction in these species during dehydration was the pulling of air through the pores in the cell walls of vessels (pores in pit membranes) as a result of high tensions on xylem water. First, we constructed vulnerability-to-embolism curves for (i) excised branches that were increasingly dehydrated in the laboratory and (ii) hydrated branches exposed to increasing levels of external air pressure. Branches of M. laurina that were dehydrated became 50% embolized at a xylem pressure potential of -1.6 MPa, which is equal in magnitude but opposite in sign to the +1.6 MPa of external air pressure that caused 50% embolism in hydrated stems. Dehydrated and pressurized branches of H. arbutifolia reached a 50% level of embolism at -6.0 and +6.4 MPa, respectively. Secondly, polystyrene spheres ranging in diameter from 20 to 149 nm were perfused through hydrated stem segments to estimate the pore size in the vessel cell walls (pit membranes) of the two species. A 50% or greater reduction in hydraulic conductivity occurred in M. laurina at perfusions of 30, 42, 64 and 82 nm spheres and in H. arbutifolia at perfusions of 20 and 30 nm spheres. Application of the capillary equation to these pore diameters predicted 50% embolism at xylem tensions of -2.2 MPa for M. laurina and -6.7 MPa for H. arbutifolia, which are within 0.7 MPa of the actual values. Our results suggest that the size of pores in pit membranes may be a factor in determining both xylem efficiency and vulnerability to embolism in some chaparral species. H. arbutifolia, with smaller pores and narrower vessels, withstands lower water potentials but has lower transport efficiency. M. laurina, with wider pores and wider vessels, has a greater transport efficiency but requires a deeper root system to help avoid catastro-phically low water potentials.     

Comparative measurements of the xylem pressure ofNicotiana plants by means of the pressure bomb and pressure probe

Determination of the pressure in the water-conducting vessels of intactNicotiana rustica L. plants showed that the pressure probe technique gave less-negative values than the Scholander-bomb method. Even though absolute values of the order of −0.1 MPa could be directly recorded in the xylem by means of the pressure probe, pressures between zero and atmospheric were also frequently found. The data obtained by the pressure probe for excised leaves showed that the Scholander bomb apparently did not read the actual tension in the xylem vessles ofNicotiana plants. The possibility that the pressure probe gave false readings was excluded by several experimental controls. In addition, cavitation and leaks either during the insertion of the microcapillary of the pressure probe, or else during the measurements were easily recognized when they occurred because of the sudden increase of the absolute xylem tension to that of water vapour or to atmospheric, respectively. Tension values of the same order could also be measured by means of the pressure probe in the xylem vessels of pieces of stem cut from leaves and roots under water and clamped at both ends. The magnitude of the absolute tension depended on the osmolarity of the bathing solution which was adjusted by addition of appropriate concentrations of polyethylene glycol. Partial and uniform pressurisation of plant tissues or organs, or of entire plants (by means of the Scholander bomb or of a hyperbaric chamber, respectively) and simultaneous recording of the xylem tension using the pressure probe showed that a 1∶1 response in xylem pressure only occurred under a few circumstances. A 1∶1 response required that the xylem vessels were in direct contact with an external water reservoir and/or that the tissue was (pre-)infiltrated with water. Corresponding pressure-probe measurements in isolated vascular bundles ofPlantago major L. orP. lanceolata L. plants attached to a Hepp-type osmometer indicated that the magnitude of the tension in the xylem vessels was determined by the external osmotic pressure of the reservoir. These and other experiments, as well as analysis of the data using classical thermodynamics, indicated that the turgor and the internal osmotic pressure of the accessory cells along the xylem vessels play an important role in the maintenance of a constant xylem tension. This conclusion is consistent with the cohesion theory. In agreement with the literature (P.E. Weatherley, 1976, Philos. Trans. R. Soc. London Ser. B23, 435–444; 1982, Encyclopedia of plant physiology, vol. 12B, 79-109), it was found that the tension in the xylem of intact plants under normal and elevated ambient pressure (as measured with the pressure probe) under quasi-stationary conditions was independent of the transpiration rate over a large range, indicating that the conductance of the flow path must be flow-dependent.     

High Molecular Weight Organic Compounds in the Xylem Sap of Mangroves: Implications for Long-Distance Water Transport

The rise of sap in mangroves has puzzled plant physiologists for many decades. The current consensus is that negative pressures in the xylem exist which are sufficiently high to exceed the osmotic pressure of seawater (2.5 MPa). This implies that the radial reflection coefficients of the mangrove roots are equal to unity. However, direct pressure probe measurements in xylem vessels of the roots and stems of mangrove (Rhizophora mangle) grown in the laboratory or in the field yielded below-atmospheric, positive (absolute) pressure values. Slightly negative pressure values were recorded only occasionally. Xylem pressure did not change significantly when the plants were transferred from tap water to solutions containing up to 1700 mOsmol kg^?1 NaCl. This indicates that the radial reflection coefficient of the roots for salt, and therefore the effective osmotic pressure of the external solution, was essentially zero as already reported for other halophytes. The low values of xylem tension measured with the xylem pressure probe were consistent with previously published data obtained using the vacuum/leafy twig technique. Values of xylem tension determined with these two methods were nearly two orders of magnitude smaller than those estimated for mangrove using the pressure chamber technique (?3 to ?6MPa). Xylem pressure probe measurements and staining experiments with alcian blue and other dyes gave strong evidence that the xylem vessels contained viscous, mucilage- and/or protein-related compounds. Production of these compounds resulting from wound or other artifactual reactions was excluded. The very low sap flow rates of about 20–50 cm h^?1 measured in these mangrove plants were consistent with the presence of high molecular weight polymeric substances in the xylem sap. The presence of viscous substances in the xylem sap of mangroves has the following implications for traditional xylem pressure measurement techniques, development of xylem tension, and longdistance water transport: (1) high external balancing pressures in the pressure chamber are needed to force xylem sap to the cut surface of the twig; (2) stable tensions much larger than 0.1 MPa can be developed only occasionally because viscous solutions provide nucleation sites for gas bubble formation; (3) the frequent presence of small gas bubbles in viscous solutions allows water transport by interfacial, gravity-independent streaming at gas/water interfaces and (4) the increased density of viscous solutions creates (gravity-dependent) convectional flows. Density-driven convectional flows and interfacial streaming, but also the very low radial reflection coefficient of the roots to NaCl are apparently the means by which R. mangle maintains water transport to its leaves despite the high salinity of the environment.     

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.     

Bicarbonate-induced alkalinization of the xylem sap in intact maize seedlings as measured in situ with a novel xylem pH probe

In higher plants the pH of the xylem sap plays an important role in drought signaling, growth regulation, and plant nutrition. However, the interpretation of the data is very controversial. The main reason for this is that the xylem pH in intact plants was not directly accessible hitherto. We present here a novel, minimally-invasive probe based on the xylem pressure-potential probe (used for measuring directly xylem pressure and the electrical potential between root xylem sap and medium). Single-tipped, double-barreled capillaries were used, one barrel served as H(+)-selective electrode, whereas pressure and electrical potential were recorded by the other one. Upon insertion of the probe into the root xylem of maize (Zea mays) seedlings, pH values ranging between about 4.2 and 4.9 were monitored when the roots were immersed in standard nutrient solution. The pH did not respond to changes in light irradiation (up to 300 micromol m(-2) s(-1)), but increased upon exposure of the root to 5 or 20 mm bicarbonate in the bath solution. Changes in pH could also be recorded in transpiring plants when the root was cut below the insertion point of the probe and placed in media with different pH. The data support the hypothesis of Mengel ([1994] Plant Soil 165: 275-283) that upon external supply with bicarbonate Fe is immobilized in the leaf apoplast via changes in xylem pH.     

Na+-K+ Exchange at the Xylem/Symplast Boundary (Its Significance in the Salt Sensitivity of Soybean)

We investigated the mechanism of Na+ reabsorption in exchange for K+ at the xylem/symplast boundary of soybean roots (Glycine max var Hodgson). The xylem vessels of excised roots were perfused with solutions of defined composition to discriminate between entry of ions into or reabsorption from the xylem vessels. In the presence of NaCl, the transport systems released K+ into the xylem sap and reabsorbed Na+. The Na+-K+ exchange was energized by proton-translocating ATPases, enhanced by external K+ concentration, and dependent on the anion permeability. Evidence was presented for the operation of H+/Na+ and H+/K+ antiporters at the xylem/symplast interface.     

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.     

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.     

树木树液上升机理研究进展

水分在植物体内的运输一直是很多植物生理生态学家所关注的一个重要问题。介绍了内聚力学说的基本假设和其存在争议,总结了近年来这一研究领域的几个热点问题,主要包括：（1）木质部栓塞及其恢复机理;（2）木质部压力探针和压力室法测定的木质部张力值不一致的现象及其可能原因;（3）补偿压学说;（4）不同界面层张力以及输水管道的毛细作用力、薄壁细胞膨压和木质部渗透压、逆向蒸腾等在树木汁液上升中的贡献;（5）最近发现的存在于木质部导管伴胞和韧皮部薄壁细胞等质膜中的水孔蛋白在植物水分运输中的调控作用等。这些方面在解释树木的树液上升中都起着重要的作用。     

Perfusion of onion root xylem vessels: a method and some evidence of control of the pH of the xylem sap

We describe a method for perfusing the xylem in the stele of excised onion roots with solutions of known composition under a pressure gradient. Tracer studies using [¹⁴C] polyethylene glycol 4000 and the fluorescent dye, Tinopal CBSX, indicated that perfusing solutions passed exclusively through the xylem vessels. The conductance of the xylem was small over the apical 100 mm of the root axis but increased markedly between 100 and 200 mm. Unbuffered perfusion solutions supplied in the range pH 3.7–7.8 emerged after passage through the xylem adjusted to pH 5.2–6.0, indicating the presence of mechanisms for absorbing or releasing protons. This adjustment continued over many hours with net proton fluxes apparently determined by the disparity between the pH of the perfusion solution and the usual xylem sap pH of about 5.5. Mild acidification of the xylem sap by buffered perfusion solutions increased the release of ⁸⁶Rb (K⁺) and ³⁵SO₄ ^2- from the stelar tissue into the xylem stream. The ion-transporting properties of onion roots seemed little changed by excision from the bulbs, or by removal of the apical zones of the root axis. The pH of sap produced by root pressure resembles that found in the outflow solutions of perfused root segments.     

New evidence for large negative xylem pressures and their measurement by the pressure chamber method

Pressure probe measurements have been interpreted as showing that xylem pressures below c. –0.4 MPa do not exist and that pressure chamber measurements of lower negative pressures are invalid. We present new evidence supporting the pressure chamber technique and the existence of xylem pressures well below –0.4 MPa. We deduced xylem pressures in water-stressed stem xylem from the following experiment: (1) loss of hydraulic conductivity in hydrated stem xylem (xylem pressure = atmospheric pressure) was induced by forcing compressed air into intact xylem conduits; (2) loss of hydraulic conductivity from cavitation and embolism in dehydrating stems was measured, and (3) the xylem pressure in dehydrated stems was deduced as being equal and opposite to the air pressure causing the same loss of hydraulic conductivity in hydrated stems. Pressures determined in this way are only valid if cavitation was caused by air entering the xylem conduits (air-seeding). Deduced xylem pressure showed a one-to-one correspondence with pressure chamber measurements for 12 species (woody angiosperms and gymnosperms); data extended to c. –10 MPa. The same correspondence was obtained under field conditions in Betula occidentalis Hook., where pressure differences between air- and water-filled conduits were induced by a combination of in situ xylem water pressure and applied positive air pressure. It is difficult to explain these results if xylem pressures were above –0.4 MPa, if the pressure chamber was inaccurate, and if cavitation occurred by some mechanism other than air-seeding. A probable reason why the pressure probe does not register large negative pressures is that, just as cavitation within the probe limits its calibration to pressures above c. –0.5 MPa, cavitation limits its measurement range in situ.     

三角叶滨藜根吸水特点与其抗盐性的关系

采用压力室和冰点渗透压计测定了三角叶滨藜在不同浓度NaCl的根系环境溶液中根木质部的压力势和伤流液的渗透势,并利用原子吸收分光光度计测定了植株和伤流液以及环境溶液中Na 含量。结果表明:随着根环境溶液NaCl浓度的增加,三角叶滨藜植株和木质部伤流液中Na 含量虽呈上升趋势,但根系的过滤系数和体内Na 相对累积量逐渐降低,说明三角叶滨藜根细胞对盐分有很强的过滤作用;木质部伤流液的渗透势随着环境溶液渗透势的降低而降低,但根木质部溶液的水势则逐渐高出根外环境溶液的渗透势;表明三角叶滨藜能够利用较低的木质部负压来抵抗根外溶液的低渗透势而反渗透吸水,并利用根细胞对盐分的过滤作用来避免从环境摄取过量的盐分。