Water and solute transport along developing maize roots

Hydraulic and osmotic properties were measured along developing maize (Zea mays L.) roots at distances between 15 and 465 mm from the root tip to quantify the effects of changes in root structure on the radial and longitudinal movement of water and solutes (ions). Root development generated regions of different hydraulic and osmotic properties. Close to the root tip, passive solute permeability (root permeability coefficient, P_sr) was high and selectivity (root reflection coefficient, _sr) low, indicative of an imperfect semipermeable root structure. Within the apical 100–150 mm, P_sr decreased by an order of magnitude and _sr increased significantly. Root hydraulic conductivity (Lp_r) depended on the nature of the force (hydrostatic and osmotic). Osmotic Lp_r was smaller by an order of magnitude than hydrostatic Lp_r and decreased with increasing distance from the root tip. Throughout the root, responses in turgor of cortical cells and late metaxylem to step changes in xylem pressure applied to the base of excised roots were measured at high spatial resolution. The resulting profiles of radial and longitudinal propagation of pressure showed that the endodermis had become the major hydraulic barrier in older parts of the root, i.e. at distances from the apex ä 150 mm. Other than at the endodermis, no significant radial hydraulic resistance could be detected. The results permit a detailed analysis of the root's composite structure which is important for its function in collecting and translocating water and nutrients.Abbreviations and Symbols CPP cell pressure probe - IT root segments with intact tips; - Lp_r root hydraulic conductivity - Lp_rh hydrostatic hydraulic conductivity of root - Lp_ro osmotic hydraulic conductivity of root - P_app hydrostatic pressure applied to cut end of root - P_c cell turgor - P_{c, cor} turgor of cortical cell - P_c,xyl turgor of late metaxylem vessel - P_ro stationary root pressure - P_r0,seal stationary root pressure of sealed root segment - P_sr solute permeability coefficient of root - RPP root pressure probe - TR root segments with tip removed - _sr reflection coefficient of root Dedicated to Professor Andreas Sievers on the occasion of his retirement     

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.      

Late Maturation of Large Metaxylem Vessels in Soybean Roots: Significance for Water and Nutrient Supply to the Shoot

The large, late metaxylem (LMX) in the roots of soybean beginsdevelopment in the centre of the stele after lignification ofthe early metaxylem poles. Subsequent maturation of the firstappearing LMX elements is gradual. They were never mature inthe 8-d-old seedlings examined. In 10 to 15-d-old plants thefirst LMX matured to open vessels at a mean of 17 cm proximalto the root tip. Additional LMX vessels developed in more proximalregions of the roots and these also matured gradually. Based on calculations from relative vessel diameters, the potentialflow of xylem sap in a single central LMX vessel is 50 timesthat in the total of all the early metaxylem (EMX) vessels ofa typical primary root of soybean. There was a marked dependence of relative leaf area on the lengthof primary root with open LMX vessels. This may result fromthe predicted increased water and nutrient flow to the shoot,facilitated by the opening of the large vessels. It is suggestedthat, as in maize, the living LMX elements may function in ionaccumulation. Dicotyledonous roots, soybean, Glycine max, xylem vessels, xylem maturation, water conduction     

Radial and axial turgor pressure measurements in individual root cells of Mesembryanthemum crystallinum grown under various saline conditions

Abstract. Radial and axial turgor pressure profiles were measured with the pressure probe in untreated and salt-treated intact roots of Mesembryanthemum crystallinum. The microcapillary of the pressure probe was inserted step-wise into the root tissue 5, 25 and 50 mm away from the root cap. For evaluation of the data, only those recordings on a given root were used in which four discontinuous increases in turgor pressure occurred. These four turgor pressure increases could be related to the rhizodermal cells and to the cells in the three cortical layers. The measurements showed that a radial turgor pressure gradient of the same magnitude (directed from the third cortical layer to the external medium) existed along the root axis. The magnitude of this turgor pressure gradient decreased with increasing salinity (up to 400 mol m^-3 NaCl) in the growth medium. Addition of 10 mol m^-3 CaCl₂ to the 400 mol m^-3 NaCl medium partly reduced the salt-induced decrease in turgor pressure, but only in cells 25–50 mm away from the root tip. Combined with this effect, a small axial turgor pressure gradient was generated, therefore, in the cortex layers which was directed to the root tip. Measurements of the volumetric elastic modulus, ?, of the wall of the individual cells showed that the presence of salt considerably reduced the magnitude of this parameter and that addition of Ca²⁺ to the strongly saline medium partially diminished this decrease. This effect was strongest in cells 50 mm away from the root tip. The magnitude of ? of rhizodermal and cortical cells increased along the root axis both in untreated and in salt-treated roots. The ? value was significantly smaller for rhizodermal cells compared to the cortical cells, with the exception of cells 50 mm from the tip. In this tissue, rhizodermal and cortical cells exhibited nearly the same values. The decrease of the ?-values with salt and the increase along the root axis under the various growth conditions could be correlated with corresponding changes in cell volume. Diurnal changes in turgor pressure could not be detected in the individual root cells, with the notable exception of the rhizodermal and cortical cells located in the region 50 mm away from the root tip of the control plants. In these cells, an increase in turgor pressure was observed during the morning hours. Determination of the average osmotic pressure in tissue sections along the roots of control and salt-treated plants revealed that at 400 mol m^-3 NaCl the osmotic pressure gradient between the tissue and the medium is exo-directed, provided that the water is not (partly) immobilized.     

Ultrastructure of xylem parenchyma cells of barley roots in relation to ion transport to the xylem

Summary The structure of xylem parenchyma cells is examined in relation to transport of ions through the root. Measurement of uptake of ⁸⁶Rb⁺ and its transport through the root at different distances from the apex show that this is a general activity along the length of the root and not confined to a limited region. Thus transport through the root is not stopped by removal of that part of the root tip containing metaxylem vessels with living contents. The structure of xylem parenchyma appears to be suitable for involvement in ion transport from the stele to the xylem. At 1 cm behind the tip, where metaxylem vessels have no living contents but ion uptake and transport are going on at high rates, xylem parenchyma cells are rich in cytoplasm with extensive rough endoplasmic reticulum and well-developed mitochondria. Their cell walls contain numerous plasmodesmata, establishing the possibility of a symplastic pathway across the stele up to the vessels. The results are discussed in relation to regulation of ion transport to the xylem vessels in roots.Dedicated to Professor O. Stocker on the occasion of his 85th birthday.     

Effects of Temperature on the Development of Metaxylem in Primary Wheat Roots and Its Hydraulic Consequence

The effects of different temperatures on the development ofmetaxylem were studied in the primary seminal root of winterwheat (Triticum aestivum L.) seedlings. Xylem development wasstudied microscopically at different distances behind the rootapex after safranin staining to reveal lignification. Diameter of the central late metaxylem (LMX) and its proportionto the stele cross-sectional area increased in the acropetaldirection. Diameter of the LMX and stele decreased with an increasein growing temperature. Numbers of early metaxylem (EMX) wereseven, seven and six at 10, 20 and 30 C, respectively. EMXwas lignified much more rapidly than the LMX along the seminalroot axes. Lignification of xylem elements commenced furthertowards the root apex at the higher temperatures. The LMX vesselsof the roots grown at the higher temperature had thicker secondarywalls. The relative conductivity of seminal roots, calculated fromPoiseuille's equation, decreased as growing temperature increased.In a drought-prone environment where wheat plants rely heavilyon stored soil water, a lowered axial conductivity in the rootswould be advantageous. The plants would tend to conserve waterduring vegetative growth for use during the critical periodsof flowering and grain-filling. Breeders selecting wheat plants for altered LMX diameters shouldcontrol temperatures during primary root development, sectionthe roots at the same distance from the tip and be aware thatcross walls may exist in the LMX for up to 30 cm from the tip. Wheat, Triticum aestivum L., roots, xylem development, hydraulic conductivity, temperature     

Spatial distribution of turgor and root growth at low water potentials

Spatial distributions of turgor and longitudinal growth were compared in primary roots of maize (Zea mays L. cv FR27 × FRMo 17) growing in vermiculite at high (−0.02 megapascals) or low (−1.6 megapascals) water potential. Turgor was measured directly using a pressure probe in cells of the cortex and stele. At low water potential, turgor was greatly decreased in both tissues throughout the elongation zone. Despite this, longitudinal growth in the apical 2 millimeters was the same in the two treatments, as reported previously. These results indicate that the low water potential treatment caused large changes in cell wall yielding properties that contributed to the maintenance of root elongation. Further from the apex, longitudinal growth was inhibited at low water potential despite only slightly lower turgor than in the apical region. Therefore, the ability to adjust cell wall properties in response to low water potential may decrease with cell development.     

The effect of humidity and light on cellular water relations and diffusion conductance of leaves ofTradescantia virginiana L.

Turgor (_p) and osmotic potential (_s) in epidermal and mesophyll cells, in-situ xylem water potential (-xyl) and gas exchange were measured during changes of air humidity and light in leaves ofTradescantia virginiana L., Turgor of single cells was determined using the pressure probe. Sap of individual cells was collected with the probe for measuring the freezing-point depression in a nanoliter osmometer. Turgor pressure was by 0.2 to 0.4 MPa larger in mesophyll cells than in epidermal cells. A water-potential gradient, which was dependent on the rate of transpiration, was found between epidermis and mesophyll and between tip and base of the test leaf. Step changes of humidity or light resulted in changes of epidermal and mesophyll turgor (_p-epi, _p-mes) and could be correlated with the transpiration rate. Osmotic potential was not affected by a step change of humidity or light. For the humidity-step experiments, stomatal conductance (g) increased with increasing epidermal turgor.g/_p-epi appeared to be constant over a wide range of epidermal turgor pressures. In light-step experiments this type of response was not found and stomatal conductance could increase while epidermal turgor decreased.Symbols E transpiration - g leaf conductance - w leaf/air vapour concentration difference - -epi water potential of epidermal cells - -mes water potential of mesophyll cells - -xyl water potential of xylem - _p-epi turgor pressure of epidermal cells - _p-mes turgor pressure of mesophyll cells - _s-epi osmotic potential of epidermal cells - _s-mes osmotic potential of mesophyll cells     

The effect of abscisic acid on cell turgor pressures,solute content and growth of wheat roots

Abscisic acid (ABA) was shown to influence turgor pressure and growth in wheat (Triticum aestivum L.) roots. At a concentrations of 25 mmol·m^-3, ABA increased the turgor pressure of cells located within 1 cm of the tip by up to 450 kPa. At 4 to 5 cm from the root tip this concentration of ABA reduced the turgor pressure of peripheral cells (epidermis and the first few cortical cell layers) to zero or close to zero while that of the inner cells was increased. Increases in sap osmolality were dependent on the concentration of ABA and the effect saturated at 5 mmol·m^-3 ABA. The increase in osmolality took about 4 h and was partly the result of reducing-sugar accumulation. Levels of inorganic cations were not affected by ABA. Root growth was inhibited at ABA concentrations that caused a turgor-pressure increase. The results show that while ABA can affect root cell turgor pressures, this effect does not result in increased root growth.Abbreviation ABA abscisic acid     

Radial Turgor Pressure Profiles in Growing and Mature Zones of Wheat Roots--A Modification of the Pressure Probe

A modification of the pressure probe is described which allowsaccurate routine recording of the turgor pressure of singlecells at measured depth within a tissue. Measurements of radial profiles of turgor pressure in wheatroots grown in some simple salt solutions (0.5 mol m^–3CaCl₂, 0.5 mol m^–3 CaCI₂ plus 10 mol m^–3 NaCl, and0.5 mol m^–3 CaCl₂ plus 10 mol m^–3 KCI), are described.Turgor pressure was constant (approximately, 0.65 MPa) alonga radius within the elongation zone irrespective of the natureof the bathing solution. In mature root tissue turgor pressurein the cortex was lower than that of the growing zone in alltreatments and the pressure of the stele was on average 0.22MPa higher than that of the cortex. Potassium in the mediumbathing the root increased the turgor pressure in mature root(both cortex and stele) relative to low salt and sodium treatments. The results are discussed in relation to both root growth andion accumulation. Key words: Pressure probe, wheat roots, salt solution     

Cell water potential,osmotic potential,and turgor in the epidermis and mesophyll of transpiring leaves

Water potential, osmotic potential and turgor measurements obtained by using a cell pressure probe together with a nanoliter osmometer were compared with measurements obtained with an isopiestic psychrometer. Both types of measurements were conducted in the mature region of Tradescantia virginiana L. leaves under non-transpiring conditions in the dark, and gave similar values of all potentials. This finding indicates that the pressure probe and the osmometer provide accurate measurements of turgor, osmotic potentials and water potentials. Because the pressure probe does not require long equilibration times and can measure turgor of single cells in intact plants, the pressure probe together with the osmometer was used to determine in-situ cell water potentials, osmotic potentials and turgor of epidermal and mesophyll cells of transpiring leaves as functions of stomatal aperture and xylem water potential. When the xylem water potential was-0.1 MPa, the stomatal aperture was at its maximum, but turgor of both epidermal and mesophyll cells was relatively low. As the xylem water potential decreased, the stomatal aperture became gradually smaller, whereas turgor of both epidermal and mesophyll cells first increased and afterward decreased. Water potentials of the mesophyll cells were always lower than those of the epidermal cells. These findings indicate that evaporation of water is mainly occurring from mesophyll cells and that peristomatal transpiration could be less important than it has been proposed previously, although peristomatal transpiration may be directly related to regulation of turgor in the guard cells.     

Water relations of growing pea epicotyl segments

The water relations of growing epicotyl segments of pea (Pisum sativum L.) were studied using the miniaturized pressure probe. For epidermal cells stationary turgor pressures of P=5 to 9 bar and half-times of water exchange of individual cells T _1/2=1 to 27 s were found. The volumetric clastic modulus () of epidermal cells varied from 12 to 200 bar and the hydraulic conductivity, Lp=0.2 to 2·10^-6 cm s^-1 bar^-1. For cortical cells P=5 to 11 bar, T _1/2=0.3 to 1 s, Lp=0.4 to 9·10^-5 cm s^-1 bar^-1 and =6 to 215 bar. The T _1/2 of cortical cells was extremely low and the Lp rather high as compared to other higher plant cells. The T _1/2-values of cortical cells were sometimes observed to change from short to substantially longer values (T _1/2=3 to 20 s). Both short and long pressure relaxations showed all the characteristics of non-artifactual curves. The change is apparently due to an increase in Lp and not , but the reason for the change in cell permeability to water is not known.In osmotic exchange experiments on peeled segments using solutions of different solutes, the half-time of osmotic water exchange for the whole segment was approximately 60 s. Water exchange occurred too quickly to be rate controlled by solute diffusion in the wall space. The data suggest that the short T _1/2-values in the cortical cells are the physiologically relevant ones for the intact tissue and that a considerable component of water transport occurs in the cell-to-cell pathway, although unstirred layer effects at the boundary between the segment and solution may influence the measured half-time. Using the theory of Molz and Boyer (1978, Plant Physiol. 62, 423–429), the gradient in water potential necessary to maintain the uptake of water for cell enlargement can be calculated from the measured diffusivities to be approximately 0.2 and 1 bar for growth rates of 1% h^-1 and 5% h^-1, respectively. Thus, although the T _1/2-values are short and Lp rather high, there may be a significant osmotic disequilibrium in the most rapidly growing tissue and as a consequence the influence of water transport on the growth rate cannot be excluded.Abbreviations P turgor pressure - T _1/2 half-time of water exchange of individual cell - Lp hydraulic conductivity - volumetric elastic modulus - t _1/2 average half-time of water exchange of tissue     

The integration of whole-root and cellular hydraulic conductivities in cereal roots

The hydraulic conductivities of excised whole root systems of wheat (Triticum aestivum L. cv. Atou) and of single excised roots of wheat and maize (Zea mays L. cv. Passat) were measured using an osmotically induced back-flow technique. Ninety minutes after excision the values for single excised roots ranged from 1.6·10^-8 to 5.5·10^-8 m·s^-1·MPa^-1 in wheat and from 0.9·10^-8 to 4.8·10^-8 m·s^-1·MPa^-1 in maize. The main source of variation was a decrease in the value as root length increased. The hydraulic conductivities of whole root systems, but not of single excised roots, were smaller 15 h after excision. This was not caused by occlusion of the xylem at the cut end of the coleoptile. The hydraulic conductivities of epidermal, cortical and endodermal cells were measured using a pressure probe. Epidermal and cortical cells of both wheat and maize roots gave mean values of 1.2·10^-7 m·s^-1·MPa^-1 but in endodermal cells (measured only in wheat) the mean value was 0.5·10^-7 m·s^-1·MPa^-1. The cellular hydraulic conductivities were used to calculate the root hydraulic conductivities expected if water flow across the root was via transcellular (vacuole-to-vacuole), apoplasmic or symplasmic pathways. The results indicate that, in freshly excised roots, the bulk of water flow is unlikely to be via the transcellular pathway. This is in contrast to our previous conclusion (H. Jones, A.D. Tomos, R.A. Leigh and R.G. Wyn Jones 1983, Planta 158, 230–236) which was based on results obtained with whole root systems of wheat measured 14–15 h after excision and which probably gave artefactually low values for root hydraulic conductivity. It is now concluded that, near the root tip, water flow could be through a symplasmic pathway in which the only substantial resistances to water flow are provided by the outer epidermal and the inner endodermal plasma membranes. Further from the tip, the measured hydraulic conductivities of the roots are consistent with flow either through the symplasmic or apoplasmic pathways.Symbols L _{p, cell} cell hydraulic conductivity - L _{p, root} root hydraulic conductivity - L _{p, root} calculated root hydraulic conductivity - root reflection coefficient     

The development of the endoder mis andPhi layer of apple roots

Summary Suberin lamellae and a tertiary cellulose wall in endodermal cells are deposited much closer to the tip of apple roots than of annual roots. Casparian strips and lignified thickenings differentiate in the anticlinal walls of all endodermal andphi layer cells respectively, 4–5 mm from the root tip. 16 mm from the root tip and only in the endodermis opposite the phloem poles, suberin lamellae are laid down on the inner surface of the cell walls, followed 35 mm from the root tip by an additional cellulosic layer. Coincidentally with this last development, the suberin and cellulose layers detach from the outer tangential walls and the cytoplasm fragments. 85 mm from the root tip the xylem pole endodermis (50% of the endodermis) develops similarly, but does not collapse. 100–150 mm from the root tip, the surface colour of the root changes from white to brown, a phellogen develops from the pericycle and sloughing of the cortex begins. A few secondary xylem elements are visible at this stage.Plasmodesmata traverse the suberin and cellulose layers of the endodermis, but their greater frequency in the outer tangential and radial walls of thephi layer when compared with the endodermis suggests that this layer may regulate the inflow of water and nutrients to the stele.     

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.     

Uptake of Lucifer Yellow CH into intact barley roots: Evidence for fluid-phase endocytosis

Intact barley (Hordeum vulgare L.) roots have been shown to take up the highly fluorescent dye Lucifer Yellow CH (LYCH) into their cell vacuoles. In the apical 1 cm of root tip, differentiating and dividing cells showed a prolific uptake of LYCH into their provacuoles. The LYCH was retained during fixation, apparently becoming bound to electron-dense material in the vacuoles. The dye freely entered the apoplast of roots in which the Casparian band was not developed, being taken up into the vacuoles of cells in both the cortex and stele. However, when LYCH was applied to a 1-cm zone approx. 6 cm behind the root tip the Casparian band on the radial walls of the endodermis completely prevented the dye from entering the cells of the stele, only the cell walls and vacuoles of the cortical cells taking up the dye. The inability of LYCH to cross the plasmalemma of the endodermal cells and enter the stele via the symplast substantiates previous claims that the dye is unable to cross the plasmalemma of plant cells. The results are discussed in the light of recent demonstrations that LYCH is a particularly effective marker for fluid-phase endocytosis in animal and yeast cells. A calculation of the energetic requirements for LYCH uptake into barley roots supports the contention that LYCH is taken up into the vacuoles of plant cells by fluid-phase endocytosis.Abbreviation LYCH Lucifer Yellow CH     

Water relations of individual leaf cells ofMesembryanthemum crystallinum plants grown at low and high salinity

Summary The effects of saline conditions on the water relations of cells in intact leaf tissue of the facultative CAM plantMesembryanthemum crystallinum were studied using the pressure probe technique. During a 12-hr light/dark regime a maximum in turgor pressure was recorded for the mesophyll cells of salttreated (CAM) plants at the beginning of the light period followed 6 hr later by a pressure maximum in the bladder cells of the upper epidermis. In contrast, the turgor pressure in the bladder cells of the lower epidermis remained constant during light/dark regime. Turgor pressure maxima were not observed in untreated (C₃) plants.This finding strongly supports the assumption that water movement during malate accumulation and degradation in salttreated plants occurs predominantly between the mesophyll cells and the bladder cells of the upper epidermis. The necessary calculations take differences in the compartment volumes and in the elastic moduli of the cell walls () of the bladder cells of the lower and upper epidermis into account.Measurements of the kinetics of water transport showed that the half-time of water exchange for the two sorts of bladder cells were nearly identical in CAM plants and in C₃ plants. The absolute values of the half-times increased by about 45% in salttreated plants (about 113 sec) compared to the control plants (78 sec). Simultaneously, the half-time of water exchange of the mesophyll cells increased by about 60% from 14 sec (untreated plants) to 22 sec (salt-exposed plants). The leaves of this plant are apparently able to closely maintain the time of propagation of short-term osmotic pressure changes over a large salinity range.A cumulative plot of the data measured on both C₃ and CAM plants showed that the differences between the values of the elastic moduli of bladder cells from the lower and from the upper epidermis are due to differences in volume and suggested that the intrinsic elastic properties of the differently located bladder cells of C₃ and CAM plants were identical.A cumulative plot of the hydraulic conductivity of the membrane obtained both on mesophyll and on bladder cells of salttreated and of untreated plantsvs. the individual turgor pressure yielded a relationship well-known from giant algal cells and some higher plant cells: The hydraulic conductivity increased at very low pressure, indicating that the water permeability properties of the membrane of the various cell types of C₃ and CAM plants are pressure dependent, but otherwise identical.The results suggest that a few fundamental physical relationships control the adaptation of the tissue cells to salinity.     

Radial Turgor and Osmotic Pressure Profiles in Intact and Excised Roots of Aster tripolium: Pressure Probe Measurements and Nuclear Magnetic Resonance-Imaging Analysis

High-resolution nuclear magnetic resonance images (using very short spin-echo times of 3.8 milliseconds) of cross-sections of excised roots of the halophyte Aster tripolium showed radial cell strands separated by air-filled spaces. Radial insertion of the pressure probe (along the cell strands) into roots of intact plants revealed a marked increase of the turgor pressure from the outermost to the sixth cortical layer (from about 0.1-0.6 megapascals). Corresponding measurements of intracellular osmotic pressure in individual cortical cells (by means of a nanoliter osmometer) showed an osmotic pressure gradient of equal magnitude to the turgor pressure. Neither gradient changed significantly when the plants were grown in, or exposed for 1 hour to, media of high salinity. Differences were recorded in the ability of salts and nonelectrolytes to penetrate the apoplast in the root. The reflection coefficients of the cortical cells were approximately 1 for all the solutes tested. Excision of the root from the stem resulted in a collapse of the turgor and osmotic pressure gradients. After about 15 to 30 minutes, the turgor pressure throughout the cortex attained an intermediate (quasistationary) level of about 0.3 megapascals. This value agreed well with the osmotic value deduced from plasmolysis experiments on excised root segments. These and other data provided conclusions about the driving forces for water and solute transport in the roots and about the function of the air-filled radial spaces in water transport. They also showed that excised roots may be artifactual systems.     

Transient Responses of Cell Turgor and Growth of Maize Roots as Affected by Changes in Water Potential

Transient responses of cell turgor (P) and root elongation to changes in water potential were measured in maize (Zea mays L.) to evaluate mechanisms of adaptation to water stress. Changes of water potential were induced by exposing roots to solutions of KCl and mannitol (osmotic pressure about 0.3 MPa). Prior to a treatment, root elongation was about 1.2 mm h-1 and P was about 0.67 MPa across the cortex of the expansion zone (3-10 mm behind the root tip). Upon addition of an osmoticum, P decreased rapidly and growth stopped completely at pressure below approximately 0.6 MPa, which indicated that the yield threshold (Ytrans,1) was just below the initial turgor. Turgor recovered partly within the next 30 min and reached a new steady value at about 0.53 MPa. The root continued to elongate as soon as P rose above a new threshold (Ytrans,2) of about 0.45 MPa. The time between Ytrans,1 and Ytrans,2 was about 10 min. During this transition turgor gradients of as much as 0.15 MPa were measured across the cortex. They resulted from a faster rate of turgor recovery of cells deeper inside the tissue compared with cells near the root periphery. Presumably, the phloem was the source of the compounds for the osmotic adjustment. Turgor recovery was restricted to the expansion zone, as was confirmed by measurements of pressure kinetics in mature root tissue. Withdrawal of the osmoticum caused an enormous transient increase of elongation, which was related to only a small initial increase of P. Throughout the experiment, the relationship between root elongation rate and turgor was nonlinear. Consequently, when Y were calculated from steady-state conditions of P and root elongation before and after the osmotic treatment, Yss was only 0.21 MPa and significantly smaller compared with the values obtained from direct measurements (0.42-0.64 MPa). Thus, we strongly emphasize the need for measurements of short-term responses of elongation and turgor to determine cell wall mechanics appropriately. Our results indicate that the rate of solute flow into the growth zone could become rate-limiting for cell expansion under conditions of mild water stress.     

Xylem and cell turgor pressure probe measurements in intact roots of glycophytes: transpiration induces a change in the radial and cellular reflection coefficients

Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σ_r) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σ_r assumed values of the order of about 0·2–0·4. The σ_r values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m^?2 s^?1; relative humidity 20–40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σ_r= 1. Further increase of the light intensity to about 400 μmol m^?2 s^?1 resulted in a slight but significant decrease of the σ_r values to about 0·8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906–912) that, in intact transpiring plants at low light intensities of about 10 μmol m^?2 s^?1 and at relative humidities of 20–40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σ_r values 0·3–0·5). In contrast to wheat and barley, very high light intensities (about 700 μmol m^?2 s^?1) were needed to shift the radial reflection coefficients of maize roots to values of about 0·9. Osmotically induced xylem pressure changes were apparently linked to changes in turgor pressure in the root cortical parenchyma cells, as shown by simultaneous measurements of xylem and cell turgor pressure. In analogy to the σ_r values of the respective glycophytes, the σ_c values of the root cortical cells of wheat and barley were close to unity, whereas σ_c for maize was significantly smaller (about 0·7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m^?2 s^?1 the cellular reflection coefficient of maize roots increased to about 0·95. In contrast to the σ_r values, the σ_c values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21–31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed.