Effect of Osmotic Stress on Turgor Pressure in Mung Bean Root Cells

Turgor pressure in cells of the elongating region of intactmung bean roots was directly measured by using the pressure-probetechnique. After the external osmotic pressure had been increasedfrom 0 MPa to 0.5 MPa, turgor pressure rapidly decreased byabout 0.5 MPa from 0.65 MPa to 0.14 MPa and root elongationstopped. Subsequent turgor regulation was clearly confirmed,which followed the osmotic adjustment to maintain a constantdifference in the osmotic pressure between root-cell sap andthe external medium ( II). It took at least 6 h for turgor pressureto recover to an adjusted constant level of about 0.5 MPa dueto turgor regulation, but rootelongation resumed within onlyan hour after the osmotic treatment. Therefore, the resumptionof root elongation under osmotic stress could not have beendirectly connected with turgor regulation. Furthermore, sincethe amounts of decrease in turgor pressure just after applicationsof various degrees of osmotic stress could be interpreted inrelation to those in II, hydraulic conductivity between theinside and the outside of root cells must be large enough toattain water potential equilibrium rapidly in response to osmoticstress. We conclude that turgor pressure in the cells of theelongating region of mung bean roots is determined mainly by II because of water potential equilibrium. (Received January 27, 1987; Accepted May 21, 1987)     

Rapid Response of the Yield Threshold and Turgor Regulation during Adjustment of Root Growth to Water Stress in Zea mays

Responses of cortical cell turgor (P) following rapid changes in osmotic pressure ([pi]m) were measured throughout the elongation zone of maize (Zea mays L.) roots using a cell pressure probe and compared with simultaneously measured root elongation to evaluate: yield threshold (Y) (minimum P for growth), wall extensibility, growth-zone radial hydraulic conductivity (K), and turgor recovery rate. Small increases in [pi]m (0.1 MPa) temporarily decreased P and growth, which recovered fully in 5 to 10 min. Under stronger [pi]m (up to 0.6 MPa), elongation stopped for up to 30 min and then resumed at lower rates. Recoveries in P through solute accumulation and lowering of Y enabled growth under water stress. P recovery was as much as 0.3 MPa at [pi]m = 0.6 MPa, but recovery rate declined as water stress increased, suggesting turgor-sensitive solute transport into the growth zone. Under strong [pi]m, P did not recover in the basal part of the growth zone, in conjunction with a 30% shortening of the growth zone. Time courses showed Y beginning to decrease within several minutes after stress imposition, from about 0.65 MPa to a minimum of about 0.3 MPa in about 15 min. The data concerning Y were not confounded significantly by elastic shrinkage. K was high (1.3 x 10-10 m2 s-1 MPa-1), suggesting very small growth-induced water potential gradients.     

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     

Measurement of a growth-induced water potential gradient in tall fescue leaves

Spatial distribution of cell turgor pressure, cell osmotic pressure and relative elemental growth rate were measured in growing tall fescue leaves ( Festuca arundinacea ). Cell turgor pressure (measured with a pressure probe) was c . 0.55 MPa in expanding cells but increased steeply (+0.3 MPa) in cells where elongation had stopped. However, cell osmotic pressure (measured with a picolitre osmometer) was almost constant at 0.85 MPa throughout the leaf. The water potential difference between the growth zone and the mature zone (0.3 MPa) was interpreted as a growth-induced water potential gradient. This and further implications for the mechanism of growth control are discussed.     

Water relations and growth of tomato fruit pericarp tissue

The water relations of young tomato fruit pericarp tissue were examined and related to tissue expansion. The relationship between bulk turgor pressure and tissue expansion (as change in fresh mass or length of tissue) was determined in slices of pericarp cut from young, growing fruit by incubation in different osmotic concentrations of polyethylene glycol 6000 or mannitol. The bulk turgor of this tissue was low (about 0.2 MPa), even in fruit from plants that were otherwise fully turgid, whether measured psychrometrically or by length change in osmotic solutions. The rate of tissue growth at maximum turgor was less than that at moderate turgor unless calcium was added to the incubation medium. However, added calcium also decreased the rate of growth at lower turgor pressures. Yield turgor was < 0.1 MPa, but it was increased by the addition of calcium ions. Electrolyte leakage from tissue was greatest at maximum turgor pressure but was decreased by the addition of calcium ions or osmoticum. Tissue growth was unaffected by a range of plant growth regulators (IAA, abscisic acid, benzyladenine and GA₃) but was inhibited, particularly at high turgor, by low concentrations of malic or citric acid. The low turgor pressure of pericarp tissue could be due to the presence of apoplastic solutes within the pericarp, and evidence for this is discussed. Thus, fruit tissue may be able to maintain optimal expansion rates only at moderate turgor and low calcium concentration.     

Hydraulic propagation of pressure along immature and mature xylem vessels of roots of Zea mays measured by pressure-probe techniques

Turgor pressure was measured in cortical cells and in xylem elements of excised roots and roots of intact plants of Zea mays L. by means of a cell pressure probe. Turgor of living and hence not fully differentiated late metaxylem (range 0.6–0.8 MPa) was consistently higher than turgor of cortical cells (range 0.4–0.6 MPa) at positions between 40 and 180 mm behind the root tip. Closer to the tip, no turgor difference between the cortex and the stele was measured. The turgor difference indicated that late-metaxylem elements may function as nutrient-storage compartments within the stele. Excised roots were attached to the root pressure probe to precisely manipulate the xylem water potential. Root excision did not affect turgor of cortical cells for at least 8 h. Using the cell pressure probe, the propagation of a hydrostatic pressure change effected by the root pressure probe was recorded in mature and immature xylem elements at various positions along the root. Within seconds, the pressure change propagated along both early and late metaxylems. The half-times of the kinetics, however, were about five times smaller for the early metaxylem, indicating they are likely the major pathway of longitudinal water flow. The hydraulic signal dissipated from the source of the pressure application (cut end of the root) to the tip of the root, presumably because of radial water movement along the root axis. The results demonstrate that the water status of the growth zone and other positions apical to 20 mm is mainly uncoupled from changes of the xylem water potential in the rest of the plant.Abbreviations and Symbols CPP cell pressure probe - EMX early metaxylem - LMX Late metaxylem - P_c cell turgor - P_r root pressure - RPP root pressure probe - t_1/2,c half-time of water exchange across a single cell - t_1/2 half-time of water exchange across multiple cells We thank Antony Matista for his expert assistance in the construction and modification of instruments. The work was supported by grant DCB8802033 from the National Science Foundation and grant 91-37100-6671 from USDA, and by the award of a Feodor Lynen-Fellowship from the Alexander von Humboldt-Foundation (Germany) to J.F.     

The biophysical basis of cell elongation and organ maturation in coleoptiles of rye seedlings: implications for shoot development

The relationships between changes in irreversible and reversible organ length, turgor (P), osmotic pressure (pi), and metabolic activity of the cells were investigated in intact coleoptiles of rye seedlings ( Secale cereale L.) that were either grown in darkness or irradiated with continuous white light. Cessation of growth at day 4 after sowing was associated with an apparent mechanical stiffening of the cell walls. Turgor pressure was measured in epidermal and mesophyll cells with a miniaturized pressure probe. No gradient of turgor was found between the peripheral and internal cells. In juvenile (growing) coleoptiles, average turgor was 0.60 MPa and a negative water potential (P - pi) was established in these cells. Upon emergence of the primary leaf, turgor declined, but P was maintained at values of 0.43 and 0.52 MPa in 7-day-old light- and dark-grown coleoptiles, respectively. Water potential in non-growing cells approached zero. The rate of dark respiration and elongation growth were not correlated. Surgical removal of the mature coleoptile revealed that the erect position of the 7-day-old shoot was dependent on the presence of this sturdy, turgid organ sheath. It is concluded that, during the first week of seedling development, the pierced, metabolically active coleoptile fulfills an essential function as an elastic basal tube for the juvenile shoot.     

Analysis of growth, composition and thickness of the cell walls of transgenic tobacco plants expressing a yeast-derived invertase

Summary Transgenic tobacco (Nicotiana tabaccum L. cv. Samsun NN) expressing a yeast invertase in the vacuole provides a novel tool for studying the role of turgor, osmotic pressure, and cell wall properties during cell expansion. The plants used showed increased osmolarity and an increased cell size in young leaves. Their advantage is that they allow long-term analysis and undisturbed conditions. Cell expansion rate was maximal in leaf six of the transgenic plants and in leaf eleven of wild-type plants. Turgor rose to 0.52 ± 0.04 MPa (n=45) and 0.35 ± 0.03 MPa (n=45) in transgenic and wild-type plants, respectively. It was maximal where elongation rates were highest. Thus, elevated cell expansion rate was, at least in part, related to an enhancement in turgor. However, comparison between turgor and relative expansion rates showed that higher turgor pressures were required to achieve similar cell expansion rates in transformed plants as in the wild-type. This finding underlines the importance of the yield threshold and, thus, of the cell wall in growth regulation. This conclusion is further supported by the observation that the cell walls of transgenic plants were up to 77% thicker than the wild-type, but not qualitatively modified.     

Measurement of hyphal turgor

Cellular turgor pressure is thought to provide the driving force for hyphal extension and for a variety of other fungal processes. This study was conducted to evaluate three different approaches to the measurement of hyphal turgor in the aquatic fungus Achlya bisexualis. Turgor was determined indirectly from measurements of the osmotic potential of hyphal extracts using an osmometer and by a refined incipient plasmolysis technique. Turgor was also measured directly from individual growing hyphae using a micropipet-based pressure probe. Osmometry provided an estimate of the mean turgor of hyphae grown in liquid culture of 0.74 MPa, while the incipient plasmolysis technique indicated turgor pressures of between 1.0 and 1.2 MPa (10 to 12 bars). With the pressure probe, turgors ranging from 0.8 to 1.2 MPa were measured from 49 hyphae in the same difined medium. The low turgor estimates from the osmometric approach probably reflected dilution of the cell contents by cell wall and extracellular fluid during sample extraction. Recordings with the pressure probe showed that turgor did not vary along the length of the coenocytic hyphae and was independent of hyphal diameter. This paper presents the first report of the direct measurement of hyphal turgor pressure.     

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.     

A Comparison of Methods for Measuring Turgor Pressures and Osmotic Pressures of Cells of Red Beet Storage Tissue

Two methods for measuring the turgor pressures of cells in discsof storage tissue of red beet (Beta vulgaris L.) were compared,and a centrifugation method for extracting sap from frozen andthawed tissue was evaluated. Turgor pressures were measureddirectly using a pressure probe, or indirectly using a vapourpressure osmometer. With the latter, discs were placed directlyin the osmometer chamber and turgor was calculated as the differencein osmotic pressure before and after freezing and thawing. Turgorin freshly cut discs, measured with the pressure probe, wasbetween 0-012 MPa and 0.118 MPa with a mean ±s.d. of0.092±;0.032 MPa (n = 24). That measured with the osmometervaried between 0.08 MPa and 0.12 MPa with a mean ±s.d.of 0.09±0.10 MPa (n = 54). After vacuum infiltrationof discs with distilled water, the turgor measured with thepressure probe increased to 1.05–1.12 MPa. Turgor measuredwith the osmometer also increased after vacuum infiltrationbut was, on average, 12% lower than that measured with the pressureprobe. Overall, the results suggest that for routine measurements,the osmometer can provide reasonable estimates of the turgorof cells in beet discs. This is because a number of factorsthat, potentially, could interfere with this method have onlya small effect in this tissue. None of the measured turgorsis indicative of that occurring in intact storage roots becauseboth excision and vacuum infiltration of discs alter the concentrationsof solutes in the extracellular space. The osmotic pressureof sap extracted by centrifugation from frozen and thawed discswas not significantly different from that measured by placingfrozen and thawed discs directly in the osmometer. Solute concentrationsin the sap were not significantly different from those measuredby chemical extraction of discs. Key words: Beta vulgaris, Osmotic pressure, Turgor pressure     

Regulation of Turgor Pressure by Suspension-Cultured Tobacco Cells

Turgor regulation and effects of high NaCl and water deficiton growth and internal solutes were studied after transferringtobacco cells from control culture medium (osmotic pressure= 0.13–0.15 MPa at time of transfer) to culture mediumcontaining either 82 mol m^–3 NaCl or 150 mol m^–3melibiose (osmotic pressure of media = 0.62 MPa). Followingtransfer to media with higher osmotic pressure, expansion rateand turgor pressure were reduced. Within 24 h of imposing thewater deficit, expansion rate had returned to that of cellsin control culture medium. However, by 24 h, turgor pressurehad only risen from 0.2 MPa to 0.65 MPa in the NaCl treatmentand to 0.53 MPa in the melibiose treatment, while it was 0.73MPa in the control treatment. Furthermore, turgor pressure remainedwithin 0.05 MPa of these respective values for the rest of the(75 h) experiment. These results suggest differences in bothcell wall properties (extensibility and/or threshold turgor)and the level at which turgor is maintained for cells in thevarious treatments. Solutes contributing nearly all (82–97%) of the osmoticpressure in cells were identified. The initial (up to 24 h)increases in turgor pressure were mainly due to increases insolute concentrations caused by relatively slow expansion rates.However, increased Na⁺ and Cl ^– uptake contributed toincreased turgor pressure in the NaCl treatment and caused turgorpressure of cells in this treatment to increase faster thanin the melibiose treatment. Likewise, expansion rate rose morequickly in the NaCl than in the melibiose treatment. After 24h, maximum expansion rate was reached and concentrations ofmost internal solutes began to decrease. Nevertheless, turgorpressure remained relatively constant. The constancy of turgorpressure was due to increased glucose uptake rates relativeto controls, with consequent increases in concentrations ofsucrose, glucose and fructose and, in cells in the melibiosetreatment, of organic acids. Glucose uptake was slower in theNaCl than in the melibiose treatment but higher turgor pressurewas maintained in the NaCl treatment due to high uptake of Na⁺and Cl^–. Glucose uptake appears to respond to a systemof turgor regulation, but further experiments are required toconfirm this and to determine whether Na⁺ and Cl^– uptakealso respond to a system of turgor regulation. Key words: Salinity, water deficit, growth     

Seasonal variation in the tissue water relations of Picea glauca

Seasonal variation in water relations of 3-yearold white spruce (Picea glauca (Moench) Voss) shoots, monitored with pressure-volume curves over 28 months, was closely related to shoot phenology and was sensitive to environmental fluctuations during both summer growth and winter dormancy. Turgor maintenance capacity was lowest during rapid shoot elongation from late May to early July; this was indicated by the lowest total turgor pressures, the highest (least negative) osmotic potentials at full turgor and the turgor loss point, the smallest differences between osmotic potentials at full turgor and the turgor loss point, the highest relative water contents at turgor loss and a linear decline in cell elasticity with decreasing turgor pressure. This suggests that the high susceptibility of white spruce seedlings to growth check after transplanting is largely attributable to the poor turgor maintenance capacity of this species in early summer.     

Turgor regulation in hyphal organisms

Turgor regulation in two saprophytic hyphal organisms was examined directly with the pressure probe technique. The ascomycete Neurospora crassa, a terrestrial fungi, regulates turgor after hyperosmotic treatments when growing in a minimal medium containing K(+), Mg(2+), Ca(2+), Cl(-), and sucrose. Turgor recovery by N. crassa after hyperosmotic treatment is concurrent with changes in ion transport: hyperpolarization of the plasma membrane potential and a decline in transmembrane ion conductance. In contrast the oomycete Achlya bisexualis, a freshwater hyphal organism, does not regulate turgor after hyperosmotic treatment, although small transient increases in turgor were occasionally observed. We also monitored turgor in both organisms during hypoosmotic treatment and did not observe a turgor increase, possibly due to turgor regulation. Both hyphal organisms grow with similar morphologies, cellular expansion rates and turgor (0.4-0.7 MPa), yet respond differently to osmotic stress. The results do not support the assumption of a universal mechanism of tip growth driven by cell turgor.     

Pressure probe study of the water relations of Phycomyces blakesleeanus sporangiophores

The physical characteristics which govern the water relations of the giant-celled sporangiophore of Phycomyces blakesleeanus were measured with the pressure probe technique and with nanoliter osmometry. These properties are important because they govern water uptake associated with cell growth and because they may influence expansion of the sporangiophore wall. Turgor pressure ranged from 1.1 to 6.6 bars (mean = 4.1 bars), and was the same for stage I and stage IV sporangiophores. Sporangiophore osmotic pressure averaged 11.5 bars. From the difference between cell osmotic pressure and turgor pressure, the average water potential of the sporangiophore was calculated to be about -7.4 bars. When sporangiophores were submerged under water, turgor remained nearly constant. We propose that the low cell turgor pressure is due to solutes in the cell wall solution, i.e., between the cuticle and the plasma membrane. Membrane hydraulic conductivity averaged 4.6 x 10(-6) cm s-1 bar-1, and was significantly greater in stage I sporangiophores than in stage IV sporangiophores. Contrary to previous reports, the sporangiophore is separated from the supporting mycelium by septa which prevent bulk volume flow between the two regions. The presence of a wall compartment between the cuticle and the plasma membrane results in anomalous osmosis during pressure clamp measurements. This behavior arises because of changes in solute concentration as water moves into or out of the wall compartment surrounding the sporangiophore. Theoretical analysis shows how the equations governing transient water flow are altered by the characteristics of the cell wall compartment.     

Pressure probe and isopiestic psychrometer measure similar turgor

Turgor measured with a miniature pressure probe was compared to that measured with an isopiestic thermocouple psychrometer in mature regions of soybean (Glycine max [L.] Merr.) stems. The probe measured turgor directly in cells of intact stems whereas the psychrometer measured the water potential and osmotic potential of excised stem segments and turgor was calculated by difference. When care was taken to prevent dehydration when working with the pressure probe, and diffusive resistance and dilution errors with the psychrometer, both methods gave similar values of turgor whether the plants were dehydrating or rehydrating. This finding, together with the previously demonstrated similarity in turgor measured with the isopiestic psychrometer and a pressure chamber, indicates that the pressure probe provides accurate measurements of turgor despite the need to penetrate the cell. On the other hand, it suggests that as long as precautions are taken to obtain accurate values for the water potential and osmotic potential, turgor can be determined by isopiestic psychrometry in tissues not accessible to the pressure probe for physical reasons.     

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.     

Effects of water stress on growth, osmotic potential and abscisic acid content of maize roots

Under water stress conditions, induced by mannitol solutions (0 to 0.66 M ) applied to the apical 12 mm of intact roots of Zea mays L. (cv. LG 11), a growth inhibition, a decrease in the osmotic potential of the cell sap and a significant accumulation of abscisic acid (ABA) were observed. When the roots were placed in a humid atmosphere after the stress, the growth rate increased again, even if elongation had been totally inhibited. Under a stress corresponding to an osmotic potential of -1.09 MPa in the solution, growth was totally inhibited, which means that the root cell turgor pressure was reduced to the yield threshold. These conditions led to the largest accumulation of ABA. The effect of water stress on the level of ABA was studied for three parts of the root. The greatest increase in ABA (about 10 fold) was obtained in the growth zone and this increase was apparently independent of the hydrolysis of the conjugated form. With a mannitol treatment of 1 h equivalent to a stress level of -1.39 MPa, a 4-fold increase in ABA efflux into the medium was obtained. These results suggest that there are interactions between water stress, root growth, osmotic potential and the ABA level. The growth under conditions of stress and the role of endogenous ABA in the control of plant metabolism, specially in the growth zone, are discussed.     

Direct and indirect measurements of Phloem turgor pressure in white ash

Direct determinations and indirect calculations of phloem turgor pressure were compared in white ash (Fraxinus americana L.). Direct measurements of trunk phloem turgor were made using a modified Hammel-type phloem needle connected to a pressure transducer. Turgor at the site of the direct measurements was calculated from the osmotic potential of the phloem sap and from the water potential of the xylem. It was assumed that the water potentials of the phloem and xylem were close to equilibrium at any one trunk location, at least under certain conditions. The water potential of the xylem was determined from the osmotic potential of xylem sap and from the xylem tension of previously bagged leaves, measured with a pressure chamber. The xylem tension of bagged leaves on a branch adjacent to the site of the direct measurements was considered equivalent to the xylem tension of the trunk at that point. While both the direct and indirect measurements of phloem turgor showed clear diurnal changes, the directly measured pressures were consistently lower than the calculated values. It is not clear at present whether the discrepancy between the two values lies primarily in the calculated or in the measured pressures, and thus, the results from both methods as described here must be regarded as estimates of true phloem turgor.     

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.