Pressure Probe Technique Research Articles

A kinetic model for turgor loss in red cabbage cells during mild heat treatment

Summary Turgor changes in red cabbage leaves (Brassica oleracea L. covar. Capitata Alef. var. rubra DC.) due to mild heat treatments were measured in the epidermal and first subepidermal cell layers with the pressure probe technique. After an equilibration time in a sorbitol solution of equal water potential, cabbage discs were immersed in solutions heated to 53 °C and 65 °C for different time intervals. Turgor loss at both temperatures could be described by a first order degradation model. This could be due to the increased permeability of membranes at higher temperatures, resulting in passive efflux of solutes and an irreversible reduction of cell turgor well before membranes are completely destroyed. The residual turgor (at t = ∞) for the 53 °C treatment was significantly different from zero. Assuming an Arrhenius type model for the temperature dependency of the rate constant k, the activation energy (Ea = 41 kJ · mol-1) and reference rate constant (kref = 1.53 min-1 for a reference temperature Tref = 323 K) were calculated for the turgor loss model in the epidermal cell layer. In the second cell layer, a time shift of the commencement of turgor decrease could be observed at the lower processing temperature. Temperature profiles at different locations in the cabbage disc were calculated by use of a finite element model and related to turgor loss observations.

A patch-clamp study of ion channels in protoplasts prepared from the marine alga Valonia utricularis.

The giant marine alga Valonia utricularis is a classical model system for studying the electrophysiology and water relations of plant cells by using microelectrode and pressure probe techniques. The recent finding that protoplasts can be prepared from the giant "mother cells" (Wang, J., Sukhorukov, V.L., Djuzenova, C.S., Zimmermann, U., Müller, T., Fuhr, G., 1997, Protoplasma 196:123-134) allowed the use of the patch-clamp technique to examine ion channel activity in the plasmalemma of this species. Outside-out and cell-attached experiments displayed three different types of voltage-gated Cl(-) channels (VAC1, VAC2, VAC3, Valonia Anion Channel 1,2,3), one voltage-gated K(+) channel (VKC1, Valonia K(+) Channel 1) as well as stretch-activated channels. In symmetrical 150 mm Cl(-) media, VAC1 was most frequently observed and had a single channel conductance of 36 +/- 7 pS (n = 4) in the outside-out and 33 +/- 5 pS (n = 10) in the cell-attached configuration. The reversal potential of the corresponding current-voltage curves was within 0 +/- 4 mV (n = 4, outside-out) and 9 +/- 7 mV (n = 10, cell-attached) close to the Nernst potential of Cl(-) and shifted towards more negative values when cell-attached experiments were performed in asymmetrical 50:150 mm Cl(-) media (bath/pipette; E(Cl(-)) -20 +/- 7 mV (n = 4); Nernst potential -28 mV). Consistent with a selectivity for Cl(-), VAC1 was inhibited by 100 micronM DIDS (4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid). VAC1 was activated by a hyperpolarization of the patch. Boltzmann fits of the channel activity under symmetrical 150 mm Cl(-) conditions yielded a midpoint potential of -12 +/- 5 mV (n = 4, outside-out) and -3 +/- 6 mV (n = 9, cell-attached) and corresponding apparent minimum gating charges of 15 +/- 3 (n = 4) and 18 +/- 5 (n = 9). The midpoint potential shifted to more negative values in the presence of a Cl(-) gradient. VAC2 was activated by voltages more negative than E(Cl(-)) and was always observed together with VAC1, but less frequently. It showed a "flickering" gating. The single channel conductance was 99 +/- 10 pS (n = 6). VAC3 was activated by membrane depolarization and frequently exhibited several subconductance states. The single channel conductance of the main conductance state was 36 +/- 5 pS (n = 5). VKC1 was also activated by positive clamped voltages. Up to three conductance states occurred whereby the main conductance state had a single channel conductance of 124 +/- 27 pS (n = 6). In the light of the above results it seems to be likely that VAC1 contributes mainly to the Cl(-) conductance of the plasmalemma of the turgescent "mother cells" and that this channel (as well as VAC2) can operate in the physiological membrane potential range. The physiological significance of VAC3 and VKC1 is unknown, but may be related (as the stretch-activated channels) to processes involved in turgor regulation.

Development of a Nearly Omnidirectional Velocity Measurement Pressure Probe

The development of a nearly omnidirectional pressure probe for three-velocity-component and pressure measurements is described, focusing on the techniques employed in probe fabrication, calibration, and frequencyresponse study. The probe tip is a sphere with 18 pressure ports properly distributed over the spherical surface. The device eliminates the velocity directionality limitations of current multihole probes and constitutes a rugged tool for use in complex three-dimensional e ow mapping. An automated calibration system was used to generate a probe calibration database of approximately 10,000 individual probe orientations. Least-squares-based and neural-network-based calibration algorithms were developed. The reliability of the calibration procedures and algorithms is demonstrated e rst. Then probe utility is demonstrated in a e owe eld with e ow reversal, downstream of a backward-facing step. Last, a study of the probe frequency response is presented. I. Introduction T HE design, evaluation, and optimization of complex aerodynamicgeometriesinvolveextensivewind-tunneltestingand/or computationally intensive numerical simulations. Even in the latter case, high-quality experimental wind-tunnel results with minimal, quantie able errors are still necessary for code-validation purposes. Moreover, in aerodynamic testing facilities where large volumes of data need to be acquired in tight schedules, downtime due to poor instrumentation performance is highly undesirable. Such facilities include industrial testing wind tunnels, as well as high-productivity computational e uid dynamics code validation facilities. 1 In such environments, e ow measurement techniques such as laser Doppler velocimetry and particle image velocimetry, although powerful, usually require painstaking efforts toward their successful usage. Costly components; complex setups; troublesome e ow seeding requirements; lack of e exibility, ruggedness, and mobility; and ease of misalignment often render such techniques impractical. Moreover, in testing of complex three-dimensional geometries, accessibility of the entire e owe eld around the model is an essential issue. When employing optical techniques, large sections of the e owe eld are obstructed optically by the presence of the model. To access such regions, repositioning of the instrumentation setup is necessary, a time-consuming process with the associated potential pitfalls. Multihole pressure probes 2‐7 in many cases have provided the easiest-to-use and most cost-effective method for three-component e ow velocity measurements in research and industry environments. However, even with expanded measurement capabilities of such instruments, the current pressure probe cone gurations and techniques have a limited range of velocity inclinations that they can measure. The velocity inclination is indicated as the cone angle µ in Fig. 1. Let µmax be the maximum cone angle that can be measured reliably by the probe. In other words, a probe with a µmax of 40 deg can accurately measure any velocity vector that is contained within a cone with its apex at the probe tip, its axis along the probe axis, and with an apex included angle of 80 deg (Fig. 1). Measurable cone angles as high as 75 deg are not uncommon for seven-hole probes. 7 Following this reasoning, an omnidirectional probe is a probe that has a cone angle of 180 deg (360 deg included angle ); i.e., it can measure any velocity vector regardless of its orientation. The present work describes the development of a nearly omnidirectional probe that can measure up to cone angles of 170 deg

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.

A study of stomatal mechanics using the cell pressure probe

The relationship between stomatal aperture (a) and guard cell pressure (Pg) was measured directly in four different species (Vicia faba, Tradescantia virginiana, Ginkgo biloba and Nephrolepis exaltata) using a special cell pressure probe technique. The effect of epidermal turgor (Pep) on this relationship was also measured in T. virginiana. The relationship was sigmoidal for V. faba and T. virginiana, but entirely convex for G. biloba and N. exaltata. Epidermal turgor was found to have a pronounced closing effect on stomata of T. virginiana. Maximum aperture with full epidermal turgor (0·92 MPa) was about half that with zero epidermal turgor. Also, with full epidermal turgor stomata of T. virginiana did not begin to open until Pg was more than 1·25 MPa. These characteristics were used to develop an expression for a as a function of Pg and Pep. Results for the different species are compared and discussed in terms of possible advantages and limitations of water economy.

Water Transport Properties of Roots and Root Cortical Cells in Proton- and Al-Stressed Maize Varieties.

Root and root cell pressure-probe techniques were used to investigate the possible relationship between Al- or H+-induced alterations of the hydraulic conductivity of root cells (LPc) and whole-root water conductivity (LPr) in maize (Zea mays L.) plants. To distinguish between H+ and Al effects two varieties that differ in H+ and Al tolerance were assayed. Based on root elongation rates after 24 h in nutrient solution of pH 6.0, pH 4.5, or pH 4.5 plus 50 [mu]M Al, the variety Adour 250 was found to be H+-sensitive and Al-tolerant, whereas the variety BR 201 F was found to be H+-tolerant but Al-sensitive. No Al-induced decrease of root pressure and root cell turgor was observed in Al-sensitive BR 201 F, indicating that Al toxicity did not cause a general breakdown of membrane integrity and that ion pumping to the stele was maintained. Al reduced LPc more than LPr in Al-sensitive BR 201 F. Proton toxicity in Adour 250 affected LPr more than LPc. In this Al-tolerant variety LPc was increased by Al. Nevertheless, this positive effect on LPc did not render higher LPr values. In conclusion, there were no direct relationships between Al- or H+-induced decreases of LPr and the effects on LPc. To our knowledge, this is the first time that the influence of H+ and Al on root and root cell water relations has been directly measured by pressure-probe techniques.

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

Xylem pressure and its relative response to the imposition of an external osmotic stress (the so-called radial reflection coefficient) were recorded in roots of intact maize plants using the xylem pressure probe technique. Consecutive insertion of two probes into the same xylem vessel or into adjacent vessels of intact roots of plants exposed to high light intensity and salt stress under laboratory conditions showed that the xylem tension was not changed by vessel probing. It was also shown by using the double probe approach that the plants were capable of overcoming artificially induced leakages. This and other evidence reported in the literature convincingly demonstrated that the probe accurately reads xylem pressure and xylem pressure responses to osmotic stress. Additional experiments were performed on plants grown in a greenhouse at a subtropical latitude. Under these conditions the plants were exposed to strong diurnal fluctuations in light intensity, relative humidity and temperature. The results showed that the absolute xylem pressure in the roots of untreated plants decreased with increasing transpiration rate from positive values in the early morning to negative values around noon (average value -0.15 MPa; maximum negative value -0.57 MPa). As the day progressed and the transpiration rate decreased, xylem pressure increased again to positive values. Correspondingly, the radial reflection coefficient for NaCI increased from about zero in the early morning to about unity at noon when transpiration reached its highest value and decreased again to very low values towards the evening. The data raise questions concerning conclusions about the mechanism of water transport in intact roots drawn from the low radial reflection coefficients measured on excised roots using the root pressure probe.

In-flight Airflow Measurements of a Cessna AgHusky Agricultural Aircraft

A measurement system based on pressure probe techniques was developed and used to survey airflow speed and direction near the trailing edge of the wings and under the fuselage of a Cessna AgHusky agricultural aircraft. The main feature of the system was an airflow measuring rake comprised of eight, 5-tube probes designed to measure total pressure and airflow direction and two stream pressure probes to measure static pressure. The probes were encased in a 0.74-m long airfoil that was positioned vertically at numerous lateral positions 0.23 m behind the trailing edge of the aircraft wing and horizontally at several locations beneath the aircraft fuselage. The test flight protocol included three aircraft loadings (0-, 227-, and 454-kg hopper loads) and two airspeeds (145 and 185 km/h) at an aircraft wheel height of 1.5 m. The measurements indicated that air velocity, airflow yaw angle, and airflow pitch angle were all significantly affected by variations in aircraft loading, aircraft speed, and measurement locations. The air velocity ranged up to 195 and 235 km/h in the propeller slipstream at indicated airspeeds of 145 and 185 km/h, respectively. Velocities of 64 to 97 km/h were recorded near the underside of the fuselage where the airflow was blocked by spray system hardware. Outboard from the propeller slipstream, the air velocities were lowest in the area directly behind the trailing edge of the wings and highest either 0.203 m above or 0.305 to 0.475 m below the trailing edges of the wings. Airflow yaw and pitch angles were largest for the 145 km/h indicated airspeed with the 454-kg load and in the area below the fuselage and aft of the spray system hardware. Outboard of the propeller slipstream, the yaw angles did not exceed 6. The airflow pitch angles ranged up to 7 at the wing measurement locations with the largest downward angles occurring 0.102 to 0.203 m above the wings and the largest upward angles occurring directly behind the trailing edges of the wings. These results provide a basis for selecting and locating spray delivery equipment on agricultural aircraft in order to minimize the effects of airflow on the production of small droplets that are most subject to spray drift.

System for In-flight Airflow Measurements of an Agricultural Aircraft

Airspeed and the orientation of spray nozzles relative to air direction significantly affect spray droplet size. A measurement system based on pressure probe techniques was developed to measure and record airspeed and airflow direction at spray boom and nozzle locations near the trailing edge of the wings and under the fuselage of a fixed-wing agricultural aircraft. The main feature of the system is an airflow measuring “rake” comprised of eight total pressure five-tube probes and two stream pressure probes. The probes are encased in an airfoil. A multiport rotary pneumatic valve is used to rapidly sample the differential (velocity) pressure between the total pressures and stream pressure. The system was calibrated in a wind tunnel for pitch and yaw angles up to ± 25°. Software was developed to control the rotary valve and data acquisition. The system was mounted on a Cessna AgHusky agricultural aircraft for flight testing and wake survey studies. A pitch, yaw, and Pitot tube boom extended forward from the leading edge of the aircraft wingtip provides aircraft attitude and airspeed reference for the rake measurement system. Data collected with this system will provide a basis for adjusting application hardware to obtain improved control of spray droplet size for various aircraft speeds and loadings.

How Does Water Ascend in Tall Trees and Other Vascular Plants?

Since the Cohesion Theory was first introduced, a series of incompatible observations has appeared in the literature. Direct measurements of xylem pressure in single vessels of higher plants and tall trees by means of the xylem pressure probe technique indicate that xylem tension in the leaves of intact, transpiring plants is often much smaller than that predicted for transpiration-driven water ascent through continuous water columns. We conclude that the available evidence warrants a critical reappraisal of tension-driven water transport as the exclusive mechanism of long-distance water transport in plants.

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.

Direct Measurements of the Pressure and Flow in the Xylem Vessels of Nicotiana tabacum and their Dependence on Flow Resistance and Transpiration Rate

AbstractThe relationships between xylem tension, velocity of water ascending and transpiration in tobacco plants were measured by means of the “xylem pressure probe technique” (Balling, A. and Zimmermann, U., Planta 182, 325–338, 1990). The flow velocity was determined by suction or injection of fluorescein (or FITC‐labelled dextrans of various molecular weights) from the microcapillary of the pressure probe into the punctured xylem vessel, followed by serial‐sectioning of the stem after a given propagation time. The distance travelled was defined as the distance from the injection point to the uppermost xylem section in which the dye could be detected. For a transpiration rate of 0.52 ± 0.12 ml . h−1, a linear dependence between the flow velocity and the tension gradients was found as expected from the Hagen‐Poiseuille law. The slope of the straight lines decreased with increasing molecular weight of the fluorescent labelled compound, presumably because of (partial) plugging of the pit membranes. The average value of the flow velocity (2.5 . 10−4 ± 0.9 . 10−4 m . s−1) was one magnitude smaller than the value estimated from the geometric dimensions of the xylem vessels, but agreed well with the literature value of 2.8 . 10−4 m . s−1 for herbs (determined by the heat pulse technique; Huber, B. Ber. deutsch. bot. Ges. 50, 89–109, 1932). The average pressure gradient was determined to be 0.39 ± 0.23 MPa . m−1, in agreement with the literature (Begg, J. E. and Turner, N. C. Plant Physiol. 46, 343–346, 1970).The first response of xylem pressure (or tension) and of flow velocity to a reduction of the transpiration rate (0.14 ± 0.06 ml . h−1) occurred after about 24 h, when an increase of the xylem pressure towards higher values associated with a decrease in flow velocity was observed. In contrast, re‐establishment of the normal transpiration rate brought the pressure (or tension) and the flow velocity back to normal values within half an hour.Similary, introduction of a transverse cross‐sectional cut into the stem did not lead during the first 10 h to a change in xylem tension (or velocity). However, during the following day the pressure fell to relatively low values (about −0.13 MPa). The velocity increased 10‐fold. In the next two days the xylem pressure increased again to normal values (average +0.03 MPa), whereas the flow velocity assumed higher values than normal.The data are discussed in terms of the water status and storage of the adjacent tissue cells.

Developmental changes in cell and tissue water relations parameters in storage parenchyma of sugarcane.

The osmotic pressure of the cell sap of stalk storage parenchyma of sugarcane (Saccharum spp. hybrids) increases by an order of magnitude during ontogeny to reach molar concentrations of sucrose at maturity. Stalk parenchyma cells must either experience very high turgor at maturation or have an ability to regulate turgor. We tested this hypothesis by using pressure probe techniques to quantify parameters of cell and tissue water relations of sugarcane storage parenchyma during ontogeny. The largest developmental change was in the volumetric elastic modulus, which increased from 6 bars in immature tissue to 43 bars in mature tissue. Turgor was maintained relatively low during sucrose accumulation by the partitioning of solutes between the cell and wall compartments. Membrane hydraulic conductivity decreased from about 12 x 10(-7) centimeters per second per bar down to 4.4 x 10(-7) centimeters per second per bar. The 2.7-fold decrease in membrane hydraulic conductivity during tissue maturation was accompanied by a 7.8-fold increase in wall elasticity. Integration of the cell wall and membrane properties appears to be by the opposing effects of turgor on hydraulic conductivity and elastic modulus. The changes in these properties during development of sugarcane stalk tissue may be a way for parenchyma cells to develop a capacity for expansive growth and still serve as a strong sink for storing high concentrations of sucrose.

Photoinhibition of stem elongation by blue and red light: effects on hydraulic and cell wall properties.

The underlying mechanism of photoinhibition of stem elongation by blue (BL) and red light (RL) was studied in etiolated seedlings of pea (Pisum sativum L. cv Alaska). Brief BL irradiations resulted in fast transient inhibition of elongation, while a delayed (lag approximately 60 minutes) but prolonged inhibition was observed after brief RL. Possible changes in the hydraulic and wall properties of the growing cells during photoinhibition were examined. Cell sap osmotic pressure was unaffected by BL and RL, but both irradiations increased turgor pressure by approximately 0.05 megapascal (pressure-probe technique). Cell wall yielding was analyzed by in vivo stress relaxation (pressure-block technique). BL and RL reduced the initial rate of relaxation by 38 and 54%, while the final amount of relaxation was decreased by 48 and 10%, respectively. These results indicate that RL inhibits elongation mainly by lowering the wall yield coefficient, while most of the inhibitory effect of BL was due to an increase of the yield threshold. Mechanical extensibility of cell walls (Instron technique) was decreased by BL and RL, mainly due to a reduction in the plastic component of extensibility. Thus, photoinhibitions of elongation by both BL and RL are achieved through changes in cell wall properties, and are not due to effects on the hydraulic properties of the cell.

Modification of the pressure-probe technique permits controlled intracellular microinjection of fluorescent probes

ABSTRACT The pressure probe has been widely used to study the water relations of plant cells. Here we describe a simple modification of the pressure-probe technique that permits the controlled microinjection of fluorescent probes into plant cells while simultaneously measuring cell turgor pressure. Using the pressure probe, less than 1 nl of the membrane-impermeant fluorescent dye Lucifer Yellow CH was introduced into micropipettes and subsequently injected into leaf trichome cells of Nicotiana clevelandii. Disruption of cell contents could be minimized by raising the hydrostatic pressure in the probe prior to impalement to a value approaching the anticipated cell turgor pressure. Injections to the cytosol resulted in intercellular symplastic transport of the dye in both acropetal and basipetal directions. In contrast, no symplastic transport was observed following an injection of dye into the vacuole. As measured with the pressure probe, cell turgor pressures were in the range 0.18 to 0.36 MPa; the half-time for water exchange across the cell boundary was approximately 10 s. The potential of this technique for the study of turgor-pressure-dependent intercellular transport and the hydraulic conductivities of the tonoplast, plasmalemma and plasmodesmata is discussed.

Auxin Changes Both the Extensibility and the Yield Threshold of the Cell Wall of <italic>Vigna</italic> Hypocotyls

Elongation of plant stem is governed by two simultaneous processes: irreversible yielding of the cell wall and uptake of water. Among many candidates for the parameters that redulate and/or restrict growth, we focused on the mechanical properties of the cell wall and determined those parameters that govern of IAA-induced growth by means of the pressure-jump method combined with the pressure-probe technique

Water Transport across Maize Roots

A double pressure probe technique was used to measure simultaneously water flows and hydraulic parameters of individual cells and of excised roots of young seedlings of maize (Zea mays L.) in osmotic experiments. By following initial flows of water at the cell and root level and by estimating the profiles of driving forces (water potentials) across the root, the hydraulic conductivity of individual cell layers was evaluated. Since the hydraulic conductivity of the cell-to-cell path was determined separately, the hydraulic conductivity of the cell wall material could be evaluated as well (Lp(cw) = 0.3 to 6.10(-9) per meter per second per megapascal). Although, for radial water flow across the cortex and rhizodermis, the apoplasmic path was predominant, the contribution of the hydraulic conductance of the cell-to-cell path to the overall conductance increased significantly from the first layer of the cortex toward the inner layers from 2% to 23%. This change was mainly due to an increase of the hydraulic conductivity of the cell membranes which was Lp = 1.9.10(-7) per meter per second per megapascal in the first layer and Lp = 14 to 9.10(-7) per meter per second per megapascal in the inner layers of the cortex. The hydraulic conductivity of entire roots depended on whether hydrostatic or osmotic forces were used to induce water flows. Hydrostatic Lp(r) was 1.2 to 2.3.10(-7) per meter per second per megapascal and osmotic Lp(r) = 1.6 to 2.8.10(-8) per meter per second per megapascal. The apparent reflection coefficients of root cells (sigma(s)) of nonpermeating solutes (KCI, PEG 6000) decreased from values close to unity in the rhizodermis to about 0.7 to 0.8 in the cortex. In all cases, however, sigma(s) was significantly larger than the reflection coefficient of entire roots (sigma(sr)). For KCI and PEG 6000, sigma(sr) was 0.53 and 0.64, respectively. The results are discussed in terms of a composite membrane model of the root.

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.

Gravitropism of cucumber hypocotyls: biophysical mechanism of altered growth.

The biophysical basis for the changes in cell elongation rate during gravitropism was examined in aetiolated cucumber (Cucumis sativus L.) hypocotyls. Bulk osmotic pressures on the two sides of the stem and in the epidermal cells were not altered during the early time course of gravitropism. By the pressure-probe technique, a small increase in turgor (0.3 bar, 30 kPa) was detected on the upper (inhibited) side, whereas there was a negligible decrease in turgor on the lower (stimulated) side. These small changes in turgor and water potential appeared to be indirect, passive consequences of the altered growth and the small resistance for water movement from the xylem, and indicated that the change in growth was principally due to changes in wall properties. The results indicate that the hydraulic conductance of the water-transport pathway was large (.25 h-1 bar-1) and the water potential difference supporting cell expansion was no greater than 0.3 bar (30 kPa). From pressure-block experiments, it appeared that upon gravitropic stimulation (1) the yield threshold of the lower half of the stem did not decrease and (2) the wall on the upper side of the stem was not made more rigid by a cross-linking process. Mechanical measurements of the stress/strain properties of the walls showed that the initial development of gravitropism did not involve an alteration of the mechanical behaviour of the isolated walls. Thus, gravitropism in cucumber hypocotyls occurs principally by an alteration of the wall relaxation process, without a necessary change in wall mechanical properties.

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

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.