Enlargement in chara studied with a turgor clamp : growth rate is not determined by turgor

A new method, the turgor clamp, was developed to test the effects of turgor on cell enlargement. The method used a pressure probe to remove or inject cell solution and change the turgor without altering the external environment of the cell walls. After the injections, the cells were permanently at the new turgor and required no further manipulation. Internode cells of Chara corallina grew rapidly with the pressure probe in place when growth was monitored with a position transducer. Growth-induced water potentials were negligible and turgor effects could be studied simply. As turgor was decreased, there was a threshold below which no growth occurred, and only reversible elastic/viscoelastic changes could be seen. Above the threshold, growth was superimposed on the elastic/viscoelastic effects. The rate of growth did not depend on turgor. Instead, the rate was highly dependent on energy metabolism as shown by inhibitors that rapidly abolished growth without changing the turgor. However, turgors could be driven above the maximum normally attainable by the cell, and these caused growth to respond as though plastic deformation of the walls was beginning, but the deformation caused wounding. Growth was inhibited when turgor was changed with osmotica but not inhibited when similar changes were made with the turgor clamp. It was concluded that osmotica caused side effects that could be mistaken for turgor effects. The presence of a turgor threshold indicates that turgor was required for growth. However, because turgor did not control the rate, it appears incorrect to consider the rate to be determined by a turgor-dependent plastic deformation of wall polymers. Instead, above the turgor threshold, the rapid response to energy inhibitors suggests a control by metabolic reactions causing synthesis and/or extension of wall polymers.     

Effect of Membrane Tension on Gap Junctional Conductance of Supporting Cells in Corti's Organ

The effects of turgor pressure-induced membrane tension on junctional coupling of Hensen cell isolates from the inner ear were evaluated by input capacitance or transjunctional conductance measurement techniques. Turgor pressure was altered by changing either pipette pressure or the osmolarities of extracellular solutions. Both positive pipette pressure and extracellular applications of hypotonic solutions, which caused cell size to concomitantly increase, uncoupled the cells as indicated by reduced input capacitance and transjunctional conductance. These changes were, in many cases, reversible and repeatable. Intracellular application of 50 μM H-7, a broad-based protein kinase inhibitor, and 10 mM BAPTA did not block the uncoupling effect of positive turgor pressure on inner ear gap junctions. The transjunctional conductance at a holding potential of −80 mV was 53.6 ± 5.8 nS (mean ± SEM, n = 9) and decreased ∼40% at a turgor pressure of 1.41 ± 0.05 kPa. Considering the coincident kinetics of cell deformation and uncoupling, we speculate that mechanical forces work directly on gap junctions of the inner ear. These results suggest that pathologies that induce imbalances in cochlear osmotic pressure regulation may compromise normal cochlear homeostasis.     

Origins of Escherichia coli Growth Rate and Cell Shape Changes at High External Osmolality

In Escherichia coli, a sudden increase in external concentration causes a pressure drop across the cell envelope, followed by an active recovery. After recovery, and if the external osmolality remains high, cells have been shown to grow more slowly, smaller, and at reduced turgor pressure. Despite the fact that the active recovery is a key stress response, the nature of these changes and how they relate to each other is not understood. Here, we use fluorescence imaging of single cells during hyperosmotic shocks, combined with custom made microfluidic devices, to show that cells fully recover their volume to the initial, preshock value and continue to grow at a slower rate immediately after the recovery. We show that the cell envelope material properties do not change after hyperosmotic shock, and that cell shape recovers along with cell volume. Taken together, these observations indicate that the turgor pressure recovers to its initial value so that reduced turgor is not responsible for the reduced growth rate observed immediately after recovery. To determine the point at which the reduction in cell size and turgor pressure occurs after shock, we measured the volume of E. coli cells at different stages of growth in bulk cultures. We show that cell volume reaches the same maximal level irrespective of the osmolality of the media. Based on these measurements, we propose that turgor pressure is used as a feedback variable for osmoregulatory pumps instead of being directly responsible for the reduction in growth rates. Reestablishment of turgor to its initial value might ensure correct attachment of the inner membrane and cell wall needed for cell wall biosynthesis.     

Origins of Escherichia coli Growth Rate and Cell Shape Changes at High External Osmolality

In Escherichia coli, a sudden increase in external concentration causes a pressure drop across the cell envelope, followed by an active recovery. After recovery, and if the external osmolality remains high, cells have been shown to grow more slowly, smaller, and at reduced turgor pressure. Despite the fact that the active recovery is a key stress response, the nature of these changes and how they relate to each other is not understood. Here, we use fluorescence imaging of single cells during hyperosmotic shocks, combined with custom made microfluidic devices, to show that cells fully recover their volume to the initial, preshock value and continue to grow at a slower rate immediately after the recovery. We show that the cell envelope material properties do not change after hyperosmotic shock, and that cell shape recovers along with cell volume. Taken together, these observations indicate that the turgor pressure recovers to its initial value so that reduced turgor is not responsible for the reduced growth rate observed immediately after recovery. To determine the point at which the reduction in cell size and turgor pressure occurs after shock, we measured the volume of E. coli cells at different stages of growth in bulk cultures. We show that cell volume reaches the same maximal level irrespective of the osmolality of the media. Based on these measurements, we propose that turgor pressure is used as a feedback variable for osmoregulatory pumps instead of being directly responsible for the reduction in growth rates. Reestablishment of turgor to its initial value might ensure correct attachment of the inner membrane and cell wall needed for cell wall biosynthesis.     

Theoretical and experimental exclusion of errors in the determination of the elasticity and water transport parameters of plant cells by the pressure probe technique

The volumetric elastic modulus of the cell wall and the hydraulic conductivity of the cell membranes were measured on ligatured compartments of different sizes of Chara corallina internodes using the pressure probe technique. The ratio between intact cell surface area and the area of puncture in the cell wall and membrane introduced by the microcapillary of the pressure probe was varied over a large range by inserting microcapillaries of widely varying diameters in different sized compartments. The relationship of the elastic modulus and the hydraulic conductivity to turgor pressure was independent of the ratio of intact cell surface area to the area of injury. The increase in the hydraulic conductivity below 2 bar turgor pressure and the volume dependence of the elastic modulus were shown to be the same as those observed in intact nonligatured cells. Theoretical considerations of the possible influence of injury of the cell wall and cell membrane around the inserted microcapillary on the measurement of the water transport and cell wall parameters do not explain the experimental findings. Thus, mechanical artifacts, if at all present, are too small to account for the observed dependence of the hydraulic conductivity and the elastic modulus on turgor pressure. The pressure probe technique thus represents an accurate method for measuring water transport parameters in both giant algal cells and in tissue cells of higher plants.     

A model for predicting growth responses in plants to changes in external water potential: Zea mays primary roots

In response to osmotic step changes, three distinct phases have been noted in the growth response of Zea mays primary roots. They are cessation or slowing of growth over a period of 15–20 minutes, tissue contraction, and a damped oscillatory return to nearly normal growth rate, all within a period of about one hour. A system model of the tissue response is presented to explain such behavior and to serve in a predictive capacity to govern future experiments.It is supposed that for turgor pressure in excess of a cell wall yield threshold, plastic flow is the major component of wall deformation, and that when turgor falls below yield threshold, elastic deformation is dominant. The equations of the model describe growth rate as a function of time in terms of the following properties; plastic flow, elastic deformation, permeability to water, and solute uptake. They are derived from basic equations of feedback interactions between internal osmotic pressure and growth rate, and between wall softening, turgor and growth rate.The model predicts oscillatory growth rate regulation, and phase and amplitude relationships between turgor pressure and growth rate. The simplest model which accounts for all observations is that of biphasic deformation, two modes of wall softening, and a dual feedback system involving osmotic and yield threshold control of growth rate.It should be noted that to predict the time course of turgor pressure, osmotic pressure, yield pressure, and growth rate, two initial conditions and six system parameter values are sufficient. So far only the initial values of growth rate and its derivative can be obtained for Zea mays primary roots. However, values for wall softening and hardening coefficients (including the strain and turgor independent component), plastic extensibility, water permeability and dilution rate coefficients have not been obtained as yet for Zea roots. Values for some of these parameters have been obtained for other roots, coleoptiles, and giant algal cells.Lest the reader despair, it should be pointed out that experimental observations coupled with simulation studies will help establish restricted ranges of values that the system parameters might assume. These can then be compared with known values in the literature and values experimentally obtained in the future.     

Variability of the turgor pressure of individual cells of the gram-negative heterotroph Ancylobacter aquaticus.

Cells of Ancylobacter aquaticus were observed under phase microscopy in a chamber to which a measured pressure could be applied. The initial collapse pressure (Ca), i.e., the lowest pressure needed to collapse the most pressure-sensitive gas vesicles, was measured for 69 cells. The cells were taken from cultures in low-density balanced exponential growth, and the experiments were performed quickly so that the bacteria were in a uniform physiological state at the time of measurement. The turgor pressure, Pt, is the difference between the pressure, C, that would cause collapse of vesicles when removed from the cell and Ca. In this paper we focus on the variability of Pt from cell to cell. Part of the observed variability of Ca was due to the variability of the collapse pressure of individual vesicles (standard deviation [SD] = 90 kPa), but because there were about 100 vesicles per cell and because a change in refracted light after the fifth vesicle (approximately) collapsed probably could be detected by the human eye, the pressure would only have an SD of 18.6 kPa due to this type of sampling error. The observed SD of Pt was 42 kPa, indicating that turgor pressure did vary considerably from cell to cell. However, the turgor pressure was independent of cell size. Statistical analysis showed that Pt would decrease 6.9 kPa over a cell cycle, but with too large an SD (19.9 kPa) to be significant. This implies that the observed change in Pt over the cell cycle is not statistically significant.     

An Insight into Cell Elasticity and Load-Bearing Ability. Measurement and Theory

We have studied the elasticity and load bearing ability of plant tissue at the cellular level, using onion (Allium cepa) epidermal cells. The Young's modulus and Poisson's ratio of the cells were obtained by loading a tensile force on onion epidermal peels of different turgor pressures, and measuring the elongation and the lateral contraction of the peels. The Young's moduli and the Poisson's ratios ranged from 3.5 to 8.0 MPa and 0.18 to 0.30, respectively. To determine the effects of cell elasticity and turgor pressure on the cell's ability to bear load, we loaded a small glass ball onto a cell and measured the projected contact area between the ball and the cell. Unlike previous studies, we considered the cell as a whole entity, and utilized the Boussinesq's solution to derive the relevant equations that related the elastic parameters and cell deformation. For cells with a turgor pressure > or = 0.34 MPa, the predicted contact area agreed well with the measured area. The equations could also predict cell turgor pressure with a deviation from the measured value of 0.01 MPa. This study gives strong support to ball tonometry, a new method of measuring cell turgor pressure.     

Turgor-dependent Changes in Avena Coleoptile Cell Wall Composition

The effects of reduced turgor pressure on growth, as measured by cell elongation, and on auxin-mediated changes in cell walls, as measured by analyses of wall composition, were examined using Avena coleoptile segments. Although moderate (1-4 bar) decreases in turgor resulted in a progressive decline in growth proportional to the decrease in turgor, the major auxin-induced change in wall composition, a decrease in noncellulosic wall glucose, was unaffected. Severe (5-8 bar) decreases, however, did inhibit this auxin effect on the wall, and with turgor decreases of 9 bars or more this auxin effect was no longer apparent. The results show that turgor pressure is required for this auxin-mediated wall modification and also that this modification of wall glucose occurs at turgor pressures less than those required for wall extension. Changes in other wall components were generally unaffected by altering turgor pressure.     

PECTATE LYASE LIKE12 patterns the guard cell wall to coordinate turgor pressure and wall mechanics for proper stomatal function in Arabidopsis

Plant cell deformations are driven by cell pressurization and mechanical constraints imposed by the nanoscale architecture of the cell wall, but how these factors are controlled at the genetic and molecular levels to achieve different types of cell deformation is unclear. Here, we used stomatal guard cells to investigate the influences of wall mechanics and turgor pressure on cell deformation and demonstrate that the expression of the pectin-modifying gene PECTATE LYASE LIKE12 (PLL12) is required for normal stomatal dynamics in Arabidopsis thaliana. Using nanoindentation and finite element modeling to simultaneously measure wall modulus and turgor pressure, we found that both values undergo dynamic changes during induced stomatal opening and closure. PLL12 is required for guard cells to maintain normal wall modulus and turgor pressure during stomatal responses to light and to tune the levels of calcium crosslinked pectin in guard cell walls. Guard cell-specific knockdown of PLL12 caused defects in stomatal responses and reduced leaf growth, which were associated with lower cell proliferation but normal cell expansion. Together, these results force us to revise our view of how wall-modifying genes modulate wall mechanics and cell pressurization to accomplish the dynamic cellular deformations that underlie stomatal function and tissue growth in plants.     

Auxin increases elastic wall-properties in rye coleoptiles: implications for the mechanism of wall loosening

Auxin-induced changes of wall-rheological properties during different growth rates of rye coleoptile segments (Secale cereale L.) were investigated. In addition, changes of osmotic concentration and turgor pressure were measured. Decrease of turgor and of osmotic concentration followed a synchronous time course. Auxin-incubated segments exhibited a faster decrease and eventually lower values of both parameters. Creep test extensibility measurements demonstrate that apparent plastic as well as elastic extensibility of distilled-water-incubated segments strongly decreased during 24 h. In auxin-incubated segments apparent plastic as well as elastic extensibilities were strongly increased, even in the absence of growth due to insufficient turgor pressure. The increasing effect of auxin on elastic wall properties is also reflected by an increase in relative reversible length (part of segment length by which segments shrink after freezing/thawing as referred to total length) and a complementary decrease of relative irreversible length (remaining length after turgor elimination as referred to turgid length); again the effects were independent of growth rate and turgor pressure. Cellulose synthesis inhibition of approx. 80% by dichlorobenzonitrile (DCB) had no significant effect either on growth or on wall-rheological properties. Independent of whether the changed rheological wall behaviour of auxin-incubated segments is causally related to the mechanism of auxin-induced wall loosening, it indicates changes of wall polymer properties and/or interactions which are conserved when no actual length increase occurs due to insufficient turgor pressure. The results suggest that IAA-induced wall loosening may be primarily mediated by cell wall changes other than cleavage of covalent, load-bearing bonds as hypothesized in various wall loosening models.     

Water relations of immobilized giant algal cells

Cells of Halicystis parvula, Acetabularia mediterranea, and Valonia utricularis were immobilized in a cross-linked alginate matrix (4–6% w/w) in order to simulate water-relation experiments in individual cells of higher plant tissues. The immobilization of these cells did not lead to an increase in the mechanical stability of the cell walls. This was demonstrated by measuring the volumetric elastic modulus of the cell wall and its dependence on turgor pressure with the aid of the non-miniaturized pressure probe. In immobilized cells, no changes in the absolute value of the elastic modulus of the cell wall could be detected for any given pressure. At the maximum turgor pressure at which non-immobilized cells normally burst (about 3–7 bar for V. utricularis; depending on cell size, 3 bar for A. mediterranea and 0.9 bar for H. parvula) reversible decreases in the pressure are observed which are succeeded by corresponding pressure increases. This obvervation indicates that coating the cells with the cross-linked matrix protects them from rapid water and turgor pressure loss. Turgor pressure relaxation processes in immobilized cells, which could be induced hydrostatically by means of the pressure probe, yielded accurate values for the half-times of water exchange and for the hydraulic conductivity of the cell membrane. The results demonstrate that the water transport equations derived for single cells in a large surrouding medium are valid for immobilized cells, so that any influence exerted by the unstirred layer which is caused by the presence of the cross-linked matrix can be ignored in the calculations. On the other hand, the evaluation of the half-times of water exchange and the hydraulic conductivity from turgor pressure relaxation processes, which have been induced osmotically, only yields correct values under certain circumstances. The model experiments presented here show, therefore, that the correct Lp-value for an individual cell in a higher plant tissue can probably only be obtained presently by using the pressure probe technique rather than the osmotic method. The results are also discussed in relation to the possible applications of immobilized cells and particularly of immobilized micro-organisms in catalytic reaction runs on an industrial scale.     

Growth Physics in Nitella: a Method for Continuous in Vivo Analysis of Extensibility Based on a Micro-manometer Technique for Turgor Pressure

The view that the plant cell grows by the yielding of the cell wall to turgor pressure can be expressed in the equation: rate = cell extensibility × turgor. All growth rate responses can in principle be resolved into changes in the 2 latter variables. Extensibility will relate primarily to the yielding properties of the cell wall, turgor primarily to solute uptake or production. Use of this simple relationship in vivo requires that at least 2 of the 3 variables be measured in a growing cell. Extensibility is not amenable to direct measurement. Data on rate and turgor for single Nitella cells can, however, be continuously gathered to permit calculation of extensibility (rate/turgor). Rate is accurately obtained from measurements on time-lapse film. Turgor is estimated in the same cell, to within 0.1 atm or less, by measurement of the ability of the cell to compress gas trapped in the closed end of a capillary the open end of which is in the cell vacuole. The method is independent of osmotic equilibrium. It operates continuously for several days, over a several fold increase in cell length, and has response time of less than one minute. Rapid changes in turgor brought on by changes in tonicity of the medium, show that extensibility, as defined above, is not constant but has a value of zero unless the cell has about 80% of normal turgor. Because elastic changes are small, extensibility relates to growth. Over long periods of treatment in a variety of osmotica the threshold value for extensibility and growth is seen to fall to lower values to permit resumption of growth at reduced turgor. A brief period of rapid growth (5× normal) follows the return to normal turgor. All variables then become normal and the cycle can be repeated. The cell remains essentially at osmotic equilibrium, even while growing at 5× the normal rate. The method has potential for detailed in vivo analyses of “wall softening.”     

Control of Cell Elongation in Nitella by Endogenous Cell Wall pH Gradients: MULTIAXIAL EXTENSIBILITY AND GROWTH STUDIES

The multiaxial stress of turgor pressure was stimulated in vitro by inflating isolated Nitella cell walls with mercury. The initial in vitro extension at pH 6.5, 5 atmospheres pressure, returned the wall approximately to the in vivo stressed length, and did not induce any additional extension during a 15-minute period. Upon release of pressure, a plastic deformation was observed which did not correlate with cell growth rates until the final stages of cell maturation. Since wall plasticity does not correlate with growth rate, a metabolic factor(s) is implicated. Walls at all stages of development exhibited a primary yield stress between 0 and 2 atmospheres, while rapidly growing cells (1-3% per hour) exhibited a secondary yield stress of 4 to 5 atmospheres. The creep rate and plastic deformation of young walls were markedly enhanced by acid buffers (10 millimolar, pH ≤ 5.3).     

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.     

Growth rate and turgor pressure: auxin effect studies with an automated apparatus for single coleoptiles

Because turgor pressure is regarded as the driving force for cell extension, any general theory of plant growth requires quantitative information on the relationship between steady irreversible growth rate and turgor pressure. To investigate contrasting views of this relation an automated apparatus was constructed which perfused both the outer and inner epidermis of a single coleoptile while its growth rate was continuously recorded. Turgor was altered abruptly by perfusing with solutions of varying tonicity. With specially grown rye coleoptiles the half-time of the osmo-elastic response was reduced to 2 minutes or less. After decay of this response, however, rate continued to change (so as to partially compensate the effects of the turgor shift in question) for 30 to 60 minutes. Only then could a steady rate be taken. A characterization of steady rate versus turgor covering five turgor values for a single coleoptile thus required many hours. The conclusions are as follows. (a) The change in steady rate, per unit change in turgor, was much greater +IAA than −IAA. (b) Both auxin and turgor act to reset an apparent stabilizing system whose presence is shown in the partial compensation of the initial response to turgor shifts. The above “extensibility” changes are operational only. They need not reflect changes in the immediate physical extensibility of the wall; they could reflect changes in a process acting on the wall. (c) The growth rate versus turgor relation shows some hysteresis.     

Loss of stability: a new look at the physics of cell wall behavior during plant cell growth

In this article we investigate aspects of turgor-driven plant cell growth within the framework of a model derived from the Eulerian concept of instability. In particular we explore the relationship between cell geometry and cell turgor pressure by extending loss of stability theory to encompass cylindrical cells. Beginning with an analysis of the three-dimensional stress and strain of a cylindrical pressure vessel, we demonstrate that loss of stability is the inevitable result of gradually increasing internal pressure in a cylindrical cell. The turgor pressure predictions based on this model differ from the more traditional viscoelastic or creep-based models in that they incorporate both cell geometry and wall mechanical properties in a single term. To confirm our predicted working turgor pressures, we obtained wall dimensions, elastic moduli, and turgor pressures of sequential internodal cells of intact Chara corallina plants by direct measurement. The results show that turgor pressure predictions based on loss of stability theory fall within the expected physiological range of turgor pressures for this plant. We also studied the effect of varying wall Poisson's ratio nu on extension growth in living cells, showing that while increasing elastic modulus has an understandably negative effect on wall expansion, increasing Poisson's ratio would be expected to accelerate wall expansion.     

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 × 10⁻⁷ centimeters per second per bar down to 4.4 × 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.     

Dynamic Relation between Expansion and Cellular Turgor in Growing Grape (Vitis vinifera L.) Leaves

Measurements of the growth and water relations of expanding grape (Vitis vinifera L.) leaves have been used to determine the relationship between leaf expansion rate and leaf cell turgor. Direct measurement of turgor on the small (approximately 15 micrometer diameter) epidermal cells over the midvein of expanding grape leaves was made possible by improvements in the pressure probe technique. Leaf expansion rate and leaf water status were perturbed by environmentally induced changes in plant transpiration. After establishing a steady state growth rate, a step decrease in plant transpiration resulted in a rapid and large increase in leaf cell turgor (0.25 megapascal in 5 minutes), and leaf expansion rate. Subsequently, leaf expansion rate returned to the original steady state rate with no change in cell turgor. These results indicate that the expansion rate of leaves may not be strongly related to the turgor of the leaf cells, and that substantial control of leaf expansion rate, despite changes in turgor, may be part of normal plant function. It is suggested that a strictly physical interpretation of the parameters most commonly used to describe the relationship between turgor and growth in plant cells (cell wall extensibility and yield threshold) may be inappropriate when considering the process of plant cell expansion.     

Gradients of turgor, osmotic pressure, and water potential in the cortex of the hypocotyl of growing ricinus seedlings : effects of the supply of water from the xylem and of solutes from the Phloem

To evaluate the possible role of solute transport during extension growth, water and solute relations of cortex cells of the growing hypocotyl of 5-day-old castor bean seedlings (Ricinus communis L.) were determined using the cell pressure probe. Because the osmotic pressure of individual cells (πⁱ) was also determined, the water potential (ψ) could be evaluated as well at the cell level. In the rapidly growing part of the hypocotyl of well-watered plants, turgor increased from 0.37 megapascal in the outer to 1.04 megapascal in the inner cortex. Thus, there were steep gradients of turgor of up to 0.7 megapascal (7 bar) over a distance of only 470 micrometer. In the more basal and rather mature region, gradients were less pronounced. Because cell turgor ≈ πⁱ and ψ ≈ 0 across the cortex, there were also no gradients of ψ across the tissue. Gradients of cell turgor and πⁱ increased when the endosperm was removed from the cotyledons, allowing for a better water supply. They were reduced by increasing the osmotic pressure of the root medium or by cutting off the cotyledons or the entire hook. If the root was excised to interrupt the main source for water, effects became more pronounced. Gradients completely disappeared and turgor fell to 0.3 megapascal in all layers within 1.5 hours. When excised hypocotyls were infiltrated with 0.5 millimolar CaCl₂ solution under pressure via the cut surface, gradients in turgor could be restored or even increased. When turgor was measured in individual cortical cells while pressurizing the xylem, rapid responses were recorded and changes of turgor exceeded that of applied pressure. Gradients could also be reestablished in excised hypocotyls by abrading the cuticle, allowing for a water supply from the wet environment. The steep gradients of turgor and osmotic pressure suggest a considerable supply of osmotic solutes from the phloem to the growing tissue. On the basis of a new theoretical approach, the data are discussed in terms of a coupling between water and solute flows and of a compartmentation of water and solutes, both of which affect water status and extension growth.