西鄂尔多斯地区强旱生小灌木的水分参数

应用PV技术研究了西鄂尔多斯地区绵刺、红沙、四合木和霸王柴4种超旱生灌木的水分关系参数膨压(ψ_P)、细胞弹性模量(ε)、细胞体积比(RCV)及其相互关系.结果表明：在4种荒漠旱生灌木中,红沙保持最大膨压的能力最强(a=2.4593).不同荒漠旱生灌木保持膨压的方式不同：绵刺通过弹性调节保持膨压(ε_max=8.4005 MPa);红沙通过渗透调节来保持膨压(ψ_π100=-3.1302 MPa;ψ₀=-3.5074 MPa);四合木通过渗透调节和弹性调节的协同作用来维持膨压;霸王柴通过渗透调节来保持膨压,而弹性调节能力较弱.绵刺具有柔软而高弹性的细胞壁,是构成其根茎系统快速吸收和传导水分能力的因素之一.四合木具有较柔软而高弹性的细胞壁且ψ_P的变化随RCV减小而趋于缓慢,说明四合木具有较强的持水能力和抗脱水能力.     

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 Study of the Stationary Volumetric Elastic Modulus during Dehydration and Rehydration of Stems of Pea Seedlings

The relationship between cortical-cell turgor pressure (P) and tissue water mass (W) was determined for stem segments of pea (Pisum sativum L.) seedlings subjected to hydration and dehydration. This allowed a test for elastic hysteresis in the cortical cells. The P-W curves for dehydration and hydration were not coincident. In some experiments, the P-W curves exhibited a "roll-off" at high P, similar to the "plateau effect" sometimes observed in pressure-chamber studies. When hydration was followed by a 4-h dehydration, the tissue water mass (W0) at minimum turgor was reduced. This might reflect a reduction in apoplastic water mass and/or a contraction of the symplast during dehydration. Neglecting the decrease in W0 leads to underestimates of the stationary volumetric elastic modulus ([epsilon]stat). The result of an analysis that assumes W0 was constant during hydration suggests that there was no significant difference in [epsilon]stat between dehydration and hydration and, hence, no significant elastic hysteresis. However, a 16-h dehydration increased [epsilon]stat; this might be a response to water stress.     

A membrane model of plant cell extension

A theory is presented for the mechanics of plant cell wall extension and is based on the analogy of the cell wall with a membrane structure made of material capable of large non-linear deformations. These wall deformations may be elastic, elastic-plastic or visco-elastic. Mathematical analyses of such membrane structures show that there is, generally, a critical internal pressure at which dimensional instability occurs. This instability is characterized by a sudden drop in internal pressure accompanied by a large increase in the physical proportions of the membrane structure. The theory proposes that cell wall extension occurs when the cell turgor pressure reaches this critical instability value. The cell wall thus stretched is fixed by biochemical synthesis of wall material. Osmotic regulation re-establishes the turgor pressure and the instability cycle repeats itself as long as the critical instability pressure of the cell is below the osmotic pressure of the cell contents. Equalization of these pressures stops cell extension. The rate of cell extension depends on the frequency of the instability cycle and is thus dependent on the various rate processes associated with the instability cycle. The theory appears to be able to explain most of the known facts regarding cell extension such as the influence of temperature and the action of some growth substances.     

A New Pressure Probe Method to Determine the Average Volumetric Elastic Modulus of Cells in Plant Tissue

A new in vivo method was used to determine an average volumetric elastic modulus ([epsilon]ave) for nongrowing cells in plant tissue. This method requires that both the relative transpiration rate, T, of the tissue and the average turgor pressure decay rate, (dP/dt)ave, of the cells are measured after the water source is removed from the plant tissue. Then [epsilon]ave is calculated from the equation [epsilon]ave = (-dP/dt)ave/T. This method was used to determine [epsilon]ave for cortical cells in stems of pea seedlings (Pisum sativum L.). The results demonstrate that [epsilon]ave increases from virtually zero at low P (approximately 0.01MPa) to approximately 10 MPa at high P (approximately 0.5 MPa). Analyses of the results indicate that the relationship between [epsilon]ave and P can be approximated by a linear function and more accurately approximated by a saturating exponential function: [epsilon]ave = [epsilon][infinity symbol][1 - exp {-k(P - Po)}], where Po is a plateau pressure (approximately 0.01 MPa), k is a rate constant (approximately 7 per MPa), and [epsilon][infinity symbol] (approximately 10 MPa) is the hypothetical maximum value of [epsilon]ave as P -> [infinity symbol]. Solutions for the turgor pressure decay (due to transpiration) as functions of time and symplasmic water mass (after the water source is removed) are derived.     

Demonstration of Respiration-Dependent Water Uptake and Turgor Generation in Vigna Hypocotyl

Respiration-dependent water uptake and turgor change were observedby the xylem perfusion technique. Immediate and reversible shrinkagewith anoxia were repeatedly demonstrated under appropriate osmoticstress in elongating cow pea hypocotyl segments. Such shrinkageand re-elongation were always preceded by reversible inhibitionand re-activation of the electrogenic xylem pump, respectively.In mature zone segments where cell wall extensibility had beenshown to be practically null by means of the turgor jump method,anoxia and reaeration caused elastic shrinkage and expansion,respectively. The extent of respiration-dependent turgor wascalculated from the amplitudes of the elastic volume changeinduced by pressure jump and anoxia. In such segments, the directionof water flow across the xylem-symplast interface should bedetermined solely by the cell wall elasticity and the changein apoplasmic concentration of osmotica controlled by the xylempump activity, irrespective of any change in water conductivityor cell wall extensibility. (Received December 11, 1987; Accepted February 20, 1988)     

Effects of osmotic concentrations and IAA on the rapid shortening of the cortex in the main pulvinus ofMinosa pudica

In 0.8 M mannitol solution, no rapid shortening was recorded, while normal action potential was recorded. This result suggests that motive force of rapid shortening is an elastic stretch of the cell wall caused by the turgor pressure, and that osmotic concentration of motor cell is 0.6–0.7 M. IAA increased the rapid shortening, recovery rate and Cl⁻-efflux. The magnitudes of rapid shortening in IAA were 3–10 times as large as those in APW. The mean and maximum values of the rapid shortening in IAA were 87.0±2.2μm and 207 μm, respectively, or 1.6 and 3.9% of the whole length of motor tissue. When the large rapid shortening occurred, the large recovery rate was observed. These results suggest that both mechanisms, expulsion and re-entry of cell sap, are enhanced by IAA treatment. IAA-induced hyperpolarization was observed with a short time lag, which suggests that IAA enhances the electrogenic ion pump.     

Plasma membrane and cell wall properties of an aspen hybrid (Populus tremula × tremuloides) parenchyma cells under the influence of salt stress

The effect of salinity on cell turgor, plasma membrane permeability and cell wall elasticity has been measured in petioles of an aspen hybrid using the cell pressure probe. Control plants were grown in soil without the addition of NaCl and treated plants were grown in soil with 50 mM of NaCl for 1, 2, 3 and 4 weeks. In parenchyma cells from Populus tremula × tremuloides petioles with an increased level of NaCl in the soil: (a) turgor pressure was reduced after 1 week of treatment but afterward it was similar to untreated plants, (b) the value of elastic modulus of the cell walls increased, and (c) hydraulic conductivity of the plasma membrane of treated plants decreased in comparison to untreated ones. No histological differences and distribution of JIM5 antibody between the petioles of plants grown under salinity and the untreated were found. In cell walls of parenchyma and collenchyma from plants grown under salinity, the presence of pectic epitopes recognized by JIM7 antibodies was increased in comparison to the control plants. The obtained results indicate that under salt stress the permeability of water through plasma membrane is disturbed, cell walls became more rigid but the turgor pressure did not change.     

MECHANICAL BEHAVIOR OF PLANT TISSUES AS INFERRED FROM THE THEORY OF PRESSURIZED CELLULAR SOLIDS

The mechanical behavior of plant tissues and its dependency on tissue geometry and turgor pressure are analytically dealt with in terms of the theory of cellular solids. A cellular solid is any material whose matter is distributed in the form of beamlike struts or complete “cell” walls. Therefore, its relative density is less than one and typically less than 0.3. Relative density is the ratio of the density of the cellular solid to the density of its constitutive (“cell wall”) material. Relative density depends upon cell shape and the density of cell wall material. It largely influences the mechanical behavior of cellular solids. Additional important parameters to mechanical behavior are the elastic modulus of “cell walls” and the magnitude of internal “cell” pressure. Analyses indicate that two “stiffening” agents operate in natural cellular solids (plant tissues): 1) cell wall infrastructure and 2) the hydrostatic influence of the protoplasm within each cellular compartment. The elastic modulus measured from a living tissue sample is the consequence of both agents. Therefore, the mechanical properties of living tissues are dependent upon the magnitude of turgor pressure. High turgor pressure places cell walls into axial tension, reduces the magnitude of cell wall deformations under an applied stress, and hence increases the apparent elastic modulus of the tissue. In the absence of turgid protoplasts or in the case of dead tissues, the cell wall infrastructure will respond as a linear elastic, nonlinear elastic, or “densifying” material (under compression) dependent upon the magnitude of externally applied stress. Accordingly, it is proposed that no single tangent (elastic) modulus from a stress-strain curve of a plant tissue is sufficient to characterize the material properties of a sample. It is also suggested that when a modulus is calculated that it be referred to as the tissue composite modulus to distinguish it from the elastic modulus of a noncellular solid material.     

Estimation of the volumetric elastic modulus and membrane hydraulic conductivity of willow sieve tubes

Severed aphid stylets were used to follow the kinetics of sieve tube turgor and osmotic pressure (π) responses following step changes in water potential applied to the cambial surface of willow (Salix exigua Nutt.) bark strips. The kinetics of the turgor response were monitored with a pressure transducer. In separate experiments, the kinetics of the π response were followed by freezing point determinations on stylet exudate. The sieve tube volumetric elastic modulus in the bark strips was about 21 bars, but may be higher in intact stems. The membrane hydraulic conductivity was about 5 × 10⁻³ centimeters per second per bar; several factors make it difficult to estimate its value accurately. Differences in the turgor pressure (P) and π responses, as well as the relatively more rapid initial turgor response to a water potential (ψ) change, suggested a time-dependent component in sieve tube wall elasticity.

Our observations were generally not supportive of the idea that sieve tubes might osmoregulate. However, the bark strip system may not be suitable for addressing that question.

Separate measurements of ψ, P, and π demonstrate that the relationship predicted by the fundamental cell water potential equation, ψ = P − π, is applicable within experimental error (± 0.4 bar) to sieve tube water relations.

     

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.     

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.     

Water uptake by growing cells: an assessment of the controlling roles of wall relaxation, solute uptake, and hydraulic conductance

Growing plant cells increase in volume principally by water uptake into the vacuole. There are only three general mechanisms by which a cell can modulate the process of water uptake: (a) by relaxing wall stress to reduce cell turgor pressure (thereby reducing cell water potential), (b) by modifying the solute content of the cell or its surroundings (likewise affecting water potential), and (c) by changing the hydraulic conductance of the water uptake pathway (this works only for cells remote from water potential equilibrium). Recent studies supporting each of these potential mechanisms are reviewed and critically assessed. The importance of solute uptake and hydraulic conductance is advocated by some recent studies, but the evidence is indirect and conclusions remain controversial. For most growing plant cells with substantial turgor pressure, it appears that reduction in cell turgor pressure, as a consequence of wall relaxation, serves as the major initiator and control point for plant cell enlargement. Two views of wall relaxation as a viscoelastic or a chemorheological process are compared and distinguished.     

Transient changes in length and growth of wheat coleoptile segments following treatments with osmotica and auxin

The dependence of elongation on the osmotic potential of the medium was investigated, using coleoptile segments (CS) of Triticim aestivum L. (cv. Hartri) and an optoelectronic device. The study aimed at separating the osmoelastic response from the irreversible growth response when an osmoticum (mannitol) was added, and to compare both processes in order to consider the possibility of growth-induced reduction in turgor pressure. The prompt inhibition of elongation registered just after addition of 50 mM mannitol as well as the subsequent resumption of the original elongation rate could be quantitatively explained by the extent and the kinetics of the osmoelastic relaxation. An initial reduction in the irreversible elongation component by mild osmotic stress could not be demonstrated. Above a critical value, the irrevesible growth was insensitive to a further increase in water potential. The minimum turgor pressure required to drive steady growth was not far from zero in both the presence and absence of auxin. The rate (r) of osmotically caused shortening per unit change of water potential was determined from the kinetics of CS shortening induced by addition of mannitol at nearly isotonic concentration (300 mM). This parameter relates a fractional change in length to the difference in water potential between inside and outside, and was assumed to depend largely on the hydraulic resistance of the tissue and cuticle. It was found to be independent of IAA. The relatively low value of Γ suggests significant reduction of turgor at high growth rates. In accordance with this conclusion, the extent of osmoelastic shortening after a transfer to 300 mM mannitol (dependent on wall strain) was significantly decreased in the presence of IAA. Addition of 100 μM IAA to CS growing at a constant rate induced pronounced oscillations in the rate of elongation, which may be connected with the change in elastic cell wall strain. Whereas the steady state growth rate before the addition of IAA was the same in the presence and in the absence of 50 mM mannitol, the maximum growth rate found after addition of IAA was substantially reduced in the mannitol variant.     

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.     

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.     

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.     

Turgor and net ion flux responses to activation of the osmotic MAP kinase cascade by fludioxonil in the filamentous fungus Neurospora crassa

The internal hydrostatic pressure (turgor) of the filamentous fungus Neurospora crassa is regulated at about 400–500 kiloPascals, primarily by an osmotic MAP kinase cascade which activates ion uptake from the extracellular medium and glycerol synthesis. In the absence of hyperosmotic stress, the phenylpyrrole fungicide fludioxonil activates the osmotic MAP kinase cascade, resulting in cell death. Turgor, the electrical potential and net ion fluxes were measured after treatment with fludioxonil. In wildtype, fludioxonil causes a hyperpolarization of the plasma membrane and net H⁺ efflux from the cell, consistent with activation of the H⁺-ATPase. At the same time, net K⁺ uptake occurs, and turgor increases (about 2-fold above normal levels). None of these changes are observed in the os–2 mutant (which lacks a functional MAP kinase, the last of the three kinases in the osmotic MAP kinase cascade). Tip growth ceases as hyperpolarization, net ion flux changes, and turgor increases begin. The inappropriate turgor increase is the probable cause of eventual lysis and death. The results corroborate a multi-pathway response to hyperosmotic stress that includes activation of plasma membrane transport. The relation to cell expansion (tip growth) is not direct. Increases in turgor due to ion transport might be expected to increase growth rate, but this does not occur. Instead, there must be a complex regulatory interplay between the growth and the turgor driving force, possibly mediated by regulation of cell wall extensibility.     

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.     

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.