Changes In Turgor Pressure Research Articles

Cell elongation, turgor and osmotic pressure in developing sunflower hypocotyls

The relationship between cell elongation, change in turgor and cell osmotic pressure was investigated in the sub-apical region of hypocotyls of developing sunflower seedlings (Helianthus annuus L.) that were grown in continuous white light. Cell turgor was measured with the pressure probe. The same hypocotyl sections were used for determination of osmotic pressure of the tissue sap. Acceleration of cell elongation during the early phase of growth was accompanied by a 25% decrease in both turgor and osmotic pressure. During the linear phase of growth both pressures remained largely constant. The difference between turgor and osmotic pressure (water potential) was -0.10 to -0.13 MPa

Synergistic activation of plasma membrane H+– ATPase in Arabidopsis thaliana cells by turgor decrease and by fusicoccin

The regulation of the H+‐ATPase of plasma membrane is a crucial point in the integration of transport processes at this membrane. In this work the regulation of H+‐ATPase activity induced by changes in turgor pressure was investigated and compared with the stimulating effect of fusicoccin (FC). The exposure of cultured cells of Arabidopsis thaliana L. (ecotype Landsberg 310–14‐2) to media containing mannitol (0. 15 or 0. 3 M) or polyethylene glycol 6000 (PEG) (15. 6% or 22% w/v) resulted in a decrease in the turgor pressure of the cells and in a strong stimulation of H+ extrusion in the incubation medium. The osmotica‐induced H+ extrusion was (1) inhibited by the inhibitor of plasma membrane H+‐ATPase, erythrosin B (EB), (2) dependent on the external K+ concentration, (3) associated with a net K+ influx, and (4) lead to an increase of cellular malate content. These results show that the reduction of external osmotic potential stimulates the activity of plasma membrane H+‐ATPaseThe effect of mannitol was only partially inhibited by treatments with cycloheximide (CH) and cordycepin, which block protein and mRNA synthesis, respectively. All the effects of osmotica were qualitatively and quantitatively similar to those induced by 5 μM FC. However, when FC and mannitol (or PEG) were fed together, their effects on H+ extrusion appeared synergistic, irrespective of whether FC was present at suboptimal or optimal concentrations. This behaviour suggests that the modes of action of FC and of the osmotica on H+‐ATPase activity differ at least in some step(s)

Osmotic Biosensors. How to Use a Characean Internode for Measuring the Alcohol Content of Beer

Isolated internodes of Chara corallina and Nitella flexilis have been used to determine the concentration of one passively permeating solute in the presence of non-permeating solutes. The technique was based on the fact that the shape of the peaks of the biphasic responses of cell turgor (as measured in a conventional way using the cell pressure probe) depended on the concentration and composition of the solution and on the permeability and reflection coefficients of the solutes. Peak sizes were proportional to the concentration of the permeating solute applied to the cell. Thus, using the selective properties of the cell membrane as the sensing element and changes of turgor pressure as the physical signal, plant cells have been used as a new type of biosensor based on osmotic principles. Upon applying osmotic solutions, the responses of cell turgor (P) exactly followed the P(t) curves predicted from the theory based on the linear force/flow relations of irreversible thermodynamics. The complete agreement between theory and experiment was demonstrated by comparing measured curves with those obtained by either numerically solving the differential equations for volume (water) and solute flow or by using an explicit solution of the equations. The explicit solution neglected the solvent drag which was shown to be negligible to a very good approximation. Different kinds of local beers (regular and de-alcoholized) were used as test solutions to apply the system for measuring concentrations of ethanol. The results showed a very good agreement between alcohol concentrations measured by the sensor technique and those obtained from conventional techniques (enzymatic determination using alcohol dehydrogenase or from measurement of the density and refraction index of beer). However, with beer as the test solution, the characean internodes did show irreversible changes of the transport properties of the membranes leading to a shift in the responses when cells were treated for longer than 1 h with diluted beer. The accuracy and sensitivity of the osmotic biosensor technique as well as its possible applications are discussed.

Mechanogenetic regulation of transcription

In many biological systems mechanical forces regulate gene expression: in bacteria changes in turgor pressure cause a deformation of the membrane and induce the expression of osmoregulatory genes; in plants gravity regulates cell growth (‘geotropism’); in mammals stretching a muscle induces hypertrophy which is accompanied by qualitative changes in protein synthesis. Consequently, the term ‘mechanogenetic control’ seems to be a suitable common name for all these processes. The mechanism by which mechanical factors modulate transcriptional activity is still unknown. The purpose of this review is to bring together data from different fields in order to obtain a better understanding of the mechanogenetic control of cell growth.

Effects of repeated stress on turgor pressure and cell elasticity changes in black spruce seedlings

One-year-old black spruce (Piceamariana (Mill.) B.S.P.) seedlings were preconditioned by exposing them to either one or two dehydration–rehydration cycles by using the osmoticum polyethylene glycol 3350. Preconditioned and unconditioned seedlings were then subjected to a more severe osmotic (water) stress by exposing them to a higher concentration of polyethylene glycol. Effects of repeated dehydration–rehydration cycles on cell-water relations were studied after 3, 7, and 13 days of stress relief using pressure–volume curve analysis. Repeated dehydration–rehydration cycles caused a cumulative increase in turgor potentials at full saturation. In these preconditioned plants there was also a progressive lowering of osmotic potentials and relative water contents at zero turgor, which increased over time with stress relief. The decline in osmotic potentials at zero turgor in osmotically stressed black spruce was associated with increased cell wall relaxation, followed by increased turgor potentials, in preconditioned but not in unconditioned seedlings. Saturated osmotic potentials were not altered by repeated, short-term conditioning stresses, suggesting that tissue elasticity was more important for turgor regulation than osmotic adjustment.

Short- and Long-Term Effects of Salinity on Leaf Growth in Rice (Oryza sativaL.)

Addition of 50 mM NaCl to Oryza sativa L. had little effect upon the time of leaf initiation, but leaf mortality prior to the normal phase of senescence was increased and the onset of senescence was advanced. There was no significant effect upon the day-to-day pattern of growth, nor upon the ultimate length, of leaves that were developing at the time of, or shortly after, salinization with 50 mM NaCI. Leaves that developed after prolonged exposure of the plants to salinity were shorter. Addition of NaCl, KC1 or mannitol to the root medium brought about a cessation of leaf elongation within one minute. Growth at a reduced rate restarted abruptly after a lag period that depended upon the external concentration. Elongation rate recovered its original value within 24 h after exposure to 50 mM NaCl, though not at higher concentrations. Addition of NaCl at concentrations up to 100 mM elicited no short-term effects upon photosynthetic gas exchange. Na uptake contributed to osmotic adjustment of the growing zone. When plants were rapidly exposed to 50 mM NaCl, no change in turgor pressure was detectable in the growing zone with the resolution of the miniature pressure probe used (about 70 kPa). It is concluded that the initial growth reduction in rice caused by salinization is due to a limitation of water supply. A clear distinction is made between the initial effects of low salinity which are recoverable and the long-term effects which result from the accumulation of salt within expanded leaves.

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, ɛ, 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 ɛ of rhizodermal and cortical cells increased along the root axis both in untreated and in salt‐treated roots. The ɛ 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 ɛ‐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.

Structural Comparison between Free and Anchored Roots inTillandsia(Bromeliaceae) Species

SUMMARYThe loss of the absorbing function by aerial roots observed in Tillandsia (Bromeliaceae) is a display of extremely specialized epiphytism. The roots of the «atmospheric» Tillandsia usually anchor the plant to the support: however, some of these roots grow free. The structure of both free and anchoring roots is compared in this paper. Analyses by SEM, TEM and LM were carried out on the atmospheric species: T. latifolia Meyen, T. macdougalli Smith. The following features emerged: in the rootcap of both adhering and free roots the dead and degenerating cells remain to form a protective cover; no ultrastructural differences can be noted. In contrast, the underlying living rootcap cells show ultrastructural differences. In the free roots and in the non contact side of the anchored roots the rootcap cells appear to be involved in a utilization process of the cytoplasmic content of the outer degenerating cells; in the adhering side of the anchored roots they show large cytoplasmic zones which appear to be modified into amorphous material. Cytochemical reactions indicate that this material has a protein-polysaccharidic nature, therefore, since it is hydrophilic, contact with the support can be facilitated by means of changes in turgor pressure. Some epidermal cells of the adhering side of the anchored roots form unicellular hairs. The ultrastructural features of these are consistent with those found in cells which secrete lipo-polysaccharidic material. In fact, some similar material is observed on the outer side of the hairs. All of the above characteristics of the anchoring roots may be regarded as morphologic features which contribute to their adhering function.

Water Flow Across the Sieve Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. IV. Root Tips and Seed Coats

In the present paper, the theory developed in Part I of this series is applied to seed coats of Phaseolus vulgaris and some combined data on root tips of Hordeum distichum and Hordeum vulgare. Because of the large back-pressures implied, it is concluded that phloem transport into these primary sinks would be physiologically impossible in the absence of a symplastic pathway for the unloading of water from sieve elements. In this case, unloading of water and sucrose will occur predominantly as a pressure-driven flow of solution through plasmodesmata, although diffusion can contribute significantly to the plasmodesmatal sucrose flux. At least 20% of the plasmodesmata connecting sieve elements and adjacent cells must be unobstructed if large changes in turgor and osmotic pressure are to be avoided. Depending on the membrane area available for water fluxes, it is possible that the difference in water potential across the sieve-tube plasmalemma can lead to significant errors when axial turgor gradients are estimated from gradients of osmotic pressure and external water potential. The magnitude and even the sign of these errors is uncertain, but it is possible that sieve-tube turgor pressures will be significantly underestimated in primary sinks.

Water Flow Across the Sieve-Tube Boundary: Estimating Turgor and Some Implications for Phloem Loading and Unloading. III. Phloem in the Leaf

The theory described in Part I of this series is applied here to the loading of water and sucrose into the sieve-element-companion-cell (se-cc) complexes of minor veins. It is concluded that symplastic phloem loading cannot account for large increases in osmotic pressure from mesophyll to se-cc complex or the dilution of sieve tube sap which is evident in leaves. By contrast, loading from the free space into a symplastically isolated se-cc complex can account for both these observations. Within the se-cc complex, sucrose and water will be transported from the companion cells to the sieve elements by a pressure-driven flow of solution through the cytoplasmic annuli of plasmodesmata. The associated changes in turgor and osmotic pressure are small, and so the se-cc complex can be regarded as a single compartment with respect to water potential. The assumption that this compartment is at water flux equilibrium will lead to significant overestimates of turgor gradients. However, if the trans-membrane water potential difference and the external water potential are taken into account, the correlation of such gradients with sieve-element dimensions and / or transport velocities provides one means of testing the Munch hypothesis of phloem transport

A New Method of Measurement of Changes in Turgor Pressure in the Mesophyll Cells of Leaves

Summary The aim of this paper is to report a convenient indicator of turgor changes in the mesophyll cells of the leaves of Vicia faba L. It has been demonstrated that Young's modulus of the leaf blade being a measure if its stiffness may be such an indicator. A new method of a fast measurement of the changes in Young's modulus has been proposed. The new method made it possible to carry out measurements which could not be realized by other techniques available. In the discussion the presented method is compared with the commonly applied «pressure probe» of Husken et al. (1978). As an example of the application of the method the results of measurements of light-induced turgorpressure changes are presented.

Turgor Pressure and Phototropism inSinapis albaL. Seedlings

Rich, T. C. G. and Tomos, A. D. 1988. Turgor pressure and phototropism in Sinapis alba L. seedlings.—J. exp. Bot 39: 291-299. Phototropic responses were studied in light-grown mustard hypocotyls. Phototropism was induced by adding 0.27 μmol m−2 s−1 unilateral blue light to a background of low pressure sodium (SOX) lamp light. Curvatures of some 6° from the vertical were reached by 60 min, the curvature rate between 20 min and 60 min being 0.14° min−1. From the axial growth rate and tissue geometry the local growth rates of illuminated and shaded sides of the hypocotyl were calculated to be 1.5 and 4.5 μmin−1 respectively. Turgor pressures of expanding cells in control plants and in the shaded and illuminated sides of the blue light illuminated hypocotyls were measured to be 0.40-0.55 MPa with a pressure probe. No changes in turgor pressure were observed on initiation of curvature. The decay of pressure in the cells of non-transpiring plants following excision indicated that the yield stress threshold of the tissue may be as low as 0.1 MPa. These results indicate that the phototropic growth response in this tissue is not mediated by changes in turgor pressure.

Leaf Diffusive Conductance and Tap Root Cell Turgor Pressure of Sugarbeet

Abstract. The interrelationships of leaf diffusive conductance, tap root cell turgor pressure and the diameter of the tap root of sugarbeet were studied. The study was conducted on well‐watered plants growing in pots under artificial light in the glasshouse. In a typical experiment, on illumination (400 μmol m−2 s−1) leaf conductance increased from 0.6 to 7.4 mm s−1. Cell turgor pressure in the tap root decreased from 0.8 MPa to 0.45 MPa and the root diameter (9.0 cm) contracted by 145μm. Removal of light resulted in the reversal of each of the above parameters to their previous values. Quantitively similar results were obtained when sugar beet plants were uprooted and the response of each of the parameters was measured. The sequence of events however was different. On stimulation by light, changes in leaf diffusive conductance preceded the turgor and root diameter changes (which were simultaneous) by some 15–20min. In contrast, on uprooting the simultaneous changes in root turgor pressure and diameter preceded the changes in leaf conductance. The lag times between changes in diffusive conductance and turgor pressure in the root were between 20 and 30 min.Tap root turgor pressure and diameter correlated strongly and permitted the calculation of an apparent whole root volumetric elastic modules (55–63 MPa). The small changes in tissue volume relative to the transpiration rate suggest that the tap root is not a significant source of transpirational water during the day.

The relationship between turgor pressure and titratable acidity in mesophyll cells of intact leaves of a Crassulacean-acid-metabolism plant, Kalanchoe daigremontiana Hamet et Perr

Day/night changes in turgor pressure (P) and titratable acidity content were investigated in the (Crassulacean-acid-metabolism (CAM) plant Kalanchoe daigremontiana. Measurements of P were made on individual mesophyll cells of intact attached leaves using the pressure-probe technique. Under conditions of high relative humidity, when transpiration rates were minimal, changes in P correlated well with changes in the level of titratable acidity. During the standard 12 h light/12 h dark cycle, maximum turgor pressure (0.15 MPa) occurred at the end of the dark period when the level of titratable acidity was highest (about 300 μeq H(+)·g(-1) fresh weight). A close relationship between P and titratable acidity was also seen in leaves exposed to perturbations of the standard light/dark cycle. (The dark period was either prolonged, or else only CO2-free air was supplied in this period). In plants deprived of irrigation for five weeks, diurnal changes in titratable acidity of the leaves were reduced (ΔH=160 μeq H(+)·g(-1) fresh weight) and P increased from essentially zero at the end of the light period to 0.02 MPa at the end of the dark period. Following more severe water stress (experiments were made on leaves which had been detached for five weeks), P was zero throughout day and night, yet small diurnal changes in titratable acidity were still measured. These findings are discussed in relation to a hypothesis by Lüttge et al. 1975 (Plant Physiol. 56,613-616) for the role of P in the regulation of acidification/de-acidification cycles of plants exhibiting CAM.

Mechanics of root elongation and the effects of 3,5-diiodo-4-hydroxybenzoic acid (DIHB)

This paper reports the results of two series of experiments. In the first the effects of DIHB on the rate of root elongation were compared on unstressed roots and on roots stressed by mechanical impedance and by inadequate levels of aeration. Barley plants were grown in beds of small glass spheres through which nutrient solution was circulated. Mechanical impedance of 25 kPa was applied by subjecting the beds to a confining pressure. Inadequate aeration was obtained by reducing the oxygen concentration in the nutrient solution to 5%. The second series examined possible effects of DIHB on the elastic modulus of root tips of wheat and pea. Elastic modulus gives an indication of the behaviour of roots in structured soil where penetration of peds can be limited by the buckling of root tips. The elastic modulus was measured in experiments of the static cantilever type on roots previously immersed in solutions of polyethylene glycol of different osmotic potential. Elastic modulus measurements can also detect any changes in turgor pressure and wilting characteristics of roots and can therefore help to identify the mechanisms of action of DIHB. DIHB caused increases in root elongation relative to controls in all cases: 26±5.7% in unstressed roots, 14±6.4% in mechanically impeded roots and 54±9.8% in roots growing in 5% oxygen. DIHB had no effect on the elastic modulus, osmotic or turgor pressure of the roots. It is concluded that DIHB acts by modifying the cell wall extensibility factor.

The regulation of turgor pressure during sucrose mobilisation and salt accumulation by excised storage-root tissue of red beet

The changes in turgor pressure that accompany the mobilisation of sucrose and accumulation of salts by excised disks of storage-root tissue of red beet (Beta vulgaris L.) have been investigated. Disks were washed in solutions containing mannitol until all of their sucrose had disappeared and then were transferred to solutions containing 5 mol·m(-3) KCl+5 mol·m(-3) NaCl in addition to the mannitol. Changes in solute contents, osmotic pressure and turgor pressure (measured with a pressure probe) were followed. As sucrose disappeared from the tissue, reducing sugars were accumulated. For disks in 200 mol·m(-3) mannitol, the final reducing-sugar concentration equalled the initial sucrose concentration so there was no change in osmotic pressure or turgor pressure. At lower mannitol concentrations, there was a decrease in tissue osmotic pressure which was caused by a turgor-driven leakage of solutes. At concentrations of mannitol greater than 200 mol·m(-3), osmotic pressure and turgor pressure increased because reducing-sugar accumulation exceeded the initial sucrose concentration. When salts were provided they were absorbed by the tissue and reducing-sugar concentrations fell. This indicated that salts were replacing sugars in the vacuole and releasing them for metabolism. The changes in salf and sugar concentrations were not equal because there was an increase in osmotic pressure and turgor pressure. The amount of salt absorbed was not affected by the external mannitol concentration, indicating that turgor pressure did not affect this process. The implications of the results for the control of turgor pressure during the mobilisation of vacuolar sucrose are discussed.

Day/night variations in turgor pressure in individual cells of Mesembryanthemum crystallinum L.

Using a pressure probe, turgor pressure was directly determined in leaf-mesophyll cells and the giant epidermal bladder cells of stems, petioles and leaves of the halophilic plant Mesembryanthemum crystallinum. Experimental plants were grown under non-saline conditions. They displayed the photosynthetic characteristics typical of C3-plants when 10 weeks old and performed weak CAM when 16 weeks old. In 10 week old plants, the turgor pressure (P) of bladder cells of stems was 0.30 MPa; of bladder cells of petioles 0.19 MPa, and of bladder cells of leaves 0.04 MPa. In bladder cells from leaves of 16 week old plants, marked changes in turgor pressure were observed during day/night cycles. Maximum turgor occurred at noon and was paralleled by a decrease in the osmotic pressure of the bladder cell sap. Similar changes in the cell water relations were observed in plants in which traspirational water loss was prevented by high ambient relative humidity. Turgor pressure of mesophyll cells also increased during day-time showing macimum values in the early morning. No such changes in turgor pressure and osmotic pressure were observed in bladder and mesophyll cells of the 10 week old plants not showing the diurnal acid fluctuation typical of CAM.

Changes in turgor pressure in response to increases in external NaCl concentration in the gas-vacuolate cyanobacterium Microcystis sp.

Changes in cell turgor pressure have been followed in cells of Microcystis sp. transferred to culture medium containing added NaCl at osmolalities of 30–1,500 mosmol kg-1 (≡ 74–3,680 kPa). Upon upshock turgor decreased, due to osmotically-induced water loss from the cell. However, partial recovery of turgor was then observed in illuminated cells, with maximum turgor regain in media containing 30–500 mosmol kg-1 NaCl. The lightdependent recovery of turgor pressure was completed within 60 min, with no evidence of further changes in cell turgor up to 24 h. This is the first direct evidence that turgor regulation may occur in a prokaryotic organism. Short-term increases in cell K+ content were also observed upon upshock in NaCl, indicating that turgor regain may involve a turgorsensitive K+ uptake system. Estimation of internal K+ concentration in cells transferred to 250 mosmol kg-1 NaCl showed that changes in cell K+ may account for at least half of the observed turgor regain up to 60 min.

Osmoregulation of gene expression in Salmonella typhimurium: proU encodes an osmotically induced betaine transport system.

Previous evidence has indicated that a gene, proU, is involved in the response of bacterial cells to growth at high osmolarity. Using Mu-mediated lacZ operon fusions we found that transcription of the proU gene of Salmonella typhimurium is stimulated over 100-fold in response to increases in external osmolarity. Our evidence suggests that changes in turgor pressure are responsible for these alterations in gene expression. Expression of proU is independent of the ompR gene, known to be involved in osmoregulation of porin expression. Thus, there must be at least two distinct mechanisms by which external osmolarity can influence gene expression. We show that there are relatively few genes in the cell which are under such osmotic control. The proU gene is shown to encode a high-affinity transport system (Km = 1.3 microM) for the osmoprotectant betaine, which is accumulated to high concentrations in response to osmotic stress. Even when fully induced, this transport system is only able to function in medium of high osmolarity. Thus, betaine transport is regulated by osmotic pressure at two levels: the induction of expression and by modulation of activity of the transport proteins. We have previously shown that the proP gene encodes a lower-affinity betaine transport system (J. Cairney, I. R. Booth, and C. F. Higgins, J. Bacteriol., 164:1218-1223, 1985). In proP proU strains, no saturable betaine uptake could be detected although there was a low-level nonsaturable component at high substrate concentrations. Thus, S. typhimurium has two genetically distinct pathways for betaine uptake, a constitutive low-affinity system (proP) and an osmotically induced high-affinity system (proU).

Changes in Water Relations and Elastic Properties of Apple Fruit Cells during Growth and Development

Abstract Water relations parameters of individual cells in the cortex of apple fruit (Malus domestica Borkh., ‘Cox's Orange Pippin’) were determined during 2 growing seasons between the end of June and the end of September. Apple and cell diameters doubled during this time. Cell turgor pressure (P) of sliced tissue incubated in a nutrient solution ranged between 6 and 12 bar (0.6 and 1.2 MPa). The elastic cell modulus (∈) increased with both cell turgor and cell volume. For high turgor pressures (P > 5 bar), the average values of ∈ ranged between 30 and 180 bar (3.0 and 18.0 MPa). The half-times (T1/2) for the water exchange of individual cells with their adjacent tissue increased during the seasons from 8–10 s to 20–25 s which was partly due to a decrease of the hydraulic conductivity of the cell membrane (Lp) during ripening. Lp decreased from about 3 to about 1 · 10−6cm · s−1 · bar−1 (from 3 to 1 · 10−7m · s−1 · MPa−1). The probe data allowed estimates of the diffusion coefficient for the propagation of a change in water potential, volume and turgor pressure across the tissue (Dc) · Dc ranged between 3 and 9 · 10−7 cm2 · s−1 (3 and 9 · 10−11m2 · s−1) and increased continuously during the season. The results indicate that the reduction in the elastic extensibility at the cell level (1/∈) does not correspond with the reduction of the firmness of the fruit as maturity is approached. On the contrary, the fruit become softer. Therefore, the physiological meaning of tissue Young's moduli as determined by other methods has to be questioned. The results are discussed with respect to the development of fruit disorders.