Water relations of Picea abies. II. Determination of the apoplastic water content and other water relations parameters of needles by means of capillary microcryoscopy and the pressure-volume

Using the pressure volume analysis (PV analysis) on the shoots of Norway spruce (Picea abies [L.] Karst.) and the here presented capillary microcryoscopy of the needle press sap of the same shoots, it was possible to determine the amount of apoplastic water in the needles (W_an) as well as in the defoliated shoots (W_as). Additionally, the bulk osmotic pressure at full water saturation in the symplast of the needles and defoliated shoots (π_on and π_os) was determined. The dependence of the bulk-averaged turgor pressure (P_t) on the water content and the relationship between the bulk modulus of elasticity of the needles (?_n) and the bulk-averaged needle turgor pressure (P_tn) was shown with help of the PV analysis on the whole shoots and defoliated shoots. The study was conducted at the end of the vegetation period in 1987 and during winter 1988. The proportion of W_an in the total needle water content (W_tn) was 14% in September 1987 and 12.5% in March 1988. The respective percentage of W_as in W_ts were 27% and 25%. The amount of apoplastic water depended on the ratio of the dry weight of defoliated shoots to the dry weight of the whole shoots. A standard mean value for the amount of W_an in the total water content of the shoots (W_t) was therefore not possible. The bulk osmotic pressure at full water saturation in the needle symplasts was –1.9 MPa in September 1987 and –2.2 MPa in winter 1988. The respective values of the bulk osmotic pressures in the symplast of the defoliated shoots (π_os) were –1.5 MPa and –1.7 MPa. Thus π_on was 0.1 MPa lower and π_os 0.3–0.4 MPa higher than the average osmotic pressure during full water saturation in the symplast of the whole shoots (π_o). The relation between bulk-averaged turgor pressure and water content showed that during water loss P_tn dropped more rapidly than the turgor pressure of defoliated shoots (P_ts). Consequently the needles were less elastic than the defoliated shoots. The turgor values of whole shoots followed an intemediate course between P_tn and P_ts. The flat course of P_ts seems to be the main reason for the often observed “plateau” of ψ-isotherms of whole shoots near full turgor.     

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

Summary 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 CAMAbbreviations CAM crassulacean acid metabolism - V volume of the cells (mm³) - P turgor pressure (MPa) - volumetric elastic modulus (MPa) - ⁱ osmotic pressure of the cell sap (MPa) - T _1/2 half-time of water exchange (s) - Lp hydraulic conductivity of the cell membrane (m·s^-1·MPa^-1) - A surface area of cells (mm²) - P pressure changes (MPa) - V volume changes (mm³) - nocturanal nighttime - diurnal daytime     

An artificial osmotic cell: a model system for simulating osmotic processes and for studying phenomena of negative pressure in plants

Abstract An artificial osmotic cell has been constructed using reverse osmosis membranes. The cell consisted of a thin film of an osmotic solution (thickness: 100 to 200 μm) containing a non-permeating solute and was bounded between the membrane and the front plate of a pressure transducer which continuously recorded cell turgor. The membrane was supported by metal grids to withstand positive and negative pressures (P). At maximum, negative pressures of up to –0.7 MPa (absolute) could be created within the film on short-term and pressures of up to –0.3 MPa could be maintained without cavitation for several hours. As with living plant cells, the application of osmotic solutions of a non-permeating solute resulted in monophasic relaxations of turgor pressure from which the hydraulic conductivity of the membrane (Lp) and the elastic modulus of the cell (?) could be estimated. The application of solutions with permeating solutes resulted in biphasic pressure relaxation curves (as for living cells) from which the permeability (P_s) and reflection (σ_s) coefficients could be evaluated for the given membrane. Lp, P_s, and σ_s were independent of P and did not change upon transition from the positive to the negative range of pressure. It is concluded that the artificial cell could be used to simulate certain transport properties of living cells and to study phenomena of negative pressure as they occur in the xylem and, perhaps, also in living cells of higher plants.     

Diurnal changes in xylem pressure and mesophyll cell turgor pressure of the lianaTetrastigma voinierianum: The role of cell turgor in long-distance water transport

Summary Long-term xylem pressure measurements were performed on the lianaTetrastigma voinierianum (grown in a tropical greenhouse) between heights of 1 m and 9.5 m during the summer and autumn seasons with the xylem pressure probe. Simultaneously, the light intensity, the temperature, and the relative humidity were recorded at the measuring points. Parallel to the xylem pressure measurements, the diurnal changes in the cell turgor and the osmotic pressure of leaf cells at heights of 1 m and 5 m (partly also at a height of 9.5 m) were recorded. The results showed that tensions (and height-varying tension gradients) developed during the day time in the vessels mainly due to an increase in the local light intensity (at a maximum 0.4 MPa). The decrease of the local xylem pressure from positive, subatmospheric or slightly above-atmospheric values (established during the night) to negative values after daybreak was associated with an almost 1 1 decrease in the cell turgor pressure of the mesophyll cells (on average from about 0.4 to 0.5 MPa down to 0.08 MPa). Similarly, in the afternoon the increase of the xylem pressure towards more positive values correlated with an increase in the cell turgor pressure (ratio of about 1 1). The cell osmotic pressure remained nearly constant during the day and was about 0.75–0.85 MPa between 1 m and 9.5 m (within the limits of accuracy). These findings indicate that the turgor pressure primarily determines the corresponding pressure in the vessels (and vice versa) due to the tight hydraulic connection and thus due to the water equilibrium between both compartments. An increase in the transpiration rate (due to an increase in light intensity) results in very rapid establishment of a new equilibrium state by an equivalent decrease in the xylem and cell turgor pressure. From the xylem, cell turgor, and cell osmotic pressure data the osmotic pressure (or more accurately the water activity) of the xylem sap was calculated to be about 0.35–0.45 MPa; this value was apparently not subject to diurnal changes. Considering that the xylem pressure is determined by the turgor pressure (and vice versa), the xylem pressure of the liana could not drop to — in agreement with the experimental results — less than -0.4 MPa, because this pressure corresponds to zero turgor pressure.     

Plumbing the depths: extracellular water storage in specialized leaf structures and its functional expression in a three‐domain pressure –volume relationship

A three‐domain pressure–volume relationship (PV curve) was studied in relation to leaf anatomical structure during dehydration in the grey mangrove, Avicennia marina. In domain 1, relative water content (RWC) declined 13% with 0.85 MPa decrease in leaf water potential, reflecting a decrease in extracellular water stored primarily in trichomes and petiolar cisternae. In domain 2, RWC decreased by another 12% with a further reduction in leaf water potential to ?5.1 MPa, the turgor loss point. Given the osmotic potential at full turgor (?4.2 MPa) and the effective modulus of elasticity (~40 MPa), domain 2 emphasized the role of cell wall elasticity in conserving cellular hydration during leaf water loss. Domain 3 was dominated by osmotic effects and characterized by plasmolysis in most tissues and cell types without cell wall collapse. Extracellular and cellular water storage could support an evaporation rate of 1 mmol m^?2s^?1 for up to 54 and 50 min, respectively, before turgor loss was reached. This study emphasized the importance of leaf anatomy for the interpretation of PV curves, and identified extracellular water storage sites that enable transient water use without substantive turgor loss when other factors, such as high soil salinity, constrain rates of water transport.     

Determination of free space, growth, solute concentrations and parameters of water relations of suspension-cultured tobacco cells

Abstract Methods were developed for measuring water content of the free space of suspension-cultured tobacco cells using ³H- or ¹⁴C-sorbitol. Sorbitol was not taken up by cells in significant quantities over the 3 min taken to label free space. Free space accounted for 50–60% of the water content of cell pellets irrespective of whether ³H- or ¹⁴-C-sorbitol was used. ¹⁴C-inulin labelled 13.5% less of the water in cell pellets than ³H-sorbitol, probably due to inadequate penetration by inulin into the free space in the cell wall matrix and within clumps of cells. Measurement of free space is necessary for measuring growth on a fresh or dry weight basis, solute concentrations and parameters of water relations of cells. Techniques for making these measurements on tobacco cells were also developed in this study. Solutes were measured after extraction from cells by expressing sap or by boiling cells in ethanol. Similar solute concentrations were found using both methods of extraction. By expressing sap from cells grown in culture medium with an osmotic pressure of 0.24 MPa, the cells were found to have an internal osmotic pressure of 0.70 MPa. Glucose, fructose, sucrose, amino acids and K⁺ accounted for 60% of this osmotic pressure. Elastic moduli were estimated using the Boyle-Van't Hoff relationship after suspending cells in solutions with different osmotic pressures and assessing their water content or internal osmotic pressure. For two different lines of tobacco cells, elastic modulus varied between 1 MPa and 5.4 MPa at turgor pressures of 0.15–0.52 MPa (line 1) and between 0.2 MPa and 4.2 MPa at turgor pressures of 0.04–0.26 MPa (line 2).     

Direct turgor pressure measurements in individual leaf cells of Tradescantia virginiana

The water relations of leaves of Tradescantia virginiana were studied using the miniaturized pressure probe (Hüsken, E. Steudle, Zimmermann, 1978 Plant Physiol. 61, 158–163). Under well-watered conditions cell turgor pressures, P _o, ranged from 2 to 8 bar in epidermal cells. In subsidiary cells P _o was about 1.5 to 4.5 bar and in mesophyll cells about 2 to 3.5 bar. From the turgor pressure, relaxation induced in individual cells by changing the turgor pressure directly by means of the pressure probe, the half-time of water exchange was measured to be between 3 and 100 s for the epidermal, subsidiary, and mesophyll cells. The volumetric elastic modulus, , of individual cells was determined by changing the cell volume by a defined amount and simultaneously measuring the corresponding change in cell turgor pressure. The values for the elastic modulus for epidermal, subsidiary, and mesophyll cells are in the range of 40 to 240 bar, 30 to 200 bar, and 6 to 14 bar, respectively. Using these values, the hydraulic conductivity, L _p, for the epidermal, subsidiary, and mesophyll cells is calculated from the turgor pressure relaxation process (on the basis of the thermodynamics of irreversible processes) to be between 1 and 55·10^-7 cm s^-1 bar^-1. The data for the volumetric elastic modulus of epidermal and subsidiary cells indicate that the corresponding elastic modulus for the guard cells should be considerably lower due to the large volume changes of these cells during opening or closing. Recalculation of experimental data obtained by K. Raschke (1979, Encycl. Plant Physiol. N.S., vol. 7, pp 383–441) on epidermal strips of Vicia faba indicates that the elastic modulus of guard cells of V. faba is in the order of 40–80 bar for closed stomata. However, with increasing stomatal opening, i.e., increasing guard cell volume, decreases. Therefore, in our opinion Raschke's results would indicate a relationship between guard cell volume and which would be inverse to that for plant cells known in the literature. assumes values between 20–40 bar when the guard cell colume is soubled.     

Tissue water relations in elfin cloud forest tree species of Serrania de Macuira,Guajira, Colombia

Summary Diurnal courses of stomatal conductance, leaf water potential, and the components of tissue water potential were measured in six canopy species in an elfin cloud forest. High values of stomatal conductance were measured on cloudy days and during early morning and late afternoon of sunny days. Decreases in stomatal conductance with increases in vapour pressure deficit may have been a response to avoid further water deficits and suggested a stomatal response to changes in relative humidity. Daily transpiration varied between 470 and 1014 g m^-2 day^-1 during cloudy days and between 532 and 944 g m^-2 day^-1 during clear days. Stomatal conductance may have also responded to changes in leaf water potential, which was minimum at noon. The minimum tissue water potential measured in the field was -1.8 MPa in Myrcianthes fragrans, and the minimum turgor pressure was 0.49 MPa also in M. fragrans. There was a correlation between the osmotic potential and the minimum tissue water potential, suggesting that osmotic potential plays a major role in the maintenance of turgor in these species, in spite of the great variability in the elastic properties of leaf tissues. Turgor pressure decreased during the day following the course of water potential but never approached the turgor loss point, as it has been measured in some lowland rain forest trees. This is a strong indication that elfin cloud forest trees do not suffer severe water deficits, and that small tree stature is not directly related to water shortage.     

Cell water potential,osmotic potential,and turgor in the epidermis and mesophyll of transpiring leaves

Water potential, osmotic potential and turgor measurements obtained by using a cell pressure probe together with a nanoliter osmometer were compared with measurements obtained with an isopiestic psychrometer. Both types of measurements were conducted in the mature region of Tradescantia virginiana L. leaves under non-transpiring conditions in the dark, and gave similar values of all potentials. This finding indicates that the pressure probe and the osmometer provide accurate measurements of turgor, osmotic potentials and water potentials. Because the pressure probe does not require long equilibration times and can measure turgor of single cells in intact plants, the pressure probe together with the osmometer was used to determine in-situ cell water potentials, osmotic potentials and turgor of epidermal and mesophyll cells of transpiring leaves as functions of stomatal aperture and xylem water potential. When the xylem water potential was-0.1 MPa, the stomatal aperture was at its maximum, but turgor of both epidermal and mesophyll cells was relatively low. As the xylem water potential decreased, the stomatal aperture became gradually smaller, whereas turgor of both epidermal and mesophyll cells first increased and afterward decreased. Water potentials of the mesophyll cells were always lower than those of the epidermal cells. These findings indicate that evaporation of water is mainly occurring from mesophyll cells and that peristomatal transpiration could be less important than it has been proposed previously, although peristomatal transpiration may be directly related to regulation of turgor in the guard cells.     

Turgor and Longitudinal Tissue 12Helianthus annuus

The quantitative relationship between turgor and the pressureexerted by the inner tissues (cortex, vascular tissue, and pith)on the peripheral cell walls (longitudinal tissue pressure)was investigated in hypocotyls of sunflower seedlings (Helianthusannuus L.) In etiolated hypocotyls cell turgor pressures, asmeasured with the pressure probe, were in the range 0·38to 0·55 MPa with an average of 0·48 MPa. In irradiatedhypocotyls turgor pressures varied from 0·40 to 0·57MPa with a, mean at 0·49 MPa. The pressure exerted bythe inner tissues on the outer walls was estimated by incubatingpeeled sections in a series of osmotic test solutions (polyethyleneglycol 8000). The length change was measured with a transducer.In both etiolated and irradiated hypocotyls an external osmoticpressure of 0·5 MPa was required to inhibit elongationof the inner tissues, i.e. the average cell turgor and the longitudinaltissue pressure are very similar quantities. The results indicatethat the turgor of the inner tissues is displaced to and borneby the thick, growth-limiting peripheral cell walls of the hypocotyl. Key words: Helianthus annuus, hypocotyl growth, tissue pressure, turgor pressure, wall stress     

Salt tolerance in the halophyte Suaeda maritima L. Dum.

Osmotic potentials and individual epidermal cell turgor pressures were measured in the leaves of seedlings of Suaeda maritima growing over a range of salinities. Leaf osmotic potentials were lower (more negative) the higher the salt concentration of the solution and were lowest in the youngest leaves and stem apices, producing a gradient of osmotic potential towards the apex of the plant. Epidermal cell turgor pressures were of the order of 0.25 to 0.3 MPa in the youngest leaves measured, decreasing to under 0.05 MPa for the oldest leaves. This pattern of turgor pressure was largely unaffected by external salinity. Calculation of leaf water potential indicated that the gradient between young leaves and the external medium was not altered by salinity, but with older leaves, however, this gradient diminished from being the same as that for young leaves in the absence of NaCl, to under 30% of this value at 400 mM NaCl. These results are discussed in relation to the growth response of S. maritima.     

Phototropic Response of the Bean Pulvinus: Movement of Water and Ions

Segmental analysis of the laminar pulvinus of Phaseolus vulgaris L. showed that its phototropic curvature is accompanied by efflux of inorganic ions and water from its contracting sector and a comparable influx into its expanding one. All the major ions, except Na⁺, contributed to this transport, suggesting that the response to light involves changes in the driving force, or conductivity of a wide range of solutes. During the curvature, K⁺ and CI^? made the greatest and equivalent contributions to efflux, but only Cl^? exhibited a matching influx into the expanding sector, while K⁺ influx was much less. Use of the cell pressure probe showed that, as the laminar angle of elevation changed between ?40° to +40°, turgor pressure in the expanding motor cells increased by 0.48 MPa and decreased in the contracting cells by 0.32 MPa. Picoliter osmometry of single-cell samples showed that during this movement vacuolar osmotic pressure remained constant. Thus, changes in turgor pressure resulted from changes in apoplastic, rather than the protoplastic osmotic pressure. Volumetric modulus of elasticity of pulvinar motor cells is very low, showing that their walls are very elastic. These properties increase the effectiveness of converting osmotic work into the large-scale, reversible volume changes responsible for leaf movements.     

Replicase proteins as determinants of phloem-dependent long-distance movement of tobamoviruses in tobacco

Summary The turgor pressure of growing pollen tubes of the lily (Lilium longiflorum Thunb.) has been recorded using a turgor pressure probe. Insertion of the probe's micropipette was routinely accomplished, providing recording periods of 20 to 30 min. Probe insertion did not affect tube growth. The stable turgor values ranged between 0.1 and 0.4 MPa, the mean value being 0.209 ± 0.064 MPa (n=106). A brief increase in turgor, generated by injection of oil through the pressure probe, caused the tube to burst at its tip. Burst pressures ranged between 0.19 and 0.58 MPa, that is, individual lily pollen tubes do not withstand turgor pressure approaching twice their regular turgor pressure. In contrast, parallel experiments using the incipient plasmolysis technique yielded a mean putative turgor pressure of 0.79 MPa either using sucrose (n=24) or mannitol (n=25). Surprisingly, turgor pressure was not significantly correlated with tube growth rate which ranged from zero to 13 m/min. Nor did it correlate with tube length over the tested range of 100 to 1600 m. In addition the influence of the medium's osmolality was surprisingly low: raising the external osmotic pressure from 0.36 to 1.08 MPa, with sucrose or mannitol, only caused mean turgor pressure to decline from 0.27 to 0.18 MPa. We conclude that growing lily pollen regulates its turgor pressure remarkably well despite substantial variation in tube growth rate, tube length, and osmotic milieu.     

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 eq 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 (H=160 eq 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.Abbreviations CAM crassulacean acid metabolism - FW fresh weight - P turgor pressure     

Turgor pressure changes trigger characteristic changes in the electrical conductance of the tonoplast and the plasmalemma of the marine alga Valonia utricularis

The giant marine alga Valonia utricularis is capable of regulating its turgor pressure in response to changes in the osmotic pressure of the sea water. The turgor pressure response comprises two phases, a fast, exponential phase arising exclusively from water shifting between the vacuole and the external medium (time constant about 10 min) and a second very slow, almost exponential phase adjusting (but not always) the turgor pressure near to the original value by release or uptake of KCl (time constant about 5 h). The changes in the vacuolar membrane potential as well as in the individual conductances of the tonoplast and plasmalemma accompanying turgor pressure regulation were measured by using the vacuolar perfusion assembly (with integrated microelectrodes, pressure transducers and pressure‐regulating valves) as described by Wang et al. (J. Membrane Biology 157, 311–321, 1997). Measurements on pressure‐clamped cells gave strong evidence that the turgor pressure, but not effects related to water flow (i.e. electro‐osmosis or streaming potential) or changes in the internal osmotic pressure and in the osmotic gradients, triggers the cascade of osmotic and electrical events recorded after disturbance of the osmotic equilibrium. The findings definitely exclude the existence of osmosensors as postulated for other plant cells and bacteria. There was also evidence that turgor pressure signals were primarily sensed by ion transporters in the vacuolar membrane because conductance changes were first recorded in the many‐folded tonoplast and then significantly delayed in the plasmalemma independent of the direction of the osmotic challenge. Consistently, turgor pressure up‐regulation (but not down‐regulation) could be inhibited reversibly by external addition of the K⁺ transport inhibitor Ba²⁺ and/or by the Cl^– transport inhibitor 4,4′‐diisothiocyanatostilbene‐2,2′‐disulfonic acid (DIDS). Extensive studies under iso‐, hyper‐ and hypo‐osmotic conditions revealed that K⁺ and Cl^– contribute predominantly to the plasmalemma conductance. Addition of 0.3 mm NaCN showed further that part of the K⁺ and Cl^– transporters depended on ATP. These transporters are apparently up‐regulated upon hyper‐osmotic, but not hypo‐osmotic challenge. These findings explain the strong increase of the K⁺ influx upon lowering turgor pressure and the less pronounced pressure‐dependence of the Cl^– influx of V. utricularis reported in the literature. The data derived from the blockage experiments under hypo‐osmotic conditions were also equally consistent with the experimental findings that the K⁺ efflux is solely passive and progressively increases with increasing turgor pressure due to an increase of the volumetric elastic modulus of the cell wall. However, despite unravelling some of the sequences and other components involved in turgor pressure regulation of V. utricularis the co‐ordination between the ion transporters in the tonoplast and plasmalemma remains unresolved because of the failure to block the tonoplast transporters by addition of Ba²⁺ and DIDS from the vacuolar side.     

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.To whom correspondence should be addressed.     

Dynamics of leaf water relations components in co‐occurring iso‐ and anisohydric conifer species

Because iso‐ and anisohydric species differ in stomatal regulation of the rate and magnitude of fluctuations in shoot water potential, they may be expected to show differences in the plasticity of their shoot water relations components, but explicit comparisons of this nature have rarely been made. We subjected excised shoots of co‐occurring anisohydric Juniperus monosperma and isohydric Pinus edulis to pressure‐volume analysis with and without prior artificial rehydration. In J. monosperma, the shoot water potential at turgor loss (Ψ_TLP) ranged from ?3.4 MPa in artificially rehydrated shoots to ?6.6 MPa in shoots with an initial Ψ of ?5.5 MPa, whereas in P. edulis mean Ψ_TLP remained at ～ ?3.0 MPa over a range of initial Ψ from ?0.1 to ?2.3 MPa. The shoot osmotic potential at full turgor and the bulk modulus of elasticity also declined sharply with shoot Ψ in J. monosperma, but not in P. edulis. The contrasting behaviour of J. monosperma and P. edulis reflects differences in their capacity for homeostatic regulation of turgor that may be representative of aniso‐ and isohydric species in general, and may also be associated with the greater capacity of J. monosperma to withstand severe drought.     

Derivation of a weighted-average reflection coefficient for mesophyll cell membranes of Kalanchoë daigremontiana

In a study with the crassulacean acid metabolism (CAM) plant Kalanchoë daigremontiana Hamet et Perr. using the pressure probe, Rygol et al. (1987, Planta 172, 487–4493) calculated a value for the reflection coefficient () for malate of 0.6. This value was derived from the relationship between measured changes in cell turgor pressure and malic-acid concentration, and would imply that malate was a relatively ineffective osmoticum. Here we show that the calculation of Rygol et al. (1987) involved the implicit assumption that xylem tension was constant with changing cell turgor pressure and osmotic pressure. This has been shown not to be the case using the pressure-chamber technique. We present an alternative method of deriving a weighted-mean value of a for K. daigremontiana and show that it is not significantly different from 1.0.Part of this work was carried out at the University of Edinburgh, to whom we are grateful for facilities, with funding from the Agricultural and Food Research Council, UK. Murphy is grateful to the board of management of the Glasstone Benefaction for financial support at the University of Oxford. We thank Prof. U. Zimmermann for his comments on an earlier version of this paper.     

Osmotic relations of the drought-tolerant shrub Artemisia tridentata in response to water stress

Turgor maintenance, solute content and recovery from water stress were examined in the drought-tolerant shrub Artemisia tridentata. Predawn water potentials of shrubs receiving supplemental water remained above ?2 MPa throughout summer, while predawn water potentials of untreated shrubs decreased to ?5 MPa. Osmotic potentials decreased in conjunction with water potentials maintaining turgor pressures above 0 MPa. The decreases in osmotic potentials were not the result of osmotic adjustment (i.e. solute accumulation). Leaf solute contents decreased during drought, but leaf water volumes decreased more than 75% from spring to summer, thereby passively concentrating solutes within the leaves. The maintenance of positive turgor pressures despite decreases in leaf water volumes is consistent with other studies of species with elastic cell walls. Inorganic ion, organic acid, and carbohydrate contents of leaves declined during drought. The only solutes accumulating in leaves of A. tridentata with water stress were proline and a cyclitol, both considered compatible solutes. Total and osmotic potentials recovered rapidly following rewatering of shrubs; solute contents did not change except for a decrease in proline. Maintaining turgor through the passive concentration of solutes may be advantageous compared to synthesis of new solutes for osmotic adjustment in arid environments.     

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

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