The identification of aluminium-resistance genes provides opportunities for enhancing crop production on acid soils.

Acid soils restrict plant production around the world. One of the major limitations to plant growth on acid soils is the prevalence of soluble aluminium (Al(3+)) ions which can inhibit root growth at micromolar concentrations. Species that show a natural resistance to Al(3+) toxicity perform better on acid soils. Our understanding of the physiology of Al(3+) resistance in important crop plants has increased greatly over the past 20 years, largely due to the application of genetics and molecular biology. Fourteen genes from seven different species are known to contribute to Al(3+) tolerance and resistance and several additional candidates have been identified. Some of these genes account for genotypic variation within species and others do not. One mechanism of resistance which has now been identified in a range of species relies on the efflux of organic anions such as malate and citrate from roots. The genes controlling this trait are members of the ALMT and MATE families which encode membrane proteins that facilitate organic anion efflux across the plasma membrane. Identification of these and other resistance genes provides opportunities for enhancing the Al(3+) resistance of plants by marker-assisted breeding and through biotechnology. Most attempts to enhance Al(3+) resistance in plants with genetic engineering have targeted genes that are induced by Al(3+) stress or that are likely to increase organic anion efflux. In the latter case, studies have either enhanced organic anion synthesis or increased organic anion transport across the plasma membrane. Recent developments in this area are summarized and the structure-function of the TaALMT1 protein from wheat is discussed.

Plant aquaporins: multifunctional water and solute channels with expanding roles.

There is strong evidence that aquaporins are central components in plant water relations. Plant species possess more aquaporin genes than species from other kingdoms. According to sequence similarities, four major groups have been identified, which can be further divided into subgroups that may correspond to localization and transport selectivity. They may be involved in compatible solute distribution, gas-transfer (CO2, NH3) as well as in micronutrient uptake (boric acid). Recent advances in determining the structure of some aquaporins gives further details on the mechanism of selectivity. Gating behaviour of aquaporins is poorly understood but evidence is mounting that phosphorylation, pH, pCa and osmotic gradients can affect water channel activity. Aquaporins are enriched in zones of fast cell division and expansion, or in areas where water flow or solute flux density would be expected to be high. This includes biotrophic interfaces between plants and parasites, between plants and symbiotic bacteria or fungi, and between germinating pollen and stigma. On a cellular level aquaporin clusters have been identified in some membranes. There is also a possibility that aquaporins in the endoplasmic reticulum may function in symplasmic transport if water can flow from cell to cell via the desmotubules in plasmodesmata. Functional characterization of aquaporins in the native membrane has raised doubt about the conclusiveness of expression patterns alone and need to be conducted in parallel. The challenge will be to elucidate gating on a molecular level and cellular level and to tie those findings into plant water relations on a macroscopic scale where various flow pathways need to be considered.

Ammonia and amino acid transport across symbiotic membranes in nitrogen-fixing legume nodules.

Biological nitrogen fixation involves the reduction of atmospheric N2 to ammonia by the bacterial enzyme nitrogenase. In legume-rhizobium symbioses, the nitrogenase-producing bacteria (bacteroids) are contained in the infected cells of root nodules within which they are enclosed by a plant membrane to form a structure known as the symbiosome. The plant provides reduced carbon to the bacteroids in exchange for fixed nitrogen, which is exported to the rest of the plant. This exchange is controlled by plant-synthesised transport proteins on the symbiosome membranes. This review summarises our current understanding of these transport processes, focusing on ammonia and amino acid transport.

Malate-permeable channels and cation channels activated by aluminum in the apical cells of wheat roots.

Aluminum (Al(3+))-dependent efflux of malate from root apices is a mechanism for Al(3+) tolerance in wheat (Triticum aestivum). The malate anions protect the sensitive root tips by chelating the toxic Al(3+) cations in the rhizosphere to form non-toxic complexes. Activation of malate-permeable channels in the plasma membrane could be critical in regulating this malate efflux. We examined this by investigating Al(3+)-activated channels in protoplasts from root apices of near-isogenic wheat differing in Al(3+) tolerance at a single locus. Using whole-cell patch clamp we found that Al(3+) stimulated an electrical current carried by anion efflux across the plasma membrane in the Al(3+)-tolerant (ET8) and Al(3+)-sensitive (ES8) genotypes. This current occurred more frequently, had a greater current density, and remained active for longer in ET8 protoplasts than for ES8 protoplasts. The Al(3+)-activated current exhibited higher permeability to malate(2-) than to Cl(-) (P(mal)/P(Cl) > or = 2.6) and was inhibited by anion channel antagonists, niflumate and diphenylamine-2-carboxylic acid. In ET8, but not ES8, protoplasts an outward-rectifying K(+) current was activated in the presence of Al(3+) when cAMP was included in the pipette solution. These findings provide evidence that the difference in Al(3+)-induced malate efflux between Al(3+)-tolerant and Al(3+)-sensitive genotypes lies in the differing capacity for Al(3+) to activate malate permeable channels and cation channels for sustained malate release.

Channel-mediated permeation of ammonia gas through the peribacteroid membrane of soybean nodules.

Ammonia permeability of the peribacteroid membrane (PBM) from N(2)-fixing soybean nodules was measured (8x10(-5) m/s) using isolated PBM in a stopped-flow spectrofluorimeter. Ammonia (NH(3)) uptake into PBM vesicles was inhibited by up to 42% by HgCl(2) (EC(50)=2.9 microM, mercaptoethanol-reversible) and reduced by ATP pre-incubation. The activation energy of NH(3) uptake (52 kJ/mol) increased (118 kJ/mol) with HgCl(2). Water transport was also HgCl(2)-sensitive (EC(50)=52.6 microM), but increased by ATP pre-incubation. NH(3) and H(2)O may permeate via different pathways through Nodulin 26 or there is another protein on the PBM that is permeable to NH(3).

Fast activation of a time-dependent outward current in protoplasts derived from coats of developing Phaseolus vulgaris seeds.

An outward current that appeared to activate instantaneously in response to depolarising voltage pulses at low sampling frequencies predominated in the plasma membrane of ground-parenchyma protoplasts derived from coats of developing Phaseolus vulgaris L. (cv. Redland Pioneer) seeds. However, the outward current showed time-dependent activation when higher sampling frequencies were used to measure the current. Activation of the current was best described as a double-exponential time course with the fast and slow time constants being 1 and 20 ms, respectively. The current also exhibited a rapid deactivation that followed a double-exponential time course with time constants of approximately 2 and 30 ms, respectively. "Tail-current" analysis allowed us to show that this current exhibited a low selectivity between K- and Cl- (PK:Cl = 1.8). Such a fast-activating current may account for some of the reports of time-independent, instantaneous currents that have been observed in plasma membranes of plant cells digitised at low sampling frequencies. Therefore, when "instantaneous" currents appear it is advisable to characterise these currents using higher sampling frequencies with correspondingly higher filtering frequency cut-offs.

Tolerance of salinized floodplain conditions in a naturally occurring Eucalyptus hybrid related to lowered plant water potential.

Rising saline groundwater and reduced flooding frequency are causing dieback of Eucalyptus largiflorens F. Muell. along the Murray River in Australia. A green-leaved variant of E. largiflorens, which is probably a hybrid with a local mallee species (E. gracilis F. Muell.), tolerates saline conditions better than the more common grey-leaved variant. The green variant exhibited more negative water potentials than the grey variant, and comparison with soil water potential profiles indicated that the green variant extracted water from slightly higher up the soil profile where the salt content was lower but the soil was drier. However, the stable isotopes of water (2H and 18O) in the xylem did not differ significantly between paired green and grey trees, suggesting that both variants used the same water source. The green variant may be able to extract water for a longer period from a given point in the soil profile and tolerate a higher salt concentration around its roots than the grey variant. Predawn leaf water potentials of both variants decreased with increasing salinity of groundwater and decreasing depth to the groundwater, probably because the roots were being progressively confined to soil with lower matric potential as groundwater discharge through transpiration progressively salinized soil up the profile. The green variant had a lower assimilation rate and stomatal conductance than the grey variant, although the differences were not statistically significant during most of the year. Discrimination of 13C indicated that the green variant had a higher leaf internal CO2 concentration than the grey variant, indicative of a greater biochemical limitation on photosynthesis, perhaps resulting from the effects of operating at lower water potentials. The green variant had significantly lower stem hydraulic conductivity than the grey variant, probably because of its smaller xylem vessel diameter and higher degree of embolism. The more conservative water use of the green variant and its ability to operate at lower water potential than the grey variant appear to underlie its ability to tolerate conditions of reduced useable water above the saline groundwater. This advantage appears to outweigh the costs of increased xylem embolism and reduced assimilation.

Characterization of an ammonium transport protein from the peribacteroid membrane of soybean nodules.

Nitrogen-fixing bacteroids in legume root nodules are surrounded by the plant-derived peribacteroid membrane, which controls nutrient transfer between the symbionts. A nodule complementary DNA (GmSAT1) encoding an ammonium transporter has been isolated from soybean. GmSAT1 is preferentially transcribed in nodules and immunoblotting indicates that GmSAT1 is located on the peribacteroid membrane. [14C]methylammonium uptake and patch-clamp analysis of yeast expressing GmSAT1 demonstrated that it shares properties with a soybean peribacteroid membrane NH4^{+ channel described elsewhere. GmSAT1 is likely to be involved in the transfer of fixed nitrogen from the bacteroid to the host.}

Aluminum activates an anion channel in the apical cells of wheat roots.

We describe an anion channel in the plasmalemma of protoplasts isolated from wheat (Triticum aestivum L.) roots that is activated by aluminum (Al3+). In the whole-cell configuration, addition of 20-50 microM AlCl3 to the external solution depolarized the membrane and activated an inward current that could remain active for more than 60 min. The activation by Al3+ was rapid in 20% of protoplasts examined, whereas in another 30% a delay of more than 10 min occurred after Al3+ was added. Once the current was activated, changing the external Cl- concentration shifted the membrane reversal potential with ECl, showing that the channel is more selective for anions than cations (Ca2+, K+, tetraethylammonium+). The channel could be activated by Al3+, but not by La3+, and was observed in protoplasts isolated from the root apex but not in protoplasts isolated from mature root tissue. The anion channel antagonist niflumate inhibited the current in whole cell measurements by 83% at 100 microM. Outside-out patch recordings revealed a multistate channel with single-channel conductances of between 27 and 66 pS.

Effect of low oxygen concentration on the electrical properties of cortical cells of wheat roots.

Short-term effect of oxygen-deficiency on the membrane potential difference (PD), membrane resistance of cortical cells and electrical coupling between cortical cells was investigated using excised wheat roots. Hypoxia rapidly depolarised the membrane potential of the cortical cells by about 60 mV, while hypoxia had little effect on the membrane resistance of the cells. No significant change in membrane resistance by potassium channel blockers, TEA+ and verapamil, under hypoxia was observed. The electrical coupling ratio, which is a measure of plasmodesmatal resistance, between cortical cells of wheat roots was 5.9 % in aerated solution and was not affected by the low oxygen treatment, suggesting that solute transport through cytoplasmic annulus of plasmodesmata could not be affected. The possible involvement of the endoplasmic reticulum in intercellular transport of solute and water is discussed.

Pathways for the permeation of Na+ and Cl- into protoplasts derived from the cortex of wheat roots.

Sodium permeation into cortex cells of wheat roots was examined under conditions of high external NaCI and low Ca(2+). Two types of K(+) inward rectifier were observed in some cells. The time-dependent K(+) inward rectifier was Ca(2+)-sensitive, increasing in magnitude as external Ca(2+) was decreased from 10 mM to 0.1 mM, but did not show significant permeability to Na(+). However, the spiky inward rectifier showed significant Na+ permeation at Ca(2+) concentrations of 1 and 10 mM. In cells that initially did not show K(+) inward rectifier channels, fast and sometimes slowly activating whole-cell inward currents were induced at membrane potentials negative of zero with high external Na(+) and low Ca(2+) concentrations. With 1 mM Ca(2+) in the external solution, large inward currents were carried by Rb(+), Cs(+), K(+), Li(+), and Na(+). The permeability sequence shows that K(+), Rb(+) and Cs(+) are all more permeant than Na(+), which is about equally as permeant as Li(+). When some K(+) was present with high concentrations of Na(+) the inward currents were larger than with K(+) or Na(+) alone. About 60% of the inward current was reversibly blocked when the external Ca(2+) activity was increased from 0.03 mM to 2.7 mM (half inhibition at 0.31 mM Ca(2+) activity). Changes in the characteristics of the current noise indicated that increased Ca(2+) reduced the apparent single channel amplitude. In outside-out patches inward currents were observed at membrane potentials more positive than the equilibrium potentials for K(+) and Cl(-) when the external Na(+) concentration was high. These channels were difficult to analyse but three analysis methods yielded similar conductances of about 30 pS.

Characterization of Water Channels in Wheat Root Membrane Vesicles.

The functional significance of water channels in wheat (Triticum aestivum L.) root membranes was assessed using light scattering to measure vesicle shrinking in response to osmotic gradients rapidly imposed in a stopped flow apparatus. Vesicles were obtained from both a plasma membrane fraction and a plasma membrane-depleted endomembrane fraction including tonoplast vesicles. Osmotic water permeability (Pos) in the endomembrane fraction was high (Pos= 86.0 [mu]m s-1) with a low activation energy (EA= 23.32 kJ mol-1 [plus or minus] 3.88 SE), and was inhibited by mercurials (K1= 40 [mu]M HgCl2, where K1 is the inhibition constant for half-maximal inhibition), suggesting participation of water channels. A high ratio of osmotic to diffusional permeability (Pd) (using D2O as a tracer, Pos/Pd = 7 [plus or minus] 0.5 SE) also supported this view. For the endomembrane fraction there was a marked decrease in Pos with increasing osmotic gradient that was not observed in the plasma membrane fraction. Osmotic water permeability in the plasma membrane fraction was lower (Pos= 12.5 [mu]m s-1) with a high activation energy (EA= 48.07 kJ mol-1 [plus or minus] 3.63 SE) and no mercury inhibition. Nevertheless, Pos/Pd was found to be substantially higher than one (Pos= 3 [plus or minus] 0.2 SE), indicating that water channels mediated water flow in this fraction, too. Possible distortion of the Pos/Pd value by unstirred layer effects was shown to be unlikely.

Ion channels in the plasma membrane of protoplasts from the halophytic angiosperm Zostera muelleri.

Patch clamp studies show that there may be as many as seven different channel types in the plasma membrane of protoplasts derived from young leaves of the halphytic angiosperm Zostera muelleri. In wholecell preparations, both outward and inward rectifying currents that activate in a time- and voltage-dependent manner are observed as the membrane is either depolarized or hyperpolarized. Current voltage plots of the tail currents indicate that both currents are carried by K+. The channels responsible for the outward currents have a unit conductance of approximately 70 pS and are five times more permeable to K+ than to Na+. In outside-out patches we have identified a stretch-activated channel with a conductance of 100 pS and a channel that inwardly rectifies with a conductance of 6 pS. The reversal potentials of these channels indicate a significant permeability to K+. In addition, the plasma membrane contains a much larger K+ channel with a conductance of 300 pS. Single channel recordings also indicate the existence of two Cl- channels, with conductances of 20 and 80 pS with distinct substates. The membrane potential difference of perfused protoplasts showed rapid action potentials of up to 50 mV from the resting level. The frequency of these action potentials increased as the external osmolarity was decreased. The action potentials disappeared with the addition of Gd3+, an effect that is reversible upon washout.

Pump and K+ inward rectifiers in the plasmalemma of wheat root protoplasts.

An electrogenic pump, a slowly activating K+ inward rectifier and an intermittent, "spiky," K+ inward rectifier, have been identified in the plasmalemma of whole protoplasts from root cortical cells of wheat (Triticum) by the use of patch clamping techniques. Even with high external concentrations of K+ of 100 mM, the pump can maintain the membrane potential difference (PD) down to -180mV, more negative than the electrochemical equilibrium potentials of the various ions in the system. The slowly activating K+ inward rectifier, apparent in about 23% of protoplasts, allows inward current flow when the membrane PD becomes more negative than the electrochemical equilibrium potential for K+ by about 50 mV. The current usually consists of two exponentially rising components, the time constant of one about 10 times greater than the other. The longer time constant is voltage dependent, while the smaller time constant shows little voltage dependence. The rectifier deactivates, on return of the PD to less negative levels, with a single exponential time course, whose time constant is strongly voltage dependent. The spiky K+ inward rectifier, present in about 68% of protoplasts, allows intermittent current, of considerable magnitude, through the plasmalemma at PDs usually more negative than about -140mV. Patch clamp experiments on detached outside-out patches show that a possibly multi-state K+ channel, with maximum conductance greater than 400 pS, may constitute this rectifier. The paper also considers the role of the pump and the K+ inward rectifiers in physiological processes in the cell.

Multiple conductances in the large K+ channel from Chara corallina shown by a transient analysis method.

The large conductance K+ channel in the tonoplast of Chara corallina has subconductance states (substates). We describe a method that detects substates by monitoring the time derivative of channel current. Substates near to the full conductance tend to have long durations and high probabilities, while those of smaller amplitude occur with less probability and short duration. The substate pattern is similar in cell-attached, inside-out and outside-out patches over a range of temperatures. The pattern changes at high Ca2+ concentration (10 mol m-3) on the cytoplasmic face of inside-out patches. One substate at approximately 50% of the full conductance is characterized by a high frequency of transitions from the full conductance level. This midstate conductance is not a constant proportion of the full conductance but changes as a function of membrane potential difference (p.d.) showing strong inward rectification. We suggest that the channel is a single pore that can change conformation and/or charge profile to give different conductances. The mean durations of the full conductance level and the midstate decrease as the membrane p.d. becomes more negative. Programs for analysis of channel kinetics based on an half-amplitude detection criterion are shown to be unsuitable for analysis of the K+ channel.

The k/na selectivity of a cation channel in the plasma membrane of root cells does not differ in salt-tolerant and salt-sensitive wheat species.

The characteristics of cation outward rectifier channels were studied in protoplasts from wheat root (Triticum aestivum L. and Triticum turgidum L.) cells using the patch clamp technique. The cation outward rectifier channels were voltage-dependent with a single channel conductance of 32 +/- 1 picosiemens in 100 millimolar KCl. Whole-cell currents were dominated by the activity of the cation outward rectifiers. The time- and voltage-dependence of these currents was accounted for by the summed behavior of individual channels recorded from outside-out detached patches. The K(+)/Na(+) permeability ratio of these channels was measured in a salt-sensitive and salt-tolerant genotype of wheat that differ in rates of Na(+) accumulation, using a voltage ramp protocol on protoplasts in the whole-cell configuration. Permeability ratios were calculated from shifts in reversal potentials following ion substitutions. There were no significant differences in the K(+)/Na(+) permeability ratios of these channels in root cells from either of the two genotypes tested. The permeability ratio for K(+)/Cl(-) was greater than 50:1. The K(+)/Na(+) permeability ratio averaged 30:1, which is two to four times more selective than the same type of channel in guard cells and suspension culture cells. Lowering the Ca(2+) concentration in the bath solution to 0.1 millimolar in the presence of 100 millimolar Na(+) had no significant effect on the K(+)/Na(+) permeability ratios of the channel. It seems unlikely that the mechanism of salt tolerance in wheat is based on differences in the K(+)/Na(+) selectivity of these channels.

Ion channels in the plasma membrane of Amaranthus protoplasts: one cation and one anion channel dominate the conductance.

This report details preliminary findings for ion channels in the plasma membrane of protoplasts derived from the cotyledons of Amaranthus seedlings. The conductance properties of the membrane can be described almost entirely by the behavior of two types of ion channel observed as single channels in attached and detached patches. The first is a cation-selective outward rectifier, and the second a multistate anion-selective channel which, under physiological conditions, acts as an inward rectifier. The cation channel has unit conductance of approx. 30 pS (symmetrical 100 K+) and relative permeability sequence K+ greater than Na+ much greater than Cl- (1:0.16:0.03): whole-cell currents activate in a time-dependent manner, and both activation and deactivation kinetics are voltage dependent. The anion channel opens for hyperpolarized membrane potentials, has a full-level conductance of approx. 200 pS and multiple subconductance states. The number of subconductances does not appear to be fixed. When activated the channel is open for long periods, though shuts if the membrane potential (Vm) is depolarized; at millimolar levels of [Ca2+]cyt this voltage dependency disappears. Inward current attributable to the anion channel is not observed in whole-cell recordings when MgATP (2 mM) is present in the intracellular solution. By contrast the channel is active in most detached patches, whether MgATP is present or not on the cytoplasmic face of the membrane. The anion channel has a significant permeability to cations, the sequence being NO3- greater than Cl- greater than K+ greater than Aspartate (2.04:1:0.18 to 0.09:0.04). The relative permeability for K+ decreased at progressively lower conductance states. In the absence of permeant anions this channel could be mistaken for a cation inward rectifier. The anion and cation channels could serve to clamp Vm at a preferred value in the face of events which would otherwise perturb Vm.

Determination of Solute Permeability in Chara Internodes by a Turgor Minimum Method : Effects of External pH.

The analysis of Sha'afi et al. (Sha'afi, Rich, Mickulecky, Solomon 1970 J Gen Physiol 55: 427-450) for determining solute permeability in red blood cells has been modified and applied to turgid plant cells. Following the addition of permeating solute to the external medium, a biphasic response of cell turgor can be measured with the pressure probe in isolated internodes of Chara corallina. After an initial decrease in turgor due to water flow (water phase), turgor increases due to the uptake of the solute (solute phase) until the original turgor is reattained. From the pressure/time course in the neighborhood of the minimum turgor, the permeability of the osmotic solute can be determined. The data obtained by the minimum method for rapidly permeating (ethanol, methanol) and slowly permeating (formamide, dimethylformamide) solutes are similar to those calculated from the half-time of pressure changes during the solute phase and to data obtained by other workers using radioactive tracers. The methods employing the pressure probe were applied to examine the effect of high pH (up to pH 11) on the membrane permeability. There appeared to be no effect of high pH on the permeability coefficients, reflection coefficients, and hydraulic conductivity.

The effect of different growing conditions on water relations parameters of leaf epidermal cells of Tradescantia virginiana L.

Tradescantia virginiana L. plants were cultivated under contrasting conditions of temperature, humidity, light quality and intensity, and nutrient status in order to investigate the effect of growth conditions on the water relations parameters of the leaf epidermal cells. Turgor pressure (P), volumetric elastic modulus (ɛ), half-time of water potential equilibration (T 1/2), hydraulic conductivity (L p ) were measured with the miniaturized pressure probe in single cells of the upper and lower epidermis of leaves. Turgor differed (range: 0.1 bar to 7.2 bar) between treatments with lowest values under warm and humid conditions and additional supply of fertilizer, and highest values under conditions of low air humidity and low nutrient supply. The volumetric elastic modulus changed by 2 orders of magnitude (range: 3.0 bar to 350 bar, 158 cells), but ɛ was only affected by the treatments, in as much as it was dependent on turgor. The turgor dependence of ɛ, measured on intact leaves of T. virginiana, was similar to that for cells of the isolated (peeled) lower epidermis, where ɛ as a function of turgor was linear over the whole range of turgors. This result has implications for the discussion of pressure/volume curves as measured by the pressure bomb where changes in "bulk leaf ɛ" are frequently discussed as "adaptations" to certain treatments. The measurements of the hydraulic conductivity indicate that this parameter varies between treatments (range of means: 2.4×10-6 cm s-1 bar-1 to 13.4×10-6 cm s-1 bar-1). There was a negative correlation for L p in cells of intact leaves as a function of turgor which was altered by the growing conditions. However, a correlation with turgor could not be found for cells from isolated epidermis or cells from a uniform population of plants. The large variation in L p from cell to cell observed in the present and in previous studies was accounted for in a study of 100 cells from a uniform population of plants by the propagation of measurement errors in calculating L p . The results suggest that in T. virginiana cellular water relations are changed mainly by the turgor dependence of ɛ.

Water Relations of Seagrasses: STATIONARY VOLUMETRIC ELASTIC MODULUS AND OSMOTIC PRESSURE OF THE LEAF CELLS OF HALOPHILA OVALIS, ZOSTERA CAPRICORNI, AND POSIDONIA AUSTRALIS.

The stationary volumetric elastic modulus (epsilon(s)) of the leaf cells of three seagrasses (Halophila ovalis (R.Br.) Hook, Zostera capricorni Aschers, and Posidonia australis Hook f.) was evaluated from estimates of epsilon(s) plus intracellular osmotic pressure (epsilon(s) + II(i)) and II(i). The estimates of (epsilon(s) + II(i)) were made using a linear displacement transducer to measure very small changes in thickness of leaf tissue produced by changes in external osmotic pressure (II(o)). epsilon(s) increases with increasing turgor pressure in each of the species and the maximum values of epsilon(s) are: 22 megapascals for H. ovalis, 17 megapascals for Z. capricorni, and 51 megapascals for P. australis.There is a hysteresis in thickness changes versus changes in II(o) which indicates a hysteresis in the relationship between volume and turgor pressure. The hysteresis results in epsilon(s) being different for swelling and for shrinking cells over the same range of II(o) and this may be important in other aspects of plant-water relations.A new design of an apparatus employing a linear displacement transducer for measuring very small changes in tissue thickness is described. The new design has the advantages of virtually frictionless movement and a precision of 0.05 micrometer.