The effect of abscisic acid on cell turgor pressures,solute content and growth of wheat roots

Abscisic acid (ABA) was shown to influence turgor pressure and growth in wheat (Triticum aestivum L.) roots. At a concentrations of 25 mmol·m^-3, ABA increased the turgor pressure of cells located within 1 cm of the tip by up to 450 kPa. At 4 to 5 cm from the root tip this concentration of ABA reduced the turgor pressure of peripheral cells (epidermis and the first few cortical cell layers) to zero or close to zero while that of the inner cells was increased. Increases in sap osmolality were dependent on the concentration of ABA and the effect saturated at 5 mmol·m^-3 ABA. The increase in osmolality took about 4 h and was partly the result of reducing-sugar accumulation. Levels of inorganic cations were not affected by ABA. Root growth was inhibited at ABA concentrations that caused a turgor-pressure increase. The results show that while ABA can affect root cell turgor pressures, this effect does not result in increased root growth.Abbreviation ABA abscisic acid     

Generation of action potentials in Chara corallina by turgor pressure changes

A depolarisation of the membrane potential difference (about-170 mV) of Chara corallina is observed in response to changes in cell turgor pressure using the pressure probe technique. The depolarisation occurs in phase with the pressure pulse (0.2 s duration) and is independent of the direction of the applied pressure gradient. This latter finding is in contradiction to results predicted on the basis of electro-kinetic phenomena. Pressure induced electrical leakages were ruled out by measuring the depolarisation in response to pressure in individual segments of the internode which were electrically isolated from one another. The changes in potential were recorded by external electrodes and an internal electrode which was positioned close to the micropipette of the pressure probe inserted through one of the electrically isolated nodes. The depolarisation in response to increasing positive or negative pressure gradients in the intact node region and in the intact middle segments was comparable to that monitored in the node region containing the pressure probe. Action potentials were initiated when the pressure gradients exceeded at least 2 bar. The action potentials were elicited at random in one of the two compartments adjacent to the node regions, but were never found to be initiated in the node regions themselves. The pressure-induced action potentials are explained in terms of an electro-mechanical compression (or expansion) of a local membrane area and discussed in their relevance to the propagation of pressure signals in response to water and salt stress in higher plants.Abbreviations PD potential difference     

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.     

Does turgor limit growth in tall trees?

The gravitational component of water potential contributes a standing 0.01 MPa m^?1 to the xylem tension gradient in plants. In tall trees, this contribution can significantly reduce the water potential near the tree tops. The turgor of cells in buds and leaves is expected to decrease in direct proportion with leaf water potential along a height gradient unless osmotic adjustment occurs. The pressure–volume technique was used to characterize height‐dependent variation in leaf tissue water relations and shoot growth characteristics in young and old Douglas‐fir trees to determine the extent to which growth limitation with increasing height may be linked to the influence of the gravitational water potential gradient on leaf turgor. Values of leaf water potential (Ψ_l), bulk osmotic potential at full and zero turgor, and other key tissue water relations characteristics were estimated on foliage obtained at 13.5 m near the tops of young (approximately 25‐year‐old) trees and at 34.7, 44.2 and 55.6 m in the crowns of old‐growth (approximately 450‐year‐old) trees during portions of three consecutive growing seasons. The sampling periods coincided with bud swelling, expansion and maturation of new foliage. Vertical gradients of Ψ_l and pressure–volume analyses indicated that turgor decreased with increasing height, particularly during the late spring when vegetative buds began to swell. Vertical trends in branch elongation, leaf dimensions and leaf mass per area were consistent with increasing turgor limitation on shoot growth with increasing height. During the late spring (May), no osmotic adjustment to compensate for the gravitational gradient of Ψ_l was observed. By July, osmotic adjustment had occurred, but it was not sufficient to fully compensate for the vertical gradient of Ψ_l. In tall trees, the gravitational component of Ψ_l is superimposed on phenologically driven changes in leaf water relations characteristics, imposing potential constraints on turgor that may be indistinguishable from those associated with soil water deficits.     

Combined solvent and water activity stresses on turgor regulation and membrane adaptation in Oceanimonas baumannii ATCC 700832

Oceanimonas baumannii ATCC 700832 is a Gram negative marine bacterium capable of utilising phenol as a sole carbon source. The ability of the bacterium to tolerate low water activity when utilising either succinate or phenol as a substrate in minimal medium was studied. The membrane lipid and protein composition showed two discreet adaptive phases as salinity increased. Firstly, when NaCl concentration was increased from 0.15% (w/v), the minimum at which growth was observed, to 1% NaCl (w/v), the ratio of zwitterionic to anionic phospholipids in the membrane increased significantly. At the same time the ratio of saturated to unsaturated fatty acids and the total membrane protein decreased significantly. The second phase was observed when salinity was increased from 1% to 7% NaCl (w/v) as the ratio of zwitterionic to anionic phospholipids decreased and membrane protein increased. However, the ratio of saturated to unsaturated fatty acids was unaffected. Salinity also affected the tolerance of cultures to elevated levels of phenol. Cultures grown in 0.15% NaCl (w/v) could tolerate 12 mM phenol, whereas in the presence of 1% NaCl (w/v) cultures continued to grow in up to 20 mM phenol and in 7% NaCl (w/v) cultures 8 mM phenol could be tolerated. Changes to the composition of the membrane phospholipids and fatty acids were also observed when phenol concentrations were at the maximum that could be tolerated. Under such conditions the ratio of zwitterionic to anionic phospholipids decreased twofold compared to cultures utilising 4 mM phenol as the substrate, in all salinities except in 7% NaCl (w/v) cultures, where there was no significant effect. The ratio of saturated to unsaturated fatty acids increased significantly in all salinities compared to cultures grown with 4 mM phenol. This revised version was published online in June 2006 with corrections to the Cover Date.     

Graphical evaluation and partitioning of turgor responses to drought in leaves of durum wheat

The relationship between relative water content (R) and turgor potential (_p) may be derived from pressure-volume (PV) curves and analyzed in various ways. Fifty PV curves were measured with the pressure chamber on leaves of durum wheat (Triticum durum L.). The plots of _p versus R were highly variable and could not be adequately described by a single mathematical function. The area below the curve was therefore determined by means of an area meter. This procedure gave the integral of turgor from full saturation to the turgor-loss point. Responses to drought treatment could thus be quantified and partitioned into effects of osmotic adjustment and elastic adjustment. These two adjustment responses, which are probably of different metabolic origin, together improve turgor maintenance in durum wheat considerably.Abbreviations and symbols PV pressure-volume - R relative water content - T_i turgor integral between full saturation and turgor-loss point - _p turgor (pressure) potential     

Growth at reduced turgor: irreversible and reversible cell-wall extension of maize coleoptiles and its implications for the theory of cell growth

The relationship between steady-state elongation rate (G) and turgor pressure (P; G/P curve) was investigated using isolated segments of maize (Zea mays L.) coleoptiles incubated in osmotic solutions of a water potential range of 0 to -10 bar (polyethylene glycol 6000 as osmoticum). Short-term elongation measurements revealed curvilinear G/P curves with a steep slope at high turgor and a shallow slope at low turgor. Owing to a decrease of osmotic pressure and turgor, there was a tendency for straightening of the G/P curves during long-term elongation. An elongation rate of zero was adjusted by lowering the turgor by 4.5 bar at a constant osmotic pressure of 6.7 bar. Auxin increased — whereas abscisic acid decreased — the slope of the G/P curve but these hormones had no effect on the threshold turgor of growth (Y = 2.2 bar). It is concluded that extensibility of the growing cell walls represented by the yielding coefficient of Lockhart's growth equation is turgor-dependent and therefore decreases to a very low value as the turgor approaches Y. When the turgor was kept at Y, a constant segment length was maintained over at least 6 h. However, separation of reversible (l_rev) and irreversible (l_irr) components of total (in vivo) length (l_tot = l_rev + l_irr) W measuring segment length before and after freezing/thawing revealed that l_irr increased continuously and l_rev decreased continuously at constant l_tot. After a step-down in turgor the segments grew in l_irr although they shrank in l_tot over the whole turgor range of 0irr irreversible length - l_rev reversible length - l_tot total length (= l_irr + l_rev) - _i osmotic pressure of cell sap - _i water potential of tissue - _o water potential of incubation medium - ABA abscisic acid - G growth rate - m yielding coefficient - P turgor pressure - PEG polyethylene glycol 6000 - Y yield threshold Supported by Deutsche Forschungsgemeinschaft (SFB 206). We thank R. Hertel for helpful comments.     

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     

Influence of cell turgor on sucrose partitioning in potato tuber storage tissues

Sucrose uptake and partitioning in potato (Solanum tuberosum L.) tuber discs were examined under a range of mannitol and ethylene-glycol concentrations. Mannitol caused the same changes in turgor over a wide range of incubation periods (90 min-6 h), indicating that it did not penetrate the tissue. In comparison, ethylene glycol reduced turgor losses but did not eliminate them, even after 6 h. Between 100 mM and 300 mM mannitol, turgor fell by 350 kPa, compared with 35 kPa in ethylene glycol. Uptake experiments in mannitol alone showed that total sucrose uptake was strongly correlated with both osmotic potential and with turgor potential. In subsequent experiments sucrose uptake and partitioning were examined after 3 h equilibration in 100 mM and 300 mM concentrations of mannitol and ethylene glycol. Total sucrose uptake and the conversion of sucrose to starch were enhanced greatly only at 300 mM mannitol, indicating an effect of turgor, rather than osmotic potential on sucrose partitioning. The inhibitors p-chloromercuribenzenesulfonic acid and carbonylcyanide m-chlorophenylhydrazone (CCCP) both reduced sucrose uptake, but in quite different ways. p-Chloromercuribenzenesulfonic acid reduced total sucrose uptake but did not affect the partitioning of sucrose to starch. By contrast, CCCP inhibited total uptake and virtually eliminated the conversion of sucrose to starch. Despite this, sucrose uptake in the presence of CCCP continued to increase as the mannitol concentration increased, indicating an increase in passive transport at higher mannitol concentrations. Increased sucrose uptake above 400 mM mannitol was shown to be the result of uptake into the free space. The data show that starch synthesis is optimised at low but positive turgors and the relation between sucrose partitioning and the changing diurnal water relations of the tuber are discussed.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - PCMBS p-chloromercuribenzenesulfonic acid     

Osmotic adjustment and the inhibition of leaf,root, stem and silk growth at low water potentials in maize

The expansion growth of plant organs is inhibited at low water potentials ( _w), but the inhibition has not been compared in different organs of the same plant. Therefore, we determined elongation rates of the roots, stems, leaves, and styles (silks) of maize (Zea mays L.) as soil water was depleted. The _w was measured in the region of cell expansion of each organ. The complicating effects of transpiration were avoided by making measurements at the end of the dark period when the air had been saturated with water vapor for 10 h and transpiration was less than 1% of the rate in the light. Growth was inhibited as the _w in the region of cell expansion decreased in each organ. The _w required to stop growth was-0.50,-0.75, and-1.00 MPa, in this order, in the stem, silks, and leaves. However, the roots grew at these _w and ceased only when _w was lower than-1.4 MPa. The osmotic potential decreased in each region of cell expansion and, in leaves, roots and stems, the decrease was sufficient to maintain turgor fully. In the silks, the decrease was less and turgor fell. In the mature tissue, the _w of the stem, leaves and roots was similar to that of the soil when adequate water was supplied. This indicated that an equilibrium existed between these tissues, the vascular system, and the soil. At the same time, the _w was lower in the expanding regions than in the mature tissues, indicating that there was a _w disequilibrium between the growing tissue and the vascular system. The disequilibrium was interpreted as a _w gradient for supplying water to the enlarging cells. When water was withheld, this gradient disappeared in the leaf because _w decreased more in the xylem than in the soil, indicating that a high flow resistance had developed in the xylem. In the roots, the gradient did not decrease because vascular _w changed about the same amount as the soil _w. Therefore, the gradient in _w favored water uptake by roots but not leaves at low _w. The data show that expansion growth responds to low _w differently in different growing regions of the plant. Because growth depends on the maintenance of turgor for extending the cell walls and the presence of _w gradients for supplying water to the expanding cells, several factors could have been responsible for these differences. The decrease of turgor in the silks and the loss of the _w gradient in the leaves probably contributed to the high sensitivity of these organs. In the leaves, the gradient loss was so complete that it would have prevented growth regardless of other changes. In the roots, the maintenance of turgor and _w gradients probably allowed growth to continue. This difference in turgor and gradient maintenance could contribute to the increase in root/shoot ratios generally observed in water-limited conditions.Symbols _s osmotic potential - _w water potential     

Effects of blue light on electrical potential and turgor in pulvinar motor cells ofPhaseolus

Pulvinar motor cells ofPhaseolus vulgaris L display transient depolarization of the membrane potential and a turgor pressure decrease when exposed to a pulse of blue light. To analyze the mechanism of the transient depolarization, the effects of some factors such as anoxia, metabolic inhibitors and specific inhibitors of H⁺-ATPase have been examined. The findings have led to the conclusion that blue light inactivates the electrogenic H⁺-pumping ATPase in the plasma membrane of the motor cells. This inactivation seems to suppress ion uptake and decrease the turgor pressure of the motor cells.     

Auxin action on proton influx in corn roots and its correlation with growth

At concentrations inhibitory to the elongation of corn (Zea mays L.) roots, the auxins, indole-3-acetic acid (IAA) and α-naphthaleneacetic acid (α-NAA), cause an increase in the pH of the bathing medium; this increase occurs with an average latent period shorter than the latent period for the inhibitory effect of these auxins on elongation. Indole-2-carboxylic acid, an inactive structural analogue of IAA, and β-naphthaleneacetic acid, an inactive analogue of α-NAA, affect neither growth nor the pH of the medium. Since acid pH is known to promote and basic pH to inhibit root elongation, the data are consistent with the hypothesis that hormone-induced modification of cell-wall pH plays a role in the control of elongation of roots, as has been proposed for elongation of stems and coleoptiles.     

Effects of turgor potential on 1-aminocyclopropane-1-carboxylic acid uptake into the vacuolar compartment and ethylene production in tomato pericarp slices

While solute transport and ethylene production by plant tissue are sensitive to the osmotic concentration of the solution bathing the tissue, the influence of tissue water relations and specifically tissue turgor potential on the kinetics of 1-aminocyclopropane-1-carboxylic acid (ACC) uptake into the vacuolar compartment and ethylene production have not been examined. 1-Aminocyclopropane-1-carboxylic acid transport and ethylene production were examined in tomato (Lycopersicon esculentum Mill. cv. Liberty) pericarp slices incubated in solutions having a range of mannitol, polyethylene glycol 3350 and ethylene glycol concentrations known to affect tissue water relations. Tissue osmotic and turgor potentials were derived from osmolality measurements of cell saps recovered by freeze-thawing and corrected for the contribution of the free-space solution. When relatively nonpermeable (mannitol or polyethylene glycol 3350) osmotica were used, both ACC uptake and ethylene production were greatest at a solution osmolality of 230 milliosmolal where tissue turgor potential ranged between 120 and 140 kPa. At higher and lower turgor potentials, the high-affinity saturating component of ACC uptake and ethylene production were inhibited, and ACC efflux from the vacuolar compartment was increased. The inhibition of ACC uptake was evident as a decrease in V_max with no effect on K_m. Turgor potential changes caused by adjusting solution osmolality with mannitol or polyethylene glycol 3350 were accompanied by changes in the osmotic potential and water potential of the tissue. The effects of turgor potential vs the osmotic and water potentials of tomato pericarp slices were differentiated by comparing responses to nonpermeable osmotica and mixtures of nonpermeable and permeable osmotica. Ethylene glycol-mannitol mixtures had effects on the osmotic potential and water potential of the tissue similar to those of nonpermeable osmotica but had less effect on tissue turgor, ACC transport and ethylene production. Incubating tissue in solutions without nonpermeable osmotica osmotically shocked the tissue. Increasing solution osmolality with ethylene glycol in the absence of nonpermeable osmotica increased tissue turgor and ethylene production. The present study indicates that tissue turgor is an important factor affecting the kinetics of ACC uptake into the vacuolar compartment and ethylene production in tomato pericarp slices.     

Combined effect of NaCl-salinity and hypoxia on growth,photosynthesis, water relations and solute accumulation in Phragmites australis plants

Aim of the present study was to investigate the effects of two key environmental factors of estuarine ecosystems, salinity and hypoxia, on the physiological attributes in reed plants (Phragmites australis (Cav.) Trin. ex Steudel). Growth, leaf gas exchange, water (and ion) relations, and osmotic adjustment were determined in hydroponically grown plants exposed to hypoxia at varying NaCl-salinity concentrations (0, 50, 100, and 200 mM). Plants grew well under hypoxia treatment with standard nutrient solution without added salt and at NaCl concentrations up to 100 mM. Reed plants were able to produce and allocate phytomass to all their organs even at the highest salt level (200 mM NaCl). In plants subjected to hypoxia at various water potentials no clear relationships were found between growth and photosynthetic parameters except for g_s, whereas growth displayed a highly significant correlation with plant–water relations. A and g_s of reed plants treated with hypoxia at varying water potential of nutrient solutions were positively correlated and the former variable also had a strong positive relationship with E. Leaf Ψ_w and Ψ_π followed a similar trend and declined significantly as water potential of watering solutions was lowered. Highly significant positive correlations were identified between leaf Ψ_w and photosynthetic parameters. At all NaCl concentrations, the increase in total inorganic ions resulted from increased Na⁺ and Cl⁻ while K⁺, Ca²⁺, and Mg²⁺ concentrations decreased with increasing osmolality of nutrient solutions. Common reed has an efficient mechanism of Na⁺ exclusion from the leaves and exhibited a high leaf K⁺/Na⁺ selectivity ratio over a wide range of salinities under hypoxia treatment. In Phragmites australis grown in 200 mM NaCl, K⁺ contributed 17% toΨ_π, whereas Na⁺ and Cl⁻ accounted for only 11% and 6%, respectively. At the same NaCl concentration, the estimated contribution of proline to Ψ_π was less than 0.2%. Changes in leaf turgor occurred with a combined effect of salinity and hypoxia, suggesting that reed plants could adjust their water status sufficiently.     

The pressure-jump technique shows maize leaf growth to be enhanced by increases in turgor only when water status is not too high

This study on expansive growth of the first leaf of maize has two goals: one is to determine how the sensitivity of growth to changes in water status varies with the initial water status of the leaf, and the other is to adapt the pressure-jump technique of Okamoto et al. (1989 , Plant and Cell Physiology 30, 979–985), developed for studying growth of excised stem segments, for use on whole seedlings. Initial water status was varied by using: transpiring vs. non-transpiring conditions, seedlings differing in emerged leaf length and hence transpiring area, and root medium without mannitol vs. medium with added mannitol (to –0·3 MPa). The results show that growth changed with changes in plant water status when the water status was low, but was unaffected when water status was very high. A stepwise change in hydrostatic pressure on the root medium was quickly and fully transmitted to the base of the leaf. The increase in leaf elongation due to a pressure step of 0·025 MPa was negligible under conditions of high plant water status and became substantial under conditions of low water status. In adapting the pressure-jump method to the whole seedling, there was some loss of resolution, and the yield threshold Y of the Lockhart equation could not be estimated directly. Nonetheless, the data were suitable for the calculation of volumetric extensibility m and the estimation of growth effective turgor (turgor above Y ). Extensibility was shown to increase 3- to 4-fold when leaf water status was reduced from the maximum to the point where elongation rate was halved, while growth effective turgor was calculated to diminish even more markedly.     

Correlation between loss of turgor and accumulation of abscisic acid in detached leaves

Mature leaves of Phaseolus vulgaris L. (red kidney bean), Xanthium strumarium L. (cocklebur), and Gossypium hirsutum L. (cotton) were used to study accumulation of abscisic acid (ABA) during water stress. The water status of individual, detached leaves was monitored while the leaves slowly wilted, and samples were cut from the leaves as they lost water. The leaf sections were incubated at their respecitive water contents to allow ABA to build up or not. At least 8 h were required for a new steady-state level of ABA to be established. The samples from any one leaf covered a range of known water potentials (), osmotic pressures (), and turgor pressures (p). The and p values were calculated from pressure-volume curves, using a pressure bomb to measure the water potentials. Decreasing water potential had little effect on ABA levels in leaves at high turgor. Sensitivity of the production of ABA to changes in progressively increased as turgor approached zero. At p=1 bar, ABA content averaged 4 times the level found in fully turgid samples. Below p=1 bar, ABA content increased sharply to as much as 40 times the level found in unstressed samples. ABA levels rose steeply at different water potentials for different leaves, according to the at which turgor became zero. These differences were caused by the different osmotic pressures of the leaves that were used; must cqual - for turgor to be zero. Leaves vary in , not only among species, but also between plants of one and the same species depending on the growing conditions. A difference of 6 bars (calculated at =0) was found between the osmotic pressures of leaves from two groups of G. hirsutum plants; one group had previously experienced periodic water stress, and the other group had never been stressed. When individual leaves were subsequently wilted, the leaves from stress-conditioned plants required a lower water potential in order to accumulate ABA than did leaves from previously unstressed plants. On the basis of these results we suggest that turgor is the critical parameter of plant water relations which controls ABA production in water-stressed leaves.Abbreviations ABA abscisic acid - me-ABA abscisic-acid methyl ester - leaf water potential - osmotic pressure - p volumeaveraged turgor - volumetric modulus of elasticity     

Flowers under pressure: ins and outs of turgor regulation in development

Background

Turgor pressure is an essential feature of plants; however, whereas its physiological importance is unequivocally recognized, its relevance to development is often reduced to a role in cell elongation.

Scope

This review surveys the roles of turgor in development, the molecular mechanisms of turgor regulation and the methods used to measure turgor and related quantities, while also covering the basic concepts associated with water potential and water flow in plants. Three key processes in flower development are then considered more specifically: flower opening, anther dehiscence and pollen tube growth.

Conclusions

Many molecular determinants of turgor and its regulation have been characterized, while a number of methods are now available to quantify water potential, turgor and hydraulic conductivity. Data on flower opening, anther dehiscence and lateral root emergence suggest that turgor needs to be finely tuned during development, both spatially and temporally. It is anticipated that a combination of biological experiments and physical measurements will reinforce the existing data and reveal unexpected roles of turgor in development.     

Xylem and cell turgor pressure probe measurements in intact roots of glycophytes: transpiration induces a change in the radial and cellular reflection coefficients

Xylem probe measurements in the roots of intact plants of wheat and barley revealed that the xylem pressure decreased rapidly when the roots were subjected to osmotic stress (NaCl or sucrose). The magnitude of the xylem pressure response and, in turn, that of the radial reflection coefficients (σ_r) depended on the transpiration rate. Under very low transpiration conditions (darkness and high relative humidity), σ_r assumed values of the order of about 0·2–0·4. The σ_r values of excised roots were also found to be rather low, in agreement with data obtained using the root pressure probe of Steudle. For transpiring plants (light intensities at least 10 μmol m^?2 s^?1; relative humidity 20–40%) the response was nearly 1:1, corresponding to radial reflection coefficients of σ_r= 1. Further increase of the light intensity to about 400 μmol m^?2 s^?1 resulted in a slight but significant decrease of the σ_r values to about 0·8. Similar measurements on maize roots confirmed our previous results (Zhu et al. 1995, Plant, Cell and Environment 18, 906–912) that, in intact transpiring plants at low light intensities of about 10 μmol m^?2 s^?1 and at relative humidities of 20–40% as well as in excised roots, the xylem pressure response was much less than expected from the external osmotic pressure (σ_r values 0·3–0·5). In contrast to wheat and barley, very high light intensities (about 700 μmol m^?2 s^?1) were needed to shift the radial reflection coefficients of maize roots to values of about 0·9. Osmotically induced xylem pressure changes were apparently linked to changes in turgor pressure in the root cortical parenchyma cells, as shown by simultaneous measurements of xylem and cell turgor pressure. In analogy to the σ_r values of the respective glycophytes, the σ_c values of the root cortical cells of wheat and barley were close to unity, whereas σ_c for maize was significantly smaller (about 0·7) under laboratory conditions. When the light intensity was increased up to about 700 μmol m^?2 s^?1 the cellular reflection coefficient of maize roots increased to about 0·95. In contrast to the σ_r values, the σ_c values of the three species investigated remained almost unchanged when the leaves were exposed to darkness and humidified air or when the roots were cut. The transpiration-dependent (species-specific) pattern of the cellular and radial reflection coefficients of the root compartment of the three glycophytes apparently resulted from (flow-dependent) concentration-polarization and sweep-away effects in the roots of intact plants. The data could be explained straightforwardly terms of theoretical considerations outlined previously by Dainty (1985, Acta Horticulturae 171, 21–31). The far-reaching consequences of this finding for root pressure probe measurements on excised roots, for the occurrence of pressure gradients under transpiring conditions, and for the non-linear flow-force relationships in roots found by other investigators are discussed.     

Ascorbate peroxidase activity, not the mRNA level, is enhanced in salt-stressed Raphanus sativus plants

Important functions of water relations are considered to be related to genome size diversity in gymnosperms. We investigated relationships among genome size, dimensional characteristics of conductive cells, and water relations parameters by using young seedlings of six Pinus species. Xylem hydraulic conductivity was not correlated with genome size and dimensional characteristics of conductive cells, but the water potential at the turgor loss point was. Pinus species with large genome sizes had thick cell walls and small ratios of lumen radii to cell wall thickness in their conductive cells, and those species lost their turgor to tissue dehydration al low water potentials. The characteristics observed in the present study may contribute to pine drought tolerance.