Half-times Of Water Exchange Research Articles

The Relationship between Turgor Pressure Change and Cell Hydraulics of Midrib Parenchyma Cells in the Leaves of Zea mays.

Leaf dehydration decreases water potential and cell turgor pressure. Therefore, changes in cell turgor pressure may regulate water transport across plant cell membranes. Using a cell pressure probe, the hydraulic properties of parenchyma cells in the midrib of maize (Zea mays L.) leaves were measured (half time of water exchange in cells as a measure of hydraulic conductivity Lp). Using intact plants with root systems encased in a pressure chamber, the root systems were pressurized and the turgor pressure in leaf cells increased by increments up to 0.3 MPa. However, the increase in the cell turgor did not increase but stabilized values. Increased water potential in leaf cells seemed to have stabilizing effects on the probably due to enhanced water availability. When the cell turgor decreased by 0.1 MPa to 0.3 MPa with releasing the pressure in the pressure chamber, was temporarily increased to a large degree, a factor of up to 13 within 30 min.

Effect of nutrient deficiencies on the water transport properties in figleaf gourd plants

Effects of nitrogen, phosphorus, and potassium deficiencies on water transport properties in figleaf gourd plants were studied. Plants were treated for different period of deficiency and physiological parameters such as stomatal conductance, photosynthesis and transpiration were measured. Cell and root pressure probes were utilized to measure turgor and root pressures, half-times of water exchange and hydraulic conductivities to analyze water transport properties. When plants were grown in nitrogen or phosphorus deficient nutrient solutions, they became insensitive to mercury, suggesting that aquaporin was closed resulting in reduced hydraulic conductivity. Inclusion of tungstate, however, restored the sensitivity of cells to mercury, indicating the importance of internal nutrient concentration, not the incoming nutrient supply. The hydrostatic hydraulic conductivity of roots grown in nitrogen deficient solution, representing apoplastic pathway of water transport, was reduced but this reduction was dramatically recovered by the application of tungstate, indicating the importance of nutrient availability from storage pools in relation to water status of plants.

Early response in water relations influenced by NaCl reflects tolerance or sensitivity of barley plants to salinity stress via aquaporins

Barley varieties, K305 and I743, which are sodium chloride (NaCl) tolerant and sensitive respectively, were hydroponically grown to determine the short-term effects of NaCl on the cell water relations in roots using a cell pressure probe, and on the transcript levels of 10 barley PIP aquaporin genes (HvPIPs) in roots. Stomatal conductance, as an indicator of sensitivity to NaCl, was decreased to less than half values of control upon exposure to 100 mmol L–1 NaCl for 24 h in I743 whereas tolerant variety, K305, was able to maintain original conductance. Osmotic half-times of water exchange in cortical cells allowed for a clear distinction between the two varieties up to 200 mmol L–1 NaCl. With treatment duration of up to 12 h with 100 mmol L–1 NaCl, the elastic modulus was reduced in I743 but increased in K305. Hydrostatic half-times of water exchange in K305 increased rapidly, whereas this value remained unchanged in I743. Application of abscisic acid (ABA) after 1 h NaCl treatment restored the hydraulic conductivity of cells (Lp) in K305 but not in I743 whereas the opposite results were obtained when mercury chloride (HgCl2) was applied, verifying the contrasting gating response of aquaporins in two varieties. Reduced expression of HvPIPs was consistent with the reduction of hydraulic conductivity of both varieties after 24 h NaCl, but without any significant differences between them, indicating the importance of the activities of existing aquaporins rather than de novo synthesis to cope with short-term effects of salt stress.

Osmotic effects of NaCl on cell hydraulic conductivity of corn roots

In this study, whether the effect of salt (NaCl) stress on cell hydraulic conductivity (L(p)) is via osmotic pressure or ion toxicity and whether abscisic acid (ABA) can release the salt adverse effect were tested. Immediate effects of NaCl and ABA on root cortical cell L(p) of maize (Zea mays L.) were detected by measuring changes in half time of water exchange (T(1/2)) and turgor of individual single cells with a cell pressure probe for at least 1 h. The results showed that stepwise additions of NaCl (50 mM) significantly (P < 0.01) reduced the water permeability. One-step addition of 50 mM NaCl even more drastically decreased L(p). ABA was not able to instantaneously reverse the low water permeability induced by the salt stress. Long-term effects of NaCl, mannitol and sorbitol, and ABA on L(p) were measured for 6 days. Both NaCl and a mixture of mannitol and sorbitol, with the same osmotic strength of 0.25 MPa, significantly reduced L(p) at the early stage of the treatments. The declined L(p) in the salinized cell gradually and partially recovered after 2 days, whereas the L(p) with the mannitol and sorbitol mixture treatment was all time inhibited. With long-time treatment, ABA (500 nM) significantly (P < 0.01) increased turgor and L(p) of the NaCl-treated cells. In general, NaCl reduced water permeability of corn root cortical cells most likely by an osmotic stress. ABA could not instantaneously change water permeability of the corn root cortical cell subjected to NaCl stress; however, with long-time treatment, ABA was able to in part relieve the salt stress likely by osmotic adjustment.

Gating of aquaporins by light and reactive oxygen species in leaf parenchyma cells of the midrib of Zea mays

Changes of the water permeability aqùaporin (AQP) activity of leaf cells were investigated in response to different light regimes (low versus high). Using a cell pressure probe, hydraulic properties (half-time of water exchange, T1/2 ∞ 1/water permeability) of parenchyma cells in the midrib tissue of maize (Zea mays L.) leaves have been measured. A new perfusion technique was applied to excised leaves to keep turgor constant and to modify the environment around cells by perfusing solutions using a pressure chamber. In response to low light (LL) of 200 μmol m−2 s−1, T1/2 decreased during the perfusion of a control solution of 0.5 mM CaCl2 by a factor of two. This was in line with earlier results from leaf cells of intact maize plants at a constant turgor. In contrast, high light (HL) at intensities of 800 μmol m−2 s−1 and 1800 μmol m−2 s−1 increased the T1/2 in two-thirds of cells by factors of 14 and 35, respectively. The effects of HL on T1/2 were similar to those caused by H2O2 treatment in the presence of Fe2+, which produced ·OH (Fenton reaction; reversible oxidative gating of aquaporins). Treatments with 20 mM H2O2 following Fe2+ pre-treatments increased the T1/2 by a factor of 30. Those increased T1/2 values could be partly recovered, either when the perfusion solution was changed back to the control solutuion or when LL was applied. 3mM of the antioxidant glutathione also reversed the effects of HL. The data suggest that HL could induce reactive oxygen species (ROS) such as ·OH, and they affected water relations. The results provide evidence that the varying light climate adjusts water flow at the cell level; that is, water flow is maximized at a certain light intensity and then reduced again by HL. Light effects are discussed in terms of an oxidative gating of aquaporins by ROS.

Light‐induced transpiration alters cell water relations in figleaf gourd (Cucurbita ficifolia) seedlings exposed to low root temperatures

Water relation parameters including elastic modulus (epsilon), half-times of water exchange (T(w)(1/2)), hydraulic conductivity and turgor pressure (P) were measured in individual root cortical and cotyledon midrib cells in intact figleaf gourd (Cucurbita ficifolia) seedlings, using a cell pressure probe. Transpiration rates (E) of cotyledons were also measured using a steady-state porometer. The seedlings were exposed to low ambient (approximately 10 micromol m(-2) s(-1)) or high supplemental irradiance (approximately 300 micromol m(-2) s(-1) PPF density) at low (8 degrees C) or warm (22 degrees C) root temperatures. When exposed to low irradiance, all the water relation parameters of cortical cells remained similar at both root temperatures. The exposure of cotyledons to supplemental light at warm root temperatures, however, resulted in a two- to three-fold increase in T(w)(1/2) values accompanied with the reduced hydraulic conductivity in both root cortical (Lp) and cotyledon midrib cells (Lp(c)). Low root temperature (LRT) further reduced Lp(c) and E, whether it was measured under low or high irradiance levels. The reductions of Lp as the result of respective light and LRT treatments were prevented by the application of 1 microM ABA. Midrib cells required higher concentrations of ABA (2 microM) in order to prevent the reduction in Lp(c). When the exposure of cotyledons to light was accompanied by LRT, however, ABA proved ineffective in reversing the inhibition of Lp. LRT combined with high irradiance triggered a drastic 10-fold reduction in water permeability of cortical and midrib cells and increased epsilon and T(w)(1/2) values. Measurement of E indicated that the increased water demand by the transpiring plants was fulfilled by an increase in the apoplastic pathway as principal water flow route. The importance of water transport regulation by transpiration affecting the hydraulic conductivity of the roots is discussed.

Light and turgor affect the water permeability (aquaporins) of parenchyma cells in the midrib of leaves of Zea mays

In response to light, water relation parameters (turgor, half-time of water exchange, T(1/2), and hydraulic conductivity, Lp; T(1/2) proportional 1/Lp) of individual cells of parenchyma sitting in the midrib of leaves of intact corn (Zea mays L.) plants were investigated using a cell pressure probe. Parenchyma cells were used as model cells for the leaf mesophyll, because they are close to photosynthetically active cells at the abaxial surface, and there are stomata at both the adaxial and abaxial sides. Turgor ranged from 0.2 to 1.0 MPa under laboratory light condition (40 micromol m(-2) s(-1) at the tissue level), and individual cells could be measured for up to 6 h avoiding the variability between cells. In accordance with earlier findings, there was a big variability in T(1/2)s measured ranging from 0.5 s to 100 s, but the action of light on T(1/2)s could nevertheless be worked out for cells having T(1/2)s greater than 2 s. Increasing light intensity ranging from 100 micromol m(-2) s(-1) to 650 micromol m(-2) s(-1) decreased T(1/2) by a factor up to five within 10 min and increased Lp (and aquaporin activity) by the same factor. In the presence of light, turgor decreased due to an increase in transpiration, and this tended to compensate or even overcompensated for the effect of light on T(1/2). For example, during prolonged illumination, cell turgor dropped from 0.2 to 1.0 MPa to -0.03 to 0.4 MPa, and this drop caused an increase of T(1/2) and a reduction of cell Lp, i.e. there was an effect of turgor on cell Lp besides that of light. To separate the two effects, cell turgor (water potential) was kept constant while changing light intensity by applying gas pressure to the roots using a pressure chamber. At a light intensity of 160 micromol m(-2) s(-1), there was a reduction of T(1/2) by a factor of 2.5 after 10-30 min, when turgor was constant within +/-0.05 MPa. Overall, the effects of light on T(1/2) (Lp) were overriding those of turgor only when decreases in turgor were less than about 0.2 MPa. Otherwise, turgor became the dominant factor. The results indicate that the hydraulic conductivity increased with increasing light intensity tending to improve the water status of the shoot. However, when transpiration induced by light tends to cause a low turgidity of the tissue, cell Lp was reduced. It is concluded that, when measuring the overall hydraulic conductivity of leaves, both the effects of light and turgor should be considered. Although the mechanism(s) of how light and turgor influence the cell Lp is still missing, it most likely involves the gating of aquaporins by both parameters.

Gating of water channels (aquaporins) in cortical cells of young corn roots by mechanical stimuli (pressure pulses): effects of ABA and of HgCl2.

Hydraulic properties (half-time of water exchange, T1/2, and hydraulic conductivity, Lp; T1/2 approximately 1/Lp) of individual cells in the cortex of young corn roots were measured using a cell pressure probe for up to 6 h to avoid variations between cells. When pulses of turgor pressure of different size were imposed, T1/2 (Lp) responded differently depending on the size. Pulses of smaller than 0.1 MPa, which induced a small proportional water flow, caused no changes in T1/2 (Lp). Medium-sized pulses of between 0.1 and 0.2 MPa caused an increase in T1/2 (decrease in Lp) by a factor of 4 to 23. The effects caused by medium-sized pulses were reversible within 5-20 min. When larger pulses of more than 0.2 MPa were employed, changes were not reversible within 1-3 h, but could be reversed within 30 min in the presence of 500 nM of the stress hormone ABA. Cells with a short T1/2 responded to the aquaporin blocker mercuric chloride (HgCl2). The treatment had no effect on cells which exhibited long T1/2 following a mechanical inhibition by the large-pulse treatment. Step decreases in pressure resulted in the same inhibition as step increases. Hence, the treatment did not cause a stretch-inhibition of water channels and was independent of the directions of both pressure changes and water flows induced by them. It is concluded that inhibition is caused by the absolute value of intensities of water flow within the channels, which increased in proportion to the size of step changes in pressure. Probable mechanisms by which the mechanical stimuli are perceived are (i) the input of kinetic energy to the channel constriction (NPA motif of aquaporin) which may cause a conformational change of the channel protein (energy-input model) or (ii) the creation of tensions at the constriction analogous to Bernoulli's principle for macroscopic pores (cohesion-tension model). Estimated rates of water flow within the pores were a few hundred micro m s-1, which is too small to create sufficient tension. They were much smaller than those proposed for AQP1. Based on literature data of single-channel permeability of AQP1, a per channel energy input of 200 kBxT (kB=Boltzmann constant) was estimated for the energy-input model. This should be sufficient to initiate changes of protein conformation and an inactivation of channels. The data indicate different closed states which differ in the amount of distortion and the rates at which they relax back to the open state.

Hydraulic resistance to radial water flow in growing hypocotyl of soybean measured by a new pressure-perfusion technique

Hydraulic resistances to water flow have been determined in the cortex of hypocotyls of growing seedlings of soybean (Glycine max L. Merr. cv. Wayne). Data at the cell level (hydraulic conductivity, Lp; half-time of water exchange, T 1/2; elastic modulus, ɛ; diffusivity for the cell-to-cell pathway, D c) were obtained by the pressure probe, diffusivities for the tissue (D t) by sorption experiments and the hydraulic conductivity of the entire cortex (Lpr) by a new pressure-perfusion technique. For cortical cells in the elongating and mature regions of the hypocotyls T 1/2=0.4-15.1 s, Lp=0.2·10(-5)-10.0·10(-5) cm s(-1) bar(-1) and D c=0.1·10(-6)-5.5·10(-6) cm(2) s(-1). Sorption kinetics yielded a tissue diffusivity D t=0.2·10(-6)-0.8·10(-6) cm(2) s(-1). The sorption kinetics include both cell-wall and cell-to-cell pathways for water transport. By comparing D c and D t, it was concluded that during swelling or shrinking of the tissue and during growth a substantial amount of water moves from cell to cell. The pressure-perfusion technique imposed hydrostatic gradients across the cortex either by manipulating the hydrostatic pressure in the xylem of hypocotyl segments or by forcing water from outside into the xylem. In segments with intact cuticle, the hydraulic conductance of the radial path (Lpr) was a function of the rate of water flow and also of flow direction. In segments without cuticle, Lpr was large (Lpr=2·10(-5)-20·10(-5) cm s(-1) bar(-1)) and exceeded the corticla cell Lp. The results of the pressure-perfusion experiments are not compatible with a cell-to-cell transport and can only the explained by a preferred apoplasmic water movement. A tentative explanation for the differences found in the different types of experiments is that during hydrostatic perfusion the apoplasmic path dominates because of the high hydraulic conductivity of the cell wall or a preferred water movement by film flow in the intercellular space system. For shrinking and swelling experiments and during growth, the films are small and the cell-to-cell path dominates. This could lead to larger gradients in water potential in the tissue than expected from Lpr. It is suggested that the reason for the preference of the cell-to-cell path during swelling and growth is that the solute contribution to the driving force in the apoplast is small, and tensions normally present in the wall prevent sufficiently thick water films from forming. The solute contribution is not very effective because the reflection coefficient of the cell-wall material should be very small for small solutes. The results demonstrate that in plant tissues the relative magnitude of cell-wall versus cell-to-cell transport could dependent on the physical nature of the driving forces (hydrostatic, osmotic) involved.

Water relations of the epidermal bladder cells ofOxalis carnosa Molina

All of the cells of the upper (adaxial) epidermis of the leaves ofOxalis carnosa are transformed into large bladders, while in the lower epidermis the bladder cells are interrupted by "normal" cells with stomata. The epidermal bladders contain a high concentration of free oxalic acid (pH approx. 1). Water-relations parameters of these epidermal bladder cells have been determined using the pressure probe. Original cell turgor (P0) of the closely packed bladders of theupper epidermis was P0=0.7 to 2.9 bar ([Formula: see text]; mean±SD;N=25 cells) and lower than that in the club-shaped bladders of thelower epidermis (P0=1.3 to 3.7 bar;[Formula: see text];N=25 cells). Large differences in the elastic modulus (ε) and the hydraulic conductivity (Lp) of the two different types of cells were observed. For the lower epidermal bladders, ε=18 to 166 bar and was similar to that of other higher plant cells. Also, for these cells it was found that ε was increasing with both, cell turgor and cell volume. By contrast, ε of the cells of the upper epidermis was by one order of magnitude smaller (ε=1.9 to 17.0 bar) and no dependence of ε on cell volume could be detected. The Lp values of the cell membranes were also different (lower epidermis:[Formula: see text]; upper epidermis:[Formula: see text]). These differences seem to be too large to be caused by errors in determining the exchange area for water (A) between cells and adjacent tissue. The half-times of water exchange between bladders and leaf (T1/2) were, on average, somewhat longer for the upper than for the lower epidermis (lower epidermis: T1/2=7 to 38 s; upper epidermis: T1/2=22 to 213 s), but the differences in the T1/2 values were not as distinct as for ε and Lp. This is because of the compensatory effects of ε, Lp and the different ratios of volume to exchange area. Since the bladders make up about 75% of the entire volume of the leaf, it is assumed that the rate of response of the leaf to changes in the water potential should be similar to that of the bladder cells. The results are discussed in terms of a possible function of the bladders in the leaf.

Water relations of immobilized giant algal cells

Cells of Halicystis parvula, Acetabularia mediterranea, and Valonia utricularis were immobilized in a cross-linked alginate matrix (4-6% w/w) in order to simulate water-relation experiments in individual cells of higher plant tissues. The immobilization of these cells did not lead to an increase in the mechanical stability of the cell walls. This was demonstrated by measuring the volumetric elastic modulus of the cell wall and its dependence on turgor pressure with the aid of the non-miniaturized pressure probe. In immobilized cells, no changes in the absolute value of the elastic modulus of the cell wall could be detected for any given pressure. At the maximum turgor pressure at which non-immobilized cells normally burst (about 3-7 bar for V. utricularis; depending on cell size, 3 bar for A. mediterranea and 0.9 bar for H. parvula) reversible decreases in the pressure are observed which are succeeded by corresponding pressure increases. This obvervation indicates that coating the cells with the cross-linked matrix protects them from rapid water and turgor pressure loss. Turgor pressure relaxation processes in immobilized cells, which could be induced hydrostatically by means of the pressure probe, yielded accurate values for the half-times of water exchange and for the hydraulic conductivity of the cell membrane. The results demonstrate that the water transport equations derived for single cells in a large surrouding medium are valid for immobilized cells, so that any influence exerted by the unstirred layer which is caused by the presence of the cross-linked matrix can be ignored in the calculations. On the other hand, the evaluation of the half-times of water exchange and the hydraulic conductivity from turgor pressure relaxation processes, which have been induced osmotically, only yields correct values under certain circumstances. The model experiments presented here show, therefore, that the correct Lp-value for an individual cell in a higher plant tissue can probably only be obtained presently by using the pressure probe technique rather than the osmotic method. The results are also discussed in relation to the possible applications of immobilized cells and particularly of immobilized micro-organisms in catalytic reaction runs on an industrial scale.

Water relations of growing pea epicotyl segments

The water relations of growing epicotyl segments of pea (Pisum sativum L.) were studied using the miniaturized pressure probe. For epidermal cells stationary turgor pressures of P=5 to 9 bar and half-times of water exchange of individual cells T 1/2=1 to 27 s were found. The volumetric clastic modulus (ɛ) of epidermal cells varied from 12 to 200 bar and the hydraulic conductivity, Lp=0.2 to 2·10(-6) cm s(-1) bar(-1). For cortical cells P=5 to 11 bar, T 1/2=0.3 to 1 s, Lp=0.4 to 9·10(-5) cm s(-1) bar(-1) and ɛ=6 to 215 bar. The T 1/2 of cortical cells was extremely low and the Lp rather high as compared to other higher plant cells. The T 1/2-values of cortical cells were sometimes observed to change from short to substantially longer values (T 1/2=3 to 20 s). Both short and long pressure relaxations showed all the characteristics of non-artifactual curves. The change is apparently due to an increase in Lp and not ɛ, but the reason for the change in cell permeability to water is not known.In osmotic exchange experiments on peeled segments using solutions of different solutes, the half-time of osmotic water exchange for the whole segment was approximately 60 s. Water exchange occurred too quickly to be rate controlled by solute diffusion in the wall space. The data suggest that the short T 1/2-values in the cortical cells are the physiologically relevant ones for the intact tissue and that a considerable component of water transport occurs in the cell-to-cell pathway, although unstirred layer effects at the boundary between the segment and solution may influence the measured half-time. Using the theory of Molz and Boyer (1978, Plant Physiol. 62, 423-429), the gradient in water potential necessary to maintain the uptake of water for cell enlargement can be calculated from the measured diffusivities to be approximately 0.2 and 1 bar for growth rates of 1% h(-1) and 5% h(-1), respectively. Thus, although the T 1/2-values are short and Lp rather high, there may be a significant osmotic disequilibrium in the most rapidly growing tissue and as a consequence the influence of water transport on the growth rate cannot be excluded.

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, ε, 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, ε decreases. Therefore, in our opinion Raschke's results would indicate a relationship between guard cell volume and ε which would be inverse to that for plant cells known in the literature. ε assumes values between 20-40 bar when the guard cell colume is soubled.