Water transport across plant tissue: Role of water channels

The contribution of water-filled, selective membrane pores (water channels) is integrated into a general concept of water transport in plant tissue. The concept is based on the composite anatomical structure of tissues which results in a composite transport pattern. Three main pathways of water flow have been distinguished, ie the apoplastic, symplastic and transcellular (vacuolar) paths. Since the symplastic and transcellular components can not be distinguished experimentally, these components are summarized as a cell-to-cell component. Water channel activity may control the overall water flow across tissues provided that the contribution of the apoplastic component is relatively low. The composite transport model has been applied to roots where most of the data are available. Comparison of the hydraulic conductivity at the root cell and organ levels shows that, depending on the species, there may be a dominating cell-to-cell or apoplastic water flow. Most remarkably, there are differences in the hydraulic conductivity of roots which depend on the nature of the force used to drive water flows (osmotic or hydrostatic pressure gradients). This is predicted by the model. The composite transport model explains low reflection coefficients of roots, the variability in root hydraulic resistance and differences between herbaceous and woody species. It is demonstrated that there is also a composite transport of water at the membrane level (water channel arrays vs bilayer arrays). This results in low reflection coefficients of plasma membranes for certain test solutes as derived for isolated internodes of Chara. The titration of water channel activity in this alga with mercurials and its dependence on changes in temperature or external concentration show that water channels do not exclusively transport water. Rather, they are permeable to relatively big uncharged organic solutes. The result indicates that, at least for Chara, the concept of an exclusive transport of water across water channels has to be questioned.     

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

Summary From equilibrium thermodynamics an equation is given to show that in a liquid negative pressures (tensions) are physical reality and may reliably be recorded from any point of the aqueous phase within the xylem conduit by the xylem pressure probe introduced by Balling et al. (Naturwissenschaften 75: 409–411, 1988).     

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

Summary The calculation of absolute-pressure values on the basis of measurements with differential-gauge pressure sensors, as described by Thürmer et al. (Protoplasma 206: 152–162, 1999), leads to discrepancies with the definition of absolute pressure when negative values are reached. From previous experiments with the xylem pressure probe we can conclude that the recorded pressure signal belongs not only to the xylem pressure, as stated by the authors, but also to the capillary pressure.     

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

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

The maize Brown midrib1 locus affects cell wall composition and plant development in a dose-dependent manner

The four brown midrib (bm) mutants of maize have a reduced content and altered subunit composition of the cell wall polymer lignin. The bm mutations have traditionally been considered completely recessive, because the brown midrib phenotype is only apparent in plants homozygous for the mutation. In addition to an effect on cell wall composition, some bm mutations have been shown to affect flowering time. We had preliminary evidence for a dosage effect of the Bm1 locus on flowering time, which prompted this detailed study on the Bm1 locus. In this study, near-isogenic lines (in an A619 background) with zero, one or two bm1 mutant alleles were compared. The bm1 heterozygotes flowered significantly earlier than both the wild-type plants and bm1 mutants. This difference can at least be partly attributed to an accelerated growth rate in the later stages of plant development. Furthermore, Fourier transform infrared spectroscopy revealed that the cell wall composition of the bm1 heterozygous plants is distinct from both the bm1 and wild-type homozygotes. The combination of the data on flowering time and the data on cell wall composition provide evidence for a dosage effect at the Bm1 locus.     

Comparison of plant cell turgor pressure measurement by pressure probe and micromanipulation

The conventional method of measuring plant cell turgor pressure is the pressure probe but applying this method to single cells in suspension culture is technically difficult and requires puncture of the cell wall. Conversely, compression testing by micromanipulation is particularly suited to studies on single cells, and can be used to characterise cell wall mechanical properties, but has not been used to measure turgor pressure. In order to demonstrate that the micromanipulation method can do this, pressure measurements by both methods were compared on single suspension-cultured tomato (Lycopersicon esculentum vf36) cells and generally were in good agreement. This validates further the micromanipulation method and demonstrates its capability to measure turgor pressure during water loss. It also suggests that it might eventually be used to estimate plant cell hydraulic conductivity.     

The transport of ions across plant cell members

Procedures developes to transform the genome of animasl have improves our fundamental understanding of the mechanisms of gene expression. Techniques in molecular biology are now allowing transformation with foreign genes that code for proteins of high value in this exciting area, and some prospects for this technology in the future.     

Complete turgor maintenance at low water potentials in the elongating region of maize leaves

Leaf elongation rate, water potential, and osmotic potential were measured in the fifth leaf of maize (Zea mays L.) plants growing in soil from which water was withheld for varying times. Elongation occurred in the basal region, which was enclosed by other leaf sheaths. When water was withheld from the soil, leaf elongation decreased and eventually ceased even though enough solutes accumulated in the elongating region to maintain turgor virtually constant. In the exposed blade, however, turgor was lost and wilt symptoms developed. If the night was prolonged, the elongating region lost much of its ability to accumulate solute, which suggests that the accumulating solutes were of recent photosynthetic origin. Under these conditions, leaf elongation was restricted to higher water potentials than under the usual photoperiodic regime.     

Water transport through plant tissue: the apoplasm and symplasm pathways

A detailed quantitative analysis of water flow through the apoplasm and symplasm of plant tissue is presented. The analysis results in two coupled diffusion equations which describe water transport in the two pathways. Various parameters entering the analysis identify the physical properties of the tissue which control the transport process as the resistance to water flow per cell in the two parallel pathways, the resistance per cell between pathways, and the water capacity per cell in the two pathways. Values for the several resistances and water capacities are estimated from available data, and a model problem is solved wherein a sheet of tissue at an initial water potential of — δ bars is immersed in a container of water. The resulting solutions show that depending on the values assigned to the controlling parameters, local water potential equilibrium between each cell and its cell wall may or may not obtain. In the special case of local equilibrium (water potential in the symplasm and apoplasm pathways essentially equal), the transport process can be described by a single diffusion equation which is derived along with an expression for the tissue diffusivity. It is concluded that the weakest link in the analysis is the estimated value for the permeability of the plasmodesma membrane, and that a logical extension of the theory would be to include the effects of a diffusable solute.     

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.     

Seed coat cell turgor in chickpea is independent of changes in plant and pod water potential

Turgor pressure in cells of the pod wall and the seed coat of chickpea (Cicer arietinum L.) were measured directly with a pressure probe on intact plants under initially dry soil conditions, and after the plants were irrigated. The turgor pressure in cells of the pod wall was initially 0.25 MPa, and began to increase within a few minutes of irrigation. By 2-4 h after irrigation, pod wall cell turgor had increased to 0.97 MPa. This increase in turgor was matched closely by increases in the total water potential of both the pod and the stem, as measured by a pressure chamber. However, turgor pressure in cells of the seed coat was relatively low (0.10 MPa) and was essentially unchanged up to 24 h after irrigation (0.13 MPa). These data demonstrate that water exchange is relatively efficient throughout most of the plant body, but not between the pod and the seed. Since both the pod and the seed coat are vascularized tissues of maternal origin, this indicates that at least for chickpea, isolation of the water relations of the embryo from the maternal plant does not depend on the absence of vascular or symplastic connections between the embryo and the maternal plant.     

Apoplastic transport across young maize roots: effect of the exodermis

The uptake of water and of the fluorescent apoplastic dye PTS (trisodium 3-hydroxy-5,8,10-pyrenetrisulfonate) by root systems of young maize (Zea mays L.) seedlings (age: 11–21 d) has been studied with plants which either developed an exodermis (Casparian band in the hypodermis) or were lacking it. Steady-state techniques were used to measure water uptake across excised roots. Either hydrostatic or osmotic pressure gradients were applied to induce water flows. Roots without an exodermis were obtained from plants grown in hydroponic culture. Roots which developed an exodermis were obtained using an aeroponic (=mist) cultivation method. When the osmotic concentration of the medium was varied, the hydraulic conductivity of the root (Lp _r in m³ · m⁻² · MPa⁻¹ · s⁻¹) depended on the osmotic pressure gradient applied between root xylem and medium. Increasing the gradient (i.e. decreasing the osmotic concentration of the medium; range: zero to 40 mM of mannitol), increased the osmotic Lp _r. In the presence of hydrostatic pressure gradients applied by a pressure chamber, root Lp _r was constant over the entire range of pressures (0–0.4 MPa). The presence of an exodermis reduced root Lp _r in hydrostatic experiments by a factor of 3.6. When the osmotic pressure of the medium was low (i.e. in the presence of a strong osmotic gradient between xylem sap and medium), the presence of an exodermis caused the same reduction of root Lp _r in osmotic experiments as in hydrostatic ones. However, when the osmotic concentration of the medium was increased (i.e. the presence of low gradients of osmotic pressure), no marked effect of growth conditions on osmotic root Lp _r was found. Under these conditions, the absolute value of osmotic root Lp _r was lower by factors of 22 (hydroponic culture) and 9.7 (aeroponic culture) than in the corresponding experiments at low osmotic concentration. Apoplastic flow of PTS was low. In hydrostatic experiments, xylem exudate contained only 0.3% of the PTS concentration of the bathing medium. In the presence of osmotic pressure gradients, the apoplastic flow of PTS was further reduced by one order of magnitude. In both types of experiments, the development of an exodermis did not affect PTS flow. In osmotic experiments, the effect of the absolute value of the driving force cannot be explained in terms of a simple dilution effect (Fiscus model). The results indicate that the radial apoplastic flows of water and PTS across the root were affected differently by apoplastic barriers (Casparian bands) in the exodermis. It is concluded that, unlike water, the apoplastic flow of PTS is rate-limited at the endodermis rather than at the exodermis. The use of PTS as a tracer for apoplastic water should be abandoned. Received: 9 October 1997 / Accepted: 5 February 1998     

Molecular characterization of a brown midrib3 deletion mutation in maize

The caffeic acid O-methyltransferase (COMT) gene plays an important role in the synthesis of lignin. We have used the polymerase chain reaction in conjuction with genomic analysis to characterize deletion mutations of this gene in maize. In addition, we have analyzed and compared regions of the COMT gene from three distinct heterotic groups. Both PCR and Southern analysis indicate that the active wild-type COMT gene can be polymorphic. We suggest that the intron domain of at least one heterotic inbred can contribute to the alteration of the wild-type gene. In addition, multiple deletion mutations have occurred at this locus. We have found a previously uncharacterized deletion mutation in which segments of both the intron and exon have been deleted and replaced by other sequences. Precise knowledge of its sequence has allowed us to develop an assay by which we can follow this mutation in a breeding program.     

Induction changes in photosystems I and II in plant leaves upon modulation of membrane ion transport

The steady-state regime of linear photosynthetic electron transport implies concerted operation of photosystems I and II (PSI and PSII) in plant leaves. Acidification of the thylakoid lumen is known to cause down-regulation of PSII photochemical activity but it is not yet clear how the proton accumulation in the lumen affects the PSI activity and coordinated operation of the two photosystems in intact leaves. Chlorophyll fluorescence and absorbance of oxidized chlorophyll P700 in the near-infrared region ΔA _810–870 (ΔA ₈₁₀) are convenient noninvasive indicators of the redox state of PSII and PSI components, respectively. Simultaneous measurements of chlorophyll fluorescence and ΔA ₈₁₀ in pea leaves revealed that some kinetic stages in the induction curves occur synchronously both in dark-adapted and preilluminated leaves. After the treatment of leaves with ionophores promoting or inhibiting the light-induced thylakoid pH gradient (valinomycin, nigericin, monensin), the induction curves of ΔA ₈₁₀ and chlorophyll fluorescence were consistently modified. The results suggest that characteristic stages of ΔA ₈₁₀ induction curve, representing the second and the third waves of P700 photooxidation, are closely related to ΔpH generation, although the bases of ΔpH dependence differ for these two stages. The second wave of ΔA ₈₁₀ depends presumably on stroma alkalinization as a precondition for photoactivation of electron flow from PSI to terminal acceptors. The third wave of ΔA ₈₁₀ is apparently due to retardation of electron flow between PSII and PSI upon acidification of the lumen.     

Water transport in plant cuticles: an update

The scale, mechanism, and physiological importance of cuticular transpiration were last reviewed in this journal 5 and 10 years ago. Progress in our basic understanding of the underlying processes and their physiological and structural determinants has remained frustratingly slow ever since. There have been major advances in the quantification of cuticular water permeability of stomata-bearing leaf and fruit surfaces and its dependence on leaf temperature in astomatous surfaces, as well as in our understanding of the respective roles of epicuticular and intracuticular waxes and molecular-scale aqueous pores in its physical control. However, understanding the properties that determine the thousand-fold differences between permeabilities of different cuticles remains a huge challenge. Molecular biology offers unique opportunities to elucidate the relationships between cuticular permeability and structure and chemical composition of cuticles, provided care is taken to quantify the effects of genetic manipulation on cuticular permeability by reliable experimental approaches.     

Water relations of turgor recovery and restiffening of wilted cabbage leaves in the absence of water uptake

A novel phenomenon in which wilted cabbage leaves appeared to regain positive turgor pressures without additional water uptake has been previously reported (J Levitt [1986] Plant Physiol 82: 147-153). These experiments were replicated and the biophysical nature of turgor recovery characterized. Leaf water potential and its components were assayed in hydrated, wilted, and desiccated leaves which appeared to regain turgor after wilting. The hypotheses that turgor recovery was due to an increased volumetric elastic modulus (ε), or alternatively the result of solute redistribution were tested. Quantitative evidence that turgor recovery occurs in excised leaves was found. Leaf turgor pressure in hydrated leaves (~0.6 megapascal) decreased to zero upon wilting. After continued desiccation, turgor pressure returned to approximately 0.3 megapascal even though leaf relative water content declined. The ε of hydrated leaves was large and there was no evidence of an increased ε in the turgor-recovered leaves. Solute mobilization occurred during desiccation. The apoplastic osmotic potential decreased from −0.15 to −0.44 megapascal in hydrated and turgor-recovered leaves, respectively, and solutes were transported from the lamina to the midrib tissue. Solute redistribution coupled with the high ε may have resulted in localized turgor recovery in specific cells in the desiccated leaves.     

Etioplast-chloroplast transformation in maize leaves: Effects of tissue age and light intensity

Summary The effects of light intensity and cell age on the greening of etioplasts were studied in seedlings of maize.We could see that in the youngest tissues examined by us the etioplast greening is very fast and occurs according to a particular pattern which is characterized by the contemporary presence of grana and large non crystalline prolamellar bodies. On the contrary, in the oldest examined tissues the etioplast greening is slow and the formation of grana appears to be delayed and subsequent to the using up of the prolamellar bodies.In the young tissues the intensity of the light mainly affects the duration of the lag-phase preceding the chlorophyll accumulation, while in the old tissues it also affects the total amount of chlorophyllous pigments, the restraining effect of the light appearing amplified by a concomitant restraining effect of cell age.Supported by a grant of C.N.R.     

Regulation of potassium transport in leaves: from molecular to tissue level

Over millions of years, plants have evolved a sophisticated network of K+ transport systems. This Botanical Briefing provides an overview of K+ transporters in various leaf tissues (epidermis, mesophyll, guard cells and vascular system) at both the cellular and organelle levels. Despite the tremendous progress in our knowledge of genes encoding K+ transport systems in plants, understanding has not developed of coordinated functioning and operation of these genes or proteins in the context of whole plant physiology and plant-environment interaction. This Botanical Briefing is aimed at filling that gap by analysing electrophysiological and molecular evidence for mechanisms coordinating K+ transport between various leaf cells and tissues in changing environments.     

Effect of turgor pressure and cell size on the wall elasticity of plant cells

Direct measurements of the volumetric elastic modulus, , of cells of a higher plant were performed on the epidermal bladder cells of Mesembryanthemum crystallinum using a pressure probe technique. Measurements on giant algal cells (Valonia, Nitellopsis) are given for comparison. Giant celled algae and M. crystallinum bladders have elastic moduli, , which depend strongly on turgor pressure, P, and on cell volume, V. The values of Mesembryanthemum bladders range between 5 bar at zero pressure and 100 bar at full turgor pressure (3-4 bar). increased with cell size (volume) at a given turgor pressure, and this volume dependence was pronounced more in the high pressure range. From the (P) characteristics, complete volume-pressure curves were obtained for Mesembryanthemum bladders and giant algal cells. The results suggest that the (P) and (V) characteristics of all plant cells are similar. The significance of the pressure and volume effects for the water relations and growth processes of plant cells is discussed briefly.