Differential sensitivities of water diffusion in maize root cortex and stele to HgCl<Subscript>2</Subscript>-induced blockade of aquaporins

Spin-echo NMR comparative study of water diffusion in the cortex and stele of maize (Zea mays L.) roots was made with the aim to determine predominant pathways of radial water movement in the root. The root parts examined differed in terms of water diffusion coefficients and sensitivity to HgCl₂, the aquaporin blocker. These differences are discussed from the viewpoint of unequal contributions of separate transport pathways (apoplastic, symplastic, and transmembrane) to the overall water flow. Characteristics of water diffusion in roots with the endodermis damaged suggest an inconsiderable contribution of the endodermis into resistance to water movement.     

Effect of Water Stress and Mercuric Chloride on the Translational Diffusion of Water in Maize Seedling Roots

Water diffusion in maize roots (Zea mays L., cv. Donskaya 1) was investigated with a pulsed gradient NMR using mercuric chloride as an inhibitor of water channels in cell membranes. A novel operation program was applied that allowed selective evaluation of fractional amounts of water transported through various pathways—the apoplastic, symplasmic, and transmembrane routes. The blockage of water channels with HgCl₂ reduced the rates of water diffusion by a factor of 1.5–2. This effect was reversible and was removed by the addition of -mercaptoethanol. The coefficient of water diffusion changed with time elapsed after the HgCl₂ treatment. The effect of water stress on the rates of water diffusion was similar to that of HgCl₂. Remarkably, the water-stressed roots of maize seedlings were insensitive to the inhibitor of water channels. The results are interpreted in terms of redistribution of water flows among various routes in plant tissues. Water stress and mercuric chloride treatments decelerate the transmembrane water transport and promote water flow along the apoplastic pathway. These responses might arise from the reversible regulation of water movement along various transport pathways.     

Water uptake by plant roots: an integration of views

A COMPOSITE TRANSPORT MODEL is presented which explains the variability in the ability of roots to take up water and responses of water uptake to different factors. The model is based on detailed measurements of 'root hydraulics' both at the level of excised roots (root hydraulic conductivity, Lp_r) and root cells (membrane level; cell Lp) using pressure probes and other techniques. The composite transport model integrates apoplastic and cellular components of radial water flow across the root cylinder. It explains why the hydraulic conductivity of roots changes in response to the nature (osmotic vs. hydraulic) and intensity of water flow. The model provides an explanation of the adaptation of plants to conditions of drought and other stresses by allowing for a `coarse regulation of water uptake' according to the demands from the shoot which is favorable to the plant. Coarse regulation is physical in nature, but strongly depends on root anatomy, e.g. on the existence of apoplastic barriers in the exo- and endodermis. Composite transport is based on the composite structure of roots. A `fine regulation' results from the activity of water channels (aquaporins) in root cell membranes which is assumed to be under metabolic and other control.     

Water permeability and reflection coefficient of the outer part of young rice roots are differently affected by closure of water channels (aquaporins) or blockage of apoplastic pores

The relative contribution of the apoplastic and cell-to-cell paths to the overall hydraulic conductivity of the outer part of rice roots (LpOPR) was estimated using a pressure perfusion technique for 30-d-old rice plants (lowland cultivar, IR64, and upland cultivar, Azucena). The technique was based on the perfusion of aerenchyma of root segments from two different zones (20-50 mm and 50-100 mm from the root apex) with aerated nutrient solution using precise pump rates. The outer part of roots (OPR) comprised an outermost rhizodermis, an exodermis, sclerenchyma fibre cells, and the innermost unmodified cortical cell layer. No root anatomical differences were observed for the two cultivars used. Development of apoplastic barriers such as Casparian bands and suberin lamellae in the exodermis were highly variable. On average, matured apoplastic barriers were observed at around 50-70 mm from the root apex. Lignification of the exodermis was completed earlier than that of sclerenchyma cells. Radial water flow across the OPR was impeded either by partially blocking off the porous apoplast with China ink particles (diameter 50 nm) or by closing water channels (aquaporins) in cell membranes with 50 micro M HgCl2. The reduction of LpOPR was relatively larger in the presence of an apoplastic blockage with ink ( approximately 30%) than in the presence of the water channel blocker ( approximately 10%) suggesting a relatively larger apoplastic water flow. The reflection coefficient of the OPR (sigmasOPR) for mannitol significantly increased during both treatments. It was larger when pores of the apoplast were closed, but absolute values were low (overall range of sigmasOPR=0.1-0.4), which also suggested a large contribution of the non-selective, apoplastic path to overall water flow. The strongest evidence in favour of a predominantly apoplastic water transport came from the comparison between diffusional (PdOPR, measured with heavy water, HDO) and osmotic water permeability (PfOPR) or hydraulic conductivity (LpOPR). PfOPR was larger by a factor of 600-1400 compared with P(dOPR). The development of OPR along roots resulted in a decrease of PdOPR by a factor of three (segments taken at 20-50 and 50-100 mm from root apex, respectively). Heat-killing of living cells resulted in an increase of PdOPR for both immature (20-50 mm) and mature (50-100 mm) root segments by a factor of two. Even though both pathways (apoplast and cell-to-cell) contributed to the overall water flow, the findings indicate predominantly apoplastic water flow across the OPR, even in the presence of apoplastic barriers. Low diffusional water permeabilities may suggest a low rate of oxygen diffusion across the OPR from aerenchyma to the outer anaerobic soil medium (low PO2OPR). To date, there are no data on PO2OPR. Provisional data of radial oxygen losses (ROL) across the OPR suggest that, unlike water, rice roots efficiently retain oxygen within the aerenchyma. This ability strongly increases as roots/OPR develop.     

Radial diffusion transport of water in various zones of maize root and its sensitivity to mercury chloride

NMR-spin echo method was used for comparative study of radial water diffusion in various zones of maize (Zea mays L., cv. Donskaya 1) primary root. Coefficients of water diffusion varied strongly along the root length; the pattern of variations depended on the period during which the diffusion of water molecules was traced. Water diffusion transport in various root zones was unevently sensitive to mercury chloride, an aquaporin inhibitor. The discovered variations in the mobility of water molecules were assigned to morphological and functional features of cells and tissues in the root zones examined; they were interpreted in terms of variable contribution and redistribution of water flows along several transport pathways. The decrease in diffusional water flows could be caused by cell wall modifications (deposition of suberin) that emerge in the endoderm regions distant from the root apex and diminish the contribution of apoplastic transport.     

Water uptake by roots: effects of water deficit

The variable hydraulic conductivity of roots (Lp(r)) is explained in terms of a composite transport model. It is shown how the complex, composite anatomical structure of roots results in a composite transport of both water and solutes. In the model, the parallel apoplastic and cell-to-cell (symplastic and transcellular) pathways play an important role as well as the different tissues and structures arranged in series within the root cylinder (epidermis, exodermis, cortex, endodermis, stelar parenchyma). The roles of Casparian bands and suberin lamellae in the root's endo- and exodermis are discussed. Depending on the developmental state of these apoplastic barriers, the overall hydraulic resistance of roots is either more evenly distributed across the root cylinder (young unstressed roots) or is concentrated in certain layers (exo- and endodermis in older stressed roots). The reason for the variability of root Lp(r), is that hydraulic forces cause a dominating apoplastic flow of water around protoplasts, even in the endodermis and exodermis. In the absence of transpiration, water flow is osmotic in nature which causes a high resistance as water passes across many membranes on its passage across the root cylinder. The model allows for a high capability of roots to take up water in the presence of high rates of transpiration (high demands for water from the shoot). By contrast, the hydraulic conductance is low, when transpiration is switched off. Overall, this results in a non-linear relationship between water flow and forces (gradients of hydrostatic and osmotic pressure) which is otherwise hard to explain. The model allows for special root characteristics such as a high hydraulic conductivity (water permeability) in the presence of a low permeability of nutrient ions once taken up into the stele by active processes. Low root reflection coefficients are in line with the idea of some apoplastic bypasses for water within the root cylinder. According to the composite transport model, the switch from the hydraulic to the osmotic mode is purely physical. In the presence of heavily suberized roots, the apoplastic component of water flow may be too small. Under these conditions, a regulation of radial water flow by water channels dominates. Since water channels are under metabolic control, this component represents an 'active' element of regulation. Composite transport allows for an optimization of the water balance of the shoot in addition to the well-known phenomena involved in the regulation of water flow (gas exchange) across stomata. The model is employed to explain the responses of plants to water deficit and other stresses. During water deficit, the cohesion-tension mechanism of the ascent of sap in the xylem plays an important role. Results are summarized which prove the validity of the coehesion/tension theory. Effects of the stress hormone abscisic acid (ABA) are presented. They show that there is an apoplastic component of the flow of ABA in the root which contributes to the ABA signal in the xylem. On the other hand, (+)-cis-trans-ABA specifically affects both the cell level (water channel activity) and water flow driven by gradients in osmotic pressure at the root level which is in agreement with the composite transport model. Hydraulic water flow in the presence of gradients in hydrostatic pressure remains unchanged. The results agree with the composite transport model and resemble earlier findings of high salinity obtained for the cell (Lp) and root (Lp(r)) level. They are in line with known effects of nutrient deprivation on root Lp(r )and the diurnal rhythm of root Lp(r )recently found in roots of LOTUS.     

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.     

Water uptake by roots of maize and sunflower affects the radial transport of abscisic acid and its concentration in the xylem

The radial movement of cis-abscisic acid (ABA) has been investigated in young excised roots of Zea mays L. and Helianthus annuus L. which were grown hydroponically. In addition to the symplastic path, ABA was largely translocated across the root apoplast by solvent drag with the water in the transpiration stream. On the apoplastic path ABA may even cross the endodermis. Depending on the ABA concentration of the medium (range: 5–500 nM) and in the root apoplast, the solvent-drag component of the flow of ABA counteracted the dilution of ABA in the xylem caused by transpirational water flow. Acidification of the rhizosphere and of the root apoplast increased the apoplastic transport component. In sunflower, the apoplastic flow of ABA was significantly weaker than in maize roots. This was also indicated by the larger apparent reflection coefficient (σ_ABA) of sunflower roots for ABA (sunflower: σ_ABA = 0.97 ± 0.02, n = 6 roots; maize: σ_ABA = 0.68 ± 0.06, n = 6 roots; ±SD). For both species, σ_ABA was smaller than unity. Root reflection coefficients were affected by factors such as pH, ABA concentration of the medium, and by the suction force applied to excised root systems. Due to the complex composite structure of the permeation barrier in the root, the reflection coefficient estimated from solvent drag is also complex. Since unstirred layers affected the absolute value of the reflection coefficient, σ_ABA has been termed `apparent'. It is concluded that the pH and ABA concentration of the soil solution as well as the transpiration rate (suction force) modify the intensity of the root-to-shoot signal which is influenced by an apoplastic bypass flow of ABA. The latter may be substantially affected by the existence of Casparian bands in the exodermis, which were lacking in the roots studied in this paper. Received: 25 February 1998 / Accepted: 16 July 1998     

On the evidence of a diffusion barrier in the outer cortex apoplast of cress-roots (Lepidium sativum), demonstrated by analytical electron microscopy

To mark the apoplastic pathway of ions in the root of the dicotyledonous plant Lepidium sativum we used the heavy element lanthanum, which can be identified by analytical electron microscopy (EELS and ESI). In the front root tip, the primary walls of all meristematic cells contained lanthanum. 10-15 mm behind the root apex, lanthanum was found in the cortex cell walls up to the endodermis, but not in the stele. 20-25 mm from the tip, lanthanum was accumulated in the radial cell walls of the hypodermis, which, however, is not a complete diffusion barrier for ions, so that traces of lanthanum also were found in the cortex cell walls up to the endodermis. This study provides evidence for the presence of two apolastic diffusion barriers in the region of highest water uptake in cress roots.     

Water status and water diffusion transport in lupine roots exposed to lead

Water status and diffusion transport were studied in the roots of yellow lupine (Lupinus luteus L., cv. Juno) treated for 48 h with two selected concentrations of Pb(NO₃)₂: 150 mg l⁻¹, which inhibited root growth by about 50% (medium stress intensity), as well as 350 mg l⁻¹, which almost entirely suppressed root elongation (severe stress intensity). Relative water content (RWC), which characterizes the degree of root water saturation, slightly increased at the lower lead concentration and remained unchanged at the higher lead dose. Ultrastructure analyses under a transmission electron microscope revealed that plasmolysis was not evoked by lead in the apical part of the meristem. Moreover, direct observation of meristem cells using Nomarsky optics indicated enhanced vacuolization in the presence of both lead concentrations. These data suggest that the water status of the roots was not affected by the metal. Due to the fact that proline is involved in the maintenance of turgor in the cells, the metabolism of this amino acid was investigated. In the roots, the activity of enzymes involved in proline synthesis, such as pyrroline-5-carboxylate synthetase (P5CS) and ornithine aminotransferase (OAT), increased at 150 mg l⁻¹ Pb²⁺; nevertheless, proline content was diminished at the lower lead concentration. This effect is likely the result of proline degradation by proline dehydrogenase (PDH), since the activity of this enzyme increased at the lower lead dose. On the other hand, in the presence of 350 mg l⁻¹ Pb²⁺, a low level of proline was correlated with a decrease in the activity of P5CS and OAT, as well as unchanged PDH activity in lupine roots. These data may imply that enzymatic synthesis of proline was strongly damaged by the metal ions. The low level of proline in both experimental variants suggests that proline accumulation is inessential to maintaining the osmotic uptake of water into root cells. NMR spectroscopy showed that exposition of lupine seedlings to lead caused a deceleration in water transport in the roots due to a reduction in the water transfer rate across the membranes (transmembrane transfer) and vacuoles continuum, as well as water diffusion along the root apoplast. Fluorescence staining and immunogold labeling showed the presence of callose strands in cell walls and/or in the vicinity of them. In lead-treated lupine roots, callose was mainly localized in the parenchyma cortex placed lengthwise to the vascular cylinder. Callose deposits in the cell walls may reduce vacuolar transport, as well as increase cell wall resistance to water flow. Deceleration of diffusional water movement to the vascular system, may in turn, influence the rate of long-distance water transport to aerial parts of the plant.     

The spatially variable inhibition by water deficit of maize root growth correlates with altered profiles of proton flux and cell wall pH

Growth of elongating primary roots of maize (Zea mays) seedlings was approximately 50% inhibited after 48 h in aerated nutrient solution under water deficit induced by polyethylene glycol 6000 at -0.5 MPa water potential. Proton flux along the root elongation zone was assayed by high resolution analyses of images of acid diffusion around roots contacted for 5 min with pH indicator gel. Profiles of root segmental elongation correlated qualitatively and quantitatively (r(2) = 0.74) with proton flux along the surface of the elongation zone from water-deficit and control treatments. Proton flux and segmental elongation in roots under water deficit were remarkably well maintained in the region 0 to 3 mm behind the root tip and were inhibited from 3 to 10 mm behind the tip. Associated changes in apoplastic pH inside epidermal cell walls were measured in three defined regions along the root elongation zone by confocal laser scanning microscopy using a ratiometric method. Finally, external acidification of roots was shown to specifically induce a partial reversal of growth inhibition by water deficit in the central region of the elongation zone. These new findings, plus evidence in the literature concerning increases induced by acid pH in wall-extensibility parameters, lead us to propose that the apparently adaptive maintenance of growth 0 to 3 mm behind the tip in maize primary roots under water deficit and the associated inhibition of growth further behind the tip are related to spatially variable changes in proton pumping into expanding cell walls.     

Cycloheximide inhibits root water flow and stomatal conductance in aspen (Populus tremuloides) seedlings

Aspen (Populus tremuloides Michx.) roots were treated with cycloheximide, a protein synthesis inhibitor, to examine the role of protein synthesis in root water transport and plant water relations. Within less than 30 min following root application, cycloheximide inhibited steady‐state root water flow rates and 1 h after the application of 1 mm cycloheximide, root hydraulic conductivity had decreased by 85% compared with control roots. However, stomatal conductance showed a significant inhibition only after 2 h following cycloheximide treatment. The reduction in root hydraulic conductivity was accompanied by an almost three‐fold increase in the apoplastic water flow ratio as determined by the trisodium 3‐hydroxy‐5,8,10‐pyrenesulphonate tracer dye. Cycloheximide‐treated roots showed a decrease in the immunostaining intensity of a 32 kDa microsomal protein band that immunoreacted with the AnthPIP1; 1 antibody suggesting a decrease in the membrane aquaporin expression. These changes occurred without severe metabolic disruptions as measured by root respiration. The results point to the importance of protein‐mediated transport in roots and the rapidity of response suggests that protein synthesis may be used as a principal regulatory mechanism in root water transport in aspen.     

Review article. How does water get through roots?

On the basis of recent results with young primary maize roots, a model is proposed for the movement of water across roots. It is shown how the complex, 'composite anatomical structure' of roots results in a 'composite transport' of both water and solutes. Parallel apoplastic, symplastic and transcellular pathways play an important role during the passage of water across the different tissues. These are arranged in series within the root cylinder (epidermis, exodermis, central cortex, endodermis, pericycle stelar parenchyma, and tracheary elements). The contribution of these structures to the root's overall radial hydraulic resistance is examined. It is shown that as soon as early metaxylem vessels mature, the axial (longitudinal) hydraulic resistance within the xylem is usually not rate-limiting. According to the model, there is a rapid exchange of water between parallel radial pathways because, in contrast to solutes such as nutrient ions, water permeates cell membranes readily. The roles of apoplastic barriers (Casparian bands and suberin lamellae) in the root's endo- and exodermis are discussed. The model allows for special characteristics of roots such as a high hydraulic conductivity (water permeability) in the presence of a low permeability of nutrient ions once taken up into the stele by active processes. Low root reflection coefficients indicate some apoplastic by-passes for water within the root cylinder. For a given root, the model explains the large variability in the hydraulic resistance in terms of a dependence of hydraulic conductivity on the nature and intensity of the driving forces involved to move water. By switching the apoplastic path on or off, the model allows for a regulation of water uptake according to the demands from the shoot. At high rates of transpiration, the apoplastic path will be partially used and the hydraulic resistance of the root will be low, allowing for a rapid uptake of water. On the contrary, at low rates of transpiration such as during the night or during stress conditions (drought, high salinity, nutrient deprivation), the apoplastic path will be less used and the hydraulic resistance will be high. The role of water channels (aquaporins) in the transcellular path is in the fine adjustment of water flow or in the regulation of uptake in older, suberized parts of plant roots lacking a substantial apoplastic component. The composite transport model explains how plants are designed to optimize water uptake according to demands from the shoot and how external factors may influence water passage across roots.     

Control of water uptake by rice (<Emphasis Type="Italic">Oryza sativa</Emphasis> L.): role of the outer part of the root

A new pressure-perfusion technique was used to measure hydraulic and osmotic properties of the outer part of roots (OPR) of 30-day-old rice plants (lowland cultivar: IR64, and upland cultivar: Azucena). The OPR comprised rhizodermis, exodermis, sclerenchyma and one cortical cell layer. The technique involved perfusion of aerenchyma of segments from two different root zones (20-50 mm and 50-100 mm from the tip) at precise rates using aerated nutrient solution. The hydraulic conductivity of the OPR (Lp(OPR)=1.2x10(-6) m s(-1) MPa(-1)) was larger by a factor of 30 than the overall hydraulic conductivity (Lp(r)=4x10(-8) m s(-1) MPa(-1)) as measured by pressure chamber and root pressure probe. Low reflection coefficients were obtained for mannitol and NaCl for the OPR (sigma(sOPR)=0.14 and 0.09, respectively). The diffusional water permeability ( P(dOPR)) estimated from isobaric flow of heavy water was smaller by three orders of magnitude than the hydraulic conductivity (Lp(OPR)/ P(fOPR)). Although detailed root anatomy showed well-defined Casparian bands and suberin lamellae in the exodermis, the findings strongly indicate a predominantly apoplastic water flow in the OPR. The Lp(OPR) of heat-killed root segments increased by a factor of only 2, which is in line with the conclusion of a dominating apoplastic water flow. The hydraulic resistance of the OPR was not limiting the passage of water across the root cylinder. Estimations of the hydraulic properties of aerenchyma suggested that the endodermis was rate-limiting the water flow, although the aerenchyma may contribute to the overall resistance. The resistance of the aerenchyma was relatively low, because mono-layered cortical septa crossing the aerenchyma ('spokes') short-circuited the air space between the stele and the OPR. Spokes form hydraulic bridges that act like wicks. Low diffusional water permeabilities of the OPR suggest that radial oxygen losses from aerenchyma to medium are also low. It is concluded that in rice roots, water uptake and oxygen retention are optimized in such a way that hydraulic water flow can be kept high in the presence of a low efflux of oxygen which is diffusional in nature.     

Permeability of composite chondrocyte-culture-millipore membranes to solutes of varying size and shape.

A model connective-tissue system was developed that is amenable to the determination of permeability coefficients of diffusing solutes. The system involves the culture of 13-day chick-embryo chondrocytes on a Millipore filter (HA:0.45 micron pore size) to form what is, in effect, a confluent, extremely thin cartilage slice of uniform thickness. These cultured chondrocyte membranes were used to measure the steady-state flux of radioactively labelled low-molecular-weight solutes and micro-ions. Similarly, the permeability coefficients of either radioactively labelled or enzymically active proteins across the membranes were determined. The membrane was found to have no marked effects on the diffusional behaviour of low-molecular-weight non-electrolytes (water, proline, ribose, glucose, sorbitol, raffinose). For micro-ions (Na+, SO42-, Cl-, glutamate, glucuronate,), the diffusive behaviour was found to be markedly affected by the ionic strength of the solvent used in a manner which was consistent with a Donnan distribution resulting from the immobilized proteoglycans. Globular proteins permeated the membrane at rates which decreased as the molecular size of the diffusing solute increased. The apparent diffusion rates of fibrinogen and of collagen through the membranes were greater than would be expected on the basis of their diffusion coefficients in free solution. Reasons for this behaviour are discussed.     

Water transport in plants: Role of the apoplast

The present state of modelling of water transport across plant tissue is reviewed. A mathematical model is presented which incorporates the cell-to-cell (protoplastic) and the parallel apoplastic path. It is shown that hydraulic and osmotic properties of the apoplast may contribute substantially to the overall hydraulic conductivity of tissues (Lp_r) and reflection coefficients (67-1). The model shows how water and solutes interact with each other during their passage across tissues which are considered as a network of hydraulic resistors and capacitances (composite transport model). Emphasis is on the fact that hydraulic properties of tissues depend on the nature of the driving force. Osmotic gradients cause a much smaller tissue Lp_r than hydrostatic. Depending on the conditions, this results in variable hydraulic resistances of tissues and plant organs. For the root, the model readily explains the well-known phenomenon of variable hydraulic resistance for the uptake of water and non-linear force/flow relations. Along the cell-to-cell (protoplastic) path, water flow may be regulated by the opening and closing of selective water channels (aquaporins) which have been shown to be affected by different environmental factors. H Lambers Section editor     

Diffusion of sucrose and dextran through agar gel membranes

Mass transfer limitations severely impede the performance of bioreactions involving large molecules by gel-entrapped microorganisms. This paper describes a quantitative investigation of such diffusional limitations in agar gel membranes. Sucrose and commercial dextran fractions with (weight-average) molecular weights ranging from 10,000 to 2,000,000 Da were used as standard diffusants. For all tested solutes but sucrose, the values of the agar/water partition coefficients highlighted steric hindrance at the entrance of the membrane pores. The effective diffusivity of sucrose in agar was similar to that in water. All dextran fractions, however, displayed restricted diffusion in the agar membranes. Their effective diffusivities were a decreasing function of the agar content of the gel membrane (0.5, 1.0, or 1.5% w/v). The effective diffusivity in a given membrane decreased as the molecular weight of the diffusing molecule increased. T500 (ucbar|M_w = 470,000 Da) and ucbar|M_w = 1,950,000 Da) fractions were unable to diffuse through 1.0 or 1.5% agar membranes. The diffusion data did not agree with the classical (Renkin) model for a hard sphere diffusing through a cylindrical pore. These results are discussed in terms of gel and diffusant characteristics.     

Water transport in maize roots under the influence of mercuric chloride and water stress: a role of water channels

The influence of inhibitor of water channels, HgCl₂, on water diffusion in maize (Zea mays L.) seedling roots was investigated with the pulsed nuclear magnetic resonance (NMR) method. Blocking of water channels decreased the water permeability of cell membranes by 1.5 – 2 times. This effect of HgCl₂ was exhibited only in the roots of seedlings grown in a nutrient solution containing Ca²⁺ and was reversed with Hg-scavenging agent β-mercaptoethanol. Subsequent incubation of Ca²⁺-deficient roots in the nutrient solution with Ca²⁺ recovered the sensitivity to HgCl₂. The water stress decreased water diffusion rates similarly to HgCl₂ and the effects of water stress and HgCl₂ were not additive. The obtained data demonstrate the possibilities of the pulsed NMR method for study of the transmembrane water exchange in vivo in connection with water channel functioning.     

Nonexponential Diffusion Extinction of Spin Echo from Water Protons in Wheat Root

The method of NMR spin echo with the magnetic field pulse gradient was used for studying self-diffusion of water molecules in the radial root direction. It was shown on the basis of physiologopharmacological methods of modification of the object that the coefficients of water self-diffusion resulting from computer decomposition of nonexponential diffusion extinction of the relative echo amplitude in the root to components are related to the subcellular and supercellular organization (structure) of the root and reflect changes in water permeability of the two transport channels of plasmodesmas.     

Abscisic acid in the xylem: where does it come from, where does it go to?

Abscisic acid is a hormonal stress signal that moves in the xylem from the root to the different parts of the shoot where it regulates transpirational water loss and leaf growth. The factors that modify the intensity of the ABA signal in the xylem are of particular interest because target cells recognize concentrations. ABA(xyl), will be decreased as radial water flow through the roots is increased, assuming that radial ABA transport occurs in the symplast only. Such dilutions of the plant hormone concentration can be compensated in different ways, which help to keep the ABA-concentrations in the xylem constant: (i) apoplastic bypass flows of ABA, (ii) ABA flows between the stem parenchyma and the xylem during transport and (iii) the action of beta-D-glucosidases that release free ABA from its conjugates to the root cortex and the leaf apoplast. The significance of reflection coefficients (sigma(ABA)), permeability coefficients of membranes (P(S)(ABA)) and apoplastic barriers for ABA is discussed.