Effects of cell swelling on fluid flow across alveolar epithelium

The effects of decreased temperature, metabolic inhibition, and hyposmolality on osmotically and hydrostatically driven water flux across bullfrog alveolar epithelium were studied. Lungs were isolated from double-pithed Rana catesbeiana and prepared as sacs. Either an osmotic (0.1 M raffinose) or hydrostatic (6.3-6.6 Torr) pressure gradient was imposed across the tissue. These gradients resulted in the volume flow of water from the alveolar to pleural bath. Control water flux and hydraulic conductivity were estimated from the rate of weight loss of the lung sac. Subsequently water flux and hydraulic conductivity were determined under one of the following conditions: 1) temperature of bathing solutions lowered to 2 degrees C; 2) 1 mM 2,4-dinitrophenol added to both alveolar and pleural baths, or 3) both baths changed to half-isosmotic Ringer solution. The control hydraulic conductivities for osmotic (Lpo) and hydrostatic (Lph) pressure gradients were 3.65(+/- 0.94) X 10(-12) and 2.14(+/- 0.63) X 10(-10) ml/dyn X s, respectively. Under conditions of metabolic inhibition, hyposmolality, and decreased temperature, Lph decreased by 77, 83, and 92%, and Lpo decreased by 56, 34, and 59%, respectively. These results are most consistent with the hypothesis that the decrements in hydraulic conductivity under our experimental conditions are due to epithelial cell swelling and perhaps to changes in the characteristics of the paracellular pathway.     

Permeability of Iris germanica's multiseriate exodermis to water, NaCl, and ethanol

The exodermis of Iris germanica roots is multiseriate. Its outermost layer matures first with typical Casparian bands and suberin lamellae. But as subsequent layers mature, the Casparian band extends into the tangential and anticlinal walls of their cells. Compared with roots in which the endodermis represents the major transport barrier, the multiseriate exodermis (MEX) was expected to reduce markedly radial water and solute transport. To test this idea, precocious maturation of the exodermis was induced with a humid air gap inside a hydroponic chamber. Hydraulic conductivity (Lp(pc)) was measured on completely submerged roots (with an immature exodermis) and on air-gap-exposed root regions (with two mature exodermal layers) using a pressure chamber. Compared with regions of roots with no mature exodermal layers, the mature MEX reduced Lp(pc) from 8.5×10(-8) to 3.9×10(-8) m s(-1) MPa(-1). Puncturing the MEX increased Lp(pc) to 19×10(-8) m s(-1) MPa(-1), indicating that this layer constituted a substantial hydraulic resistance within the root (75% of the total). Alternatively, a root pressure probe was used to produce pressure transients from which hydraulic conductivity was determined, but this device measured mainly flow through the endodermis in these wide-diameter roots. The permeability of roots to NaCl and ethanol was also reduced in the presence of two mature MEX layers. The data are discussed in terms of the validity of current root models and in terms of a potential role for I. germanica MEX during conditions of drought and salt stress.     

Heteropore populations of bullfrog alveolar epithelium

Diffusional fluxes of a large number of hydrophilic solutes and water across bullfrog (Rana catesbeiana) alveolar epithelium were measured in the Ussing-type flux chamber. Lungs were isolated from double-pithed animals and studied as flat sheets. Radioactive solutes and water were added to the upstream reservoir, and the rate of change of downstream reservoir radioactivity was monitored. A permeability coefficient was estimated for each substance from a linear relationship between radiotracer concentration in the downstream reservoir and time. These permeability data were used to analyze the equivalent water-filled pore characteristics of the alveolar epithelial barrier. The data reveal that the alveolar epithelium is best characterized by two distinct pore populations rather than by a single homogeneous pore population. The large-pore population consists of pores with a radius of about 5 nm and occupies 4% of the available pore area. The small-pore population consists of pores with a radius of about 0.5 nm and occupies 96% of the available pore area. The number of small pores to large pores is 2.68 X 10(3). After the alveolar surface was damaged by acid, a large-pore population with a radius of about 27 nm was seen, allowing nearly free diffusion of solutes. A major implication of the presence of two populations of pores in the alveolar epithelium is that hydrostatically driven bulk water flow occurs predominantly through the large pores, while osmotically driven bulk water flow takes place predominantly through the small pores. As a result, in general, hydrostatic and osmotic gradients may not be equally effective driving forces for water flow across this tissue.     

Type I cell-like morphology in tight alveolar epithelial monolayers

The pulmonary alveolar epithelium separates air spaces from a fluid-filled interstitium and might be expected to exhibit high resistance to fluid and solute movement. Previous studies of alveolar epithelial barrier properties have been limited due to the complex anatomy of adult mammalian lung. In this study, we characterized a model of isolated alveolar epithelium with respect to barrier transport properties and cell morphology. Alveolar epithelial cells were isolated from rat lungs and grown as monolayers on tissue culture-treated Nuclepore filters. On Days 2-6 in primary culture, monolayers were analyzed for transepithelial resistance (Rt) and processed for electron microscopy. Mean cell surface area and arithmetic mean thickness (AMT) were determined using morphometric techniques. By Day 5, alveolar epithelial cells in vitro exhibited morphologic characteristics of type I alveolar pneumocytes, with thin cytoplasmic extensions and protruding nuclei. Morphometric data demonstrated that alveolar pneumocytes in vitro develop increased surface area and decreased cytoplasmic AMT similar to young type I cells in vivo. Concurrent with the appearance of type I cell-like morphology, monolayers exhibited high Rt (greater than 1000 omega.cm2), consistent with the development of tight barrier properties. These monolayers of isolated alveolar epithelial cells may reflect the physiological and morphological properties of the alveolar epithelium in vivo.     

Analysis of the introduction and removal of glycerol in rabbit kidneys using a Krogh cylinder model

In 1982, Rubinsky and Cravalho described a Krogh cylinder model for the analysis of cryoprotectant transport in a perfused organ. By application of the Kedem-Katchalsky equations, changes in tissue volume caused by movements of water and solute were used to predict changes in capillary radius (Cryobiology 19, 70-82, 1982). We have now measured the changes in vascular resistance that are produced when sucrose or glycerol is introduced into the perfusate flowing through rabbit kidneys at 10 degrees C, and have analyzed these data by means of the Rubinsky-Cravalho semiempirical model. The sucrose data provided an estimate of hydraulic conductivity and the dimensions of the Krogh tissue units. Three rates of addition of glycerol, 10, 30, and 90 mM/min to a final concentration of 3 M, were studied. The vascular resistance fell to approximately 40% of its initial value (radius approximately 128% of initial value) with all three rates of addition, and then returned toward its normal value while the glycerol concentration was still increasing. This behavior could be explained either by a sudden change in solute permeability at that capillary radius, or by an inverse dependence of reflection coefficient upon solute concentration. Evidence is presented that favors the latter interpretation. The best fits for the apparent hydraulic conductivity and apparent solute permeability for glycerol are 1 X 10(-6) cm/sec atm and 6 X 10(-8) cm/sec, respectively, with the reflection coefficient falling from 1.0 when the glycerol concentration is zero to 0.1 when it is 3 M. The model is used to predict tissue concentrations of glycerol throughout each experiment.     

A mathematical model of solute coupled water transport in toad intestine incorporating recirculation of the actively transported solute

A mathematical model of an absorbing leaky epithelium is developed for analysis of solute coupled water transport. The non-charged driving solute diffuses into cells and is pumped from cells into the lateral intercellular space (lis). All membranes contain water channels with the solute passing those of tight junction and interspace basement membrane by convection-diffusion. With solute permeability of paracellular pathway large relative to paracellular water flow, the paracellular flux ratio of the solute (influx/outflux) is small (2-4) in agreement with experiments. The virtual solute concentration of fluid emerging from lis is then significantly larger than the concentration in lis. Thus, in absence of external driving forces the model generates isotonic transport provided a component of the solute flux emerging downstream lis is taken up by cells through the serosal membrane and pumped back into lis, i.e., the solute would have to be recirculated. With input variables from toad intestine (Nedergaard, S., E.H. Larsen, and H.H. Ussing, J. Membr. Biol. 168:241-251), computations predict that 60-80% of the pumped flux stems from serosal bath in agreement with the experimental estimate of the recirculation flux. Robust solutions are obtained with realistic concentrations and pressures of lis, and with the following features. Rate of fluid absorption is governed by the solute permeability of mucosal membrane. Maximum fluid flow is governed by density of pumps on lis-membranes. Energetic efficiency increases with hydraulic conductance of the pathway carrying water from mucosal solution into lis. Uphill water transport is accomplished, but with high hydraulic conductance of cell membranes strength of transport is obscured by water flow through cells. Anomalous solvent drag occurs when back flux of water through cells exceeds inward water flux between cells. Molecules moving along the paracellular pathway are driven by a translateral flow of water, i.e., the model generates pseudo-solvent drag. The associated flux-ratio equation is derived.     

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.     

土壤-植物系统水分运移过程的阻容电模拟

把土壤-植物系统水分运移作为一维水流运动由阻容电路进行模拟,在于将D arcy-R ichards方程从对单点的描述扩展到对一段流路的描述。由此出发,考虑到水流的非稳态性,某一流路的水阻定义为其水势差与平均流量之比,水容为其贮水量对平均水势的导数。与D arcy-R ichards方程相对应,水阻、时间常数分别为导水度、水分扩散度的倒数,相应地单位化的水阻率、比时间常数分别为导水率、水分扩散率的倒数。把SP系统沿水流通道分为若干部分,每一局部的水阻与其水容相并联,各局部间相串联。在此基础上,文章给出了土壤-植物系统水流模拟通式、总水容与分水容间的关系式、总水阻与分水阻间的关系式及特定条件下叶水势随时间变化的关系式。     

Water Flow in Beta vulgaris Storage Tissue

The relative magnitudes of the hydraulic resistances, water capacities, and water potential equilibration time constants for the single cell, for the apoplast, and for the symplast in higher plant tissue are assessed. Swelling of beetroot (Beta vulgaris, var. `Detroit Red') storage tissue sections in pure water is measured using a displacement transducer. This method of measurement avoids the difficulty of solute diffusion in the apoplast. Theoretical analysis of the experimental results shows that the main path of water flow into the tissue is the apoplast rather than the symplast, that the main resistance to water flow into the cells is usually the cell membrane rather than the apoplast, but that in some cases the apoplast resistance and water capacity can contribute significantly to the water potential equilibration time constant of the tissue.     

Water transport in barley roots

Radial transport of water in excised barley (Hordeum distichon, cv. Villa) roots was measured using a new method based on the pressure-probe technique. After attaching excised roots to the probe, root pressures of 0.9 to 2.9 bar were developed. They could be altered either by changing the root pressure artificially (with the aid of the probe) or by changing the osmotic pressure of the medium in order to induce water flows across the root. The hydraulic conductivity of the barley roots (per cm² of outer root surface) was obtained in different types of experiments (initial water flow, pressure relaxations, constant water flow) and was (0.3–4.3)·10^-7 cm s^-1 bar^-1. The hydraulic conductivity of the root was by an order of magnitude smaller than the hydraulic conductivity of the cell membranes of cortical and epidermal cells (0.8–2.2)·10^-6 cm s^-1 bar^-1. The half-times of water exchange of these cells was 1–21 s and two orders of magnitude smaller than that of entire excised roots (100–770 s). Their volumetric elastic modulus was 15–305 bar and increased with increasing turgor. Within the root cortex, turgor was independent of the position of the cell within a certain layer and turgor ranged between 3 and 5 bar. The large difference between the hydraulic conductivity of the root and that of the cell membranes indicates that there is substantial cell-to-cell (transcellular plus symplasmic) transport of water in the root. When it is assumed that 10–12 membrane layers (plasmalemma plus tonoplast) in the epidermis, cortex and endodermis form the hydraulic resistance to water flow, a value for the hydraulic conductivity of the root can be calculated which is similar to the measured value. This picture for water transport in the root contradicts current models which favour apoplasmic water transport in the cortex.     

Primary responses of root and leaf elongation to water deficits in the atmosphere and soil solution

Plants experience drought by a limitation of water supply andby enhanced transpiration. Both processes tend to decrease theplant's water potential, but affect growth responses in theroot and leaf differently. The evaluation of the underlyingmechanisms leads to a discussion of recent studies on biophysicalaspects of cell expansion at a cellular, tissue and organ level.Two processes enable roots to compensate rapidly effects ofwater deficits originating in the medium: (i) adjustment ofthe minimum pressure in cells required for expansion (yieldthreshold), and (ii) solute transport within the elongationzone. Limitations of root growth are discussed with respectto hydraulic, mechanical, and solute relations in the root elongationzone. It is argued that the variable nature of both the yieldthreshold and solute transport challenges the applicabilityof the Lockhart concept to determine growth-related parametersfrom steady conditions of turgor and growth. On a whole organlevel, the attenuation of xylem pressure along the root is importantfor the differential response of root and leaf growth. Experimentalevidence is presented for the hydraulic separation of the elongationzones, which is closely related to root development and functioning.The data obtained over the past few years have been used toextend mathematical models of growth and water transport inroots. Key words: Extension growth, hydraulic conductivity, root development (xylem, endodermis), transport (water and solute), turgor pressure, water stress, xylem pressure, Zea mays     

Water uptake profile response of corn to soil moisture depletion

The effects of soil moisture distribution on water uptake of drip‐irrigated corn were investigated by simultaneously monitoring the diurnal evolution of sap flow rate in stems, of leaf water potential, and of soil moisture, during intervals between successive irrigations. The results invalidate the steady‐state resistive flow model for the continuum. High hydraulic capacitance of wet soil and low hydraulic conductivity of dry soil surrounding the roots damped significantly diurnal fluctuations of water flow from bulk soil to root surface. By contrast, sap flow responded directly to the large diurnal variation of leaf water potential. In wet soil, the relation between the diurnal courses of uptake rates and leaf water potential was linear. Water potential at the root surface remained nearly constant and uniformly distributed. The slope of the lines allowed calculating the resistance of the hydraulic path in the plant. Resistances increased in inverse relation with root length density. Soil desiccation induced a diurnal variation of water potential at the root surface, the minimum occurring in the late afternoon. The increase of root surface water potential with depth was directly linked to the soil desiccation profile. The development of a water potential gradient at the root surface implies the presence of a significant axial resistance in the root hydraulic path that explains why the desiccation of the soil upper layer induces an absolute increase of water uptake rates from the deeper wet layers.     

压力室测定根系导水率方法探讨

用压力室连续测定了玉米根系长压和降压过程的导水率,结果表明,降压过程湍 得的根系导水率显著大于用升压过程的,并且前者的相关系数大于后者,这种差异是由于这两个过程中质外体途径细胞壁空间充水量不同造成的,开始升压时,由于细胞壁空间含水量低,质外体途径阻力大,导致非结构阻力;随着压力的升高,细胞壁空间含水量增大,质外体途径导度增大,减小甚至可以消除非结构阻力,降压法可以使根系快速复水,消除传统方法因长时间复水所致根结构的改变。建议用降压法测定根系导水率。     

Stochastic analysis of the relationships between saturated hydraulic conductivity variance and solute dispersion in heterogeneous soils

The effect of three‐dimensional heterogeneity of saturated hydraulic conductivity on the vertical transport of solutes in soils is examined by means of controlled numerical experiments. Saturated hydraulic conductivity, an important transport parameter that controls the dispersion of pollutants in heterogeneous soils, is assumed to be composed of a homogeneous mean value and a perturbation caused by the vertical variability of soil properties, producing a stochastic process in the mean flow direction. The spatial heterogeneity of porous soils is characterized by the variance and correlation scale of the saturated hydraulic conductivity in the transport domain. Numerical experiments are carried out to evaluate the extent of contaminant dispersion in Hawaiian Oxic soils when uncertainty exists as a result of the spatial heterogeneity of saturated hydraulic conductivity. Statistical analysis of the saturated hydraulic conductivity measurements on undisturbed soil cores from two locations in Hawaiian Oxic soils indicated two different soils with the same mean and different variances. The partial differential equations describing three‐dimensional transient flow and solute transport in soils with a random conductivity field were solved to evaluate the effect of these two variance levels on the transport of a contaminant plume originating from the surface. The significance of the variance on the spatial and temporal distribution of tracer concentrations is demonstrated using solute breakthrough curves at various depths in the soil profile. The longitudinal macrodispersivity resulting from tracer spreading in the heterogeneous soils with a finite local dispersivity is also analyzed. The analysis shows a similar solute dispersion behavior for the two variances. However, there is an overall reduction in the dispersion of solutes resulting from a uniform velocity field with the same mean. Macrodispersivity values in heterogeneous soils are proportional to the variance at smaller travel distances but converge to the same value at larger travel distances.     

植物根系和叶片生长对水分亏缺的原初反应

细胞扩张生长是植物受水分亏缺影响最敏感的生理过程之一。主要在对细胞水分导性、细胞壁特性和延伸组织中溶质传输结果分析的基础上 ,从细胞、组织和器官水平上对细胞扩展生长进行了探讨。根系和叶片细胞主要通过以下 2个过程来补偿水分胁迫的作用 :调节扩展生长需要的细胞临界膨压 ;溶质在延伸组织中的运移。此外 ,还探讨了植物根系和叶片生长对水分亏缺的生理适应机制     

Permeability properties of the subepithelial tissues of Necturus gallbladder

The permeability properties of the subepithelial connective tissue of Necturus gallbladder were evaluated by measurement of electrical resistance, dilution potentials and hydraulic water permeability. The gallbladder epithelial cells were removed by scraping and the underlying connective tissue placed in an Ussing chamber. The electrical resistance was 2.2 +/- 0.8 omega X cm2; the tissue was slightly cation selective relative to free solution. The subepithelial tissues restricted the rate of diffusion of small solutes to 50% of the free solution value. The hydraulic water permeability averaged 2.1 X 10(-2) cm/s per atm. We conclude that limitations of the area of subepithelium available for fluid movement are the most important factors in determining the restrictions to solute and water flow offered by the subepithelial tissues.     

Alveolar transepithelial potential difference and ion transport in adult rat lung

The complex morphology of the mammalian lung complicates characterization of solute transport across the intact alveolar epithelium. We impaled the subpleural alveolar epithelium with microelectrodes and measured the transepithelial potential difference (PD) of the liquid-filled vascular-perfused left lobe of the rat lung. When the air space was filled entirely with Krebs-Ringer-bicarbonate, the PD was 4.7 mV (lumen negative). The PD was not affected significantly by agents that modify either Na+ or Cl- transport, but replacement of luminal Cl- with gluconate resulted in a fourfold hyperpolarization, a response also noted for large airways. When the airways were blocked by an immiscible nonconducting fluorocarbon, basal PD was not different from unblocked lobes (4.0 mV) but was inhibited 73% by luminal amiloride. Cl(-)-free Krebs-Ringer-bicarbonate blocked in the alveoli with fluorocarbon did not induce hyperpolarization. This result suggests that 1) Cl- permselectivity of the alveolar epithelium is less than that of large airway epithelium and 2) airway PD dominates the voltage across the liquid-filled lung, even when measurements are made from alveoli. When airways are blocked by fluorocarbon, the PD across the alveolar epithelium is largely dependent on Na+ flow through a path with amiloride-sensitive channels.     

Transport of Water and Solutes across Maize Roots Modified by Puncturing the Endodermis (Further Evidence for the Composite Transport Model of the Root)

The effects of puncturing the endodermis of young maize roots (Zea mays L.) on their transport properties were measured using the root pressure probe. Small holes with a diameter of 18 to 60 [mu]m were created 70 to 90 mm from the tips of the roots by pushing fine glass tubes radially into them. Such wounds injured about 10-2 to 10-3% of the total surface area of the endodermis, which, in these hydroponically grown roots, had developed a Casparian band but no suberin lamellae. The small injury to the endodermis caused the original root pressure, which varied from 0.08 to 0.19 MPa, to decrease rapidly (half-time = 10-100 s) and substantially to a new steady-state value between 0.02 and 0.07 MPa. The radial hydraulic conductivity (Lpr) of control (uninjured) roots determined using hydrostatic pressure gradients as driving forces was larger by a factor of 10 than that determined using osmotic gradients (averages: Lpr [hydrostatic] = 2.7 x 10-7 m s-1 MPa-1; Lpr [osmotic] = 2.2 x 10-8 m s-1 MPa-1; osmotic solute: NaCl). Puncturing the endodermis did not result in measurable increases in hydraulic conductivities measured by either method. Thus, the endodermis was not rate-limiting root Lpr: apparently the hydraulic resistance of roots was more evenly distributed over the entire root tissue. However, puncturing the endodermis did substantially change the reflection ([sigma]sr) and permeability (Psr) coefficients of roots for NaCl, indicating that the endodermis represented a considerable barrier to the flow of nutrient ions. Values of [sigma]sr decreased from 0.64 to 0.41 (average) and Psr increased by a factor of 2.6, i.e. from 3.8 x 10-9 to 10.1 x 10.-9 m s-1(average). The roots recovered from puncturing after a time and regained root pressure. Measurable increases in root pressure became apparent as soon as 0.5 to 1 h after puncturing, and original or higher root pressures were attained 1.5 to 20 h after injury. However, after recovery roots often did not maintain a stable root pressure, and no further osmotic experiments could be performed with them. The Casparian band of the endodermis is discontinuous at the root tip, where the endodermis has not yet matured, and at sites of developing lateral roots. Measurements of the cross-sectional area of the apoplasmic bypass at the root tip yielded an area of 0.031% of the total surface area of the endodermis. An additional 0.049% was associated with lateral root primordia. These areas are larger than the artificial bypasses created by wounding in this study and may provide pathways for a "natural bypass flow" of water and solutes across the intact root. If there were such a pathway, either in these areas or across the Casparian band itself, roots would have to be treated as a system composed of two parallel pathways (a cell-to-cell and an apoplasmic path). It is demonstrated that this "composite transport model of the root" allows integration of several transport properties of roots that are otherwise difficult to understand, namely (a) the differences between osmotic and hydrostatic water flow, (b) the dependence of root hydraulic resistance on the driving force or water flow across the root, and (c) low reflection coefficients of 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.     

Apoplastic transport of abscisic acid through roots of maize: effect of the exodermis

The exodermal layers that are formed in maize roots during aeroponic culture were investigated with respect to the radial transport of cis-abscisic acid (ABA). The decrease in root hydraulic conductivity (Lp_r) of aeroponically grown roots was stimulated 1.5-fold by ABA (500 nM), reaching Lp_r values of roots lacking an exodermis. Similar to water, the radial flow of ABA through roots (J_ABA) and ABA uptake into root tissue were reduced by a factor of about three as a result of the existence of an exodermis. Thus, due to the cooperation between water and solute transport the development of the ABA signal in the xylem was not affected. This resulted in unchanged reflection coeffcients for roots grown hydroponically and aeroponically. Despite the well-accepted barrier properties of exodermal layers, it is concluded that the endodermis was the more effective filter for ABA. Owing to concentration polarisation effects, ABA may accumulate in front of the endodermal layer, a process which, for both roots possessing and lacking an exodermis, would tend to increase solvent drag and hence ABA movement into the xylem sap at increased water flow (J_Vr). This may account for the higher ABA concentrations found in the xylem at greater pressure difference. Received: 26 January 1999 / Accepted: 26 May 1999