Computer Simulation of the Rapid Adaptation of Elongation Growth to Osmotic Stress in Segments of Cowpea Stem by Application of the Apoplast Canal Model

The transport of water during the adaptive rapid recovery ofelongation growth upon additional osmotic stress was examinedin the model stem segments of cowpea (Vigna unguiculata L.)by numerical solution of the extended canal equation, whichconsists of a set of time-dependent non-linear partial differentialequations. The calculated dynamic behaviour of the depletionof solute within the apoplast canal effectively explained thereported transient changes in water flow and, therefore, ingrowth during the adaptation to stress if the stress-inducedenhancement of net uptake of solute from the apoplast canalwas assumed. The extended canal model was also adequate forsimulation of the elastic shrinkage of hypocotyl segments uponexposure to osmotic stress which took place when the supplyof energy was limited. It appeared that the function of thecanal system in absorbing water is intrinsically stable againstperturbations of the water environment. (Received September 5, 1990; Accepted January 11, 1991)     

A model for radial water transport across plant roots

Summary A model based on the canal theory (Katou andFurumoto 1986 a, b) is proposed for the absorption of solute and water at the root periphery. The present canal model in the periphery and the model which was previously proposed for the exudation in the stele (Katou et al. 1987), are organized into a model for radial transport across excised plant roots, in the light of anatomical and physiological knowledge of maize roots. The canal equations for both canals are numerically solved to give quite a good explanation for the observed exudation of maize roots. It is found that the regulation of solute transport has a primary importance in the regulation of water transport across excised roots. The internal cell pressure of the symplast adjusts the water absorption at the root periphery to the water secretion into the vessels. There seems no need for this explanation of the radial water transport across roots to assume cell membranes with low reflection coefficient or variable water permeability. It would seem that the apoplast wall layers play a crucial role in metabolic control of water transport in roots as well as in hypocotyls.Abbreviations J _s ^ex* the theoretically estimated rate of solute exudation per unit surface area of model maize roots - J that of volume exudation per unit surface area of model maize roots - the reflection coefficient of the cell membrane against solutes     

Mechanism of pressure-induced water flow across plant roots

Summary Pressure-induced non-linear water flow across plant roots was analyzed theoretically. The double-canal model of radial water transport shown lately explained accurately the observed non-linear water flow in maize roots. The driving force rather than the hydraulic permeability caused the non-linear flow of water. The conclusion was drawn that non-linearity in pressure-induced water flow was an inherent property of the apoplast canal system in roots. Net solute transport plays a primary part for water transport.     

A model for water transport in the stele of plant roots

Summary The mechanism of water movement across roots is, as yet, not well understood. Some workable black box theories have already been proposed. They, however, assumed unrealistic cell membranes with low values of , or were based on a poor anatomical knowledge of roots. The role of root stele in solute and water transport seems to be especially uncertain. An attempted explanation of the nature of root exudation and root pressure by applying the apoplast canal theory (Katou andFurumoto 1986 a, b) to transport in the root stele is given. The canal equations are solved for boundary conditions based on anatomical and physiological knowledge of the root stele. It is found that the symplast cell membrane, cell wall and net solute transport into the wall apoplast are the essential constituents of the canal system. Numerical analysis shows that the canal system enables the coupled transport of solutes and water into a xylem vessel, and the development of root pressure beyond the level predicted by the osmotic potential difference between the ambient medium and the exudate. Observations on root exudation and root pressure previously reported seem to be explained quite well. It is concluded that the movement of water in the root stele although apparently active is essentially osmotic.Abbreviations J _v ^ex volume exudation per root surface - J₀ non-osmotic exudation - L^r overall radial hydraulic conductivity of an excised root - reflection coefficient - C_s difference in the osmotic concentration between the bathing medium and the exudate - R gas constant - T absolute temperature - C_K molar concentration of K⁺ - C_Cl molar concentration of Cl^– - C_j molar concentration of ion species j - P_j membrane permeability of ion j - z_j valence of ion j - F Faraday constant - V_ix intracellular electric potential with reference to the canal     

Temperature and growth-induced water potential

When the steins of dark-grown soybean [Glycine max (L.) Merr.] seedlings grew rapidly at favorable temperatures in saturating humidities, a water potential of about 0·2 MPa was induced by growth ($pS_o-$pS_w, where $pS_o is the water potential of the basal nonelongating tissue and $pS_w is the water potential of the elongating tissue). If this water potential was caused by high concentrations of solute in the apoplast, as has been proposed, lowering the temperature should have little effect on the potential. On the other hand, if the water potential was caused by apoplast tensions generated by growth, then the tensions should disappear as growth is inhibited by low temperatures. We observed that the growth-induced water potential became too small to detect when growth was inhibited by temperatures as low as 13—5 °C. The disappearance was observed as a rise in apoplast water potential using a thermocouple psychrometer for intact plants, a rise in cell turgor using a miniature pressure probe and a decrease in apoplast tensions using a pressure chamber. The disappearance was not caused by a loss of solute from the apoplast because the tensions fully accounted for the growth-induced water potential at all temperatures. The results are consistent with the lack of solute measured directly in the apoplast solutions at high temperatures (Nonami & Boyer 1987). Therefore, it was concluded that little solute was present in the apoplast at any temperature, and the growth-induced water potential was associated mostly with a tension that moved water from the xylem and into the surrounding cells to meet the demand of cell enlargement.     

Regulation of plant elongation growth by surface and xylem proton pumps

The roles of plasmalemma electrogenic proton pumps in elongation growth of plant stems are discussed on the basis of growth-electrophysiological studies on hypocotyl segments ofVigna unguiculata. Plant stems usually have two spatially separated electrogenic proton pumps: the surface proton pump which is located on the surface membrane of the symplast and the xylem proton pump, on the cell membrane of the symplast/xylem apoplast boundary. The surface proton pump excretes protons into the surface cell wall layer and causes the loosening of the cell wall. The xylem proton pump excretes protons into the xylem apoplast and drives the uptake of solute and water into the symplastvia secondary and/or tertiary active mechanisms: the proton cotransport system and the apoplast canal system. Both the surface and the xylem proton pumps are active during elongation growth because both the yielding of cell wall loosening and the uptake of water are necessary for continued elongation growth.     

Origin of growth-induced water potential : solute concentration is low in apoplast of enlarging tissues

We developed a new method to measure the solute concentration in the apoplast of stem tissue involving pressurizing the roots of intact seedlings (Glycine max [L.] Merr. or Pisum sativum L.), collecting a small amount of exudate from the surface of the stem under saturating humidities, and determining the osmotic potential of the solution with a micro-osmometer capable of measuring small volumes (0.5 microliter). In the elongating region, the apoplast concentrations were very low (equivalent to osmotic potentials of −0.03 to −0.04 megapascal) and negligible compared to the water potential of the apoplast (−0.15 to −0.30 megapascal) measured directly by isopiestic psychrometry in intact plants. Most of the apoplast water potential consisted of a negative pressure that could be measured with a pressure chamber (−0.15 to −0.28 megapascal). Tests showed that earlier methods involving infiltration of intercellular spaces or pressurizing cut segments caused solute to be released to the apoplast and resulted in spuriously high concentrations. These results indicate that, although a small amount of solute is present in the apoplast, the major component is a tension that is part of a growth-induced gradient in water potential in the enlarging tissue. The gradient originates from the extension of the cell walls, which prevents turgor from reaching its maximum and creates a growth-induced water potential that causes water to move from the xylem at a rate that satisfies the rate of enlargement. The magnitude of the gradient implies that growing tissue contains a large resistance to water movement.     

High apoplastic solute concentrations in leaves alter water relations of the halophytic shrub, Sarcobatus vermiculatus

Predawn plant water potential (Psi(w)) is used to estimate soil moisture available to plants because plants are expected to equilibrate with the root-zone Psi(w). Although this equilibrium assumption provides the basis for interpreting many physiological and ecological parameters, much work suggests predawn plant Psi(w) is often more negative than root-zone soil Psi(w). For many halophytes even when soils are well-watered and night-time shoot and root water loss eliminated, predawn disequilibrium (PDD) between leaf and soil Psi(w) can exceed 0.5 MPa. A model halophyte, Sarcobatus vermiculatus, was used to test the predictions that low predawn solute potential (Psi(s)) in the leaf apoplast is a major mechanism driving PDD and that low Psi(s) is due to high Na+ and K+ concentrations in the leaf apoplast. Measurements of leaf cell turgor (Psi(p)) and solute potential (Psi(s)) of plants grown under a range of soil salinities demonstrated that predawn symplast Psi(w) was 1.7 to 2.1 MPa more negative than predawn xylem Psi(w), indicating a significant negative apoplastic Psi(s). Measurements on isolated apoplastic fluid indicated that Na+ concentrations in the leaf apoplast ranged from 80 to 230 mM, depending on salinity, while apoplastic K+ remained around 50 mM. The water relations measurements suggest that without a low apoplastic Psi(s), predawn Psi(p) may reach pressures that could cause cell damage. It is proposed that low predawn apoplastic Psi(s) may be an efficient way to regulate Psi(p) in plants that accumulate high concentrations of osmotica or when plants are subject to fluctuating patterns of soil water availability.     

Transport dependence of leaf evolution in dicots

The diversity of tissue and cell organization in the leaves of dicots is explained as the mutual effect of light and water fluxes distribution. Equally with certain data about the role of light distribution, the same influence of water flux distribution on the leaf structure is recognized. Dorsiventral leaves of woody plants have an adequate to structure dorsiventral ring of water circulation. Rising flux from the xylem allocates via leaf apoplast with intermediate accumulation in upper epiderma. Descending flux starts and returns to bundle moving from cell to cell along the symplast (ER) of spongy parenchyma, bundle sheath and terminal complexes of the phloem. Isolateral leaves of herbs have a concentric pathway of solute circulation corresponding to the structure. Xylem flux allocates via symplast with water and nitrogen accumulation in paraveinal parenchyma. Water returns to phloem by transit via the apoplast in parallels with phloem exudate formation. Structural features correlated with the model of water circulation in the leaf are described. Numerous lines of leaf evolution well-known for dicots collect to two main topics which are typical for woody and herbaceous forms of dicots. The mechanisms of cell and tissue differentiation under the control of transport fluxes are discussed with special attention to ontogenetic and phylogenetic trends.     

The Mechanism of Isotonic Water Transport

The mechanism by which active solute transport causes water transport in isotonic proportions across epithelial membranes has been investigated. The principle of the experiments was to measure the osmolarity of the transported fluid when the osmolarity of the bathing solution was varied over an eightfold range by varying the NaCl concentration or by adding impermeant non-electrolytes. An in vitro preparation of rabbit gall bladder was suspended in moist oxygen without an outer bathing solution, and the pure transported fluid was collected as it dripped off the serosal surface. Under all conditions the transported fluid was found to approximate an NaCl solution isotonic to whatever bathing solution used. This finding means that the mechanism of isotonic water transport in the gall bladder is neither the double membrane effect nor co-diffusion but rather local osmosis. In other words, active NaCl transport maintains a locally high concentration of solute in some restricted space in the vicinity of the cell membrane, and water follows NaCl in response to this local osmotic gradient. An equation has been derived enabling one to calculate whether the passive water permeability of an organ is high enough to account for complete osmotic equilibration of actively transported solute. By application of this equation, water transport associated with active NaCl transport in the gall bladder cannot go through the channels for water flow under passive conditions, since these channels are grossly too impermeable. Furthermore, solute-linked water transport fails to produce the streaming potentials expected for water flow through these passive channels. Hence solute-linked water transport does not occur in the passive channels but instead involves special structures in the cell membrane, which remain to be identified.     

Analysis of apoplastic solutes in the cortex of soybean nodules

Various techniques were used to extract solutes from the free space of intact soybean [ Glycine max (L.) Merr.] nodules. A variety of solutes (carbohydrates, amino acids, organic acids, ions) was found, but the major solute obtained with all methods was allantoic acid. Most work was done with a technique involving vacuum infiltration of intact detached nodules with water. This approach provided rapid sampling of the apoplastic solutes, and the results indicated that solutes were not derived from the xylem and phloem of ruptured vascular bundles. Infiltration of intact nodules with Fast Green showed dye penetration only to the barrier in the inner cortex, indicating that infected tissues did not contribute to solute composition. Although allantoic acid was the only ureide which could be detected in solute samples, no evidence was obtained for the presence of allantoinase in the cortical apoplast. The results suggest the transport of allantoic acid by an apoplastic route in nodules or the release of allantoic acid to the cortical apoplast in response to treatments which disrupt ureide export. Calculated values for solute concentrations in the cortical apoplast were in the hundred millimolar range, suggesting that apoplastic solutes may represent a significant osmotic component in the nodule cortex.     

Arbuscular mycorrhizal symbiosis and plant aquaporin expression

Almost all land plants have developed a symbiosis with arbuscular mycorrhizal fungi. Establishment of the association is accompanied by structural changes in the plant root. During arbuscule formation fungal hyphae penetrate the root apoplast and install highly specialized interfaces for solute transport between plant and fungus. The periarbuscular membrane which is part of the plant plasma membrane surrounding arbuscular structures was shown to harbour a high density of different transport systems. Among these also expression of aquaporins was described, which potentially can act as a low affinity transport system for ammonia or ammonium. The present study provides data for expression, localization and function of plant aquaporins in the periarbuscular membrane of mycorrhizal Medicago truncatula plants.     

Analysis of time hierarchy in plant cell volume changes

A simple model of plant cell volume changes is presented. It is based on Kedem-Katchalsky equations for water and solute transport and on linear approximation of the dependence of intracellular hydrostatic pressure on the cell volume. Active transport of solute is also included. The time hierarchy within the system is analyzed by appropriate normalization of variables and by the assessment of the numerical values of model coefficients. The dynamics of the system comprises a slow process of solute exchange and a fast process of water transport. This explains the wellknown biphasic response of the cell volume to a sudden change in external conditions. An approximation of equations describing the system behaviour on the basis of the Tikhonov's theorem is proposed. The approximative solution is compared with the exact numerical solution of the original equations. The approximation is very good under physiological conditions, but it ceases to hold when the solute permeability of the cell membrane increases causing the breakdown of the entire time hierarchy within the system.     

植物根系质外体屏障研究进展

根是植物吸收水分和矿质营养以维持生命活动的重要器官。根系的构型和超微结构具有物种特异性, 对水分和矿质营养的吸收有不同程度的影响。其中, 内、外皮层的木栓层和凯氏带是2种重要的质外体屏障, 可非定向地阻断水分和离子运输, 在植物生长发育及响应逆境胁迫中发挥重要作用。尽管如此, 植物根系质外体屏障的结构、化学组成、生理功能、生物合成及其调控仅在模式植物拟南芥(Arabidopsis thaliana)中被广泛研究。近年来, 关于作物大麦(Hordeum vulgare)、水稻(Oryza sativa)以及部分牧草的根系质外体屏障研究报道逐渐增多。该文系统比较了拟南芥、大麦、水稻以及部分牧草根系质外体屏障的异同, 提出今后的研究方向, 以期为深入探索禾本科作物和牧草根系质外体屏障在生长发育和逆境适应中的作用奠定理论基础, 并为作物和牧草育种工作提供新思路。     

Differential interferometry in the analytical ultracentrifuge. II. General applications

Various ways of applying differential interferometry to ultracentrifugal analyses are examined and several analytical techniques are established. In transport and moving boundary methods, the sedimentation coefficient is more precisely determined in the differential interference system than in the schlieren optical system because fringe measurement accuracy is much higher in the former system. Compared to interference and absorption optics, the differential interferometer provides a more exact s value in the transport method since an accurate calculation procedure can be adopted. Moreover, the following advantages of differential interferometry are noted. Determination of the initial solute concentration, which must be done in the usual interference method, is unnecessary in this sedimentation equilibrium method. Regardless of the partial loss of solute from the observed system due to rapid precipitation or adsorption to the cell wall during centrifugation, the molecular weight of the rest of the solute can be determined exactly. The diffusion coefficient can be determined accurately by fringe displacement analysis at the hinge point during the transient state. Together with the molecular weight and diffusion coefficient, the partial specific volume and sedimentation coefficient of a solute can be obtained from the result of a single low-speed centrifugation when the sample solutions in H₂O and D₂O are compared.     

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 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.     

Accumulation of sugars in the xylem apoplast observed under water stress conditions is controlled by xylem pH

Severe water stress constrains, or even stops, water transport in the xylem due to embolism formation. Previously, the xylem of poplar trees was shown to respond to embolism formation by accumulating carbohydrates in the xylem apoplast and dropping xylem sap pH. We hypothesize that these two processes may be functionally linked as lower pH activates acidic invertases degrading sucrose and inducing accumulation of monosaccharides in xylem apoplast. Using a novel in vivo method to measure xylem apoplast pH, we show that pH drops from ~6.2 to ~5.6 in stems of severely stressed plants and rises following recovery of stem water status. We also show that in a lower pH environment, sugars are continuously accumulating in the xylem apoplast. Apoplastic carbohydrate accumulation was reduced significantly in the presence of a proton pump blocker (orthovanadate). These observations suggest that a balance in sugar concentrations exists between the xylem apoplast and symplast that can be controlled by xylem pH and sugar concentration. We conclude that lower pH is related to loss of xylem transport function, eventually resulting in accumulation of sugars that primes stems for recovery from embolism when water stress is relieved.     

Transport and action of ascorbate at the plant plasma membrane

The plasmalemma is both a bridge and a barrier between the cytoplasm and the outside world. It is a dynamic interface that perceives and transmits information concerning changes in the environment to the nucleus to modify gene expression. In plants, ascorbate is an essential part of this dialogue. The concentration and ratio of reduced to oxidized ascorbate in the apoplast, for example, possibly modulates cell division and growth. The leaf apoplast contains millimolar amounts of ascorbate that protect the plasmalemma against oxidative damage. The apoplastic ascorbate-dehydroascorbate redox couple is linked to the cytoplasmic ascorbate-dehydroascorbate redox couple by specific transporters for either or both metabolites. Although evidence about the mechanisms driving ascorbate or dehydroascorbate transport remains inconclusive, these carrier proteins potentially regulate the level and redox status of ascorbate in the apoplast. The redox coupling between compartments facilitated by these transport systems allows coordinated control of key physiological responses to environmental cues.     

Ion accumulation in the cell walls of rice plants growing under saline conditions: evidence for the Oertli hypothesis

Abstract. When plants of rice ( Oryza saliva L.) are subjected to mildly saline (50mol m⁻³ NaCl) conditions, the leaves show symptoms of water deficit, even though ion accumulation has been more than sufficient to adjust to the decrease in external water potential. After a few days of exposure to salt, there is a negative correlation, in a population of leaves, between the leaf water concentration (g water per g dry weight) and their sodium concentration (mmol Na per g dry weight). Ion concentrations in the cell walls and the cytoplasm of cells of plants grown in low salinity were measured by X-ray microanalysis. The NaCl concentration in solution in the apoplast was calculated to be around 600mol m⁻³ in leaves of plants whose roots were exposed to only 50 mol m⁻³ NaCl. This constitutes strong evidence that an important factor in salt damage in rice is dehydration due to the extracellular accumulation of salt as suggested in the Oertli hypothesis. The implication, that changes in tissue ion concentration and solute potentials equivalent to the external medium is not evidence of plant osmotic adjustment to salinity, is discussed.