A plumber's-eye view of xylem water transport in woody plants

We present a practical for university-level students aimed at measuring and comparing xylem hydraulic properties of co-existing plant species. After sampling branches of several woody species in the field, their main hydraulic properties were measured using a simple set-up. Hydraulic conductivity (K_h ) was calculated as the ratio between water flow through a plant segment and the pressure gradient driving the flow. The percent reduction in conductivity due to xylem embolism (i.e. air-filled conduits) was estimated by comparing K_h before and after flushing the measure segments to remove all native embolism. Raw hydraulic conductivity was standardised by cross-sectional wood area or supported leaf area to obtain more meaningful measures of conducting capacity. The results showed differences among study species, particularly between conifers and angiosperms. These differences are briefly discussed in terms of wood anatomy and the general biology of the species. Overall the practical provides a good opportunity for students to appreciate the main aspects of xylem water transport and the constraints it imposes on plant water relations.     

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 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     

Abscisic acid and water transport in sunflowers

The role of abscisic acid (ABA) in the transport of water and ions from the root to the shoot of sunflower plants (Helianthus annuus) was investigated by application of ABA either to the root medium or to the apical bud. The exudation at the hypocotyl stump of decapitated seedlings was measured with and without hydrostatic pressure (0–0.3 MPa) applied to the root. All ABA concentrations tested (10^-10–10^-4 mol·l^-1) promoted exudation. Maximal amounts of exudate (200% of control) were obtained with ABA at 10^-6·mol·l^-1 and an externally applied pressure of 0.1 MPa. The effect was rapid and long-lasting, and involved promotion of ion release to the xylem (during the first hours) as well as an increase in hydraulic conductivity. Abscisic acid applied to the apical bud had effects similar to those of the rootapplied hormone. Increased rates of exudation were also obtained after osmotic stress was applied to the root; this treatment increased the endogenous level of ABA in the root as well as in the shoot. Water potentials of the hypocotyls of intact plants increased when the roots were treated with ABA at 5°C, whereas stomatal resistances were lowered. The results are consistent with the view that ABA controls the water status of the plant not only by regulating stomatal transpiration, but also by regulating the hydraulic conductivity of the root.Abbreviations and symbols ABA abscisic acid - Tv volume flow - Lp hydraulic conductivity - PEG polyethyleneglycol - water potential - osmotic potential - osmotic value - P hydrostatic pressure     

Uphill transport of water by electroosmosis

Summary Lepismatidae are able to gain water from subsaturated atmosphere above a relative humidity of 45%, surmounting a water potential difference of at least 1.1×10⁸ Pa (1,100 bar). This extraordinary task is performed by the monolayered epithelium of the posterior rectum. The particle coat of the folded apical membrane of this epithelium suggests the presence of the electrogenic, lumen-directed cation transport, which is commonly found in insects. Assuming this kind of transport and considering the anatomy of the organ, a working hypothesis for this hyposmotic water transport has been developed: The electrogenic cation transport maintains the circulation of the transported ion species across the apical membrane; the voltagedriven inward current transfers water by electroosmosis against its chemical potential from the extracellular space into the cytoplasm. Voltage and current measurements and synchronous measurements of water flow across the epithelium of the posterior rectum ofLepisma saccharina strongly corroborate this hypothesis. The transepithelial voltage is up to 200 mV (lumen positive); the short-circuit current averages 200 A per cm² of the epithelium. Both depend acutely on oxidative metabolism as does spontaneous water uptake. Exogenous transepithelial current (I) induces, independently of anoxia, a proportional change in volume flow (J _v). The induced flow has the direction of the cation flow. Its mean coupling ratio (J _v/I) is 1.5×10^–9m³/A·sec corresponding to 7 to 8 H₂O per positive unit charge. Critical evaluation of experimental data reveals that water uptake by electroosmosis may quantitatively account forin vivo performance without requiring any unusual assumption.     

Amino acid transport systems in animal cells: Interrelations and energization

After summarizing the discrimination of the several transport systems of neutral amino acids in the cell of the higher animal, I discuss here the ways in which 2 dissimilar transport systems interact, so that one tends to run forward for net entry and the other backwards for net exodus. An evaluation of the proposals for energization shows that uphill transport continues when neither alkali-ion gradients nor ATP levels are favorable. Evidence is presented that under these conditions a major contribution is made by another mode of energization, which may depend on the fueling of an oxidoreductase in the plasma membrane. This fueling may involve the export by the mitochondrion of the reducing equivalents of NADH by one of the known shuttles, e.g., the malate-aspartate shuttle. After depletion of the energy reseves in the Ehrilich cell by treating it with dinitrophenol plus iodoacetate concentrative uptake of test amino acids is restoration by pyruvate but in poor correlation with the restoration of alkali-ion gradients and ATP levels. This restoration by pyruvate but not by glucose is highly senstitive to rotenone. A combination of phenazine methosulfate and ascorbate will also produce transport restoration, before either the alkali-ion gradients or ATP levels have begun to rise. The restoration of transport applies to a model amino acid entering by the Na⁺-independent system, as well as to one entering by the principal Na⁺-dependent system, restoration being blocked by ouabain, despite the weak effect of ouabain on the alkali-ion gradients in the Ehrlich cell. Quinacrine terminates very quickly the uptake of model amino acids, before the alkali-ion gradients have begun to fall and before the ATP level has been halved. Quinacrine is also effective in blocking restoration of uphill transport by either pyruvate or the phenazine reagent. Preliminary results show that vesicles prepared from the plasma membrane of the Ehrlich cell quickly reduce cytochrome c or ferricyanide in the presence of NADH, and that the distribution of a test amino acid between the vesicle and its environment is influenced by NADH, quinacrine, and an uncoupling agent in ways consistent with the above proposal, assuming that a majority of the vesicles are everted.     

The electron transport systems of Fasciola hepatica mitochondria.

The electron transport systems of Fasciola hepatica mitochondria were investigated spectrophotometrically at room temperature and at −196°. The mitochondria were found to contain substrate reducible a-, b- and c-type cytochromes. All of the cytochrome components of the classical mammalian type of respiratory chain were present, although the concentration of cytochromes aa₃ was low. In addition to the mammalian type of respiratory chain, the Fasciola mitochondria contained a substrate reducible b-type cytochrome component (557 nm) which included a CO reactive o-type cytochrome. The results suggest that F. hepatica mitochondria contain a branched electron transport system including a mammalian type of chain and involving two terminal oxidases and at least two b-type cytochromes.     

三江平原沼泽湿地水系统研究浅析

综述了沼泽湿地水文研究进展,对其水系统研究的地位和作用进行讨论,并综合分析了其主要内容和研究系统,同时对沼泽湿地水系统研究前景提出了展望。     

Energetics of membrane transport in protoplasts

Examples are given to illustrate the recent use of isolated protoplasts in the study of membrane transport with the emphasis on the energetics of solute transport. A model is also presented for the mechanism of active solute transport at the plasmalemma.     

Co-ordination of vapour and liquid phase water transport properties in plants

The pathway for water movement from the soil through plants to the atmosphere can be represented by a series of liquid and vapour phase resistances. Stomatal regulation of vapour phase resistance balances transpiration with the efficiency of water supply to the leaves, avoiding leaf desiccation at one extreme, and unnecessary restriction of carbon dioxide uptake at the other. In addition to maintaining a long-term balance between vapour and liquid phase water transport resistances in plants, stomata are exquisitely sensitive to short-term, dynamic perturbations of liquid water transport. In balancing vapour and liquid phase water transport, stomata do not seem to distinguish among potential sources of variation in the apparent efficiency of delivery of water per guard cell complex. Therefore, an apparent soil-to-leaf hydraulic conductance based on relationships between liquid water fluxes and driving forces in situ seems to be the most versatile for interpretation of stomatal regulatory behaviour that achieves relative homeostasis of leaf water status in intact plants. Components of dynamic variation in apparent hydraulic conductance in intact plants include, exchange of water between the transpiration stream and internal storage compartments via capacitive discharge and recharge, cavitation and its reversal, temperature-induced changes in the viscosity of water, direct effects of xylem sap composition on xylem hydraulic properties, and endogenous and environmentally induced variation in the activity of membrane water channels in the hydraulic pathway. Stomatal responses to humidity must also be considered in interpreting co-ordination of vapour and liquid phase water transport because homeostasis of bulk leaf water status can only be achieved through regulation of the actual transpirational flux. Results of studies conducted with multiple species point to considerable convergence with regard to co-ordination of stomatal and hydraulic properties. Because stomata apparently sense and respond to integrated and dynamic soil-to-leaf water transport properties, studies involving intact plants under both natural and controlled conditions are likely to yield the most useful new insights concerning stomatal co-ordination of transpiration with soil and plant hydraulic properties.     

Physiological role of tyrosine transport systems in Halobacterium salinarium

The quantitative content of three transport systems for aromatic amino acids in cells of Halobacterium salinarium was measured: the common system (K _m is about 10^-6 M) and two tyrosine-specific systems with high and low affinity (K _m is about 10^-8 and 10^-5 M, respectively). To determine the activity of each of three systems separately, a method was developed based on the selective phenylalanine effect on these activities. When phenylalanine exeeds [¹⁴C]tyrosine by four to sixforld, it inhibits competitively the activity of the common system, and its 50- to 100-fold molar excess is inhibitory in a non-competitive way for the specific high affinity system (HAT system). The specific low affinity system (LAT system) is practically insensitive to phenylalanine. The activities of tyrosine-specific transport systems are slightly dependent on the culture age, and the observed decrease in transport activity during growth is due mainly to the decreased content of the common system. The HAT system formation is regulated by the repression type, and the effectors are aromatic amino acids especially tyrosine itself. The physiological sense of the tyrosine transport system's multiplicity in H. salinarium is discussed.     

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     

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 thyroid hormones on the gene expression of calcium transport systems in rat muscles

It has been previously shown that modification of thyroid hormone levels have a profound impact on cardiac function, predominantly through a direct regulation of the sarcoplasmic reticulum protein levels. Nevertheless, little is known about the regulation of calcium transport systems in skeletal muscle due to the altered concentration of thyroid hormones. Thus, the goal of our study was to find out whether altered thyroid status could change the gene expression of the Na(+)/Ca(2+) exchanger (NCX), the inositol 1,4,5-trisphosphate (IP(3)) receptors and ryanodine receptors (RyRs) in slow and fast skeletal muscles of rats. A hyperthyroid state was maintained in rats by triiodothyronine (T(3)) administration, while methimazole was employed for inducing hypothyroidism. After a period of 2-10 months of T(3) treatment we observed a significant increase in mRNA levels of the NCX, RyRs and IP(3) receptors. This increase was more pronounced in the slow soleus than in the fast extensor digitorum longus (EDL) muscle. It is tempting to speculate that thyroid hormones also alter calcium concentration and thus influence the process of excitation-contraction coupling in the skeletal muscle.     

Variation in material transport and water chemistry along a large ephemeral river in the Namib Desert

• 1 The chemical characteristics of floodwaters in ephemeral rivers are little known, particularly with regard to their organic loads. These rivers typically exhibit a pronounced downstream hydrological decay but few studies have documented its effect on chemical characteristics and material transport. To develop a better understanding of the dynamics of floods and associated material transport in large ephemeral rivers, floods of the ephemeral Kuiseb River in south‐western Africa were tracked and repeatedly sampled at multiple points along the river's lower 220 km.
• 2 We quantified the composition and transport of solute and sediment loads in relation to longitudinal hydrological patterns associated with downstream hydrological decay. Source and sink areas for transported materials were identified, and the composition and transport dynamics of the organic matter load were compared to those described from more mesic systems.
• 3 Concentrations of sediments and solutes transported by floods in the Kuiseb River tended to increase downstream in association with pronounced hydrological decay. The contribution of particulate organic matter to total organic load is among the highest recorded, despite our observation of unusually high levels of dissolved organic matter. Hydrological decay resulted in deposition of all transported material within the lower Kuiseb River, with no discharge of water or materials to the Atlantic Ocean.
• 4 Our results suggest that longitudinal variation in surface flow and associated patterns of material transport renders the lower Kuiseb River a sink for materials transported from upstream. The downstream transport and deposition of large amounts of labile organic matter provides an important carbon supplement to heterotrophic communities within the river's lower reaches.

     

Distribution of cytokinin-like activity in different plant parts of the water hyacinth, Eichhornia crassipes

Cytokinin-like activity in extracts of leaf laminae, petioles, shoots, roots and flowers of young plants of the water hyacinth, Eichhornia crassipes S. was analyzed following Sephadex LH-20 column chromatography using the soybean callus bioassay. In all plant parts analyzed, two prominent peaks of cytokinin activity having elution volumes similar to zeatin and zeatin riboside were detected. Putative cytokinin gluco-side-like activity was detected only in leaves and flowers. The cytokinin complements of the leaves and the roots were qualitatively different. It would appear that cytokinins supplied by the roots are metabolized in the leaves or certain cytokinins are synthesized in the leaves themselves. The possible significance and distribution of cytokinins in different plant parts in relation to roots is discussed.     

Radial transport of water across cortical sleeves of excised roots ofZea mays L.

The stationary radial volume flows across maize (Zea mays L.) root segments without steles (sleeves) were measured under isobaric conditions. The driving force of the volume flow is an osmotic difference between the internal and external compartment of the root preparations. It is generated by differences in the concentrations of sucrose, raffinose or polyethylene glycol. The flows are linear functions of the corresponding osmotic differences ( ) up to osmotic values which cause plasmolysis. The straight lines obtained pass through the origin. No asymmetry of the osmotic barrier could be detected within the range of driving forces applied ( =±0.5 MPa), corresponding to volume-flow densities of j_{v, s}=±7·10^–8 m·s^–1. Using the literature values for the reflection coefficients of sucrose and polyethylene glycol in intact roots (E. Steudle et al. (1987) Plant Physiol.84, 1220–1234), values for the sleeve hydraulic conductivity of about 1·10^–7 m·s^–1 MPa^–1 were calculated. They are of the same order of magnitude as those reported in the literature for the hydraulic conductivity of intact root segments when hydrostatic pressure is applied.Abbreviations and symbols a _s outer surface of sleeve segment - c concentration of osmotically active solute - j _{v, s} radial volume flow density across sleeve segment - Lp_s hydraulic conductivity of sleeves - Lp_r hydraulic conductivity of intact roots - _N thickness of Nernst diffusion layer - reflection coefficient of root for solute - osmotic value of bulk phase - osmotic coefficient     

Stable oscillations in mathematical models of biological control systems

Summary Oscillations in a class of piecewise linear (PL) equations which have been proposed to model biological control systems are considered. The flows in phase space determined by the PL equations can be classified by a directed graph, called a state transition diagram, on anN-cube. Each vertex of theN-cube corresponds to an orthant in phase space and each edge corresponds to an open boundary between neighboring orthants. If the state transition diagram contains a certain configuration called a cyclic attractor, then we prove that for the associated PL equation, all trajectories in the regions of phase space corresponding to the cyclic attractor either (i) approach a unique stable limit cycle attractor, or (ii) approach the origin, in the limitt→∞. An algebraic criterion is given to distinguish the two cases. Equations which can be used to model feedback inhibition are introduced to illustrate the techniques.     

Characterization of ammonium and methylamine transport systems in phototrophic sulfur bacteria

Abstract Transport of ammonium and methylamine into the cells of green sulfur bacterium Chlorobium limicola and purple sulfur bacterium Thiocapsa roseopersicina is carried out by a common transport system. This system has (for C. limicola and T. roseopersicina , respectively) pH optimum 7.0 and 7.5; V _max 0.6 and 4.2 nmol min⁻¹ (mg protein)⁻¹; K_m 5.9 × 10⁻⁵ M and 1.3 × 10⁻⁵ M, and is capable of forming 120- and 600-fold methylamine gradients. The methylamine transport can be energized by the artificially imposed transmembrane K⁺ diffusive potential and is inhibited by tetraphenylphosphonium or valinomycin and K⁺. The data presented indicate that methylamine transport in both studied species is exclusively driven by the membrane potential gradient (ΔΨ).     

Expression of substrate specificity in facilitated transport systems

Summary In facilitated transport systems the carrier reorientation step is shown to be largely independent of the forces of interaction between the substrate and the carrier site, whereas in coupled systems (obligatory exchange or cotransport) reorientation proceeds at the expense of the binding force developed in the transition state. In consequence, the expression of substrate specificity is expected to differ in the two systems. In the facilitated transport of analogs no larger than the normal substrate, the affinity but not the maximum rate of transport can vary widely; with larger analogs, both the affinity and rale can vary if steric constraints are more severe in the translocation step than in binding. In coupled transport, by contrast, the translocation step can be highly sensitive to the structure of the substrate, and binding much less sensitive. The theory agrees with published observations on facilitated systems for choline and glucose in erythrocytes, as well as on Na⁺-coupled systems for the same substrates in other cells. The following mechanism, which could account for the behavior, is proposed. In facilitated systems, the transport site fits the substrate closely and retains its shape as the carrier undergoes reorientation. In coupled systems, the site is initially looser, but during carrier reorientation it contracts around the substrate. In both systems, the carrier encloses the substrate during the translocation step, though for a different reason: in coupled but not in facilitated systems the binding force enormously increases in the enclosed state, through a chelation effect. In both systems, steric interference with enclosure retards the translocation of bulky substrate analogs.