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。     

Water storage and osmotic pressure influences on the water relations of a dicotyledonous desert succulent

Abstract Water storage and nocturnal increases in osmotic pressure affect the water relations of the desert succulent Ferocactus acanthodes, which was studied using an electrical circuit analog based on the anatomy and morphology of a representative individual. Transpiration rates and osmotic pressures over a 24-h period were used as input variables. The model predicted water potential, turgor pressure and water flow for various tissues. Plant capacitances, storage resistances and nocturnal increases in osmotic pressure were varied to determine their role in the water relations of this dicotyledonous succulent. Water coming from storage tissues contributed about one-third of the water transpired at night: the majority of this water came from the nonphotosynthetic, water storage parenchyma of the stem. Time lags of 4 h were predicted between maximum transpiration and maximum water uptake from the soil. Varying the capacitance of the plant caused proportional changes in osmotically driven water movement but changes in storage resistance had only minor effects. Turgor pressure in the chlorenchyma depended on osmotic pressure, but was fairly insensitive to doubling or halving of the capacitance or storage resistance of the plant. Water uptake from the soil was only slightly affected by osmotic pressure changes in the chlorenchyma. For this stem succulent, the movement of water from the chlorenchyma to the xylem and the internal redistribution of water among stem tissues were dominated by nocturnal changes in chlorenchyma osmotic pressure, not by transpiration.     

Evaluation of water stress control with polyethylene glycols by analysis of guttation

The water relations of pepper plants (Capsicum frutescens L.) under conditions conducive to guttation were studied to evaluate the control of plant water stress with polyethylene glycols. The addition of polyethylene glycol 6000 to the nutrient solution resulted in water relations similar to those expected in soil at the same water potentials. Specifically, xylem pressure potential in the root and leaf became more negative during a 24-hour treatment period, while osmotic potential of the root xylem sap remained constant. The decrease in pressure potential was closely correlated with the decrease in osmotic potential of the nutrient solution. In contrast, the addition of polyethylene glycol 400 to the nutrient medium resulted in a reduction of osmotic potential in the root xylem sap; this osmotic adjustment in the xylem was large enough to establish an osmotic gradient for entry of water and cause guttation at a nutrient solution osmotic potential of −4.8 bars. Pressure potential in the root and leaf xylem became negative only at nutrient solution osmotic potentials lower than −4.8 bars. About half of the xylem osmotic adjustment in the presence of polyethylene glycol 400 was caused by increased accumulation of K⁺, Na⁺, Ca²⁺, and Mg²⁺ in the root xylem. These studies indicate that larger polyethylene glycol molecules such as polyethylene glycol 6000 are more useful for simulating soil water stress than smaller molecules such as polyethylene glycol 400.     

Applications of the compensating pressure theory of water transport

Some predictions of the recently proposed theory of long-distance water transport in plants (the Compensating Pressure Theory) have been verified experimentally in sunflower leaves. The xylem sap cavitates early in the day under quite small water stress, and the compensating pressure P (applied as the tissue pressure of turgid cells) pushes water into embolized vessels, refilling them during active transpiration. The water potential, as measured by the pressure chamber or psychrometer, is not a measure of the pressure in the xylem, but (as predicted by the theory) a measure of the compensating pressure P. As transpiration increases, P is increased to provide more rapid embolism repair. In many leaf petioles this increase in P is achieved by the hydrolysis of starch in the starch sheath to soluble sugars. At night P falls as starch is reformed. A hypothesis is proposed to explain these observations by pressure-driven reverse osmosis of water from the ground parenchyma of the petiole. Similar processes occur in roots and are manifested as root pressure. The theory requires a pump to transfer water from the soil into the root xylem. A mechanism is proposed by which this pump may function, in which the endodermis acts as a one-way valve and a pressure-confining barrier. Rays and xylem parenchyma of wood act like the xylem parenchyma of petioles and roots to repair embolisms in trees. The postulated root pump permits a re-appraisal of the work done by evaporation during transpiration, leading to the proposal that in tall trees there is no hydrostatic gradient to be overcome in lifting water. Some published observations are re-interpreted in terms of the theory: doubt is cast on the validity of measurements of hydraulic conductance of wood; vulnerability curves are found not to measure the cavitation threshold of water in the xylem, but the osmotic pressure of the xylem parenchyma; if measures of xylem pressure and of hydraulic conductance are both suspect, the accepted view of the hydraulic architecture of trees needs drastic revision; observations that xylem feeding insects feed faster as the water potential becomes more negative are in accord with the theory; tyloses, which have been shown to form in vessels especially vulnerable to cavitation, are seen as necessary for the maintenance of P, and to conserve the supplementary refilling water. Far from being a metastable system on the edge of disaster, the water transport system of the xylem is ultrastable: robust and self-sustaining in response to many kinds of stress.     

三角叶滨藜根吸水特点与其抗盐性的关系

采用压力室和冰点渗透压计测定了三角叶滨藜在不同浓度NaCl的根系环境溶液中根木质部的压力势和伤流液的渗透势,并利用原子吸收分光光度计测定了植株和伤流液以及环境溶液中Na 含量。结果表明:随着根环境溶液NaCl浓度的增加,三角叶滨藜植株和木质部伤流液中Na 含量虽呈上升趋势,但根系的过滤系数和体内Na 相对累积量逐渐降低,说明三角叶滨藜根细胞对盐分有很强的过滤作用;木质部伤流液的渗透势随着环境溶液渗透势的降低而降低,但根木质部溶液的水势则逐渐高出根外环境溶液的渗透势;表明三角叶滨藜能够利用较低的木质部负压来抵抗根外溶液的低渗透势而反渗透吸水,并利用根细胞对盐分的过滤作用来避免从环境摄取过量的盐分。     

An analysis of resistance to water flow through wheat and tall fescue leaves during pressure chamber efflux experiments

Abstract. This is a physical analysis of water movement in wheat ( Triticum ) and tall fescue ( Festuca arundinacea ) leaves placed in the Scholander pressure chamber. It takes into account the efflux resistances of water movement through the xylem and water flow across the cell membranes. Xylem resistance was estimated using Poiseuille's law.
Leaves which had been pressurized in the chamber were embedded, sectioned, examined under a light microscope and photographed. Cells were intact but distorted and xylem vessels were intact. Cells in portions of the blade squeezed by the chamber sealing grommet were crushed, but xylem vessels remained intact.
By applying pressure several tenths of a megapascal in excess of the balance pressure, water was forced from each leaf through the severed end which protruded from the chamber. Efflux curves were drawn by plotting the total water expressed as a function of time after the pressure increase. Water efflux from the shortest wheat leaf lasted only 10 min while efflux from the longest continued for up to 40 min. The efflux from a tall fescue leaf which was rehydrated and cut to a shorter length was much more rapid than efflux from the original leaf.
Experiments combined with mathematical analysis suggested that the effect of leaf length on efflux is related to a high resistance to water flow through vascular bundles. Xylem resistance would be sufficient to produce this effect if it were 10 times greater than that predicted by Poiseuille's law. Both the observations of water flow from the cut end of the leaf and the mathematical model suggested very little water flows from bundles with vessels of diameter less than 12 μm. The apparent explanation is high resistance to water flow through these small diameter vessels.     

Leaf water stress in engelmann spruce: influence of the root and shoot environments

The response of xylem pressure potential of Engelmann spruce (Picea engelmannii Engelm.) to environmental factors was studied in the natural subalpine environment. Data were analyzed in the context of a leaf water potential model based upon the van den Honert model for water transport through the soil-plant-atmosphere continuum. At soil temperatures of 10 to 15 C, xylem pressure potential decreased to about −10 bars as the ratio of leaf to air absolute humidity difference to leaf diffusion resistance (an estimate of transpiration) increased. The potentials were slightly lower at all flux rates above zero when the soil temperature was 5 to 10 C, and at temperatures of 0 to 5 C the potentials decreased sharply to as low as −20.4 bars, even though the soil water supply was adequate. The relative viscosity of water and soil to leaf resistances for flow were compared for Engelmann spruce and citrus at low soil temperatures. These comparisons indicated that decreased root permeability was probably not an important factor causing higher stresses in spruce at 5 to 10 C, but for citrus, root permeability became limiting at soil temperatures as high as 13.5 C. Xylem pressure potential was correlated with net radiation during the daytime when soil temperature was above 7 C. Under other conditions, however, xylem potential and net radiation apparently had a different relationship. The relationship between flux density and potential was the same on unshaded and shaded portions of the crown, with differences in potential related to differences in flux density.     

Changes in plant–soil hydraulic pressure gradients of soybean in response to soil drying

Plant gas‐exchange response to drying soil in many instances tracks a common pattern when expressed as a function of fraction of transpirable soil water (FTSW). There is little decrease in gas exchange until FTSW decreases to a value in the range of about 0.3–0.45, then with further drying gas exchange declines approximately linearly. This unique pattern is hypothesised to reflect mainly changes in the water potential gradient between bulk soil and plant. The primary objective was to directly document the basis of this response by measuring the hydrostatic pressure gradient required in the soil to maintain leaf xylem at zero potential with decreasing FTSW. Pots in which soybean (Glycine max) plants were grown were placed in a pressure chamber and the pressure adjusted to maintain zero water potential in a leaf petiolule. These results showed a small, relatively constant hydrostatic pressure had to be applied to the soil to maintain zero leaf xylem water potential until FTSW decreased to approximately 0.3–0.45. Thereafter, the required hydrostatic pressure gradient increased as FTSW continued to decrease. Hydraulic conductance was calculated to be relatively stable early in the drying cycle, and then decrease as the soil dried to comparatively high FTSW of 0.5–0.7.     

The mechanisms of refilling of xylem conduits and bleeding of tall birch during spring

Seasonal variations in osmolality and components of xylem sap in tall birch trees were determined using several techniques. Xylem sap was extracted from branch and trunk sections of 58 trees using the very rapid gas bubble-based jet-discharge method. The 5-cm long wood pieces were taken at short intervals over the entire tree height. The data show that large biphasic osmolality gradients temporarily exist within the conducting xylem conduits during leaf emergence (up to 272 mosmol x kg(-1) at the apex). These gradients (arising mainly from glucose and fructose) were clearly held within the xylem conduit as demonstrated by (1)H NMR imaging of intact twigs. Refilling experiments with benzene, sucrose infusion, electron and light microscopy, as well as (1)H NMR chemical shift microimaging provided evidence that the xylem of birch represents a compartment confined by solute-reflecting barriers (radial: lipid linings/lipid bodies; axial: presumably air-filled spaces). These features allow transformation of osmolality gradients into osmotic pressure gradients. Refilling of the xylem occurs by a dual mechanism: from the base (by root pressure) and from the top (by hydrostatic pressure generated by xylem-bound osmotic pressure). The generation of osmotic pressure gradients was accompanied by bleeding. Bleeding could be observed at a height of up to 21 m. Bleeding rates measured at a given height decreased exponentially with time. Evidence is presented that the driving force for bleeding is the weight of the static water columns above the bleeding point. The pressure exerted by the water columns and the bleeding volume depend on the water-filling status of (communicating) vessels.     

Maize stomatal conductance in the field: its relationship with soil and plant water potentials, mechanical constraints and ABA concentration in the xylem sap

Abstract. Stomatal conductance, leaf water potential, soil water potential and concentration of abscisic acid (ABA) in the xylem sap were measured on maize plants growing in the field, in two treatments with contrasting soil structures. Soil compaction affected the stomatal conductance, but this effect was no longer observed if the soil water potential was increased by irrigation. Differences in leaf water potential did not account for the differences in conductance between treatments. Conversely, the relationship between stomatal conductance and concentration of ABA in the xylem sap was consistent during the experiment. The proposed interpretation is that stomatal conductance was controlled by the root water potential via an ABA message. Control of the stomatal conductance by the leaf water potential or by an effect of mechanical stress on the roots is unlikely.     

Gas exchange and water balance of a mistletoe species and its mangrove hosts

Summary The gas exchange and water relations of the hemiparasite Pthirusa maritima and two its mangrove host species, Conocarpus erectus and Coccoloba uvifera, were studied in an intertidal zone of the Venezuelan coast. Carbon uptake and transpiration, leaf osmotic and total water potential, as well as nutrient content in the xylem sap and leaves of mistletoes and hosts were followed through the dry and wet season. In addition, carbon isotope ratios of leaf tissue were measured to further evaluate water use efficiency. Under similar light and humidity conditions, mistletoes had higher transpiration rates, lower leaf water potentials, and lower water use efficiencies than their hosts. Potassium content was much higher in mistletoes than in host leaves, but mineral nutrient content in the xylem sap of mistletoes was relatively low. The resistance of the liquid pathway from the soil to the leaf surface of mistletoes was larger than the total liquid flow resistance of host plants. Differences in the daily cycles of osmotic potential of the xylem sap also indicate the existence of a high resistance pathway along the vascular connection between the parasite pathway along the vascular connection between the parasite and its host. P. maritima mistletoes adjust to the different physiological characteristics of the host species which it parasitizes, thus ensuring an adequate water and carbon balance.     

In Situ Estimates of Variable Plant Resistance to Water Flux in Ilex opaca Ait

Xylem pressure potentials and stomatal diffusion resistances were measured in the field in Ilex opaca Ait. during days which differed in temperature and vapor pressure deficit. Water flux into leaves was calculated by combining the field data with laboratory determinations of the relation between tissue water deficit and water potential. Estimates of apparent plant resistance were then calculated from fluxes and differences between soil water potential and xylem tension. The resistance depended strongly on water flux, dropping by a factor of over 7 from low to high water flow rates. This extends the generality of variable plant resistances measured in controlled environment studies to I. opaca as it occurs naturally in the field. The relation of apparent plant resistance to water flux as estimated in this study can be useful in simulation models which calculate water uptake to leaves as a flux driven by a difference in soil and leaf water potentials across a resistance between the bulk soil and the leaf.     

Apoplastic transport across young maize roots: effect of the exodermis

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

Sugar accumulation in roots of two grape varieties with contrasting response to water stress

Root sugar accumulation was studied in two grapevine varieties contrasting in tolerance to water stress. During a 10‐day water withholding treatment, the drought‐tolerant variety, Grenache, sustained less negative predawn and midday leaf water potentials as well as root water potential compared with the sensitive variety, Semillon. Grenache vines also maintained lower stomatal conductance and transpiration than Semillon vines throughout the drying period. In both varieties there was accumulation of sucrose in the roots and concentrations were inversely correlated to leaf and root water status. In both Grenache and Semillon, elevated root osmolality was associated with decreased soil moisture indicating that sugar accumulation may play a role in osmotic protection. Petiole xylem sap abscisic acid (ABA) concentrations increased with water deficit in both varieties and were highest for vines with the most negative root and predawn leaf water potentials. Furthermore, root sucrose concentrations were positively correlated with leaf xylem sap ABA concentrations, indicative of integration between carbohydrate metabolism and the ABA signalling system. Similar root sugar accumulation patterns between the two varieties, however, demonstrate that other factors are likely influencing the ability of the drought‐tolerant variety to remain hydrated.     

盐胁迫对大豆根系木质部压力和Na+吸收的影响

取栽培大豆的水培幼苗为材料,用木质部压力探针和原子吸收分光光度计测定了盐胁迫条件下其根木质部压力和伤流液中Na~+含量的变化,以分析大豆抗盐吸水的机制.结果表明:在25～150 mmol/L NaCl的浓度范围内,随着盐胁迫强度的增加,大豆根木质部负压力的绝对值逐渐增大,但相对负压力和根的径向反射系数则逐渐减小;木质部伤流液中Na~+含量逐渐增加,但Na~+的相对含量则逐渐降低.同时,虽然根系吸水所需的木质部负压力(压力势)及根木质部伤流液的渗透势随着盐胁迫强度的增加都有所下降,但两者共同作用使木质部水势下降的幅度远远小于根外溶液水势(渗透势)下降的幅度,即随着根外溶液盐浓度的升高,根木质部溶液的总水势逐渐高出根外溶液的水势.上述结果说明,在盐胁迫下大豆可以利用相对小的木质部负压力逆水势梯度吸水,且通过避免对Na~+的过量吸收来适应盐胁迫环境.     

Influence of Xylem Water Potential on Leaf Elongation and Osmotic Adjustment of Wheat and Lupin

The leaf elongation rate and osmotic pressure at full turgorof wheat (Triticum aestivum L.) and lupin (Lupinus cosentiniiGuss.) were measured in well watered plants, in plants thatwere allowed to dry the soil slowly over 7 d, and in plantsin which the water potential of the leaf xylem was maintainedhigh by applying pressure to the roots during the drying cycle.Maintenance of high xylem water potentials failed to preventa reduction in the rate of leaf elongation as the soil dried,while the osmotic pressure at full turgor and the degree ofosmotic adjustment increased as the soil water content decreased.The rate of leaf elongation was reduced more and the degreeof osmotic adjustment was higher in leaves with high xylem waterpotentials than in those in which leaf xylem potentials wereallowed to decrease as soil water content decreased. Osmoticadjustment was linearly correlated with the reduction in leafelongation rate in both wheat and lupin. Key words: Osmotic adjustment, leaf elongation, turgor regulation     

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.     

Unraveling the Effects of Plant Hydraulics on Stomatal Closure during Water Stress in Walnut

The objectives of the study were to identify the relevant hydraulic parameters associated with stomatal regulation during water stress and to test the hypothesis of a stomatal control of xylem embolism in walnut (Juglans regia x nigra) trees. The hydraulic characteristics of the sap pathway were experimentally altered with different methods to alter plant transpiration (Eplant) and stomatal conductance (gs). Potted trees were exposed to a soil water depletion to alter soil water potential (Psisoil), soil resistance (Rsoil), and root hydraulic resistances (Rroot). Soil temperature was changed to alter Rroot alone. Embolism was created in the trunk to increase shoot resistance (Rshoot). Stomata closed in response to these stresses with the effect of maintaining the water pressure in the leaf rachis xylem (P(rachis)) above -1.4 MPa and the leaf water potential (Psileaf) above -1.6 MPa. The same dependence of Eplant and gs on P(rachis) or Psileaf was always observed. This suggested that stomata were not responding to changes in Psisoil, Rsoil, Rroot, or Rshoot per se but rather to their impact on P(rachis) and/or Psileaf. Leaf rachis was the most vulnerable organ, with a threshold P(rachis) for embolism induction of -1.4 MPa. The minimum Psileaf values corresponded to leaf turgor loss point. This suggested that stomata are responding to leaf water status as determined by transpiration rate and plant hydraulics and that P(rachis) might be the physiological parameter regulated by stomatal closure during water stress, which would have the effect of preventing extensive developments of cavitation during water stress.     

Axial and Radial Hydraulic Resistance to Roots of Maize (Zea mays L.)

A root pressure probe was employed to measure hydraulic properties of primary roots of maize (Zea mays L.). The hydraulic conductivity (Lp_r) of intact root segments was determined by applying gradients of hydrostatic and osmotic pressure across the root cylinder. In hydrostatic experiments, Lp_r was constant along the segment except for an apical zone of approximately 20 millimeters in length which was hydraulically isolated due to a high axial resistance. In osmotic experiments, Lp_r decreased toward the base of the roots. Lp_r (osmotic) was significantly smaller than Lp_r (hydrostatic). At various distances from the root tip, the axial hydraulic resistance per unit root length (R_x) was measured either by perfusing excised root segments or was estimated according to Poiseuille's law from cross-sections. The calculated R_x was smaller than the measured R_x by a factor of 2 to 5. Axial resistance varied with the distance from the apex due to the differentiation of early metaxylem vessels. Except for the apical 20 millimeters, radial water movement was limiting water uptake into the root. This is important for the evaluation of Lp_r of roots from root pressure relaxations. Stationary water uptake into the roots was modeled using measured values of axial and radial hydraulic resistances in order to work out profiles of axial water flow and xylem water potentials.     

A model of water flow through plants incorporating shoot/root 'message' control of stomatal conductance

Abstract. A model of water flow from the soil into the plant, and from the plant to the atmosphere is described. There are three state variables in the model: the soil, root and shoot water contents. The flow rate of water from the soil to the root is calculated by dividing the gradient in water potential by a resistance, comprising the resistance from the bulk soil to the root surface, and that from the root surface to the root interior. The resistance in the soil depends on the soil hydraulic conductivity, which in turn depends on the soil water potential. The flow rate from the root to the shoot is given by the gradient in water potential divided by a resistance, which depends on the structural dry mass of the plant. Transpiration is described by the Penman-Monteith equation. The plant water characteristics can be modified to take account of osmotic and cell wall rigidity parameters. The model incorporates the concept of shoot/root ‘messages’ of water stress, which influence stomatal conductance. The message works through the generation of a hormone as the pressure potential in the shoot (mesophyll) or root falls. This hormone induces a shift of osmoticum from the guard cells to the surrounding mesophyll cells, which causes an increase (i.e. closer to zero) in the osmotic potential in these cells. This, in turn, causes a decrease in their pressure potential, and so reduces stomatal conductance. The model is used as a framework to address some of the issues that have recently been raised concerning the role of water potential in describing water flow through plants. We conclude that, with the hormone present, there is unlikely to be a unique relationship between stomatal conductance and shoot total water potential, since stomatal conductance depends on the pressure potential in the guard cells, which may differ from that in other cells. Nevertheless, this does not imply that water potential is not an important, and indeed fundamental, component for describing water flow through plants. Other aspects of water flow through plants are also considered, such as diurnal patterns of shoot, root and soil water potential components. It is seen that these may differ from the commonly held view that, as the soil dries down, they all attain the same values during the dark period, and which, as we show, is largely unsubstantiated either theoretically or experimentally.