Water Transport in Onion (Allium cepa L.) Roots (Changes of Axial and Radial Hydraulic Conductivities during Root Development)

The hydraulic architecture of developing onion (Allium cepa L. cv Calypso) roots grown hydroponically was determined by measuring axial and radial hydraulic conductivities (equal to inverse of specific hydraulic resistances). In the roots, Casparian bands and suberin lamellae develop in the endodermis and exodermis (equal to hypodermis). Using the root pressure probe, changes of hydraulic conductivities along the developing roots were analyzed with high resolution. Axial hydraulic conductivity (Lx) was also calculated from stained cross-sections according to Poiseuille's law. Near the base and the tip of the roots, measured and calculated Lx values were similar. However, at distances between 200 and 300 mm from the apex, measured values of Lx were smaller by more than 1 order of magnitude than those calculated, probably because of remaining cross walls between xylem vessel members. During development of root xylem, Lx increased by 3 orders of magnitude. In the apical 30 mm (tip region), axial resistance limited water transport, whereas in basal parts radial resistances (low radial hydraulic conductivity, Lpr) controlled the uptake. Because of the high axial hydraulic resistance in the tip region, this zone appeared to be "hydraulically isolated" from the rest of the root. Changes of the Lpr of the roots were determined by measuring the hydraulic conductance of roots of different length and referring these data to unit surface area. At distances between 30 and 150 mm from the root tip, Lpr was fairly constant (1.4 x 10-7 m s-1 MPa-1). In more basal root zones, Lpr was considerably smaller and varied between roots. The low contribution of basal zones to the overall water uptake indicated an influence of the exodermal Casparian bands and/or suberin lamellae in the endodermis or exodermis, which develop at distances larger than 50 to 60 mm from the root tip.     

An Automated Potometer

An automated potometer is described whose output voltage providesa continuous record of plant water uptake. The measurement systemis based on the capacitive transduction of meniscus positionin a potometer sensing tube, such that a change in output voltageof the capacitance-measuring circuit is proportional to watervolume uptake by the plant. Advantages of the continuous high resolution record of wateruptake made with the potometer are demonstrated by measurementsusing cut plants (a) which show the dependence of calculatedtranspiration flux density and stomatal conductance on climaticvariables and (b) to calibrate a sap flowmeter. A design fora whole plant potometer is presented.     

Evaluation of the heat pulse velocity technique for measurement of sap flow in rainforest and eucalypt forest species of south-eastern Australia

Sap flow in the stems of two cut saplings each of Eucalyptus maculata (a canopy eucalypt forest tree), Doryphora sassafras and Ceratopetalum apetalum (both canopy rainforest trees of south-eastern coastal Australia) was measured by the heat pulse velocity technique and compared with water uptake from a potometer. Scanning electron micrographs of wounding caused by implantation of temperature sensor and heater probes into the sapwood showed that wounding was similar in rainforest and eucalypt species and was elliptical in shape. A circular wound has been implicitly assumed in previous studies. Accurate measurements of sapling water use were obtained using the smaller transverse wound dimension rather than the larger longitudinal dimension because maximum disruption of sap flow through the xylem vessels occurred in the transverse plane. Accurate measurements of sap flux were obtained above a minimum threshold sap velocity. These velocities were 15·7,10·9 and 9·4 cm h^?1 for E. maculata, C. apetalum and D. sassafras, respectively. Below the threshold sap velocity, however, sap flow could not be accurately calculated from measurements of heat pulse velocity. The minimum threshold sap velocity appeared to be determined by probe construction and xylem anatomy. Despite the elliptical wounding and inaccurate measurement of sap flow below the threshold sap velocity, total sap flow over the experimental period for two saplings of each species was within 7% of water use measured by the potometer.     

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.     

Resistance to Water Flow in the Seminal Roots of Wheat

Water uptake by the seminal root system of wheat is describedby a model which incorporates radial and axial resistance toflow in the root and the resistance to flow in the soil. Inan experiment where wheat plants were dependent on subsoil waterextracted from below 45 cm by one, three, or five seminal roots,calculations indicated that the axial resistance was the dominantresistance within the soil-root system and that flow rate changedpredictably with changing axial resistance. The model also indicatedthat the observed shape of the metaxylem vessel in the seminalaxes has important effects on the pattern of water uptake.     

Soil and plant resistances to water uptake byVicia faba L.

Summary The hydraulic resistivity ofVicia faba L. roots grown in soil was estimated from steady state measurements of transpiration rate and leaf and soil water potentials. Root and stem axial resistivities, estimated from xylem vessel radii, were negligible. Root radial resistivity was estimated to be 1.3×10¹² sm⁻¹. This root radial resistivity value was used to estimate, root resistance to water uptake for a field crop ofVicia faba. Previously published results were used for root distribution and soil water contents at the drained upper limit (DUL) and the lower limit (LL) of extractable soil water. Soil resistance to water uptake was estimated from single root theory using the steady rate solution. At the DUL, root resistance was about 10⁵ times greater than soil resistance. At the LL, soil resistance exceeded root resistance for depths less than 0.3 m, but for depths greater than this soil resistance was smaller than root resistance. Estimates of possible uptake rates at given leaf water potentials indicated that overall soil resistance had a negligible influence upon uptake, even at the LL. The reliability of this result is examined in detail. It is concluded that over the complete range of extractable soil water contents soil resistanceper se would not have limited water use by this crop. This conclusion may also be valid for a wide range of soil and crop combinations.     

Root Xylem Influence on the Water Relations and Drought Resistance of Rice

In order to determine the importance of root axial resistanceto water flow for drought resistance of rice (Oryza sativa L.)aseries of glasshouse and growth chamber studies was conductedfrom 1985 to 1986. A preliminary study surveyed root distributionand histological characteristics of six cultivars grown in aerobicsoil (20x20x90cm boxes) under well–watered ormoisturedeficit conditions. Subsequently, four experiments were conductedwith plants grown in culture solution. Our results demonstratethat plant breeders can use root thickness as a selection indexfor xylem size for root diameters up to about 1–2 mm.Usingthe Poiseuille–Hagen Law for water movement in capillaries,rice root axial resistance explained differences in leaf waterpotential and transpiration when only one cultivar was used,but did not explain differences among cultivars. Thus, increasingroot xylem vessel radii probably will not directly increasedrought resistance. Key words: Rice (Oryza sativa), roots, xylem characteristics, drought resistance     

Root elongation is restricted by axial but not by radial pressures: so what happens in field soil?

Root distribution determines largely the zone of soil that roots have access to for water and nutrient uptake, and is of great importance especially if water and fertilizer input is restricted. Mechanical impedance is the major limitation to root elongation in many field soils. Until now, experiments have focused largely on the axial resistance to root growth. In a fascinating study of the radial forces exerted by the roots of chickpea, root extension, diameter change, and the radial forces that axially unimpeded roots exert are reported: Kolb et al. (this volume) record radial stresses of about 0.3?MPa that are broadly consistent with cell turgor pressures, but, interestingly, find no restriction to axial elongation. This result is in marked contrast to large decreases in elongation of pea radicles resulting from much smaller axial pressures reported elsewhere in the literature (e.g., an 85?% decrease in root elongation in response to axial pressures of?<?0.1?MPa). The situation is different also from that in homogeneous soil, where root penetration resistance pressures of 0.4-1.0?MPa are typically required to halt root elongation. Soil structure and strength properties will determine the balance of axial and radial pressures on an individual root tip, and hence the root elongation response. It appears that a degree of radial confinement may help roots to extend axially into hard soil. This result also complements recent findings that in strong field soils the availability of soil macropores has a large influence on regulating the root-elongation rates of seedlings.     

Water Movement Through Plant Roots

Mathematical analysis of the hydraulics of water movement throughplant roots, in terms of radial and axial resistances, has ledto equations which provide new insights into the effects ofthe component resistan ces on water uptake by and movement throughindividual roots and root systems. The ratio of axial to radialresistance determines the optimum length of a root and its totalresistance to water movement. The equations permit direct calculationsof the plant water potentials necessary, at the base of theplant, to sustain given flow rates through root systems withgiven characteristics. Lateral spacing and the resistance ofindividual laterals are the dominant factors determining totalflux per unit area into a root. When soil water potential increases with depth (surface layersdrier) root resistance tends to decrease with increasing flowrate; the reverse occurs when the surface is wetter than thelower layers. Calculated patterns of water movement into andthrough roots, in relation to soil water potential and flowrate through the root, indicate efflux from root to soil undercertain conditions. This is considered to reflect reality, althoughthe fluxes are probably transient or intermittent. The equations presented should be combined with equations describingwater movement through soil to define the behaviour of the wholeroot-soil system adequately.     

Influence of xylem development on axial hydraulic conductance within Prunus root systems

The effect of secondary growth on the distribution of the axial hydraulic conductance within the Prunus root system was investigated. Secondary growth resulted in a large increase in both the number (from about 10 to several thousand) and diameter of xylem vessels (from a few micrometres to nearly 150 µm). For fine roots (<3 mm), an increase in root diameter was correlated with a slight increase in the number of xylem vessels and a large increase in their diameter. Conversely, for woody roots, an increase in root diameter was associated with a dramatic increase in the number of xylem vessels, but little or no change in vessel diameter. The theoretical axial conductivity (Kh, m⁴.s^-1.MPa^-1) of root segments was calculated with the Poiseuille-Hagen equation from measurements of vessel diameter. Kh measured using the tension-induced technique varies over several orders of magnitude (7.4᎒^-11 to 5.7᎒^-7 m⁴.s^-1.MPa^-1) and shows large discrepancies with theoretical calculated Kh. We concluded that root diameter is a pertinent and useful parameter to predict the axial conductance of a given root, provided the root type is known. Indeed, the relationship between measured Kh and root diameter varies according to the root type (fine or woody), due to differences in the xylem produced by secondary growth. Finally, we show how the combination of branching pattern and axial conductance may limit water flow through root systems. For Prunus, the main roots do not appear to limit water transfer; the axial conductance of the main axes is at least 10% higher than the sum of the axial conductance of the branches.     

Root clumping may affect the root water potential and the resistance to soil-root water transport

We have appraised for clumped root systems the widely-accepted view that the resistance to water flux from soil to roots (‘soil resistance’) is low under most field conditions, so that root water potential would closely follow the mean soil water potential. Three root spatial arrangements were studied, simulating either the regular pattern generally assumed in models, or two degrees of root clumping frequently observed in the field. We used a numerical 2-dimensional model of water transfer which assumes a control of evapotranspiration by root signalling. Calculations were carried out at two evaporative demands and for two contrasting soil hydraulic properties. The rate of soil depletion, the timing of the reduction in evapotranspiration and the difference between root water potential and mean soil water potential were all affected by the root spatial arrangement, with a greater effect at high evaporative demand and low soil hydraulic conductivity. Almost all the soil water reserve was available to plants without reduction in evapotranspiration in the regular case, while only a part of it was available in clumped cases. In the regular case, calculated ‘soil resistances’ were similar to those calculated using Newman's (1969) method. Conversely they were higher by up to two orders of magnitude in clumped root spatial arrangements. These results place doubt on the generality of the view that ‘soil resistance’ is low under common field conditions. They are consistent with the results of field experiments, especially with recent data dealing with root-to-shoot communication.     

Some Observations with a New Potometer on the Absorption of Water by Young Barley Plants

A potometer, working on a syphon principle, is described foruse with single, small plants where it is undesirable to floodthe stem base. It has been shown that anoxia has very littleeffect on the water absorption of roots of intact barley plants,apart from an initial inhibition of short duration. The actionof 1 x 10^–5 M dinitrophenol (pH 6.0) is similar. Neithertreatment kills the roots. It is possible that permeabilityis maintained by fermentation reactions     

Field Estimates of Root and Xylem Resistances in Pinus contorta using Root Excision

A root excision technique was used to estimate the proportionof total resistance to water flux residing in the soil, theroot, and the xylem of lodgepole pine (Pinus contorta Douglex. Loud.) trees in the field. Root excision at mid-day alwaysresulted in rapid recovery of leaf water potential when waterwas supplied to the cut stem, suggesting a high soil-root resistance.Transpiration was unaffected if leaf water potential beforecutting was not limiting leaf conductance. By mid-June wateruptake by the excised stem always exceeded calculated crowntranspiration indicating recharge of internal sapwood storage.Predawn leaf water potential before root excision was highlycorrelated with total soil-plant resistance (r² = 0·89)and calculated root water uptake (r² = 0·92).     

A simple model for the function of proteoglycans and collagen in the response to compression of the intervertebral disc.

The nucleus pulposus of the intervertebral disc exerts a pressure which enables it to support axial compression when contained by the annulus fibrosus. The disc was modelled as a thick-walled cylindrical pressure vessel in which the nucleus was contained radially by the annulus. As a result, the stress in the annulus had radial (compressive) as well as tangential (tensile) components. The radial stress at a given point in the annulus was considered to be balanced by the internal pressure which is expected to arise from the attraction of water by proteoglycans. There was a reasonable agreement between the calculated radial stress distribution and published results on the distribution of water within the annulus. As the internal pressure is expected to be isotropic, the annulus was expected to contribute to the axial resistance to compression of the disc; this contribution would be equal, in magnitude, to the radial stress. Predictions of the pressure distribution within the annulus were similar to published experimental measurements made in the radial and axial directions. The tangential stress within the annulus was considered to arise from the restoring stress in its strained collagen fibrils.     

High resolution imaging to assess oilseed species?? root hair responses to soil water stress

An imaging method was developed to evaluate crop species differences in root hair morphology using high resolution scanners, and to determine if the method could also detect root hair responses to soil water availability. High resolution (1890 picture elements (pixels) cm^?1) desktop scanners were buried in containers filled with soil to characterize root hair development under two water availability levels (?63 and ?188?kPa) for canola (Brassica napus L. cv Clearwater), camelina (Camelina sativa L. Crantz cv Cheyenne), flax (Linum usitatissimum L. cv CDC Bethune), and lentil (Lens culinaris Medik. cv Brewer). There was notable effect of available moisture on root hair geometry (RHG). At ?188?kPa, length from the root tip to the root hair initiation zone decreased and root hair length (RHL) became more variable near the root hair initiation zone as compared to ?63?kPa. For the response of primary axial RHL, significant main effects were present for both water availability (P?<?0.05) and species (P?<?0.0001); lateral RHL showed a significant main effect for both water availability (P?<?0.05) and species (P?<?0.01) as well. For both primary axial and lateral root hair density (RHD), there was a significant effect of species (P?<?0.0001), but no significant response to water availability. No water availability x species interaction was present in any case. Low available water reduced RHL in both primary axial and lateral roots. The change in RHL due to water availability was most evident in canola and camelina. Additionally, those with greater RHL $ \left( {\text{canola} = \text{camelina} > \text{flax} = \text{lentil}} \right) $ had lower RHD $ \left( {\text{canola} = \text{camelina} < \text{flax} < \text{lentil}} \right) $ in primary axial roots and a similar trend was found in lateral RHL. Both water and species had a significant effect on primary axial root surface area (RSA) (P?<?0.05) but no significant effect was found for lateral RSA. For primary axial RSA the longest and most dense root hair had the greatest RSA. This novel approach to in situ rhizosphere imaging allowed observation of species differences in root hair development in response to water availability and should be useful in future studies of rhizosphere interactions and crop water and nutrient management.     

Understanding the Hydraulics of Porous Pipes: Tradeoffs Between Water Uptake and Root Length Utilization

The water uptake region in roots is several hundred times longer than the root diameter. The distributed nature of the uptake zone requires that the hydraulic design of roots be understood by analogy to flow through a “porous pipe.” Here we present results of an analytical and experimental investigation that allowed an in-depth analysis of root hydraulic properties. Measurements on nodal maize roots confirm the nonlinear distribution of water uptake predicted by the porous pipe model. The major design parameter governing the distribution of water uptake along a porous pipe is the ratio between its axial and radial hydraulic resistance. However, total flow is proportional to the pipe's overall resistance. These results suggest the existence of a tradeoff between the effective utilization of root length and the total capacity for water uptake.     

Water stress induced by PEG decreases the maximum growth pressure of the roots of pea seedlings

Roots of plants growing in dry soil often experience large mechanical impedance because the decreased soil water content is associated with increased in soil strength. The combined effect of mechanical impedance and water stress hinders the establishment of seedlings in many soils, but little is known about the interaction between these two stresses. A method has been designed that, for the first time, measured the maximum axial force exerted by a root growing under controlled water stress. Using this technique the axial force exerted by a pea radicle was measured using a shear beam, while the seedling was suspended in an aerate solution of polyethylene glycol 20 000 at osmotic potentials between 0 and -0.45 MPa. The maximum growth force was then divided by the cross-sectional area of the root to give the maximum axial growth pressure. The value of maximum axial growth pressure decreased linearly from 0.66 and 0.35 MPa as the osmotic potentials of the solution of PEG decreased from 0 to -0.45 MPa. In dry soil, therefore, the maximum strength of soil that a root can penetrate is decreased because of the decrease in maximum growth pressure. The elongation rates of unimpeded roots were similar whether the roots were subject to either a matric potential in soil or to an osmotic potential in a solution of PEG.Key words: Pisum sativum L, pea, mechanical impedance, axial growth pressure, water stress, PEG 20 000.      

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.     

Modelling of the Hydraulic Architecture of Root Systems: An Integrated Approach to Water Absorption--Distribution of Axial and Radial Conductances in Maize

The ‘Hydraulic Tree Model’ of the root system simulateswater uptake through root systems by coupling a root architecturemodel with laws for water flow into and along roots (Doussan,Pagès and Vercambre,Annals of Botany81: 213–223,1998). A detailed picture of water absorption in all roots comprisingthe root system is thus provided. Moreover, the influence ofdifferent distributions of radial and axial hydraulic conductancesin the root system on the patterns of water uptake can be analysed.Use of the model with Varney and Canny's data (1993) for flowalong maize roots demonstrated that a constant conductance inthe root system cannot reproduce the observed water flux profiles.Taking into account the existing data on hydraulic conductancesin maize roots, we fitted the distribution of conductances inthe root system to the observed flux data. The result is that,during root tissue maturation, the radial conductivity decreasesby one order of magnitude while the axial conductance increasesby about three orders of magnitude. Both types of conductanceexhibit abrupt changes in their evolution. Due to the conductancedistribution in the root system, appreciable water potentialgradients may develop in the roots, in both the branch rootsand main axes. An important point is that the conductance distributionin the branch roots described by the model should be relatedto the age of the tissue (and not the distance from the branchroot tip) and is therefore closely related to the developmentprocess. Thus for branch roots, which represent about 90% ofthe calculated total water uptake in 43-d-old maize, water absorptionwill depend on the opening of the metaxylem in the axes, andon the time dependent variation of the conductances in the branchroots.Copyright 1998 Annals of Botany Company Water; absorption; root system; architecture; model; hydraulic conductance;Zea maysL.