A comparison of plant hydraulic conductances in wheat and lupins

Previous studies have shown similar water use for lupins (Lupinusangustifolius L.) and wheat (Triticum aestivum L.), despitea considerably smaller root system in lupins. A field studyand an experiment under controlled conditions using pressure-fluxrelationships were conducted to examine whether higher hydraulicconductances were responsible for the greater water uptake perunit root length in lupins. In the field experiment, the fluxof water and differences in water potential through the soil-plantsystem were measured for both species and used to calculatethe hydraulic conductance through the plant and through theroot and shoot. The hydraulic conductance for the whole plantwas 3–5 times greater in lupins than in wheat. This relativedifference between the species was similar when plant hydraulicconductance was expressed per unit of root length. This occurreddespite the difference in midday water potential between soiland leaves, being consistently greater in wheat (–1.0MPa) than in lupins (–0.7 MPa). When the total plant conductancewas separated into its components, the combined soil and rootconductance and the shoot conductance were 2 and 6 times greater,respectively, in lupins than in wheat. In the experiment undercontrolled conditions, hydraulic conductance for the entireroot system was determined using a pressure chamber. The specificroot hydraulic conductances were 4 times greater in lupins thanin wheat. The results from both field and controlled conditionsexperiments suggest that the greater water uptake per unit rootlength in lupins compared to wheat results from appreciablylarger root and shoot hydraulic conductances. Key words: Lupins, wheat, hydraulic conductances, water, uptake, pressure-flux     

The Partitioning of Hydraulic Conductances within Mature Orange Trees

Sap flow (F) and leaf water potential (LWP) were followed diurnallyin mature Valencia and Shamouti orange trees in an orchard.The hydraulic conductance of these trees was computed from thediurnal relationship between the LWP and F. The driving forcefor water movement was estimated from a weighted average ofsunlit and shaded LWP, assuming that leaves in the shade transpireto some extent. LWP of covered, non-transpiring leaves was alsomeasured hourly. It was assumed to represent the xylem waterpotential within the axial conduit of the trunk. Relating coveredLWP to F on an hourly basis enables the computation of the hydraulicconductance of the root system, including axial conductances.The hydraulic conductance of the transpiring crown was computed.Its magnitude was comparable to the root system hydraulic conductance. Key words: Orange trees, hydraulic conductance, sap flow, leaf water potential     

Spatio-temporal Variations in Axial Conductance of Primary and First-order Lateral Roots of a Maize Crop as Predicted by a Model of the Hydraulic Architecture of Root Systems

Rates at which water can be transported along plant roots (axial pathway) vary through time, in part depending on xylem maturation. Because of experimental constraints, the dynamics of root functional heterogeneity under field conditions remains mostly uncharted territory. Recent advances in mechanistic modelling offer opportunities to bypass such experimental limitations. This paper examines the dynamics of local variations in axial conductance of primary and first-order lateral roots of a maize crop using the architecture-based modelling approach developed by Doussan et al. (Annals of Botany: 81, 213–223, 1998). Specifically, we hypothesised that points of major resistance to long distance water transfers could arise from discrepancies between the hydraulic maturity (or water carrying capacity) of main axes and branch roots. To test this assumption, spatial distributions of root axial conductance were tested after 30, 60 and 100 days at soil depths of 10, 50 and 100 cm under a maize (Zea mays L.) crop sown at a density of 8 plants m⁻². As the crop developed, the corresponding root populations encompassed ever increasing amounts of hydraulically mature first-order laterals (branch roots): after a 100-day growth period, the vast majority of laterals had reached their maximum axial conductance at all soil depths down to 100 cm. In contrast, the axial conductance of a large proportion of main axes (primary roots) remained low, even at shallow soil depths and after 100 days of growth. The imbalance between the hydraulic maturity of primary and lateral roots was most conspicuous at soil depths of 100 cm, where ~10% only of the former compared to ~80% of the latter, had reached their maximum axial conductance after a 100-day growth period.     

Root Distribution, Growth, Respiration, and Hydraulic Conductivity for Two Highly Productive Agaves

Cultivated Agave mapisaga and A. salmiana can have an extremelyhigh above-ground dry-weight productivity of 40 Mg ha^–1yr^–1. To help understand the below-ground capabilitiesthat support the high above-ground productivity of these Crassulaceanacid metabolism plants, roots were studied in the laboratoryand in plantations near Mexico City. For approximately 15-year-oldplants, the lateral spread of roots from the plant base averaged1.3 m and the maximal root depth was 0.8 m, both considerablygreater than for desert succulents of the same age. Root andshoot growth occurred all year, although the increase in shootgrowth at the beginning of the wet season preceded the increasein growth of main roots. New lateral roots branching from themain roots were more common at the beginning of the wet season,which favoured water uptake with a minimal biomass investment,whereas growth of new main roots occurred later in the growingseason. The root: shoot dry weight ratio was extremely low,less than 0.07 for 6-year-old plants of both species, and decreasedwith plant age. The elongation rates of main roots and lateralroots were 10 to 17 mm d^–1, higher than for various desertsucculents but similar to elongation rates for roots of highlyproductive C₃ and C₄ agronomic species. The respiration rateof attached main roots was 32 µmol CO₂ evolved kg^–1dry weight s^–1 at 4 weeks of age, that of lateral rootswas about 70% higher, and both rates decreased with root age.Such respiration rates are 4- to 5-fold higher than for Agavedeserti, but similar to rates for C₃ and C₄ agronomic species.The root hydraulic conductivity had a maximal value of 3 x 10^–7ms^–1 MPa^–1 at 4 weeks of age, similar to A. deserti.The radial hydraulic conductivity from the root surface to thexylem decreased and the axial conductivity along the xylem increasedwith root age, again similar to A. deserti. Thus, although rootsof A. mapisaga and A. salmiana had hydraulic properties perunit length similar to those of a desert agave, their highergrowth rates, their higher respiration rates, and the greatersoil volume explored by their roots than for various desertsucculents apparently helped support their high above-groundbiomass productivity Key words: Crassulacean acid metabolism, productivity, root elongation rate, root system, water uptake     

A method for measuring xylem hydraulic conductance and embolism in entire root and shoot systems

Current methods for determining the influence of xylem cavitationon hydraulic conductance are limited to unbranched stem or rootsegments with hydraulic conductances above c. 2 mmol s^–1MPa^–1. Lower conductances and/or highly branched systemsare encountered in seedlings, arid-land shrubs, herbs, and distalportions of shoot and root systems of trees. In order to quantifythe hydraulic impact of cavitation in such systems, existingtechniques have been modified. Branched shoot or root systemswere prepared for measurement by removal of leaves, or roottips, respectively. The shoot or root system was enclosed ina vacuum chamber with the proximal end protruding and suppliedwith perfusing solution. Flow through the xylem was inducedby chamber vacuum. Hydraulic conductance was determined fromthe slope of the flow rate versus pressure relationship. Xylemembolism was quantified from the increase in hydraulic conductancefollowing high pressure (100 kPa) perfusion of solution throughthe plant. Examples are provided of the application of the methodto cavitation studies in the cold desert shrub Artemisia tridentata. Key words: Hydraulic conductance, xylem cavitation, embolism, whole root/shoot system     

MAIN ROOT-LATERAL ROOT JUNCTIONS OF TWO DESERT SUCCULENTS: CHANGES IN AXIAL AND RADIAL COMPONENTS OF HYDRAULIC CONDUCTIVITY DURING DRYING

The influence of junctions between main roots and lateral roots on water flow was investigated for the desert succulents Agave deserti and Ferocactus acanthodes during 21 d of drying in soil. Under wet conditions, the junctions did not restrict xylem water flow from lateral roots to main roots, consistent with predictions of axial conductance based on vessel diameters. Embolism caused by drying reduced such axial conductance more at the junctions than in adjoining root regions. Connective tracheary elements at the junctions were abundantly pitted and had large areas of unlignified primary wall, apparently making them more susceptible to embolism than vessels or tracheids elsewhere in the roots. Unlike the decrease in axial conductance, the overall hydraulic conductivity of the junction increased during drying because of an increase in the conductivity of the radial pathway. Despite such increases, main roots may not lose substantial amounts of water to a dry soil during drought, initially because embolism at the junctions can limit xylem flow and later because soil hydraulic conductivity decreases. Moreover, the increased root hydraulic conductivity and a potentially rapid recovery from embolism by connective tracheary elements may favor water uptake near the junctions upon soil rewetting.     

Thermal and Water Relations of Roots of Desert Succulents

Two succulent perennials from the Sonoran Desert, Agave desertiEngelm. and Ferocactus acanthodes (Lem.) Britton and Rose, loselittle water through their roots during drought, yet respondrapidly to light rainfall. Their roots tend to be shallow, althoughabsent from the upper 20 mm or so of the soil. During 12–15d after a rainfall, new root production increased total rootlength by 47 per cent to 740 m for A. deserti and by 27 percent to 230 m for F. acanthodes; root dry weight then averagedonly 15 per cent of shoot dry weight. The annual carbon allocatedto dry weight of new roots required 11 per cent of shoot carbondioxide uptake for A. deserti and 19 per cent for F. acanthodes.Elongation of new roots was greatest near a soil temperatureof 30°C, and lethal temperature extremes (causing a 50 percent decrease in root parenchyma cells taking up stain) were56°C and -7°C. Soil temperatures annually exceeded themeasured tolerance to high temperature at depths less than 20mm, probably explaining the lack of roots in this zone. Attached roots immersed in solutions with osmotic potentialsabove -2·6 MPa could produce new lateral roots, with50 per cent of maximum elongation occurring near -1·4MPa for both species. Non-droughted roots lost water when immersedin solutions with osmotic potentials below -0·8 MPa,and root hydraulic conductance decreased markedly below about-1·2 MPa. Pressure-volume curves indicated that, fora given change in water potential, non-droughted roots lostthree to five times more water than droughted roots, non-droughtedleaves, or non-droughted stems. Hence, such roots, which couldbe produced in response to a rainfall, will lose the most tissuewater with the onset of drought, the resulting shrinkage beingaccompanied by reduced root hydraulic conductance, less contactwith drying soil, and less water loss from the plant to thesoil. Agave deserti, Ferocactus acanthodes, roots, soil, temperature, water stress, drought, Crassulacean acid metabolism, succulents     

Stomatal behaviour and water movement through roots of wheat plants treated with abscisic acid

Abstract Experiments with isolated roots of wheat plants suggested that when water uptake rates are low, low concentrations of abscisic acid (ABA) may increase the flux of water into roots. This increase was recorded despite an ABA-stimulated reduction in the hydraulic conductance of the whole root system. Hydraulic conductances were measured under steady-state conditions. A system is described where the stomatal behaviour and water movement through roots of a single intact plant may be concurrently monitored. Experiments with intact plants confirmed that application of ABA could increase the rate of water movement into roots when uptake rates were low. No such increase was observed at high flux rates. Application of ABA to roots caused partial stomatal closure and caused conductance to oscillate around a reduced mean value. An ABA-stimulated increase in the turgor sensitivity of stomata is postulated and the significance of this effect is discussed.     

Vascular Architecture of a Large-leafed Genotype ofTrifolium repens

The objectives of this study were to identify the vascular connectionsfrom roots to upper axial bundles in one genotype ofTrifoliumrepensL. ‘Grasslands Kopu’, identify pathways followedby the transpiration stream, and establish whether these pathwayscould account for previously-observed patterns of clonal integration.The study provides new information on vascular connections betweenroot and parent and branch stolons at nodes possessing botha root and a branch, and to the first two leaves on branch stolons.A nodal root is connected to the lower nearside axial bundleof the parent stolon but to both lower and upper nearside axialbundles of the branch. Upper sympodia provide a long-distancetransport pathway from a parent stolon to the apex of branchstolons. Lower sympodia are functionally different, providingshort-distance transport to structures in close proximity tothe source root. This is consistent with observed patterns ofclonal integration inT. repensand may provide a simple architecturalmechanism facilitating foraging.Copyright 1998 Annals of BotanyCompany Acid fuchsin, clonal integration, foraging, physiological integration, serial sections, white clover,Trifolium repens(L.), vascular architecture, xylem transport.     

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.     

Genotypic, Developmental and Drought-Induced Differences in Root Hydraulic Conductance of Contrasting Sugarcane Cultivars

Hydraulic properties of entire root systems and isolated rootsof three contrasting sugarcane clones were evaluated using transpiration-induceddifferences in hydrostatic pressure across intact root systems,root pressure-generated xylem sap exudation, and pressure-fluxrelationships. Regardless of the measurement technique employed,the clones were ranked in the same order on the basis of theirleaf area–specific total root system hydraulic conductance(C_root). All methods employed detected large developmental changesin G_rootroot with maximum values occurring in plants with approximately02 m₂ total leaf area. Genotypic ranking according to G_root,was reflected as a similar ranking according to root length-specifichydraulic conductance (L) of individual excised roots. Genotypicdifferences in G_root and L were consistent with anatomical characteristicsobserved in individual roots. Patterns of G_root, during soildrying and following re-irrigation suggested that the declinein G_root, observed during soil drying occurred within the rootsrather than at the soil–root interface and may have beencaused in part by xylem cavitation in the roots. Key words: Root hydraulic conductance, Saccharum spp, transpiration, root pressure, pressure-flux     

Rectifier-like Activities of Roots of Two Desert Succulents

Axial and radial water flows for roots in response to appliedhydrostatic pressure drops, water loss from roots after variousperiods of drying, and development of new roots after rewettingdroughted plants were examined for two sympatric desert succulents.Agave deserti Engelm. and Ferocactus acanthodes (Lemaire) Brittonand Rose. For a 40 kPa hydrostatic pressure drop applied to20 mm long root pieces, radial water flows from the epidermisto the root xylem were 2- to 5-fold greater at the tip thanat midlength and were much less than axial flows along the xylem.Upon drying detached roots in air at 20 °C and a water vapoursaturation deficit of 1.2 kPa (50% relative humidity), radialwater flow decreased more than 10-fold in 3–6 h, and couldrecover to the original level 6 h after rewetting. The rateof water loss from attached roots of plants dried in air at20 °C and a 1.2 kPa saturation deficit decreased about 200-foldin 72 h, which would greatly limit water loss from the plantto a drying soil. At 96 h after rewetting roots of A. desertithat had been exposed to air at 20 °C and a 1.2 kPa saturationdeficit for 120 h, rehydration of existing roots and developmentof new roots contributed about equally to water uptake by thewhole plant. In summary, roots of these desert succulents canreadily take up water from a wet soil but do not lose much waterto a dry soil, thus effectively acting like rectifiers withrespect to plant-soil water movement. Key words: Agave, Cactus, Drought, Root, Water flow, Xylem     

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.     

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.     

Growth and Development of Roots of Grapevine (Vitis vinifera L.) in Relation to Water Uptake from Soil

Developmental patterns of lateral roots and their vascular differentiationwere investigated for Vitis vinifera L. cv. Shiraz to assessthe likely contribution of lateral roots to total water uptakeof plants subjected to different irrigation regimes. Correlationanalyses showed a significant positive correlation between mainroot diameter and the diameter of first order lateral rootsof well-watered plants, but in water-stressed plants the twowere not significantly correlated. The correlations betweendiameters of first order lateral roots and the diameters ofmain roots were greater than correlations between the lengthsof first order laterals and the diameters of main roots. Thesuberised surface area of well-watered main roots increasedfrom 4% of total surface area at 0·25 cm to 100% at 10cm from the tip, whereas that of stressed plants increased from15% at 0·25 cm to 100% at 5 cm from the tip. In all treatmentsthe highest linear density of first order laterals was about7 laterals cm^-1 of main root. More than 50% of first order lateralshad diameters less than 0·05 cm, and more than 90% ofthem had lengths less than 5 cm. Calculations of axial resistancesbased on xylem diameter measurements suggest that the axialresistances of root segments may not be uniform along rootsas is often assumed in models of water uptake. Water flow intothe main roots via the lateral root pathway is likely to bemuch smaller than that via the direct radial flow pathway asonly about 1% of surface area of main roots is directly occupiedby lateral roots, leaving the other 99% of main root surfacearea available for the direct radial flow pathway.Copyright1994, 1999 Academic Press Axial resistance, grapevine (Vitis vinifera L. cv. Shiraz) roots, root diameter, root length, xylem vessels     

Dynamic measurements of roots hydraulic conductance using a high-pressure flowmeter in the laboratory and field

A new high-pressure flowmeter(HPFM)is described which is capableof rapid water-flow measurements. The HPFM permits dynamic determinationof hydraulic conductance of roots, K_r, and can be used in tehlaboratory or field. The base of a root is connected to theHPFM and water is perfused into the root system opposite tothe normal direction of flow during trnaspiration. The perfusionpressure is changed at a constant rate of 3–7 kPa s^–1while measuring the flow into the root every 2–4 s. Theslope of the plot of flow versus applied pressure is K_r. This paper describes the HPFM, presnents the theory of dynamicflow measurements, discusses sources of error, presnets evidencethat dynamic measurements of K_r in Ficus maclellandi (and sixother tropical species from Panama) yield the correct result,and demonstrates the use of the method under field conditionsin Panama on Cecropia obtusifolia and Palicourea guianensis. Key words: High-pressure flowmeter, root and shoot hydraulic conductance, Ficus maclellandi, Cecropia obtusifolia, Palicourea guianensis     

Changes in root hydraulic conductivity for two tropical epiphytic cacti as soil moisture varies

The tropical epiphytic cacti Epiphyllum phyllanthus and Rhipsalis baccifera experience extreme variations in soil moisture due to limited soil volumes and episodic rainfalls. To examine possible root rectification, whereby water uptake from a wet soil occurs readily but water loss to a dry soil is minimal, responses of root hydraulic conductivity (L_p) to soil drying and rewetting were investigated along with the underlying anatomical changes. After 30 d of soil drying, L_p decreased 50%–70% for roots of both species, primarily because increased suberization of the periderm reduced radial conductivity. Sheaths composed of soil particles, root hairs, and mucilage covered young roots and helped reduce root desiccation. Axial (xylem) conductance increased during drying due to vessel differentiation and maturation, and drought-induced embolism was relatively low. Within 4 d of rewetting, L_p for roots of both species attained predrought values; radial conductivity increased for young roots due to the growth of new branch roots initiated during drying and for older roots due to the development of radial breaks in the periderm. The decreases in L_p during drought reduced plant water loss to a dry soil, and yet maximal water uptake and transpiration occurred within a few days of rewetting, helping these epiphytes to take advantage of episodic rainfalls in a moist tropical forest.     

Gas and Liquids in Intercellular Spaces of Maize Roots

Oils are spontaneously absorbed by gas-filled intercellularspaces (IS) in maize root cortex. The network of these spacesin living root sections was imaged by confocal laser scanningmicroscopy using a fluorescent solution of Nile red in oil.The gas volume fraction (GVF) of root segments was quantifiedby the increase in weight (differentiated zones) or tissue density(2–3 mm root tips) due to complete vacuum infiltration.Cooling to 6 °C or inhibition of oxidative phosphorylationdiminished the GVF of root tips but did not significantly affectthe GVF of differentiated root zones. The threshold pressuredifference for measurable infiltration of isolated root segmentsis lower (10 to 15  kPa) than the threshold for infiltrationof comparable zones of attached roots or of detached roots withthe cut surface sealed (>60 kPa). In the absence of an opencut, pressure-driven infiltration of the root cortex is acceleratedby microscopic fissures within the epidermal/hypodermal barrier.The GVF of the root cortex was reduced after transferring rootsfrom sugar solutions (0.1 to 0.3M ) to water. This points toefficient water transport from the medium to sugar-containingcortical cell walls through epidermal and hypodermal protoplasts.When 2-cm-long primary roots were vacuum infiltrated in situand then allowed to grow on aerated mineral medium for a further5 d, cortical IS of the originally infiltrated root bases remainedfilled with liquid but the subsequently grown apical root zoneshad a normal GVF. Copyright 1999 Annals of Botany Company Apoplastic and protoplasmic route, maize, infiltration, intercellular spaces, oil absorption, confocal laser scanning microscope, water transport, Zea mays L.     

Uptake and Partitioning of Nitrogen by Maize Infected withStriga hermonthica

The uptake and partitioning of nitrogen (N) by maize infectedwith the parasitic angiosperm,Striga hermonthicawas investigatedin sand culture in a glasshouse. The purpose was to determinethe effect ofStrigaon N uptake and partitioning in maize. Maizewas grown at 22, 66 and 133 mg N per plant and sampled fivetimes. There was no significantStrigaxN interaction in any measuredresponse. Leaf dry matter ofStriga-infected maize, averagedover all N treatments, was 92% that of uninfected maize at thefour-leaf stage but by the 18-leaf stage it had decreased to58%. Similarly, stem dry matter of infected maize which was91% that of uninfected maize at the four-leaf stage was only42% at the 18-leaf stage. Root dry matter was similar for infectedand uninfected maize. N concentration in the leaf, stem androot declined asymptotically from the first to the last samplingdate for both infected and uninfected maize. The asymptoticvalue of N concentration inStriga-infected maize was 16% greaterin the leaf, 55% in the stem, and 21% in the root than in uninfectedmaize. The concentration of N inStrigawas higher than in maizeat the 16- and 18-leaf stages. Uptake of N was similar for infectedand uninfected plants at the four–eight leaf stage butat the eight–12 leaf stage, N uptake by infected maizewas 52% that of uninfected maize. However, the proportion oftotal plant nitrogen partitioned to the root was greater (P<0.001)forStriga-infected maize. These results showed that the extentto whichS. hermonthicareduced maize growth and N uptake, butincreased the proportion of N partitioned to the roots, didnot depend on the rate of N fertilizer applied.Copyright 1998Annals of Botany Company Maize; nitrogen; partitioning;Striga hermonthica; uptake.