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.     

Ground-penetrating radar-based automatic reconstruction of three-dimensional coarse root system architecture

Background and aims

The diverse functions of roots set requirements on specific root system architecture (RSA). Investigation on RSA holds potentials for studying the adaptation of plants to environmental stresses and to interspecific competitions. Ground-penetrating radar (GPR) has provided a non-invasive method for studying in situ RSA. However, previous GPR method relied on manually connecting root points detected between radargrams to restore each root branch, resulting in limited accuracy and efficiency of reconstructing RSA. The objective of this study is to improve the effectiveness of 3D RSA reconstruction using GPR root detection data.

Methods

A total of 213 coarse root sections (with diameter >0.5 cm) were extracted from a distribution map of a reference shrub (Arctostaphylos pungens) root system to simulate the coarse roots identified by GPR. An automatic method was established to trace each root point to its optimum growing source point. Connections between discrete root points recovered the topology of the reference RSA. A spline curve smoothing method was applied to restore the 3D morphology of the reference RSA. The proposed protocol was then tested to rebuild the 3D RSA of a shrub (Caragana microphylla) growing in the sandy soils after in situ GPR survey. The accuracy of RSA reconstruction was quantitatively evaluated by a relationship matrix method and qualitatively assessed by direct comparisons between the reconstructed and the actual RSAs after in situ excavation.

Results

For both simulated and field collected GPR detection datasets, the reconstructed RSAs strongly corresponded to the real topology of the actual root systems. When adapting the best strategy, 186 of the 213 (87.32 %) root points on the reference root system of A. pungens were interlinked with correct topology, and the relationship matrix method detected an overall similarity of 82.75 % between the reconstructed and the actual RSAs.

Conclusion

The proposed automatic RSA reconstruction method greatly enhances the interpretation of GPR detection data regarding coarse roots, making in situ non-invasive and long-term mapping and monitoring of RSA possible.     

Root hydraulic conductivity and vessel structure modification with increasing soil depth of two oak species: Quercus pubescens and Quercus robur

There are numerous studies on water transport characteristics of trees from the base to tops, but only few deal with the variation in xylem conduit diameters from shallow to deep roots. This study compares variation in root conduit properties as a function of increasing soil depths for two oak species (Quercus pubescens Willd and Quercus robur L.) growing on two different plots. We measured root vessel characteristics at three soil depths including 0, 50 and 100 cm, and calculated the associated root-specific hydraulic conductivities. Vessel diameter and specific hydraulic conductivity increased with increasing soil depth from 0 to 50 cm, but did not change in the deeper soil layer in both species. We conclude that freeze–thaw events in upper soil layer limit vessel diameters and thus hydraulic conductivity of roots.     

VAP-RELATED SUPPRESSORS OF TOO MANY MOUTHS (VST) family proteins are regulators of root system architecture

Root system architecture (RSA) is a key factor in the efficiency of nutrient capture and water uptake in plants. Understanding the genetic control of RSA will be useful in minimizing fertilizer and water usage in agricultural cropping systems. Using a hydroponic screen and a gel-based imaging system, we identified a rice (Oryza sativa) gene, VAP-RELATED SUPPRESSOR OF TOO MANY MOUTHS1 (OsVST1), which plays a key role in controlling RSA. This gene encodes a homolog of the VAP-RELATED SUPPRESSORS OF TOO MANY MOUTHS (VST) proteins in Arabidopsis (Arabidopsis thaliana), which promote signaling in stomata by mediating plasma membrane–endoplasmic reticulum contacts. OsVST1 mutants have shorter primary roots, decreased root meristem size, and a more compact RSA. We show that the Arabidopsis VST triple mutants have similar phenotypes, with reduced primary root growth and smaller root meristems. Expression of OsVST1 largely complements the short root length and reduced plant height in the Arabidopsis triple mutant, supporting conservation of function between rice and Arabidopsis VST proteins. In a field trial, mutations in OsVST1 did not adversely affect grain yield, suggesting that modulation of this gene could be used as a way to optimize RSA without an inherent yield penalty.

Root meristem size and root system architecture in both rice and Arabidopsis are regulated by proteins related to mediators plasma membrane–endoplasmic reticulum contact.     

Drought-induced root aerenchyma formation restricts water uptake in rice seedlings supplied with nitrate

Previous studies demonstrated that ammonium nutrition results in higher water uptake rate than does nitrate nutrition under water stress, and thus enhances the tolerance of rice plants to water stress. However, the process by which water uptake is related to nitrogen form under water stress remains unknown. A hydroponic experiment with simulated water stress induced by polyethylene glycol (PEG6000) was conducted in a greenhouse to study the relationship between root aerenchyma formation and water uptake rate, such as xylem sap flow rate and hydraulic conductance, in two different rice cultivars (cv. 'Shanyou 63' hybrid indica and cv. 'Yangdao 6' indica, China). The results showed that root aerenchyma tissue increased in water-stressed plants of both cultivars fed by nitrate. No significant difference was found in root hydraulic conductivity and/or xylem sap flow rate between the two rice cultivars fed by ammonium regardless of water status, whereas these parameters decreased significantly in water-stressed plants fed by nitrate. It was concluded that aerenchyma that formed in the root cortex impeded the radial transport of water in the root cylinder and decreased water uptake in water-stressed rice plants fed by nitrate. Water transport occurred mainly through Hg-sensitive water channels in rice roots supplied with ammonium.     

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.     

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.     

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.     

Hydraulic properties of living late metaxylem and interactions between transpiration and xylem pressure in maize

A new approach to study dynamic interactions between transpiration and xylem pressure in intact plants is presented. Pressure probe measurements were preformed in living (immature) late metaxylem of maize roots rather than in adjacent mature xylem. This eliminated technical limitations related to the measurement of negative pressures. Water relations of single cells showed that turgor and volumetric elastic modulus were significantly larger in living metaxylem than in cortical cells; hydraulic conductivity was similar in both types of root cells. Increasing transpiration induced an immediate decrease of xylem pressure, and vice versa. Turgor in the living metaxylem could be continuously recorded for more than 1 h. The relationship between xylem pressure and transpiration yielded a root hydraulic resistance of 1.3 x 10⁹ MPa s m^-3. Control experiments indicated that the response of living xylem in the positive pressure range essentially paralleled that of mature root xylem in the negative range. In mature xylem, pressures as low as -0.55 MPa were recorded for short periods (several minutes). Several tests verified that the pressure probe was in contact with mature xylem during the measurements of tensions. The results demonstrate convincingly that transpiration generates an effective driving force for water uptake in roots, a central feature of the cohesion theory.Key words: Hydraulic conductivity, negative pressure, root development, turgor, water transport, Zea mays.      

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     

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     

Drought resistance of Quercus pubescens as a function of root hydraulic conductance, xylem embolism and hydraulic architecture

Water relations, xylem embolism, root and shoot hydraulic conductance of both young plants in the field and potted seedlings of Quercus pubescens have been studied with the aim of investigating whether these variables may account for the well known adaptation of this oak species to arid habitats. Our data revealed that Q. pubescens is able to maintain high leaf relative water contents under water stress conditions. In fact, relative water contents measured in summer (July) did not differ from those recorded in April. This was apparently achieved by compensating water loss by an equal amount of water uptake. Such a drought avoidance strategy was made possible by the recorded high hydraulic efficiency of stems and roots under water stress. In fact, root hydraulic conductance of field-grown plants was maintained high in summer when the percentage loss of hydraulic conductance of stems was lowest. The hydraulic architecture of young plants of Q. pubescens measured in terms of partitioning of hydraulic resistances along the water pathway revealed that the highest hydraulic resistance was located in stems of the current year's growth. This hydraulic architecture is interpreted as consistent with the adaptation of Q. pubescens to arid habitats as a consequence of the recorded seasonal changes in water relation parameters as well as in root and stem hydraulics.     

Phosphorus transport to the xylem and its regulation by water flow

Summary The effects of water flow on phosphorus uptake by roots and on its subsequent translocation to shoots were separated by giving short-term pulses of ³²P-labelled nutrient to intact tomato plants. At the end of a 5 min pulse, all the ³²P taken up by the plants was confined to the roots. Only about half of this ³²P was later translocated to shoots; there was very little translocation after 4 hours.Experiments after long-term labelling showed that only a small part of the total P in the root is readily translocated to shoots. This P appears to be in part of the symplast and contributes about 75% of the P transported to the xylem sap. The rest is presumably derived by leakage from vacuoles.A slow rate of water flow reduced both uptake into the symplast and the translocation to the shoots of P which had already been absorbed by the roots. This was conclusively demonstrated by giving a ³²P pulse before reducing the rate of water flow; ³²P not translocated to shoots was partly retained by the roots and partly lost to the external solution. Water flow also accelerates transport to the xylem of previously-absorbed P in excised roots.It is concluded that the major effect of water flow on phosphorus transport to shoots occurs after phosphorus uptake by the roots, probably during radial transport to the xylem.     

Apoplastic transport of abscisic acid through roots of maize: effect of the exodermis

The exodermal layers that are formed in maize roots during aeroponic culture were investigated with respect to the radial transport of cis-abscisic acid (ABA). The decrease in root hydraulic conductivity (Lp_r) of aeroponically grown roots was stimulated 1.5-fold by ABA (500 nM), reaching Lp_r values of roots lacking an exodermis. Similar to water, the radial flow of ABA through roots (J_ABA) and ABA uptake into root tissue were reduced by a factor of about three as a result of the existence of an exodermis. Thus, due to the cooperation between water and solute transport the development of the ABA signal in the xylem was not affected. This resulted in unchanged reflection coeffcients for roots grown hydroponically and aeroponically. Despite the well-accepted barrier properties of exodermal layers, it is concluded that the endodermis was the more effective filter for ABA. Owing to concentration polarisation effects, ABA may accumulate in front of the endodermal layer, a process which, for both roots possessing and lacking an exodermis, would tend to increase solvent drag and hence ABA movement into the xylem sap at increased water flow (J_Vr). This may account for the higher ABA concentrations found in the xylem at greater pressure difference. Received: 26 January 1999 / Accepted: 26 May 1999     

An undiscovered facet of hydraulic redistribution driven by evaporation—a study from a Populus tomentosa plantation

Maintaining the activity and function of the shallow root system of plants is essential for withstanding drought stress, but the associated mechanism is poorly understood. By investigating sap flow in 14 lateral roots (LRs) randomly selected from trees of a Chinese white poplar (Populus tomentosa) plantation receiving three levels of irrigation, an unknown root water transport mode of simultaneous daytime bi-directional water flow was discovered. This mode existed in five LRs confined to the surface soil without attached sinker roots. In the longer term, the bi-directional water flow was correlated with the soil water content. However, within the day, it was associated with transpiration. Our data demonstrated that bi-directional root sap flow occurred during the day, and was driven by evaporative demand, further suggesting the existence of circumferential water movement in the LR xylem. We named this phenomenon evaporation-driven hydraulic redistribution (EDHR). A soil-root water transport model was proposed to encapsulate this water movement mode. EDHR may be a crucial drought-tolerance mechanism that allows plants to maintain shallow root survival and activity by promoting root water recharge under extremely dry conditions.     

How the roots contribute to the ability of Phaseolus vulgaris L. to cope with chilling-induced water stress

Intact plants and stem-girdled plants of Phaseolus vulgaris grown hydroponically were exposed to 5 degrees C for up to 4 d; stem girdling was used to inhibit the phloem transport from the leaves to the roots. After initial water stress, stomatal closure and an amelioration of root water transport properties allowed the plants to rehydrate and regain turgor. Chilling augmented the concentration of abscisic acid (ABA) content in leaves, roots and xylem sap. In intact plants stomatal closure and leaf ABA accumulation were preceded by a slight alkalinization of xylem sap, but they occurred earlier than any increase in xylem ABA concentration could be detected. Stem girdling did not affect the influence of chilling on plant water relations and leaf ABA content, but it reduced slightly the alkalinization of xylem sap and, principally, prevented the massive ABA accumulation in root tissues and the associated transport in the xylem that was observed in non-girdled plants. When the plants were defoliated just prior to chilling or after 10 h at 5 degrees C, root and xylem sap ABA concentration remained unchanged throughout the whole stress period. When the plants were chilled under conditions preventing the occurrence of leaf water deficit (i.e. at 100% relative humidity), there were no significant variations in endogenous ABA levels. The increase in root hydraulic conductance in chilled plants was a response neither to root ABA accretion, nor to some leaf-borne chemical signal transported downwards in the phloem, nor to low temperature per se, as indicated by the results of the experiments with defoliated or girdled plants and with plants chilled at 100% relative humidity. It was concluded that the root system contributed substantially to the bean's ability to cope with chilling-induced water stress, but not in an ABA-dependent manner.     

A technique to measure root tip hydraulic conductivity and root water potential simultaneously

A procedure for the simultaneous measurement of hydraulic conductivityand xylem water potential of roots is presented. Roots remainintact and attached to the transpiring plant during measurement.The rate of water uptake by roots is measured at different waterpotential gradients along the root radial axis, obtained byplacing them in solutions with different osmotic potentials.Hydraulic conductivity and xylem water potential are calculatedby regression analysis of the relationship between water uptakerate and osmotic potential of the bathing solution, assumingthat xylem water potential and reflection coefficient remainconstant during measurement. Results for tomato plants experiencingdrought are presented and discussed. Key words: Root, hydraulic conductivity, water potential