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Published online 1 August 2008
Published in Vadose Zone J 7:1027-1034 (2008)
DOI: 10.2136/vzj2007.0122
© 2008 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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SPECIAL SECTION: ROOTS AND ROOT FUNCTION

Water Uptake and Hydraulics of the Root Hair Rhizosphere

Eran Segala,*, Tammi Kushnirb, Yechezkel Mualema and Uri Shania

a Dep. of Soil and Water Sciences, The Hebrew Univ. of Jerusalem, Rehovot, Israel
b Diagnostic Imaging Dep., The Chaim Sheba Medical Center, Tel Hashomer, Israel. E. Segal and T. Kushnir contributed equally to this work
* Corresponding author (esegal{at}ussl.ars.usda.gov).

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher.


Received 1 July 2007.



   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 Materials and Methods
 Results and Discussion
 Summary and Conclusions
 REFERENCES
 
Plant roots consist of various functional zones; the root cap and beyond the lateral root formation region have very low water permeability due to immature water conduit and root suberization, respectively. The root hair zone, which is located between these regions, is the most permeable zone, where both radial and axial conductivities are high, and has been suggested to play a role in water uptake enhancement. The conventional understanding of root hair function is that root hairs increase root surface area, thereby enhancing water and nutrient uptake. Yet, modeling the soil water status between root hairs shows that the soil water potential there reaches a value close to that of the root in a very short time. The corresponding low water content values within the inter-root-hair domain indicates limited mass water flow and ion diffusion rate toward the root. Consequently, we conclude that when the plant transpires (daytime), root hairs do not increase water and nutrient uptake by increasing root surface area. Instead we used both magnetic resonance imaging technology, for measurements and analysis of spatial and dynamic changes in water content in the rhizosphere, and numerical modeling to show that: (i) root hairs function mostly by water uptake through the root hair tip plane; (ii) the growth of root hairs, perpendicular to the root surface, expands the apparent diameter of the cylinder that is characterized by the root water potential, thereby increasing the effective surface area of the root for water uptake; and (iii) the growth of needle-shaped root hairs requires minimal investment in biomass with less mechanical resistance compared with alternative strategies that require larger root diameter or root length.

Abbreviations: ERL, effective root length • MRI, magnetic resonance imaging • SRHR, soil–root hair rhizosphere


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
Materials and Methods
 Results and Discussion
 Summary and Conclusions
 REFERENCES
 
WHILE STUDYING soil water uptake by a single root, we have to understand that a root is not a uniform cylindrical sink, meaning that there are zones along a root segment that have different water permeabilities (Kramer and Boyer, 1995; Nobel, 1999). Plant roots consist of various functional zones that shift in the soil in accordance with the root tip advance (Esau, 2001). Water extraction by each single root mostly occurs throughout the region of elongation up to the region of lateral root formation, where suberization and lignification of the endodermis haven't reduced the lateral permeability of the root (Sanderson et al., 1983; Doussan et al., 1998a). This domain includes the root hair zone, which is located immediately after the root cell elongation area (Jaunin, 1988). Root hairs are single-wall cells, which differentiate to be an active absorbing unit and grow perpendicular to the main root surface. The length, width, and density of root hairs depend on the plant species and cultivar (Dittmer, 1949; Föhse et al., 1991; Gahoonia et al., 1999) and on environmental conditions such as P (Bates and Lynch, 1996; Ma et al., 2001), soil moisture (Uphof, 1962; Meisner and Karnok, 1991), and hormones (Schiefelbein, 2000). Kramer and Boyer (1995) showed that high radial conductance and mature xylem conduits in the root hair zone enable efficient water uptake.

An early testimony attributing enhancement of mineral uptake to the root hairs can be found in Nye and Tinker (1977); however, the uptake rate wasn't proportional to the total root hair surface area. It was suggested that overlap of the uptake zones due to the high density of hairs decreases the efficiency of each hair as an individual sink. Clarkson (1991) claimed that, due to the overlap, the effective root surface area is shifted to the root hair tips, and emphasized the increase of the extended root surface relative to the epidermis. Nye (1966) calculated the mineral concentration in the area between adjacent root hairs and showed low and steady concentrations a short time after uptake began. During the last decade, several studies confirmed these findings experimentally in relation to P uptake (Bates and Lynch, 1996; Ma et al., 2001; Gahoonia and Nielsen, 2003). In addition to the low and steady concentration between root hairs, the P depletion distance from the root–soil interface was correlated with the root hair length.

A large number of conceptual approaches have been used for modeling water uptake at the plant root scale. Most of the models (e.g., Feddes et al., 1974; Molz, 1981; van Genuchten, 1987; Dudley and Shani, 2003) consider roots as an ensemble of distributed sinks and do not consider root geometry or the existence of root hairs. Other models consider uptake by only a single root and assume the root to be a cylindrical sink (e.g., Gardner, 1960; Raats, 1974; Aura, 1996). Doussan et al. (1998b) proposed a detailed model describing the hydraulic architecture of the root. These studies, however, did not consider root hair function or specific contribution to water uptake. Simulation studies using several of these single-root models reveal water depletion patterns in the vicinity of roots that are characterized by a gradual transition from relatively low water contents near a root surface to higher values away from the root in the bulk soil. Such patterns have been confirmed experimentally (e.g., Hainsworth and Aylmore, 1986; Pierret et al., 2003).

The conventional understanding of root hair function is that root hairs increase root surface area, thereby enhancing water and nutrient uptake (Hofer, 1991; Nobel, 1999; Ridge and Emons, 2000). We will show in this study, however, that the soil water potential between root hairs reaches a value close to that of the root in a very short time. The corresponding low water content values within the inter-root-hair domain indicates limited mass water flow and ion diffusion rate toward the root (Jury and Sposito, 1985; Friedman and Mualem, 1994). Consequently, we conclude that when the plant transpires (daytime), root hairs do not increase water and nutrient uptake by increasing root surface area. Instead, we will show that: (i) root hairs function mostly by water uptake through the root hair tip domain; (ii) the growth of root hairs, perpendicular to the root surface, expands the apparent diameter of the cylinder that is characterized by the root water potential, thereby increasing the effective surface area of the root for water uptake; and (iii) the growth of needle-shaped root hairs requires minimal investment in biomass with less mechanical resistance compared with alternative strategies that require larger root diameter or root length.


   Materials and Methods
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
Results and Discussion
 Summary and Conclusions
 REFERENCES
 
Theoretical Consideration
Mathematical models for water flow in porous media inevitably consider hypothetical media with average properties based on the representative elementary volume concept. This approach allows continuity of the solutions in the soil domain. The proposed modeling will follow these basic principles, allowing us to quantify processes in the continuum soil–root hair rhizosphere (SRHR) domain.

As a preliminary phase of the research, we illustrated the water flow in the root hairs' rhizosphere. Figure 1 illustrates the biological–hydrologic domain of the root hair zone with boundary conditions. We analyzed the water status in the SRHR in three stages. The first stage (Fig. 1A) comprised a numerical solution of the water flow in an axisymmetrical geometry of the SRHR domain based on the radial form (no gravity) of the Richards equation with constant water pressure heads at the root–soil interface and bulk soil:

Formula 1[1]
where {theta} is soil water content (m3 m–3), h is the soil water pressure head (m), K({theta}) is the unsaturated hydraulic conductivity (m s–1), r is the radial distance from the root axis (m), and t is time (s). In the second stage (Fig. 1B), we were focused on a single root hair domain, and simulated h between two adjacent root hairs as a function of time during a water uptake period. In the third stage, we simulated the root water uptake of a single cylindrical root as a function of the effective root radius. The solution followed the Herkelrath et al. (1977) model, which incorporated the wetted fraction of the surface area of the main root segment. Substitution of the hydraulic model of Brooks and Corey (1964) yielded

Formula 2[2]
where Q* is the discharge per unit root length (m3 s–1 m–1), {theta}s is the saturated soil water content (m3 m–3), hw is the soil air-entry value (m), Ks is the saturated hydraulic conductivity (m s–1), and {eta} is a soil type dependent parameter. The subscripts soil and root describe the radial distances from the root axis of the soil, which is not influenced by the root water uptake, and the root radius, respectively. The superscript eff relates to the effective root radius, which is located at the root hair tip plane.


Figure 1
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  		FIG. 1. Scheme of the biological–hydrologic domain with boundary conditions: (A) root hair geometry by an axisymmetrical domain; (B) intra-root-hair rhizosphere; hroot and hsoil are the root soil interface and bulk soil water pressure head (m) and qrh is the water discharge per root hair length unit (m3 s–1 m–1).

 
Water depletion in the SRHR domain during a water uptake period was simulated using HYDRUS-2D with an axisymmetrical geometry platform (Simunek et al., 1999). Two domains of sandy soil were compared, each was 10 by 2 mm. One corresponds to a root without root hairs and included 2 mm of main root. The second represents the root hair zone, with five sequential root hairs with dimensions of 0.75 by 0.005 mm on the main root (2 mm), which generated a disk pattern. The average half distance between adjacent root hairs on the main root (rrh) was calculated assuming they are uniformly distributed:

Formula 3[3]
where nrh is the number of root hairs per unit length of the main root, Rr is the main root radius, and RL is main root length. Initial h was –25 cm and constant soil–root interface water pressure head (hroot) was –60 cm and applied along the root and root hair surfaces. Simulations continued for 60 min.

A similar domain as described above was used to study the effect of root hair density on water depletion in the SRHR during water uptake. The average half distance between adjacent root hairs was altered from 0.1 to 0.15 and 0.2 mm, calculated according to 25, 50, and 100 root hairs per millimeter length of the main root (1 mm in diameter). Initial h and hroot were –25 and –60 cm, respectively. Simulations continued for 1 min. Soil water pressure head at half distance between adjacent root hairs was calculated from the numerical simulations.

Two soils served for this demonstration, a sand and a silt loam. The hydraulic properties of the soils were estimated from the ROSETTA code (Schaap et al., 2001). The initial h and root–soil interface h were –25 and –200 cm for the sand and –60 and –1000 cm for the silt loam. The single-root model introduced by Herkelrath et al. (1977) was used to simulate the water uptake rate as a function of effective root radius. The effective root length (ERL) can be calculated from the relationship between the water discharge of a unit root length (Q*, cm) and the total water uptake rate (Qp, cm3 h–1):

Formula 4[4]
Sand, sandy loam, and silt loam soils were simulated while hsoil was –25, –100, and –250 cm and the radius of the bulk soil was 1 cm. The soil–root interface water pressure head was –150, –500, and –1000 cm and Qp was set to 1 cm3 h–1 for solving the ERL. The hydraulic properties of these soils were estimated from the ROSETTA code (Schaap et al., 2001).

Experimental Setup
The MRI technique (Signa 3.0T, GE Medical Systems, Milwaukee, WI) served as a quantitative and noninvasive tool. A small 4.5- by 11-cm transmit–receive volume coil (3T Small Animal Coil, Bioengineering, Inc., Minneapolis, MN) was used for soil and plant studies. A detailed description of the imaging system is given in Segal et al. (2008). Briefly, two MRI sequences were used:

  1. Three-dimensional spoiled gradient-recalled, gradient echo T1 weighted protocol with repetition time/echo time (TR/TE) 40/13 ms, and bandwidth (BW) 31.23 KHz, was used to visualize the morphology of the roots in the soil. Sequence parameters (TE, TR, and BW) were optimized to achieve the highest signal/noise ratio in our plant–soil system.
  2. Fast spin echo T1 weighted proton density weighted sequence, TR/TE, 2000/12 ms (TR was long enough to avoid T1 dependence at high water content of the signal intensity), BW 15.63 KHz, and a resolution of 140 to 250 µm, was acquired for the quantification of water in the roots and soil. Sequence parameters (TE, TR, and BW) were optimized to achieve the highest signal/noise ratio in our plant–soil system.

Soil water content was calibrated in each acquisition using sealed capsules of soil with known water content attached to each sample. Direct positive correlation was found between soil water content and image intensity, which is represented by a gray scale. Since roots have a cylindrical shape and soil water flow toward roots is described as radial flow immediate to the root, a voxel comprised different radial layers that could exhibit different water contents. Therefore, post-recording analysis included conversion from the original MRI Cartesian coordinates system to intensity as a function of radial distance around a cylindrical, single root. Furthermore, the sandy soils in our study had a particle size equal to or greater than the imaging resolution, i.e., one pixel does not accurately describe the {theta}. Thus, the measured {theta} values as a function of distance from the root surface were an average of 5 to 10 contiguous pixels at the same distance from the root surface.

Two-week-old seedlings of barley (Hordeum vulgare L., cv. Pallas), grown in 50-cm3 minilysimeters filled with sandy soil, were imaged to follow soil water content (the soil's hydraulic parameters were summarized in Segal et al., 2008, Table 1). Two genotypes were compared: a wild type with average root hair length of 0.8 mm, and a spontaneous, apparently monogenic, mutant with no root hairs (bald root type, Gahoonia and Nielsen, 2003).

Since inducing plant transpiration inside the MRI system was impractical due to low temperatures and lack of radiation, a portable growth chamber was developed to induce constant radiation, temperature, and wind conditions. Plants were moved between the MRI system during imaging and the growth chamber for day or night conditions. Daytime (8 h) conditions were achieved by using a 400 W metal halide light, located 30 cm above the plant canopy and a suction fan (600 cm3 min–1). The average daytime temperature was 32°C. Darkness, no wind, and a constant low temperature (20°C) were maintained at night (16 h). During daytime, the minilysimeters were covered with aluminum foil to avoid soil surface evaporation.

The procedure for root sampling consisted of initial three-dimensional imaging in relatively dry soil that highlighted the roots due to their high water content. This depiction was used to select a single root, i.e., a root that was isolated from neighboring roots and lysimeter walls and that included a root tip and root hair zone. Each imaging session started with soil saturation, which was followed by 12 h of drainage. Dynamic water distribution was obtained by a series of successive image acquisitions. Samples were taken at 24-h intervals at the end of the day transpiration period.

Growth of root and root hairs of wild and bald root genotypes were studied in a preliminary experiment through a narrow growth chamber filled with a transparent medium (Agar, Fisher Scientific, Pittsburgh, PA). Binocular (Olympus, Tokyo) images showed that wild-type root hairs initiated at 3 mm from the root cap and reached a final length at 13 mm. No root hairs were found on the bald root genotype. Root hair growth was not necessarily perpendicular to the main root. Analysis of various MRI images showed that the average root diameter of both the bald root and wild types was 1.8 mm.


   Results and Discussion
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
Summary and Conclusions
 REFERENCES
 
The results of water content profiles from numerical simulation are presented in Fig. 2 . Depictions include a series of water content profiles as a function of radial distance from a main root for wild and bald (no root hairs) roots. The main roots are located at the left-hand side (as the y axis) of each profile and root hairs are extended perpendicular to the main root. Initial conditions are of uniform and high water content in the whole profile. These conditions can be assumed for predawn. The rhizosphere between root hairs dried within 1 min. The drying profile did not respond to the individual root hair; instead, the root hair tips created a common boundary that practically increased the main root radius.


Figure 2
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  		FIG. 2. Soil water depletion in the root rhizosphere (HYDRUS-2D) of wild and bald root (no root hairs) barley roots.

 
Figure 3a demonstrates the effect of the increasing radius due to the root hairs on root water uptake efficiency by illustrating the water uptake rate per unit length of a single root as a function of the effective root radius. Data are depicted for three soils. Curves show increasing water uptake as the root radius increased. For instance, a single root 1 mm in diameter with root hairs 0.5 or 1 mm in average length, growing in sand, will improve the soil water extraction rate by 30 and 55%, respectively, relative to a bald root. As root hairs increase the effective root diameter, the actual absorbing surface increases, and therefore enhancement in discharge is calculated. The water uptake efficiency of the root hair zone can be translated into the ERL, which is needed to calculate the potential transpiration rate. Figure 3b presents the ERL as a function of the effective root radius for three soils. An increase in root radius decreases the ERL. The ratio between water uptake enhancement and ERL reduction due to root hairs is linear, therefore the saving of root biomass and energy sources is proportional to the root effective radius.


Figure 3
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  		FIG. 3. Simulation of (a) water uptake per unit root length and (b) effective root length vs. root radius. Calculations were based on a single root uptake model (Herkelrath et al., 1977).

 
Figure 4 presents a series of water content profiles as a function of radial distance from a main root for three root hair densities (rrh). Similar drying patterns of the SRHR were obtained as a result of altering the root hair density, representing different plant species. The rhizosphere between root hairs dried within 1 min for each density. Figure 5 presents the drying rate of the soil between two adjacent root hairs. The soil water pressure head of two soils is presented as a function of time since root water uptake began. The model predicted that the sandy soil would be dried in <1 s and the silt loam soil in a few minutes. The drying time of the soil between the root hairs is very short relative to the daily plant water uptake duration.


Figure 4
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  		FIG. 4. Soil water depletion in the root hair rhizosphere (HYDRUS-2D). Average half distance between adjacent root hairs (rrh) was calculated for 25, 50, and 100 root hairs per 1 mm of main root length.

 

Figure 5
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  		FIG. 5. Soil water pressure head (h) at half distance between adjacent root hairs as a function of time since root water uptake was started. Solid and dashed lines represent sand and silt loam soils, respectively.

 
A series of MRI acquisitions of the bald barley root at three intervals after soil saturation is presented in Fig. 6 . Decreased water content is represented by a darker gray color. The first image in Fig. 6 illustrates the fine sand 12 h after saturation, showing uniform soil water distribution with no water depletion along the root surface. The consecutive days are characterized by a depleted zone (darker color) near the root and a gradual decrease in water content in the bulk soil away from the root's influence.


Figure 6
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  		FIG. 6. A time series of magnetic resonance images of the same bald root barley single root in a minilysimeter filled with fine sand: 12 h represents the time after saturation, the end of the drainage process; Days 1 and 2 represent consecutive days, each an 8-h transpiration period. The white square represents the area of interest including the sampled root and rhizosphere.

 
Transformation of the water intensities obtained from MRI acquisitions (Fig. 6) to water content as a function of radial distance from the root axis [{theta}(r)] at 30 mm from the root cap during 2 d of water uptake is given in Fig. 7 . Water content of the bulk soil ({theta}soil) was high at the end of the drainage period ({theta}soil ? 0.32) and decreased gradually to {theta}soil ? 0.26 after 2 d. A decrease in {theta} between {theta}soil and the water content near the root ({theta}rhiz), typical of water depletion near a cylindrical sink (Gardner, 1960; Cowan, 1965; Hillel et al., 1975), was measured during the following 2 d, where {theta}rhiz decreased to about 0.17 and 0.11 at the end of the first and second days, respectively. The drawdown distance affected by the root uptake shifted from 0.25 mm from the root surface at the end of the drainage period to 0.5 and 0.7 mm at the end of Days 1 and 2, respectively. Figure 8 describes water content along the bald root axis as a function of the distance from the root surface. The {theta}(r) distribution pattern differed between the various zones along the root. As expected (Kramer and Boyer, 1995), almost no depletion was measured near the root tip, and depletion intensity increased along the root axis above the tip.


Figure 7
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  		FIG. 7. Water content in the rhizosphere as a function of radial distance from the bald root surface at 30 mm from the root cap in a fine sand lysimeter: 12 h represents the time after saturation, the end of the drainage process; Days 1 and 2 represent consecutive days, each an 8-h transpiration period. Horizontal bars represent the image sampling.

 

Figure 8
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  		FIG. 8. Water content in the rhizosphere as a function of radial distance from the root surface along the main bald root axis at the end of an 8-h transpiration day in a coarse sand lysimeter. Tip represents the root cap and 3, 10, and 16 mm are the distances from the root cap along the main root axis. Horizontal bars represent the image sampling.

 
A definite effect of the root hairs on water content profiles in the rhizosphere is depicted in Fig. 9 and 10 , where a series of MRI acquisitions of the wild-type barley root is presented. The first image represents the soil at the end of the drainage period, 12 h after saturation. The drained soil showed uniform soil water distribution, with no water depletion along the root surface. The consecutive days are characterized by a depleted zone (darker color) near the root and a gradual decrease in the bulk soil water content. The MRI artifact of dark strips parallel to the root axis are due to the sharp change in signal intensity— a high signal from the root adjacent to a weak signal generated from the dry soil. It was pronounced mainly under dry soil conditions. Since our measurements were focused on the soil immediate to the root surface (1.5 mm from the root surface), this reflection of the dry strip does not affect the accuracy of our measurements. Moreover, the first black strip is shown only for the wild type. Transformation of the water intensity obtained by the MRI acquisitions (Fig. 9) to {theta} is given as a function of radial distance from the root surface, r [L], at 35 mm from the root cap, during 2 d of water uptake. Water content in the soil at large distances from the root ({theta}soil) was high at the end of the drainage period (r > 1 mm, {theta}soil ? 0.32) and decreased gradually to {theta}soil ? 0.23 after 2 d. In contrast to the typical water depletion toward a sink that was illustrated in Fig. 7 for the bald root genotype, a plateau in water content immediate to the main root ({theta}rhiz) was measured for the "hairy" root of the wild type. The typical increase to {theta}soil started farther from the root (r > 0.45 mm). The size of the plateau fits the length of root hairs. Similar patterns were measured at the end of the first and second days, although water depletion intensified with time and {theta}rhiz decreased to 0.05. The drawdown distance affected by the root uptake shifted from 0.45 mm from the root surface at the end of the drainage period to 0.6 and 0.75 mm at the end of Days 1 and 2, respectively.


Figure 9
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  		FIG. 9. A time series of magnetic resonance images of the same single wild barley root in a minilysimeter filled with fine sand: 12 h represents the time after saturation, the end of the drainage process; Days 1 and 2 represent consecutive days, each an 8-h transpiration period. The white square represents the area of interest including the sampled root and rhizosphere.

 

Figure 10
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  		FIG. 10. Water content in the rhizosphere as a function of radial distance from the wild barley root surface at 35 mm from the root cap in a fine sand lysimeter: 12 h represents the time after saturation, the end of the drainage process; Days 1 and 2 represent consecutive days, each an 8-h transpiration period. Horizontal bars represent the image sampling.

 
The distribution of {theta}(r) along the root axis 21 h after saturation and 9 h after transpiration started is described in Fig. 11 . The water distribution pattern differed, as expected (Kramer and Boyer, 1995), between the various zones along the root. No water depletion was detected around the root cap, and the depletion intensity increased with axial distance from the tip within the studied range of 20 mm. The plateau in {theta}rhiz that started 10 mm from the root tip, and was more pronounced 20 mm from the root tip, characterizes the root hair zone that dried the rhizosphere between the main root surface and the root hair tips.


Figure 11
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  		FIG. 11. Water content in the rhizosphere as a function of radial distance from the wild barley root surface along the main root axis at the end of a 9-h transpiration day in a coarse sand lysimeter. Tip represents the root cap and 3, 10, and 20 mm are the distances from the root cap along the main root axis. Horizontal bars represent the image sampling.

 

   Summary and Conclusions
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Summary and Conclusions
REFERENCES
 
Magnetic resonance imaging was found to be an efficient and quantitative tool to follow root and soil water content distributions across space and time. Low water content values were measured at a high resolution at the root–soil interface. Moreover, successive daily imaging of the same root and rhizosphere showed the effects of water uptake and of root zones on patterns of water depletion. The comparison between wild and bald root genotypes enabled us to explore the role of root hairs in water uptake. Although it was impossible to image an individual root hair (below MRI resolution), a plateau in low water content adjacent to the root reveals the root hair's existence. A comparison of depletion patterns between wild and bald roots reveals: (i) lower {theta}rhiz in the wild type; (ii) a distinct plateau in {theta}rhiz, probably within the root hair domain; and (iii) slightly lower {theta}soil in the wild type. Evaluation of these differences between bald and wild root types implies a larger water uptake by the single wild-type root than by the bald type. Figure 12 show roots of the same studied plants at the end of the experiment that were washed of soil. A larger number of secondary roots are evident for the bald type, which could account for the less efficient uptake per unit root length as described above.


Figure 12
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  		FIG. 12. Washed wild-type roots (left) and bald-type roots (right).

 
Theoretical considerations reveal that water content within the root hair zone reaches a value close to that of the root in a very short time. Consequently, we conclude that when the plant transpires, root hairs do not increase water and nutrient uptake by increasing the actual root surface area. Instead, it functions in water uptake mostly through the root hair tip plane, i.e., increasing water uptake by expanding the apparent diameter of the cylinder that is characterized by the root water potential. These conclusions were supported by characteristic water content profiles near the absorbing root. The new conceptual model of root water uptake in the root hair rhizosphere is comprised from direct water uptake by the root hairs, mainly in the tip zone, and water flow toward the epidermis through the dry soil and along the root hairs, based on the hydraulic resistance of each pathway. We may assume that water flow in the dry soil path is negligible due to the low hydraulic gradients and conductivities.


   ACKNOWLEDGMENTS
 
We would like to thank Dr. Tara Singh Gahoonia, from the Royal Veterinary and Agricultural University, Copenhagen, Denmark, for the bald and wild barley seed.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 Materials and Methods
 Results and Discussion
 Summary and Conclusions
 REFERENCES
 




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