BIO1BL Root-Soil Contact for Desert Succulent Agave Deserti Article Essay


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Section 1. (5 points) Summary (synopsis) paragraph, not copy of abstract.

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Future direction

. What else should they look at next based on results. Present some of your ideas with support from their findings.

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Throughout the paper just make sure proper citations are being made [ex: (Hank, 2019)]


Root-Soil Contact for the Desert Succulent Agave deserti in Wet and Drying Soil
Author(s): Gretchen B. North and Park S. Nobel
Source: The New Phytologist, Vol. 135, No. 1 (Jan., 1997), pp. 21-29
Published by: Wiley on behalf of the New Phytologist Trust
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New Phytol. (1997), 135, 21-29
Root-soil contact for the desert succulent
Agave deserti in wet and drying soil
Department of Biology, University of California, Los Angeles, California 90095-1606,
(Received 29 January 1996; accepted 21 August 1996)
To investigate the extent and size of root-soil air gaps that develop during soil drying, resin casts of roots of the
desert succulent Agave deserti Engelm. were made in situ for container-grown plants and in the field. Plants that
were droughted in containers for 7 and 14 d had 24 and 34 % root shrinkage, respectively, leading to root-soil air
gaps that would reduce the hydraulic conductivity at the root-soil interface by a factor of about 5. When containers
were vibrated during drought, root-soil air gaps were greatly diminished, and the predicted conductivity at the
interface was similar to that of the control (moist soil). For plants in the field (4 and 6 wk after the last rainfall),
root shrinkage was greater than for container-grown plants, but root-soil contact on the root periphery was greater,
which led to a higher predicted hydraulic conductivity at the root-soil interface. To test the hypothesis that
root-soil air gaps would help to limit water efflux from roots in drying soil, the water potentials of the soil, root,
and shoot of plants from vibrated containers (with gaps eliminated or reduced) and non-vibrated containers were
compared. The soil water potential was lower for vibrated containers after 14 d of drought, suggesting more rapid
depletion of soil water due to better root-soil contact, and the root water potential was lower as well, suggesting
greater water loss by roots in the absence of root-soil air gaps. Thus, air gaps could benefit A. deserti by helping
to maintain a higher root water potential in the early stages of drought and later by limiting root water loss at the
root-soil interface when the water potential exceeds that of the soil.
Key words: Hydraulic conductivity, root shrinkage, root-soil interface, root water uptake.
1984; Nobel & Cui, 1992a). In soils of intermediate
moisture levels, e.g. with water potentials of about
Root contact with the soil is essential for water and
-0-2 to -2-0 MPa in the case of desert succulents
nutrient absorption by crops as well as native plants.
(Nobel & North, 1993), water movement is limited
Soil properties, such as the degree of compaction and
by the hydraulic conductivity of the root-soil
the average particle size, and root properties, such as
interface, where air gaps between the root and the
root diameter and relative hydration, can influence
soil can arise due to root shrinkage (Faiz &
the extent of root-soil contact (Tinker, 1976; Nye,
Weatherley, 1982; Nobel & Cui, 1992a, b; Nye,
1994). In heavily compacted or waterlogged soil,
problems with root gas exchange may be exacerbated
Roots can shrink radially by as much as 40 0 in
by the absence of air spaces between roots and soil
response to increases in transpirational demand
particles (Veen et al., 1992). Conversely, incomplete
(Huck, Klepper & Taylor, 1970; Faiz & Weatherley,
root-soil contact due to loose soil structure or root
1982). Similar shrinkage can occur for roots under
shrinkage can reduce the uptake of water and
drying conditions caused by exposure to solutions of
nutrients (Faiz & Weatherley, 1982; Veen et al.,
high osmotic pressures (Cole & Alston, 1974; Taylor
1992). The primary limitation on root water uptake
& Willatt, 1983; Nye, 1994). Roots of the succulents
in moist soils is the root hydraulic conductivity
Agave deserti, Ferocactus acanthodes, and Opuntia
(Passioura, 1988; Hamza & Aylmore, 1992), whereas
ficus-indica shrink by about 200% after 4 or 5 d of
under dry conditions the conductivity of the soil is
exposure to an atmosphere with a water potential of
most limiting (Bristow, Campbell & Calissendorff,
-10 MPa (Nobel & Cui, 1992 a). Although changes
in root diameter with changing water status have
* To whom correspondence should be addressed.
E-mail: psnobel(
been observed for roots in soil (Huck et al., 1970;
Taylor & Willatt, 1983), changes in root-soil contact
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22 G. B. North and P. S. Nobel
in response to soil drying apparently have not been
based on moisture release curves for Agave Hill soil
reported. Using resin casts of soil and roots in situ,
(Young & Nobel, 1986). No precipitation was
the extent of root-soil contact and the dimensions of
recorded at Agave Hill in April or May 1995, and
air gaps between the roots and the soil were
Tsoil at a depth of 10 cm was -17 MPa on 24 April
investigated for the common desert agave A. deserti
and -34 MPa on 8 May.
under various conditions of soil moisture in the field
and in containers.
The effects of incomplete root-soil contact on
Laboratory experiments
plant water uptake have been explored through
Ramets (vegetative offshoots) c. 10 cm tall with four
models that allow the hydraulic conductivity of the
to five unfolded leaves were removed from mature
root-soil interface to vary with changes in soil water
plants of A. deserti growing in a glasshouse at the
content and root diameter (Herkelrath, Miller &
University of California, Los Angeles, and grown in
Gardner, 1977; Fernandez & McCree, 1991; Nobel
containers of equal portions of washed quartz sand
& Cui, 1992a,b; Jensen et al., 1993; Nye, 1994).
and soil from Agave Hill. After 30 d, when each
According to such models, the decrease in the
ramet had six to 10 roots averaging 12 cm in length,
conductivity at the interface due to poor soil contact
the plants were transferred to cylindrical polystyrene
can reduce water uptake by up to 99 0. Vibrating
containers 15 cm tall and 10 cm in diameter, which
qontainers of plants to eliminate air gaps between
were filled with Agave Hill soil sieved to remove
roots and soil increases water uptake, leading to
particles larger than 3 mm across. An aquarium-type
higher leaf water potentials than for non-vibrated
airstone had been placed in the soil near the base of
plants (Faiz & Weatherley, 1982). Air gaps can thus
each container and fitted to Tygon? tubing that
help to limit water loss from the roots to a drier soil,
extended through a hole drilled in the side of the
which is particularly important for desert species
container, allowing a partial vacuum to be applied to
(Nobel & Cui, 1992a). Under drought conditions,
insure complete infiltration of the soil by resin.
the water potential of roots surrounded by air gaps
Plants were maintained in the glasshouse, receiving
should be higher than that of roots in full contact
water twice weekly, with daily maximum/minimum
with the soil. Vibration experiments were therefore
air temperatures averaging 28 ?C/16 ?C, daily
performed on container-grown plants of A. deserti to
maximum/minimum r.h. of 70 %o/40 00, and a
examine the relationship between root-soil contact
mean photosynthetic photon flux density of
and the water potentials of the soil, roots and shoot.
30 mol m-2 d-.
Measurements of water potential, root diameter and
After 14 d, six plants were randomly assigned to
gap width were then used to calculate the overall
each of five groups and maintained for an additional
hydraulic conductivity of the root-soil pathway for
14 d: (1) control, which was watered twice weekly;
field and container-grown plants under a range of
(2) vibrated, in which containers were watered twice
soil moistures to help understand the role of the
weekly while placed on a thin aluminium plate that
root-soil interface in controlling water movement
was struck four times with a mallet twice daily; (3)
droughted by withholding water for the last 7 d; (4)
between roots and soil.
droughted for the entire 14 d; and (5) droughted and
vibrated. At the end of the treatments, two containers
from each group were infiltrated with resin, and six
were used for determinations of water potential. For
Field measurements
the first and second groups, containers had been
Field measurements were made at the University of
watered 3 d before measurements. The root water
California Philip L. Boyd Deep Canyon Desert
potential (Troot) was measured by excavating roots,
Research Center near Palm Desert, CA at Agave Hill
wrapping them in ParafilmO, and removing the distal
(330 38′ N, 1160 24′ W, 850 m elevation). Agave
15-mm segments in a humidified chamber; ‘root was
deserti Engelm. (Agavaceae) is the dominant species
determined after the segments equilibrated for 2 h in
at the site, which has a loamy-sand soil with a non-
a thermocouple psychrometer (Decagon Devices,
gravel portion consisting of 73 00 sand (particle sizes
Pullman, WA). Shoot water potential (Tshoot) was
of 0 05-1 0 mm) by mass (Nobel, 1977). Out-
measured for 9-mm cores removed from the base of
croppings of disintegrating granite are abundant,
unfolded leaves, also using the thermocouple psy-
resulting in numerous rocks at the soil surface and in
chrometer. v’soi1 was determined gravimetrically
the upper 20 cm of soil, where the roots of A. deserti
(Young & Nobel, 1986), using a soil bulk density of
are concentrated (Nobel, 1988; Nobel, Miller &
1 57 Mg m-3 as determined for containers of sieved
Graham, 1992). Resin casts of roots and soil were
soil; soil volume and hence bulk density were not
made in situ in two undisturbed level locations on 24
changed significantly by withholding water or by
April and 8 May 1995. The soil moisture content
vibration. Gravimetric determinations of v’so11 were
within the root zone was determined gravimetrically,
within +8% 0?f v’soi1 measured with the thermo-
and the soil water potential (Ws0il) was calculated
couple psychrometer.
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Resin infiltration and sectioning
An acrylic resin that hardened in a few hours, even
in the presence of moisture, was used to make
sections of soil and roots, both in the field and in
containers (Moran, McBratney & Koppi, 1989). For
each batch, 34 g of Araldite GY 509 resin, 34 g of
Araldite RD-2 diluent, and 32 g of HY 956 hardener
(Ciba-Geigy, Hawthorne, NY) were used. Three
batches of liquid resin were applied in sequence from
a large syringe fitted with a pipette tip and suspended
2 cm above the soil surface, which dispensed the
rroot L V’ef
resin at approximately two drops s-1 (total time
1-5 h). For plants in containers, a partial vacuum was
applied during the application of the third resin
batch ‘to improve infiltration. One hour after
infiltration, the resin was sufficiently hardened to
allow removal of the soil block, and the block was
further infiltrated with two batches of resin under
partial vacuum in the laboratory. Within 24-48 h,
the blocks were trimmed and cut in 1-cm-thick
transverse and longitudinal sections with a diamond
lapidary saw. Sections were viewed and photo-
graphed under a dissecting microscope at magnificationsof x10-x30.
Measurements of root-soil contact characteristics
were made from sections examined with the
dissecting microscope using an ocular micrometer
and from photographs enlarged to final magnifications of x 15- x 60. Data were statistically analysed
using one-way ANOVA followed by StudentNewman-Keuls pairwise testing (Sigmastat, Jandel
Scientific, San Rafael, CA).
Figure 1. Cross section of root (light stippling) after
shrinkage during drying, eccentrically located in an air
space (no stippling) in soil (dark stippling). The radii of
the root (rroot) and air space (rgap) are indicated, along with
the eccentricity (e, the distance between the centre of the
air space and the centre of the root). Arrows indicate the
components of the overall hydraulic conductivity (Loverall):
the effective soil hydraulic conductivity (L”.), the hydraulic conductivity of the air gap (Lgap) and the root
hydraulic conductivity (L,). Wedge-shaped segment with-
out an air gap shows probable effect of vibration.
1976), which was about 15 mm for plants in
Hydraulic conductivity of roots, soil, and air gaps
The hydraulic conductivity (m s-‘ MPa-‘) of the
The hydraulic conductivity of the air gap, Lgap,
overall root-soil pathway (Loverall) for water move-
was calculated assuming isothermal conditions and
ment was based on that of its three components (the
allowing the root to be located eccentrically within
soil, the root-soil air gap, and the root) in series (Fig.
the gap, where the eccentricity e (m) equals the
1; Nobel & Cui, 1992 a):
distance between the geometric centre of the air
space and that of the root (Fig. 1; Mills, 1992; Nobel
& Cui, 1992a):
Loverall soil Lgap Le’ (1)
where L ell is the effective hydraulic
the soil, Lgap is the hydraulic conductivity of an air
gap between the root and the soil, and Lp is the
hydraulic conductivity of the root (Fig. 1). L ell was
calculated as follows, assuming radial symmetry for
Tsoil (Nobel & Cui, 1992b):
gap root
L’ equals v2 Dwv Pv/(RT)2, where Vw is the
molal volume of water (m3 mol-h), DWV is the
diffusion coefficient of water vapour in air (M2 S-1),
P*v is the saturation partial pressure of water (MPa),
and RT is the gas constant times the absolute
temperature (m3 MPa mol-h); at 25 ?C, L’ equals
x 10-12 m2 s1
MPa-1 (Nobel
& Cui, 1992a).
is 418the
is a function of Tjsoil as has been determined for
When the root is concentrically located within the air
Agave Hill soil (Young & Nobel, 1986); rgap and
space, e equals zero, and cosh-1 (r2ap ? r_00t/2rg p rroot)
rroot(m) are the radii of the air space and root,
equals loge (rgap/rroot); when e equals the gap width (a
respectively (Fig. 1); and rjistant is set to the lesser of
root touches the soil), eqn 3 is not applicable. For the
30 mm and half of the inter-root spacing (Caldwell,-
intermediate cases considered here, mean values of e
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24 G. B. North and P. S. Nobel
Figur 2 Micrgah frsnscin froso gv eet nsis()Cosscinfo oto
14gud of dricingr()aphs ofore(i) buctiafte of iraotion treatment (esrinsol.() Cross section from fildon24Aril(f
Longitudinal section from field on 5 Mavx Bars =1 mm.
were always less than the mean gap width. Lp was
(maintained in moist soil) were white and generally
based on previous measurements for young main
circular in cross-section (Fig. 2a), whereas roots
roots of A. deserti (North & Nobel, 1995).
subjected to drought were more irregular in outline,
with a darker endodermal cylinder evident within
the cortex (Fig. 2b-d). Most roots in sections made
in the field were older and darker (Fig. 2e,f) than
Root shrinkage and root-soil contact
the 4-wk-old to 8-wk-old roots of the container-
grown plants. Air gaps between the roots and the
Roots showed no evidence of shrinkage or distortion
soil appeared dark in the resin sections, which were
due to resin embedding (Fig. 2). Control roots
kept wet under the microscope to improve contrast.
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Table 1. Characteristics of root-soil contact for Agave deserti in containers in the glasshouse and on two dates
in the field
Gap width Root radius Root-soil Root
Treatment (mm) (mm) contact (%) shrinkage (%)
Control 0 07 +0 02 0 86 +0 15 94+ 2 8 + 2
Vibrated 0 05 + 0 03 0-89 + 0-13 95 + 6 10 + 3
Droughted 7 d 0 25 + 0-04 a 0 79 +0 10 24+7 a 24+4 a
Droughted 14 d 034+005 b 0-64+006 21+7 a 34+3 a
Droughted/vibrated 0 07 + 003 0 71 + 013 90 + 3 9 + 2
Field (24 April) 011 +002 036+005 a 59+9 b 26+3 a
Field (5 May) 0-12+002 026+004a 46+8b 40+5b
Different letters within a column indicate significant differences from the control and between treatments (P < 0 05
for pairwise testing).
The width of the air gap was determined by averaging four measurements from the outer edge of the root in transverse
section to the edge of the soil. The percentage of root contact with the soil was determined from photographs by outlining
the perimeter of the root with thread and marking regions where no gap was evident. The percentage of root ‘shrinkage’
was calculated from the mean gap width divided by the sum of the gap width plus the root radius (= percentage
difference between air space diameter and root diameter). Data are means+ SE for n= 10 roots.
60 -(a) Control (b) Droughted 7 d (c) Droughted 14 d
a0e 1 , , , , , 1R1 I S 01 E , , E I l 1X I1 , , , I
z40 _-:mA
0 20
o 60 – (d) Droughted! (e) Field-24 April (f) Field-5 May
Z 40Co
0 20 40 60 80 100 0 20 4060 801000 20 40 60 80100
Root-soil contact (% of root periphery)
Figure 3. Root-soil contact for roots of Agave deserti. (a) Control (in moist soil). (b) After 7 d of drying in
container. (c) As for (b), but after 14 d of drying. (d) As for (c), but after vibration treatment. (e) From field
on 24 April. (f) From field on 5 May. Data represent n = 12 roots.
In some cases where air gaps were present, some soil
vibrated were similar to the control roots with
particles still adhered to the root surface but had
respect to gap width, root-soil contact, and root
separated from the bulk soil (Fig. 2b, c).
shrinkage (Table 1).
Only small air gaps were present between roots
After 7 d of drought, a significantly larger air gap
and the moist soil for the control, as average root
developed between roots of container-grown plants
contact with the soil then exceeded 90 0 (Table 1,
and the soil, along with a loosening of soil particles in
Fig. 3 a). Such control roots showed only 8 0
the vicinity of the root (Fig. 2b), and root shrinkage
shrinkage (which for all treatments is more ac-
increased to 240% (Table 1). Mean root-soil contact
curately described as the percentage difference
decreased to 24 0, with 45 00 of the roots having less
between air space diameter and root diameter; Table
than 20 00 root-soil contact (Fig. 3 b). After 14 d of
1), and all of the roots had at least 80 0 contact with
drought, the mean width of the root-soil air gap
the soil along their cross-sectional perimeter (Fig.
increased to 034 mm (Table 1), and 66 % of the
3 a). Roots from plants maintained in moist soil and
roots had less than 20 0 root-soil contact (Fig. 3 c).
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26 G. B. North and P. S. Nobel
Table 2. Water potentials of the soil and organs of Agave deserti in
containers in the glasshouse
Water potential (MPa)
Control -0.1 + 0.1 -0 45 + 0-02 -0-42 +0 03
Vibrated -0 1 +0 1 -0 47 +0 03 -0 44+0 02
Droughted -4 2+004 a -1 04 + 0 03 a -1 01 + 0-06 a
Droughted/vibrated -6-4+0 7 b -1 13 +0 02 b -0-96 + 0-06 a
Different letters within a column indicate significant differences from the
control and between treatments (P < 0 05 for pairwise testing).
Data are means+ SE for n = 6 plants treated for 14 d.
Table 3. Hydraulic conductivities of the soil (Lsff),
Water relations
root-soil air gap (Lgap), root (Lp), and overall root-soil
Soil water potential (Tsoil) for the control treatment
pathway (Loverall) for Agave deserti
did not differ from that for watered containers that
Hydraulic conductivity
(10-8 m s-‘ MPa-‘)
were vibrated (Table 2). After 14 d of drought, P5011
decreased to -4 2 MPa for containers that were not
vibrated and to – 6-4 MPa for those that were (Table
Treatment Lff Lgap L Lovera
Control 2458 6-21 20 4 65
Droughted 7 d 12 7 1 95 8 1 40
2). Vibration had no effect on root water potential
(Troot) for containers of moist soil, but Troot was
about 8 0 lower for vibrated than for non-vibrated
Droughted 14 d 5 64 1 52 5 0 97
containers after 14 d of drought (Table 2). Shoot
Droughted/vibrated 2-22 6 26 5 1 23
water potential (Tshoot) was unaffected by vibration.
Field (24 April) 33 1 422 10 2-72
Field (5 May) 12-6 4-21 5 1-94
For droughted, non-vibrated plants TProot and TPshoot
were similar, whereas for droughted, vibrated plants
TIroot was lower than TPshoot (P < 0 05; Table 2).
Data were calculated using Tsoil from the text and Table
Except for plants that were both droughted and
2, root and gap radii from Table 1, and Lp for young main
vibrated, the hydraulic conductivity of the air gap
roots of A. deserti (North & Nobel, 1991) in eqns 1-3,
assuming isothermal conditions and concentric location of
(Lgap) was the lowest and thus the most limiting
the roots within the air spaces.
conductivity in the root-soil pathway (Table 3). The
hydraulic conductivity of the soil (Lo1) was the least
limiting conductivity, except for container-grown
Roots from containers that were vibrated daily
plants droughted for 14 d; as Tsoil decreased from
during 14 d of drought did not differ from the
-0 1 MPa (control) to -6 4 MPa (droughted
control roots with respect to gap width or root
14 d/vibrated), L eof decreased by more than 103.
shrinkage (Table 1), and all had at least 80 % contact
Root hydraulic conductivity (Lp) decreased by a
factor of 4 after 14 d in drying soil, and Loverall
with the soil (Fig. 3 d).
For plants in the field, resin sections made on 24
decreased by a factor of 5 (Table 3).
April and 5 May (c. 4 and 6 wk after the last rainfall,
To quantify the effect of eccentric location of roots
respectively) included roots that were thinner on
within air spaces on hydraulic conductivity, the
average than the roots from container-grown plants
eccentricity (e) was measured. For three roots from
(Table 1). On both dates the gap width represented
containers droughted for 14 d, the mean e was
a greater fraction of the air space occupied by the
0 19 mm. Using this e in eqn 3 along with values for
roots, as reflected in the greater root shrinkage
root and gap radii from Table 1, Lgap for eccentrically
(Table 1). The mean root-soil contact for field roots
located roots was 1-84 x 108 m s-1 MPa-1, which is
was intermediate to that for control roots and
21 % greater than Lgap for concentrically located
droughted roots for the container-grown plants
roots after 14 d of drought (Table 3). For three roots
(Table 1). On both field dates, root-soil contact of
in the field (5 May), an average e of 010 mm led to
individual root cross sections ranged from 0 to
an Lgap of 7 63 x 1 08 m s-1 MPa-1, which is 81 %
100 %, with more than 5000 of the roots having at
greater than for the concentric case (Table 3).
least 60 0 contact on the earlier date, compared with
Averaged for all treatments, incorporating measured
only 150% on the later date (Fig. 3e,f). In many
values of e into eqn 3 led to values of Lgap that were
cases, roots were located adjacent to rocks, usually
29 0 0 10 0 higher than Lgap for concentrically
with full contact between root and rock.
located roots.
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when growing in pre-existing channels or macro-
pores (Tinker, 1976; van Noordwijk et al., 1992; van
Resin casts of Agave deserti roots in situ
Noordwijk, Schoonderbeek & Kooistra, 1993). The
demonstrated progressively greater root shrinkage
most extreme case of eccentricity, in which the root
and loss of root-soil contact during soil drying.
touches the soil along its periphery, can occur more
Based on the root radius and the average width of
frequently than would be predicted assuming the
root-soil air gaps, root diameter during drying
random positioning of roots within air spaces
apparently decreased by up to 34%0 in
(Kooistra et al., 1992). When the root touches the
and 40 0 in the field. Such shrinkage was somewhat
soil, the eccentricity equals the mean gap width, and
greater than the 20 0 reduction measured for roots
Lgap becomes infinite according to eqn 3 (actually,
of A. deserti in a drying atmosphere (Nobel & Cui,
the isothermal assumption is then not valid, and
1992a), in part, presumably, because of incorpor-
eqn 3 is no longer appropriate). In this regard,
ation of air gaps (accounting for c. 8 oo) that were
measured Lgap for roots of A. deserti touching a filter-
present even for well hydrated roots, such as the
paper cylinder (simulating the soil) at one point on
control. After 14 d of drying, the mean root diameter
their perimeter is only 2-4-fold greater than for roots
of container-grown plants was 26 % less than the
concentrically located in the cylinder, similar to the
control root diameter, representing a decrease similar
increase predicted by a graphical flux-plot method
to that measured previously. The greater shrinkage
that allows for non-radially symmetric pathways for
for roots in the field than in containers might partly
water movement from the soil to a root (Nobel &
reflect greater average root age in the field, as cortical
Cui, 1992b).
cells shrink and die during ageing, a process that is
hastened by drought (North & Nobel, 1995).
Models of radial water uptake by roots in soil
Substantial root-soil contact, such as for the
control roots, might thus greatly increase the
hydraulic conductivity of the root-soil interface
assume that the hydraulic conductivity of the
compared with that predicted based on mean gap
root-soil interface is inversely proportional to the
width, thereby leading to a greater overall hydraulic
width of a root-soil air gap, whether water moves as
conductivity (Loverall). However, the value of
a liquid or a vapour (Cowan & Milthorpe, 1968;
4-7 x 10o’ m s-‘ MPa-1 for Loverall for the control
Nobel & Cui, 1992 a, b; Nye, 1994). Assuming
agrees well with a value of 4-8 x 108 m s-1 MPa-1
isothermal conditions and a concentric location of
calculated using a model of water uptake
roots within air spaces, the hydraulic conductivity of
incorporating root-soil contact (Herkelrath et al.,
the gap (Lgap) for container-grown plants of A.
1977), in which the root hydraulic conductivity
deserti was predicted to decrease by a factor of 4 as
(20 x 10o8 m s-1 MPa-1) is multiplied by a contact
the gap width increased by a factor of 5. For plants
fraction equal to the volumetric soil water content
droughted in containers, as well as for plants in the
(0 24 for saturated Agave Hill soil; Young & Nobel,
field, Lgap was the lowest and therefore the most
1986). Large decreases in root-soil contact occurred
limiting hydraulic conductivity in the root-soil
during soil drying for A. deserti in containers, with
pathway, in agreement with previous calculations of
over 50 00 of the roots having less than 10 00 contact
Lgap for soils of similar water potentials (Nobel &
with the soil after 14 d. Greater root-soil contact was
Cui, 1992a). Despite greater relative shrinkage for
maintained in the field, probably because of rela-
roots in the field, the smaller gap widths and smaller
tively smaller root diameters and greater soil het-
root radii led to a predicted Lgap that was more than
erogeneity. In particular, roots were appressed
double that for droughted container-grown plants.
against rocks, where greater water availability occurs
Similarly, thinner roots of Zea mays maintain better
due to condensation and water channelling (Nobel et
contact with a drying soil and therefore take up more
al., 1992).
water per unit area according to a model based on
Another assumption, that the temperature was
changes in the root-soil contact angle (Nye, 1994).
constant from the root surface to the soil, led to an
For A. deserti in the field, thin roots should be more
over-prediction for Lgap. Because of evaporative
effective in water uptake than thick roots during the
cooling at the root surface when water is leaving, the
early stages of soil drying, but they would also tend
actual driving force for water vapour movement
to lose water more readily to the soil when the soil
across air gaps to the soil is less than that predicted
water potential decreased below that of the roots.
on the basis of root and soil water potentials (Cowan
Roots of A. deserti that were eccentrically located
& Milthorpe, 1968; Nobel & Cui, 1992b). To allow
in air spaces had values for Lgap that were on average
for thermal effects, Lgap should be reduced by
c. 200% higher than Lgap for the concentric case forc. 65-70 o (Cowan & Milthorpe, 1968, Nobel & Cui,
container-grown plants. For roots of A. deserti in the
1992 b; Nye, 1994). When combined with the average
field, a greater relative eccentricity than in containers
3000/ increase in Lgap caused by the average eccentric
resulted in a predicted increased in L gap about
location of roots, Lgap calculated for the isothermal,
500?0 greater. In most field situations, roots are
concentric case should be multiplied by 0 6 to predict
eccentrically located within air spaces, particularly –
water conduction across a gap better.
This content downloaded from on Tue, 26 Feb 2019 14:46:44 UTC
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28 G. B. North and P. S. Nobel
Whatever the exact value of Lgap’ root-soil air gaps
had a major impact on water exchange between roots
of A. deserti and the soil, as demonstrated by
The authors thank Ram Alkaly for cutting the soil sections
differences between plants in vibrated and non-
and Michael A. North for Figure 1. Financial support
vibrated containers. The soil vibration treatment
from the US National Science Foundation, grant IBN-94-
had no effect on well hydrated plants, but plants
19844, is gratefully acknowledged.
vibrated during 14 d of soil drying had substantially
smaller root-soil air gaps and greater root-soil
contact than non-vibrated plants. The soil water
potential (Tsoil) was lower for vibrated plants, in
agreement with results for water-stressed Helianthus
annuus (Faiz & Weatherley, 1982), and might reflect
the greater water uptake by roots with greater soil
contact, at least in the early stages of soil drying
Bristow KL, Campbell GS, Calissendorff C. 1984. The effects
of texture on the resistance to water movement within the
rhizosphere. Soil Science Society of America Journal 48:
Caldwell MM. 1976. Root extension and water absorption. In:
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when ‘soil exceeded ‘root. In the transitional period
of soil drying when T0soil became less than Troot,
water loss by the roots was apparently retarded by
root-soil air gaps, leading to a higher Troot for plants
that were not vibrated. Alternatively, the lower ‘root
for vibrated plants could have resulted from the
quicker depletion of soil water, exposing roots to a
larger soil-root water potential gradient for a longer
period. In any case, ‘root decreased below Tshoot for
vibrated plants, favouring water movement from the
shoot to the root. Such a shoot-root gradient did not
occur for non-vibrated plants, suggesting that wider
air gaps (and a smaller Lgap) might help to limit water
loss from the roots and thus from the shoot. No
difference in Tshoot occurred for the succulent shoots
of vibrated and non-vibrated A. deserti, perhaps
because greater water uptake by vibrated plants in
the first few days of drying was counterbalanced by
greater shoot water loss later. For desert succulents
in the field, as for many plants in drying soil,
root-soil air gaps might initially reduce water uptake
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but might also postpone root dehydration, thereby
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prolonging the period of root extension into wetter
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In summary, an increase in the width of root-soil
air gaps and a decrease in root-soil contact were
demonstrated for roots of A. deserti in drying soil.
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of about 5 lower than that of the control, or a factor
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of 3 lower when taking into consideration thermal
effects and the eccentric location of roots. Similarly,
water uptake is reduced by a factor of 3 in a model
incorporating root shrinkage and a decrease in the
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York: Cambridge University Press.
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root-soil air gap, and the root: changes for desert succulents in
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root-soil contact angle (Nye, 1994). When gap
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formation was reduced by vibrating the containers,
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soil water was depleted more rapidly and root water
potential was lower than for non-vibrated containers.
For succulent plants such as A. deserti, the potential
benefit of higher relative root water potential might
outweigh the drawback of reduced water uptake
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drying, and the lower overall hydraulic conductivity
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subsequent stages of drought.
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