Annals of Botany 86: 687±703, 2000
doi:10.1006/anbo.2000.1236, available online at http://www.idealibrary.com on
Oxygen Distribution in Wetland Plant Roots and Permeability Barriers to Gas-exchange
with the Rhizosphere: a Microelectrode and Modelling Study with Phragmites australis
W. A R M S T RO N G * {, D . CO U SI N S {, J . A R M S T RO N G {, D. W. T U R N E R { and P. M . B E C K E T T }
{Department of Biological Sciences, University of Hull, Hull, HU6 7RX, UK, {Plant Sciences, Faculty of Agriculture,
University of Western Australia, Nedlands, WA 6907, Australia and }Department of Mathematics, University of Hull,
Hull, HU6 7RX, UK
Received: 20 April 2000
Returned for revision: 5 June 2000 Accepted: 19 June 2000
Adventitious roots of intact Phragmites plantlets were secured horizontally 2±3 mm below the surface of an oxygendepleted ¯uid agar across which oxygen-free nitrogen was gently streamed to create a constant oxygen sink; the leafy
shoot was fully exposed to air. Radial oxygen pro®les through rhizosphere and root at di€erent distances from the
apex were obtained polarographically using Clark-type bevelled microelectrodes servo-driven in steps of 10 mm (root)
or 10±50 mm (rhizosphere). The pattern of radial oxygen loss (ROL) typical of wetland plants, viz. high at the apex
and declining sharply sub-apically, was related to synergism between ROL, and oxygen consumption and increasing
impedance to di€usion within the epidermal/hypodermal cylinder rather than to a surface resistance. The smallest
oxygen de®cit (2 kPa) to develop across the 80 mm thick epidermal/hypodermal cylinder was within the apical 10 mm
and was consistent with tissue oxygen di€usivities similar to water. At 100 mm from the apex, consumption and
impedance had increased the de®cit to about 15 kPa and reduced ROL almost to zero. The developing impedance
within the epidermal/hypodermal cylinder was least in cell layers immediately adjoining the cortex and increased most
in the hypodermal cell layer abutting the epidermis. The sub-apical decline in ROL appeared to coincide with the
appearance of aerenchyma in the cortex but thin walled `passage areas' (windows) in the hypodermal/epidermal
cylinder persisted locally and remained leaky to oxygen to some degree. It is through these windows that lateral roots
emerge and the cortex in line with the windows remains non-aerenchymatous. The radial and longitudinal oxygen
pro®les were consistent with modelling predictions. The shapes of the stelar oxygen pro®les were consistent with a
higher oxygen demand in the outer region (viz. pericycle, phloem, protoxylem and early metaxylem cylinder) than in
the inner core (late metaxylem cylinder and medulla), but the de®cits were relatively small (43 kPa) and consistent
with minimal wall thickening in the endodermis and narrowness of stele. The possible relevance of the results to entry
of methane and other products from the rhizosphere into root and to the mechanism of aerenchyma formation are
# 2000 Annals of Botany Company
discussed.
Key words: Transport, oxygen permeability, Phragmites, wetland roots, rhizosphere, modelling.
I N T RO D U C T I O N
Transport of oxygen from shoot to root via the cortical gasspace continuum can occur in both wetland and nonwetland plants. However, in wetland plants it is especially
e€ective due to the presence of aerenchyma (Armstrong,
1979; Drew et al., 1985; Justin and Armstrong, 1987) and
because waterlogged soils are mostly anaerobic, wetland
plants are normally totally reliant upon it to support root
growth (Armstrong and Webb, 1985; Waters et al., 1989).
Oxygen is essential for much nutrient and water uptake
(Gibbs et al., 1998). In addition, oxygen di€uses from root
to rhizosphere (radial oxygen loss; ROL) where it can
re-oxidize chemically-reduced phytotoxins (Trolldenier,
1988; Conlin and Crowder, 1989; St-Cyr and Crowder,
1989; Armstrong et al., 1992; Begg et al., 1994; Christensen
et al., 1994; Saleque and Kirk, 1995; Wang and Peverly,
1999) and support some aerobic microbial (Ueckert et al.,
1990) and even fungal activity (Pedersen et al., 1995).
However, the use of sleeving polarographic electrodes and
* For correspondence. Fax ‡44 1482-465458, e-mail w.armstrong@
biosci.hull.ac.uk
0305-7364/00/090687+17 $35.00/00
redox dyes has revealed that radial oxygen loss from
adventitious roots of wetland plants such as rice and
Phragmites is typically highest in the apical (elongating)
regions and declines basipetally (Armstrong, 1971; Conlin
and Crowder, 1989; Colmer et al., 1998; Gilbert and
Frenzel, 1998). It is not unusual for radial oxygen loss to be
virtually undetectable further than a few centimetres
(sometimes only a few millimetres) behind the root tip,
and the greatest oxygen release to the sediment is often
from basally-borne laterals (Armstrong et al., 1990, 1992,
1996b). At the same time, however, since the shoot is the
source of oxygen supply, it follows that oxygen concentrations within a root must necessarily decrease from shoot
base to root apex.
Whilst the phenomenon of basipetal decline in ROL is
well documented for wetland species, the nature and location
of the impedances responsible have never been fully established. There is the possibility that deposits of a material
such as suberin on the root surface could be responsible, and
previous modelling studies have invoked such a barrier to
help mimic the observations (Armstrong, 1979; Armstrong
and Beckett, 1987; Sorrell, 1994). However, to eliminate
# 2000 Annals of Botany Company
688
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
radial oxygen loss totally, such a surface barrier would have
to have remarkable properties: it would need to o€er in®nite
resistance. On the other hand the radial and tangential cell
walls of the hypodermal (exodermal) and epidermal cell
cylinders can often develop quite obvious secondary wall
deposits sub-apically, and it can be deduced readily from
di€usion theory that resistances spread throughout the
radial path from the outer perimeter of the cortex to the root
surface might collectively provide a more e€ective barrier to
ROL than a thin resistance at the root surface. This is
because they could act synergistically with oxygen consumption within the cells along the di€usion path and a very steep
radial decline in oxygen concentration might be achieved
without the need for any individual resistance to be
abnormally high.
The study described in this paper was undertaken (1) to
discover whether the sub-apical decline in radial oxygen loss
in a wetland species could be explained in terms of increasing
resistance within the epidermal/hypodermal cylinder, or was
mainly a surface feature; (2) to determine whether thinnerwalled regions in the epidermal-hypodermal cylinder which
lie in-line with developing laterals and sectors of nonaerenchymatous cortex behave as passage cells to ROL; (3)
to demonstrate experimentally that there is a basipetal
increase in oxygen concentration coincident with the subapical decline in radial oxygen loss (only the ROL pattern
has hitherto been measured experimentally for wetland
roots, and the basipetal increase in cortical oxygen concentration has only been deduced and modelled); and (4) to
further our knowledge of oxygen distribution in roots in
relation to anatomical features. Published work so far
contains detailed information for non-wetland plants only
viz. root nodules (Witty et al., 1987) and adventitious roots
(Armstrong et al., 1994; Gibbs et al., 1998).
The oxygen measurements were made using Clark-type
oxygen microelectrodes, and a mathematical model was
used to help interpret the data particularly in the case of the
developing impedance to radial oxygen loss sub-apically.
M AT E R I A L S A N D M E T H O D S
Plant material
Horizontal rhizomes of Phragmites australis (Cav.) Trin ex
Steud., collected from the north bank of the River Humber
(nr. Welton, E. Yorks., UK) in February, were immersed
just below the surface in non-aerated tap water in narrow
darkened tanks in a growth room (T ˆ 238C) until new
shoots and roots had sprouted. The emerged shoots
received PAR at approx. 80 mmol m ÿ2 s ÿ1. Plantlets were
obtained by excising rhizome nodes bearing vigorous new
shoots and their attendant root(s); each nodal plantlet had
about 5 mm of internodal tissue remaining on either side of
the nodal diaphragm. The shoots selected were approx.
20 cm tall and the roots 10±20 cm long.
Oxygen measurements
Microelectrodes: construction, operation and performance.
Clark-type oxygen microelectrodes with gold-tipped
cathodes (Fig. 1) were prepared as described by Armstrong
(1994) except that the alloy base (MCP47: Mining &
Chemical Products, London, UK) used to ®ll the cathode
and comprising 44.7 % bismuth, 22.6 % lead, 5.3 %
cadmium, 8.3 % tin and 19.1 % indium, has a much
lower melting point than the Wood's metal previously
employed. Borosilicate glass capillaries (OD 2 mm,
ID 1.16 mm, L 100 mm, and OD 1 mm, ID 0.58 mm,
L 150 mm: Clark Electromedical Instruments, Pangbourne,
UK) were used to fabricate the inner and outer electrode
bodies, respectively. A Flaming/Brown type programmable
micropipette puller (Sutter Instrument Co., USA: Model
P-87) was used to pull the tips to the desired speci®cations.
The tip of the outer capillary was ®lled with Si-rubber (Dow
Corning 734).
These electrodes have a much longer shelf-life than has
been reported previously and, notwithstanding breakages,
this can be from several weeks to months; malfunction
eventually arises because of gas bubbles inexplicably
forming in the electrolyte and these eventually sever contact
between anode and cathode. Even then, however, the
electrodes are salvageable. Unfortunately, very narrow tips
proved to be unsuitable for penetrating the Phragmites
roots in subapical parts and those used had tip diameters of
12±18 mm. Electrodes were bevelled to aid penetration and
the bevelling was accomplished using a small ®ne-grade
DIY honing wheel (diameter 45 mm) driven by a modelmaker's 12 V electric drill (Radiospares); the lower part of
the wheel was positioned so that it spun through a small
trough of water. The Si-rubber plugged tip of the
microelectrode outer body was gently applied to the honing
wheel for 1±2 min at an angle of approx. 458 using a
mechanical micromanipulator.
The microelectrodes were mounted on a 3-way motorized
micro-driver (DC3001R: World Precision Instruments)
and polarized by a high impedance polarograph (Barman
Electronics, Skipsea, E. Yorks., UK). Current±voltage
curves ( polarograms) to determine limiting potentials for
oxygen reduction, data collection at the chosen limiting
potential and electrode movement were all computer
controlled (Barman Electronics). Polarograms and radial
oxygen concentration pro®les were displayed during data
collection and, on completion, the data ®les could be
immediately downloaded into SigmaPlot (Jandel Scienti®c,
40699 Erkrath, Germany) for hard-copy presentation and
editing.
Although occasionally a€ected by electrical disturbance
in the mains electricity and moving static charges, signal
output from the electrodes was usually very stable and the
response to oxygen concentration linear. Electrodes were
calibrated by exposing sequentially to air and to oxygenfree nitrogen. In air (or air-saturated water) current output
was in the range 100±300 pA, 90 % response times to
changes in oxygen concentration were 51 s and the
di€erence in reading between moist air and stagnant airsaturated water was no more than 2.6 % (Fig. 2). This small
di€erence is a function of di€usive resistance within the tip
of the electrode itself (electrolyte ‡ Si-rubber) which is
considerably greater than the collecting zone beyond the tip
and it ensures that no major discontinuities in partial
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
689
Electrolysis current (pA)
F I G . 1. Oxygen microelectrodes. Sectional diagram of the tip (A) and high (B) intermediate (C) and low (D) power photographs of the
microelectrode used to collect data shown in Figs 2, 5, 7, 8 and 10±13. A, Note the inner electrode body of drawn-out glass capillary ( ®lled with
low melting point alloy and gold tipped) forming the cathode and the outer electrode body of drawn-out glass capillary with bevelled Si-rubber
®lled tip and containing the supporting electrolyte of potassium chloride (1 M). The anode (not shown) is a silver wire immersed in the KCl and
located towards the basal end of the electrode. B, High power view of tip: the inner glass-sleeved alloy with its gold tip (arrow), the bevel and the
Si-rubber (arrow) can be clearly seen. 430. C, Shows much of the drawn-out electrode tip: the Si-rubber is still clearly visible at the tip. 50.
D, Shows the whole of the drawn-out tip of electrode, the shoulder of the outside body and the inner body visible as a ¯exed black core. 15.
133.24
140
129.81
120
Air - Water
interface
100
80
0
20
40
60
80
100 120 140 160
Distance (µm)
F I G . 2. Change in microelectrode electrolysis current at an air±water
interface. Note: the interface from air into still, air-saturated, water was
at 70 mm and from water to air the current increase is only 2.6 %
(means of ®ve points on each side of interface). (s) Above water
surface; (d) below water surface. Dwell time at each step was 10 s.
pressure reading within the plant can be attributed per se to
the electrode passing from liquid to gas phases.
Experimental assembly. Plants were positioned as shown
in Fig. 3 with one root gently but ®rmly secured (using
metal bands and Terostat putty) onto a Perspex rack. The
root lay horizontally 2±3 mm below the surface of a ¯uid
nutrient agar (0.05 % w : v agar : 1/4-strength Hoaglands
solution) contained in a narrow and shallow glass trough.
The presence of the agar was to prevent any convective
streaming within the supporting medium. Oxygen-free
nitrogen was gently and continuously streamed over the
surface of the medium to impose an external oxygen sink on
the root, to encourage ROL, and to ensure that the root
obtained its oxygen only via the shoot. In this sense the
assembly mimicked, to some extent, the oxygen demand
present in waterlogged soil. Small glass plates were
arranged along the top of the trough to ensure that oxygen
concentration in the head space above the agar remained
690
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
EXPERIMENTAL ASSEMBLY
(–)
(+)
Microelectrode (servo-driven)
Root
Moveable
glass plates
Rhizome
screen
N2
De-oxygenated
agar (0.05%)
+ nutrients
Perspex
support
guides
Terostat
putty
F I G . 3. Experimental assembly for microelectrode measurements with root secured horizontally in ¯uid agar in a glass vessel with an oxygen-free
nitrogen atmosphere maintained above agar surface; air was circulated around the exposed shoot at all times.
zero or at least extremely low. These plates could be moved,
removed or replaced with narrower ones as required to open
up a path for insertion of a microelectrode. The shoot
emerged into the atmosphere of the growth room and was
isolated from the stream of oxygen-free nitrogen by a plastic
ba‚e/screen which dipped into the agar medium and
slotted over the root.
For radial pro®le measurements, the electrodes were
positioned vertically above the root using manual and
servo-assisted movements until it was visually adjudged that
the electrode would take a radial path through the root. The
plateau voltage having previously been determined and set,
the electrode was advanced towards and into the root
(in-track) in predetermined steps of either 10 mm or, within
the rhizosphere sometimes 50 mm. The dwell (equilibration)
time after each step was 7 s. It was usual to record the
rhizosphere pro®le as one data set which brought the
electrode up to the root surface; the pro®le through the root
was then started as a second set of measurements. The root,
and the tip of the electrode (before entering), were observed
throughout using a travelling vernier microscope. Once the
electrode had passed through the root, the drive was put
into reverse and a set of out-track measurements taken.
In-track and out-track data were usually in very close
agreement. This indicates how localized are the measurements made by the electrode and how localized is any
damage caused; it also suggests that the track e€ectively
seals and that there is no oxygen leakage past the electrode.
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
However, deviations sometimes arose because of the root
being pushed (in-track) or dragged (out-track) by the
electrode. In these cases, the electrode remained at the same
location within the root for a number of oxygen measurements while the servo-motor continued to step in or out.
The movement of the root was observed using the cross
wires of the microscope and the step numbers coincident
with the spurious data were recorded thus enabling them to
be edited out subsequently.
Servo-driven measurements were also made axially along
the root surface in the search for any oxygen-permeable
regions within those sub-apical parts where ROL was
largely absent. The electrode was positioned at about
10±20 mm above the root surface and the motorized
manipulator triggered to move stepwise in the z-plane.
Radial oxygen loss. Radial oxygen loss from the root to
rhizosphere can be calculated from the slope of oxygen
gradient from the root surface into the bathing medium
using the following equation:
oxygen flux …ROL† ˆ
60Dw …Cs ÿ Cr †
g cm ÿ2 min ÿ1 …1†
rs …loge rr =rs †
where Dw is the oxygen di€usion coecient in the rooting
medium (approximates that in water at the ambient
temperature: 2.267 10 ÿ5 cm2 s ÿ1 at 238C), Cs is the
oxygen concentration at the root surface (g cm ÿ3)
calculated from the partial pressure, Cr is the oxygen
concentration at a small distance from the root surface
where the radial distance (cm) from the centre of the root is
rr , and rs is the radius of the root.
Oxygen de®cits across the epidermal-hypodermal cylinder.
The oxygen de®cit across the epidermal-hypodermal
cylinder, Ccortex ÿ Cs (g cm ÿ3), is a function of the di€usive
resistance within the cylinder, the rate of oxygen consumption within it and the radial oxygen transfer across it to the
rhizosphere. It can be expressed mathematically as follows:
Cc ÿ Cs ˆ ‰Mr2s =4D…r2c =r2s ‡ 2 loge rs =rc ÿ 1†Š
‡ ROL…rc loge rs =rc †=D
…2†
where Cc is the oxygen concentration at the outer cortical
perimeter, M is the overall rate of oxygen consumption
within the epidermal-hypodermal cylinder (g cm3 s ÿ1), rc is
the cortical radius, ROL is the radial oxygen throughput to
the rhizosphere (g cm ÿ2 s ÿ1) and D is the overall oxygen
di€usion coecient of the epidermal-hypodermal cylinder.
This equation assumes uniformity in distribution of oxygen
demand and di€usivity across the cylinder. The oxygen
permeability (cm s ÿ1) of the cylinder is given by the
expression D=…rc loge rs =rc †:
Mathematical modelling
In order to help interpret the longitudinal and
radial oxygen distribution data obtained by the microelectrodes some modelling simulations were attempted.
The mathematical model has been detailed elsewhere
691
(Armstrong and Beckett, 1987; Beckett and Armstrong,
1992; Beckett et al., 2000). The model is di€usion-based
and the root and rhizosphere are treated as a set of
concentric cylinders: inner stele, outer stele, cortex,
epidermis-hypodermis and rhizosphere. Longitudinal gasphase di€usion occurs in the cortex, and oxygen is supplied
from the cortex to the other cylinders by radial liquidphase di€usion. The data used in the simulation are
summarized in the Appendix.
Anatomy
In its passage through the root the electrode causes only
very localized damage. After 1 or 2 d, this become visually
evident as a brown staining along the line of the electrode
track probably due to the action of polyphenol oxidases.
Consequently, after the measurements had been completed,
the roots were left for 2 d for the tracks to become visible.
They were then sectioned transversely, fresh and free-hand,
in order to match the oxygen pro®les with the anatomical
features along the track. A slight brown staining usually
marked the position where the electrode entered the root
allowing one to pinpoint those parts of the root which
required sectioning. The sections were then stained with
phloroglucinol and concentrated HCl to accentuate the
track and photographed using an Olympus BX40 microscope.
`Passage cell' zones in the epidermal-hypodermal cylinder
were photographed under blue light using the ¯uorescence
facility of a Zeiss photomicroscope. They were also visible
in sections stained with phloroglucinol and concentrated
HCl: most of the cell walls in the epidermal-hypodermal
cylinder stained red because of ligni®cation whilst the
passage areas remained relatively unstained.
R E S U LT S A N D D I S C U S S I O N
Oxygen concentration changes along the root
Measurements were made on a total of ®ve roots. The
results presented are from two roots in which the greatest
number of complete radial pro®les were obtained. Data
collated from these pro®les show the changes in cortical and
root surface oxygen concentrations with distance along the
roots (Figs 4 and 5). In terms of root surface concentrations (which re¯ect relative potential for ROL) both follow
the pattern found to be typical of many wetland plant
roots viz. high near the apex and then falling steeply subapically. Using cylindrical Pt-electrodes this pattern was
®rst demonstrated for Eriophorum angustifolium, Molinia
coerulea and Menyanthes trifoliata (Armstrong, 1964) and
was subsequently found in rice (Armstrong, 1971) and
Phragmites (Armstrong, 1992; Afreen-Zobayed, 1996) and
was particularly strongly developed in Schoenus nigricans
(Armstrong, 1967). It has since been con®rmed in rice and
shown to be inducible in some rice lines when roots are
transferred from aerated to de-oxygenated medium (Colmer
et al., 1998).
The sub-apical decline in root surface oxygen as seen in
Fig. 5 is less extreme than that in Fig. 4, and there appears
692
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
20
Oxygen partial pressure (kPa)
18
16
Cortex
14
12
10
8
6
Root surface
4
2
0
0
10
20
30
40
50
60 70
80
90 100
Distance from apex (mm)
F I G . 4. Oxygen partial pressures measured within the cortex and at the
root surface of a 110-mm-long Phragmites root. The root was
immersed in oxygen depleted agar, the shoot was exposed to air. The
diameter of the root was approx. 1 mm, the diameter of the electrode
tip was 18 mm. Note the basipetal increase in cortical oxygen
concentration and the corresponding decline in root surface oxygen
concentration which signi®es a decline in radial oxygen loss.
20
Oxygen partial pressure (kPa)
18
Cortex
16
14
12
10
8
Root surface
6
4
Radial oxygen pro®les
2
0
to be a reversal of the downward trend at 40±60 mm from
the apex before a further decline towards zero further back.
This type of e€ect was noted ®rst in rice using cylindrical
root-sleeving polarographic electrodes (Armstrong, 1971)
where it seemed to be associated with (a) the presence of
developing but non-emergent laterals in the pericycle and
cortex, together with (b) sectors of cortical cells which did
not break down to form aerenchyma, at least in the short
term. Anatomical studies revealed that in rice the exodermal cell walls remained thin walled directly opposite a
developing lateral and it was assumed that they represented
oxygen permeable `windows' in the exodermis. `Window'like areas were also noted in Phragmites (Justin and
Armstrong, 1987; Armstrong, 1992; Votrubova and
Pechackova, 1996) and may be related to the passage cells
in Casparian-banded exodermis reported by others (von
Guttenberg, 1968; Wilson and Robards, 1980; Peterson and
Enstone, 1996). In the next section it will be shown that
these areas do indeed represent areas which retain some
degree of oxygen permeability. Although some window
areas were detected on root 1 (data not shown), none were
in line with the tracks chosen for crossing the root. This
does not necessarily mean that root 1 had fewer developing
unemerged laterals than root 2. Lateral roots tend to arise
in lines down the root and it may have been that the radial
tracks chosen from root 1 were not along one of the lateral
lines. There were no emergent laterals along the 0±100 mm
lengths of root sampled.
The pro®les in Figs 4 and 5 show cortical oxygen
concentration increasing basipetally in a curvilinear manner
and are similar to those predicted for wetland root types
(Luxmoore et al., 1970c; Armstrong and Wright, 1976) but
have never before been detected experimentally. Both
pro®les suggest that the oxygen concentration within the
base of the root was probably signi®cantly lower than
atmospheric and this is not unexpected. The combination of
di€usive resistances and respiratory demands within the
leaf-sheath, shoot base, root-shoot junction and extreme
base of the root would probably be sucient to account for
it. Unfortunately, because of high penetration resistance, it
was not possible to insert electrodes into the root base to
con®rm this supposition. However, the inclusion of some
extra basal resistance into the mathematical model produced just such a pro®le (Fig. 6).
0
20
40
60
80
100
120
140
160
Distance from apex (mm)
F I G . 5. Oxygen partial pressures measured within the cortex (mean
values) and at the root surface of a 160-mm-long Phragmites root. The
root was immersed in oxygen-depleted agar, the shoot was exposed to
air. The diameter of the root was approx. 1.3 mm, the diameter of the
electrode tip was 14 mm (see Fig. 1). Note the basipetal increase in
cortical oxygen concentration and the overall decline in root surface
oxygen concentration which is locally reversed in the zone from 50 to
60 mm from the apex and probably due to the persistence of individual
oxygen permeable windows in the epidermal-hypodermal cylinder at
those sampling points.
Around eight±nine radial pro®les were obtained experimentally for each of the two roots. Within each radial
pro®le the oxygen concentration was sampled at approx.
150±200 points. The examples presented below are from
root 2 (axial pro®le, Fig. 5), and they illustrate the main
features and changes accompanying maturation and
increasing distance from the root apex.
In the ®rst of the pro®les, only 7 mm from the root apex
(Fig. 7) where there was no aerenchyma in the cortex, the
epidermal-hypodermal cylinder is clearly highly permeable
to oxygen; calculation shows [eqn (2)] that oxygen was
leaking away with a ¯ux density of 60 ng cm ÿ2 min ÿ1. The
oxygen de®cit developed across the epidermal-hypodermal
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
22
20
Oxygen partial pressure (kPA)
18
Cortex
16
14
12
10
8
6
Root surface
4
2
0
0
20
40
60
80
100
120
140
160
Distance from root apex (mm)
F I G . 6. Mathematical modelling: predicted basipetal increase in
cortical oxygen concentration and basipetal decline in root surface
for a wetland root having the same morphological and anatomical
dimensions as that in Fig. 5. The di€usivity and respiratory characteristics used in the model are shown in the Appendix.
cylinder amounts to approx. 2 kPa and, if it is assumed that
respiratory activity in the epidermal-hypodermal cylinder is
approx. 300±500 ng cm ÿ3 s ÿ1 (typical values for apical
regions of wetland and non-wetland adventitious roots at
238C), then from eqn (3) this is seen to be consistent with an
e€ective di€usion coecient close to that in water, or even
perhaps a little greater, equivalent to an oxygen permeability of 30 10 ÿ4 cm s ÿ1. Such a high di€usivity might be
possible if bulk ¯ow by cytoplasmic streaming were
supplementing the di€usion of oxygen across the epidermal-hypodermal cylinder (Armstrong, 1979 pp. 239±242).
In keeping with previous predictions and with data obtained
from maize roots, there was no obvious gradient across the
cortex and, in part, we attribute this to radial gas-phase
continuity across the tissue. Within the pericycle and outer
vascular cylinder, where there is no gas-space continuum
and where there should be relatively high respiratory
demand (Armstrong et al., 1991), a relatively steep gradient
can be seen indicative of an oxygen ¯ux from cortex to stele.
The rather ¯atter pro®le across the centre of the stele might
indicate a lower oxygen demand here, but it is complicated
by the electrode having travelled slightly tangentially and
also having traversed two ( possibly immature) late metaxylem vessels. In previous studies on maize (Armstrong et al.,
1994; Gibbs et al., 1998), late metaxylem vessels often
showed apparently slightly higher oxygen pressures which
have not been fully explained. It is interesting, however, that
the largest oxygen debt at this level in the stele is not more
than 2 kPa and hence the root could presumably grow very
much longer than 160 mm before any signi®cant oxygen
stress would arise at this distance from the apex.
693
The second of the radial pro®les (Fig. 8) is at 29 mm
from the root apex, and the most obvious di€erences
between this and the pro®le at 7 mm are (a) the much lower
radial oxygen loss as evidenced by the shallower gradient in
the rhizosphere, and (b) the much steeper gradient within
the epidermal-hypodermal cylinder which can be seen at
both ends of the track. The cortical and stelar pro®les are
much the same as at 7 mm from the apex except for the
shape of the pro®les within the epidermal-hypodermal
cylinder: a shallow convex curve closest to the cortex
grading into a very steep almost linear decline in the outer
half of the cylinder contrasts somewhat with those in Fig. 7
where the gradient on the far side of the root was concave
with the steepest decline being in the inner half of the
epidermal-hypodermal cylinder. Mathematical modelling
predicts a concave gradient if di€usivity and respiratory
oxygen demand are programmed with constant values
across the epidermal-hypodermal cylinder (Fig. 9). The
most likely reason for the convex pro®le in Fig. 8 is that the
di€usive impedance of the cell walls is greater in the outer
half of the epidermal-hypodermal cylinder. This seemed to
be borne out by the anatomical preparations (e.g. Fig. 10
and others not shown). Of the ®ve outermost cell layers of
the root lying beyond the last gas spaces in the cortex, the
innermost three had thinner (less stained) walls than the
outer two. It is across the outermost of the hypodermal cell
layers that the oxygen gradient is steepest, and tentatively,
we would ascribe the site of greatest oxygen impedance at
this position on the root to the tangential walls of this subepidermal cylinder. Calculations [eqn (1)] show an ROL at
this point on the root of 29 ng cm ÿ2 min ÿ1, and if the
respiratory demand for this cell layer were 180 ng cm ÿ3 s ÿ1
(the value used in the model) then, from eqn (2), this would
indicate an overall oxygen di€usion coecient of
5.5 10 ÿ7 cm2 s ÿ1. If this is correct, the oxygen consumption within the cells themselves would account for only
25 % of the de®cit across this layer and radial oxygen loss
would account for the remainder. However, since this
hypodermal layer is obviously more active in laying down
secondary cell wall deposits than the other cells of the
hypodermis it may not be unreasonable to assume that the
respiratory demand of the cells will be higher than the
assumed ®gure. Nevertheless, even if these cells were twice
as active (respiratory demand approx. 360 ng cm ÿ3 s ÿ1),
ROL would still account for only 40 % of the partial
pressure drop across this layer.
At 100 mm from the apex, ROL appears to be zero
(Fig. 11) and the oxygen partial pressure falls steeply from
15 kPa at the inner edge of the epidermal-hypodermal
cylinder to approx. 0.3 kPa at the epidermal surface. As in
Figs 8 and 10, the pro®le is convex and, once more, the
tangential cell wall resistances in the outermost of the
hypodermal cell layers appear to be responsible for the
greater part of the fall. Again, the decline in oxygen partial
pressure is continuous across the epidermal-hypodermal
cylinder indicating that the impedance to ROL has been
caused by the combined e€ect of di€usive impedance and
respiratory demand across the whole epidermal-hypodermal cylinder rather than by impermeable material deposited
on/in the epidermal surface alone. If a surface deposit had
694
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
A
15
14
Rhizosphere
E
+
H
13
Cortex
Stele
Cortex
E
+
H
12
11
Oxygen partial pressure (kPa)
10
9
8
7
6
5
4
3
2
1
0
–400
–200
0
200
400
600
800
1000
1200
Distance (µm)
B
Oxygen partial pressure (kPa)
14
Rhizosphere
Root
12
10
8
6
4
2
0
–1600
–1200
–800
–400
0
200
400
600
800
1000
1200
Distance (µm)
F I G . 7. Phragmites. Radial oxygen pro®le through root 2 at 7 mm from the apex superimposed upon a photomicrograph at this distance showing
the track (brown-stained) taken by the electrode. A, Note the obviously concave gradient within the epidermal-hypodermal cylinder on the right
hand side of the root, the ¯atness of the cortical oxygen pro®le and the relatively small oxygen de®cit developed across the stele. B, Note the
convex gradient in the rhizosphere re¯ecting the relatively high radial oxygen loss at this point. Total root length ˆ 160 mm.
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
695
15
E
+
H
14
Cortex
Stele
E
+
H
Cortex
13
12
Oxygen partial pressure (kPa)
11
10
9
8
7
6
5
4
3
2
1
0
–200
0
200
400
600
800
1000
1200
Distance (µm)
F I G . 8. Phragmites. Radial oxygen pro®le through root 2 at 29 mm from the apex superimposed upon a photomicrograph at this distance
showing the track (brown-stained) taken by the electrode. Note (1) the convex gradient within the epidermal-hypodermal cylinder, the ¯atness of
the cortical oxygen pro®le (outlying points probably due to electrical interference) and the relatively small oxygen de®cit developed across the
stele, and (2) the relatively shallow gradient in the rhizosphere on the left of the root indicative of a lower level of ROL and lower epidermalhypodermal oxygen permeability compared to that at 7 mm from the apex.
been responsible for the decline in ROL, the oxygen pro®le
across the epidermal-hypodermal cylinder would have been
expected to have been less steep and have been followed by
an apparent discontinuity, viz. a vertical fall at the root
rhizosphere boundary (Armstrong and Beckett, 1987). In
the present case, since there is apparently no signi®cant
ROL (and even a hint of in¯ow from the low background
oxygen in the medium) it would seem that oxygen demand
within the epidermal-hypodermal cylinder must now
account for the whole of the oxygen de®cit. As evidenced
by green ¯uorescence, acridine orange dye revealed live
nuclei even in the epidermis.
The oxygen de®cit within the stele, although more than at
7 or 29 mm from the apex is, at 3 kPa, still relatively small.
Since the cell walls of the medulla are obviously thickened
and with presumably more di€usive resistance than when
thin-walled, the ¯atness of the pro®le across the medulla
would seem to indicate a low oxygen demand here.
Oxygen permeable windows in the epidermal-hypodermal
cylinder
The presence of discrete oxygen-permeable windows
in otherwise impermeable sub-apical parts was clearly
696
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
Stele
1.0
Is
Cortex
E+H Rhizosphere
Os
L x = 0 .2 * L
0.8
L x = 0 .4 * L
Cx/Co
L x = 0 .6 * L
L x = 0 .8 * L
0.6
Lx = L
0.4
0.2
0.0
0.00
0.02
0.04
0.06
0.08
0.10
Radial distance from centre of root (cm)
F I G . 9. Radial oxygen pro®le predictions based on modelling using the
input data given in the Appendix. It was assumed that within any tissue
(inner stele, outer stele, cortex, epidermal-hypodermal cylinder and
rhizosphere), the di€usivity and oxygen demand were uniformly
distributed radially. The root was assumed to be 160 mm long
(L ˆ 160 mm) like the experimental root 2 and to have similar internal
anatomy. The pro®les presented are at a number of distances Lx from
the root base ˆ 0.2L, 0.4L, 0.6L, 0.8L and L, respectively.
demonstrated both by radial pro®les from rhizosphere to
root (Fig. 12) and by tracking microelectrodes longitudinally above the root surface (Fig. 13). In Fig. 12, one
track (Track 1: open symbols) represents the more normal
sub-apical condition with the root surface oxygen almost at
zero and negligible ROL; the other (Track 2: closed
symbols), a few mm basipetally from this, shows ¯ux
through one of the `windows' in which some oxygen
permeability is retained. A third track (not shown) and
identical to Track 1 was recorded a further few mm along
the root towards the base. Anatomically, these windows are
quite distinct (Fig. 14) with obviously less thickened cell
walls and cells slightly elongated radially compared to
elsewhere in the epidermal-hypodermal cylinder. However,
whilst they are still oxygen permeable, the cell walls have
clearly developed a relatively high impedance to oxygen
release compared to that at the apex. Whether the peak ¯ux
(and permeability) represented the highest values for the
window was not determined; to do so would have required
moving from the peak value at right angles to the track and
parallel with the curved surface of the root. Unfortunately
this was not attempted.
CO N C L U S I O N S
As far as we are aware, these are the ®rst experimental data
to provide a detailed record of oxygen distribution within
the adventitious roots of a wetland grass; also, the matching
of the pro®les with anatomical features along the electrode
track is unique and enhances their usefulness. It was
unfortunate that the electrode tips were rather larger than
we would have wished. Tip diameters of approx. 5 mm,
which have been used on maize seedling roots, could have
given better resolution, but this size of electrode could not
be induced to penetrate the sub-apical parts of the
Phragmites root. However, the electrode used to obtain
the pro®les presented in Figs 5, 7, 8 and 10±13 had a tip
diameter of 14 mm (Fig. 1), and this was smaller than all but
the smallest of the cells in the epidermal-hypodermal
cylinder, whilst the bevel was only 12 mm long. Furthermore, the electrode was moved in 10 mm steps, and since the
e€ective entry point for oxygen to the electrode is at the
base of the bevel, and since most cells in the epidermalhypodermal cylinder were 420 mm in diameter, and those
in the cortex were at least 40 mm in diameter, we believe that
the data probably have a spatial resolution of 410 mm.
The shape of the cortical oxygen concentration pro®le to
be expected along the length of a wetland root was ®rst
predicted by mathematical modelling (Luxmoore et al.,
1970a,b,c) using porosity, respiratory and oxygen permeability data determined from rice. The pro®les they
obtained using these rice data very much resemble those
shown here for Phragmites, particularly Fig. 5. However,
their predictions concerning the decline in root surface
oxygen concentration with distance from the apex were not
so similar: although there was a decline in root surface
oxygen concentration (and hence) ROL in the immediate
sub-apical zone, the concentrations gradually rose again
towards the base. This was despite the programming of a
basipetal decrease in root wall permeability. In reality the
ROL from rice roots can decline just as extremely as has
been shown for Phragmites in this paper. The reason why
the predictions of Luxmoore et al. failed to match the more
extreme decline in ROL which is most typical of rice and
Phragmites, probably lies in the way in which they modelled
the root, i.e. as a single homogeneous cylinder and with the
root wall as a non-respiring boundary with basipetally
declining permeability, rather than as a cellular cylinder
having both respiratory demand as well as declining
permeability. To reduce ROL sub-apically to the extent
seen in the present study using only a decline in root surface
permeability requires that it be programmed as an
exceptionally large resistance. This was the approach
taken by Armstrong and Wright (1976) and Armstrong
(1979) using an electrical model, and by Armstrong and
Beckett (1987) in initial trials of the multicylindrical
mathematical model adopted in the current study. As can
now be seen, however, if the declining permeability is
programmed into the epidermal-hypodermal cylinder in
which there is also respiratory demand, it is perfectly
possible to obtain the observed decline in ROL without
resorting to the use of abnormally low permeabilities (and
high resistances). In the modelling example shown in Fig. 9,
the permeability coecients programmed for the
epidermal-hypodermal cylinder at the di€erent distances
from the apex (Appendix) were very similar to those used
by Luxmoore et al. in their earlier studies. Modelling of the
root wall permeability can be taken a stage further with the
epidermal-hypodermal cylinder treated as a series of
concentric cylinders representing individual cell walls and
cell contents as separate entities. Oxygen concentration is
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
697
Distance (µm)
15
14
13
0
200
E
+
H
400
Cortex
600
800
Stele
Oxygen partial pressure (kPa)
12
11
10
9
8
7
6
5
4
3
2
1
0
F I G . 10. Phragmites. Root 2 at 29 mm from the apex. Enlarged view of the radial oxygen pro®le through the epidermal-hypodermal cylinder,
cortex and stele superimposed upon a photomicrograph at this distance showing the track (brown-stained) taken by the electrode. Within the
epidermal-hypodermal cylinder the convex shoulder of the pro®le can be seen to correspond with the thinner walled (less stained) cells closest to
the cortex, while the steepest gradient corresponds with the sub-epidermal layer of the hypodermis. Within the stele the steepest decline in oxygen
concentration occurs within the outermost cells encompassing the endodermis, pericycle, protoxylem and phloem, but barely reaching to the
innermost phoem and outermost metaxylem elements.
then predicted to fall in a stepwise manner with the sharpest
falls across the successive cell walls (Armstrong et al., 1994).
In view of the results presented here, it may be necessary
to reappraise the signi®cance of the basipetal decline in ROL
in wetland roots. It had been assumed (Armstrong, 1979)
that the apparent decline in oxygen permeability subapically would be accompanied by declining permeability
to water, nutrient and phytotoxin entry. It was suggested
that declining oxygen permeability accompanied by declining permeability to phytotoxins might be necessary to
protect the plant from phytotoxin entry in the face of a
narrowing of the oxidized rhizosphere as microbial activity
increased due to the availability of oxygen and other
exudates. To some extent the declining permeability to
oxygen must be accompanied by declining permeability to
other materials, and indeed there is evidence that this does
occur although to varying degrees depending upon the ionic
species concerned, e.g. in the genus Carex (Robards et al.,
1979). However, since nutrients and phytotoxins will not
necessarily be consumed within the hypodermal-epidermal
cylinder to the same degree that oxygen is (if at all), then
they may continue to be taken up even though no oxygen is
being released. An interesting consequence of this concerns
the transfer of methane from soil to atmosphere via wetland
vegetation: the sub-apical decline in ROL must result in less
methane oxidation in the rhizosphere but if the root remains
permeable to the methane its escape to the atmosphere will
be enhanced. On the other hand it must be emphasized that
in contrast to earlier work on rice (Armstrong, 1971) the
Phragmites roots used in the current study were neither
grown in waterlogged soil nor exposed to phytotoxins.
Recent studies have shown that the presence of phytotoxins
can induce substantial cell wall ligni®cation in the epidermal-hypodermal cylinder of Phragmites and, in extreme
cases, even in the cortex (Armstrong et al., 1996a;
Armstrong and Armstrong, 1999). Similar e€ects have also
been noted in rice (Armstrong and Armstrong, unpubl. res.)
and it is conceivable that under ®eld conditions the lowering
of radial permeability in sub-apical parts may be much more
extreme than observed in the current study and be sucient
to prevent both oxygen release and nutrient, phytotoxin and
methane entry. A signi®cant degree of permeability sub-
698
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
Oxygen partial pressure (kPa)
Distance (µm)
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
0
200
400
600
800
Cortex
E
+
H
1000
1200
Stele
Rhizosphere
F I G . 11. Phragmites. Root 2 at 100 mm from the apex. Enlarged view of the radial oxygen pro®le through the epidermal-hypodermal cylinder,
cortex and stele superimposed upon a photomicrograph at this distance showing the track (brown-stained) taken by the electrode. Note (1) the
convex gradient within the epidermal-hypodermal cylinder (but much steeper than at 29 mm from the apex), the ¯atness of the cortical oxygen
pro®le and the still relatively small oxygen de®cit developed across the stele, and (2) no clear indication of any ROL giving the impression that the
root has become impermeable to oxygen. Taken together, however, (1) and (2) indicate that it is the respiratory demand of the epidermalhypodermal cylinder coupled to increasing di€usive impedance in the cell walls that is preventing ROL at this position.
Oxygen partial pressure (kPa)
18
16
14
12
E+H
Plots:
Upper = ‘window’ region
Lower = non-window
10
8
Cortex
Rhizosphere
6
4
2
0
600
800
1000
1200
1400
Distance (µm)
F I G . 12. Two microelectrode radial oxygen pro®les 320 mm apart
along root 2 at approx. 100 mm from the apex. The upper pro®le
shows signi®cant radial oxygen loss and indicates the presence of a
`window' of higher oxygen permeability in the epidermal-hypodermal
cylinder (root surface oxygen partial pressure approximately 2 kPa),
the lower pro®le has no measurable radial oxygen loss and a root
surface oxygen partial pressure slightly below background.
apically might then be con®ned only to the passage areas
located opposite developing laterals and to the laterals
themselves. The lateral roots are of course major, if not the
most important, contributors to sediment oxygenation by
means of ROL (Trolldenier, 1988; Armstrong et al., 1990,
1992, 1996b). It was noticeable in the present study that the
inner exodermis was not a signi®cant impedance to radial
oxygen loss. However, this layer can become very heavily
ligni®ed in Phragmites and it is likely that it could then be a
major resistance to oxygen loss and phytotoxin entry.
In recent years much has been published on the
occurrence of passage cells in the exodermis of roots and
their role in water and nutrient uptake (Wilson and
Robards, 1980; Peterson and Enstone, 1996). It would
seem that the oxygen-permeable windows which we have
recorded here may be an extreme form of passage cell area.
It would be interesting to know why they persist in the way
they do. They can be evident at a very early stage when no
aerenchyma (or only very little) has formed in the cortex
(Fig. 14A) but also later, when aerenchyma has formed.
They are then found radially in line with persistent nonaerenchymatous sectors (Fig. 14B). It seems almost certain
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
3
Oxygen partial pressure (kPa)
‘Window’
2
1
0
500
1000
1500
2000
Distance along root surface (µm)
F I G . 13. Microelectrode oxygen pro®le along a Phragmites root (root 2;
L ˆ 160 mm) at approx. 20 mm from the surface, and 70 mm from the
apex, in the vicinity of an oxygen-permeable `window' region in the
epidermal-hypodermal cylinder. The background oxygen partial
pressure in the bathing medium was approx. 1.1 kPa.
that they are linked to activities in the stele, particularly
to the developing laterals in the pericycle. Apart from a
role in transport to and from the root, the persistence of
thin walled areas in the epidermal-hypodermal cylinder
probably facilitates emergence of lateral roots. When the
windows become ligni®ed, e.g. in response to phytotoxins,
non-emerged laterals become trapped within the cortex
(Justin and Armstrong, 1991; Armstrong et al., 1996a;
Armstrong and Armstrong, 1999).
It would be interesting to know what triggers sub-apical
decline in oxygen permeability and there are a number of
observations which might help to answer the question.
Firstly, the start of the decline coincides (or even precedes)
the ®rst appearance of aerenchyma in the cortex. Possibly
linked to this, recent studies by Comer et al. (1998) found
that in rice the sub-apical decline could be induced if
plants were transferred from an aerobic to anaerobic (and
stagnant) rooting medium. Stagnant media usually induce
aerenchyma in species which show plasticity towards
aerenchyma formation. Secondly, some permeability persists where aerenchyma doesn't form. Thirdly, permeability
declines even more strongly in response to phytotoxins,
which is suggestive of a defence response. Fourthly, and
complicating the picture, is the apparent link between
lateral root initiation and the adjacent persistence of both
non-aerenchymatous cortical sectors and in line with these,
the persistent permeability of the epidermal-hypodermal
windows. Could it be that the decline in permeability is a
defence-type reaction to products released as a result of
apoptotic cell death, or is it a normal programmed response
which can be delayed by signals emanating from the
pericycle?
699
Our ®nal observations concern the stelar oxygen concentrations observed. Earlier work on maize seedlings showed
that, with increasing distance from the root apex, the oxygen
de®cit developed across the stele increased and was greatest
near the root base (Armstrong et al., 1994) where it could
be as much as 6 kPa. In the present study no de®cit
was greater than 3 kPa. This may have been partly because
measurements were not possible closer to the root base
where increased resistance to oxygen transport arising from
ligni®cation in the endodermis and xylem parenchyma
might cause larger de®cits. However, in the maize seedlings
not only did the stele occupy a greater proportion of the
total root cross-sectional area, but stelar diameters were
approx. 400 mm compared to 5150 mm in Phragmites. In
terms of avoiding oxygen stress in the stele and stelar
meristem, the smaller the diameter, the less should be the
de®cit. One might expect narrow stelar diameters to confer
a competitive advantage in wetland conditions; it remains
to be seen to what extent this is borne out in practice. On
the other hand it should be noted that the most vital parts
of the stele, the pericycle, phloem and xylem elements lie on
the periphery close to the cortical oxygen supply and, as
such, this will minimize their chances of becoming oxygen
stressed.
These observations concerning stelar aeration may have
some relevance to current theories relating to lysigenous
aerenchyma formation. It has been suggested that oxygen
stress in the stele may be necessary to trigger additional ACC
production in the stele, and subsequently ethylene formation
in the cortex, sucient to induce cortical aerenchyma
formation (Jackson et al., 1985; Atwell et al., 1988; Jackson,
1994; He et al., 1994, 1996a,b). There is some evidence that
to bring this about in maize requires an oxygen partial
pressure of approx. 3 kPa at the root surface and possibly
some anoxia in the stele. In the present study the Phragmites
roots were beginning to form aerenchyma at 29 mm from
the apex, yet neither here nor elsewhere did stelar oxygen
partial pressures fall below 8 kPa. Although it has not been
demonstrated experimentally that ethylene promotes
aerenchyma formation in Phragmites, we have found that
roots grown in a water-saturated atmosphere or in aerated
culture solution show very little tendency to form aerenchyma, whereas in stagnant medium aerenchyma forms
readily. No doubt the Phragmites root produces some
ethylene whether the stele is oxygen-stressed or not, and with
declining gas-permeability in the epidermal-hypodermal
cylinder and a stagnant rooting medium, its radial escape
will be hindered so encouraging accumulation. Such
accumulation may be the trigger for aerenchyma formation.
However, if lysigenous aerenchyma formation is the trigger
for declining permeability in the epidermal-hypodermal
cylinder, it may be that the initial cause of ethylene
accumulation is the stagnant regime in the rooting medium.
If declining permeability in the epidermal-hypodermal
cylinder precedes aerenchyma formation then its ethylene
trapping properties might be an important pre-requisite
for lysigeny. These are important developmental questions
to be answered but, despite the apparent coincidence
of declining epidermal-hypodermal permeability and
700
Armstrong et al.ÐOxygen Distribution and Permeability Barriers to Gas-exchange
F I G . 14. Two examples of `windows' in the epidermal-hypodermal cylinder of Phragmites roots. A, Transverse section at about 30 mm from the
apex of a Phragmites root taken by ¯uorescence microscopy under blue light. The `window' can be seen in line with a developing lateral root. The
®rst signs of aerenchyma formation can be seen in the lower part of the section. B, Transverse section at approx. 110 mm from the apex of root 2,
showing a `window' in line with a sector of non-aerenchymatous cortex. The section has been stained using phoroglucinol and concentrated HCl.
Compared with elsewhere in the epidermal-hypodermal cylinder where the cell walls have been stained red (lignin), the cells in the window have
remained relatively unstained and of a more uniform large size.
aerenchyma formation, it may be that they are not causally
linked or indeed have any commonality of cause.
AC K N OW L E D G E M E N T S
We thank the Royal Society, London, for the award of a
grant to purchase the electrode puller. DWT thanks the
University of Western Australia for ®nancial support.
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APPENDIX
Modelling input data (Fig. A1) used to provide the predictions shown in Figs 6 and 9. The data are based on published
characteristics for aerenchymatous adventitious root types to be found in Luxmoore et al. (1970b) and Armstrong et al.
(1991): D (di€usivity) ˆ overall e€ective oxygen di€usion coecient (cm2 s ÿ1); Q ˆ oxygen demand (ng cm ÿ3 s ÿ1).
Subscripts refer to tissue cylinders as follows: s(i), inner stele; s(o), outer stele; c, cortex, e-h, epidermal ‡ hypodermal cell
layers. Oxygen demand in the rhizosphere was set at a low value to mimic the relatively low sink demand in the experiment.
Diffusivity (10−5 cm2 s−1)
Ds(i)
De-h
Ds(o)
2
1
0
0
5
10
15
0
5
10
15
0
5
10
15
Distance from apex (cm)
2
Diffusivity (10−1 cm2 s−1)
Qc
400
Stele
inner = 100 µm
outer = 135 µm
Cortex = 550 µm
Root = 637 µm
1
300
200
100
0
0
5
10
15
0
Distance from apex (cm)
Oxygen demand (ng cm−3 s−1)
500
Radii
10
15
0
Distance from apex (cm)
Qs(i)
500
5
Oxygen demand (ng cm−3 s−1)
Dc
Qs(o)
Qe-h
400
300
200
100
0
0
5
10
15
0
5
10
Distance from apex (cm)
15
0
5
10
15