1021-4437/03/5006- $25.00 © 2003
Russian Journal of Plant Physiology, Vol. 50, No. 6, 2003, pp. 722–732. From Fiziologiya Rastenii, Vol. 50, No. 6, 2003, pp. 808–820.
Original English Text Copyright © 2003 by Darwent, W. Armstrong, J. Armstrong, Beckett.
Although some plants can tolerate long periods of
submergence in anoxic muds cut off from any contact
with atmospheric oxygen  and some may even
achieve some anaerobic growth , oxygen is essential
for their long-term survival. Roots are particularly sen-
sitive to anoxia [3, 4]. Recent publications which deal
with the underlying causes of this anoxia intolerance
and the various adaptations that confer tolerance to
anaerobiosis and to anaerobic habitats are [5–11].
So far as oxygen supply to roots is concerned, many
of the general principles governing their aeration in
ﬂooded and drained soils are now well known [5, 7, 12].
In well-drained soils, the soil atmosphere should
mostly be the major oxygen source and radial inward
diffusive transport the major mechanism; in ﬂooded
soils or in poorly aerated solution culture, axial (longi-
tudinal) diffusive transport from the shoot via the corti-
cal intercellular gas space continuum is necessary for
plants to survive. In the latter case, stelar oxygen
demand will induce a radial inward diffusion from the
cortex and the demands of the epidermal/hypodermal
layers and rhizosphere will induce a radial outward dif-
fusion from the cortex.
The term diffusion is applied to a net movement of
material (in this case oxygen) in a particular direction
from high to low concentration (or partial pressure).
The existence of diffusion is revealed by the gradients
of concentration, and the rate of diffusion is a function
of the steepness of the gradient. However, it is impor-
tant to realize that the directional movements of indi-
vidual molecules are random and their individual
velocities are not a function of the gradient. Oxygen
concentration gradients are created by the “capture” of
oxygen at respiratory sites (sinks) and since the cap-
tured oxygen cannot return to the system, a local drop
in concentration is created. This can only be re-dressed
by oxygen movement from a more remote site. Statisti-
Exploring the Radial and Longitudinal Aeration of Primary
Maize Roots by Means of Clark-Type Oxygen Microelectrodes*
M. J. Darwent*, W. Armstrong*, J. Armstrong*, and P. M. Beckett**
*Department of Biological Science, University of Hull, Hull, HU6 7RX, United Kingdom;
fax: +44 1482-46-5458; e-mail: firstname.lastname@example.org
**Department of Mathematics, University of Hull, Hull, HU6 7RX, United Kingdom
Received April 14, 2003
—Clark-type oxygen microelectrodes were used to measure the radial and longitudinal oxygen distri-
bution in aerenchymatous and nonaerenchymatous primary roots of intact maize seedlings. A radial intake of
oxygen from the rooting medium was restricted by embedding the roots in 1% agar causing aeration to be
largely dependent upon longitudinal internal transport from the shoot. In both root types, oxygen concentrations
declined with distance from the base, and were lower in the stele than in the cortex. Also, the bulk of the oxygen
demand was met internally by transport from the shoots, but a little oxygen was received by radial inward dif-
fusion from the surrounding agar, and in some positions the hypodermal layers received oxygen from both the
agar and the cortex. Near to the base, the oxygen partial pressure difference between the cortex and the center
of the stele could be as much as 6–8 kPa. Nearer to the tip, the differences were smaller but equally signiﬁcant.
In the nonaerenchymatous roots, cortical oxygen partial pressures near the apex were becoming very low
(< 1 kPa) as root lengths approached 100 mm, and towards the center of the stele values reached 0.1 kPa or
lower. However, the data indicated that respiratory activity did not decline until the cortical oxygen pressure
was less than 2 kPa. Mathematical modeling based on Michaelis–Menten kinetics supported this and suggested
that the respiratory decline would be mostly restricted to the stele until cortical oxygen pressures approached
very low values. At a cortical oxygen pressure of 0.75 kPa, it was shown that respiratory activity in the pericycle
and phloem might remain as high as 80–100% of maximum even though in the center of the stele it could be
less than 1% of maximum. Aerenchyma production resulted in increases in oxygen concentration throughout
the roots with cortical partial pressures of ca. 5–6 kPa and stelar values of ca. 3–4 kPa near the tips of 100 mm
long roots. In aerenchymatous roots, there was some evidence of a decline in the oxygen permeability of the
epidermal–hypodermal cylinder close to the apex; a decline in stelar oxygen permeability near the base was
indicated for both root types. There was some evidence that the mesocotyl and coleoptile represented a very
signiﬁcant resistance to oxygen transport to the root.
Key words: Zea mays - root aeration - microelectrodes - modeling - oxygen - aerenchyma - polarography
*This article was submitted by the authors in English.