Agriculture Reference
In-Depth Information
et al
., 1991b, 2000). In the following section I describe the model developed by
Armstrong and Beckett (1987). This accounts for the most important processes
within the root and has been corroborated for various wetland species using
measurements of O
2
gradients within roots with microelectrodes.
6.1.2 ARMSTRONG AND BECKETT'S MODEL OF ROOT AERATION
To summarize, the main factors influencing the O
2
budget of a non-throughflow
root in anoxic soil are as follows.
(1) The extent of aerenchyma development by the degradation of the primary
root cortex.
(2) Rates of respiration in different root tissues. The formation of aerenchyma
decreases the respiratory O
2
demand per unit root volume because there is
less respiring root tissue. Also, some plants can tolerate a degree of anoxia
in parts of the root, which substantially reduces the O
2
demand per unit
root volume.
(3) The permeability of the root wall to gases. Sub-apical parts of the root can
have permeabilities several orders of magnitude smaller than those in the
region of the tip.
(4) The proportion of fine lateral roots branching off the primary root. Having
high surface area to volume ratios, laterals tend to be O
2
-leaky.
For simplicity, the effects of lateral roots are not dealt with explicitly in Arm-
strong and Beckett's model, but they are dealt with in Section 6.2.
In the model, the internal structure of the root is described as three concentric
cylinders corresponding to the central stele, the cortex and the wall layers. Diffu-
sivities and respiration rates differ in the different tissues. The model allows for
the axial diffusion of O
2
through the cortical gas spaces, radial diffusion into the
root tissues, and simultaneous consumption in respiration and loss to the soil. A
steady state is assumed, in which the flux of O
2
across the root base equals the net
consumption in root respiration and loss to the soil. This is realistic because root
elongation is in general slow compared with gas transport. The basic equation is
D
G
θ
G
f
G
d[O
2
]
G
d
z
rD
L
d[O
2
]
L
d
r
d
d
z
1
r
d
d
r
+
−
R
root
−
R
soil
=
0
(
6
.
1
)
where the first term represents axial diffusion through the cortical gas spaces, the
second term radial diffusion through root tissues, and the third and fourth terms
the rates of O
2
consumption in tissue respiration and loss to the soil, respectively.
Here
D
G
and
D
L
are the diffusion coefficients of O
2
in air and water, respectively,
θ
G
is the gas content of root by volume,
f
G
is the impedance factor for diffusion
in the cortical gas spaces,
r
is the radial distance,
z
is the axial distance and [O
2
]
G
