Agriculture Reference
In-Depth Information
radius
of hill
r
0
zone of
laterals
r
lat
r
1
r
2
r
max
zone of
decreasing
porosity
zone of
root tip
Figure 6.5
Idealized primary root and its cylinder of laterals. The parallel lines indicate
the increasing length density of laterals along the primary root. The branching of the lat-
erals is not represented (Kirk, 2003). Reproduced by permission of Blackwell Publishing
more-permeable wall and no laterals in the region of the tip. Where the later-
als emerge from the primary root, there are generally cracks in the epidermis a
few
ยต
m wide and apparently directly connected to the primary root aerenchyma
(Butterbach-Bahl
et al
., 2000). It seems likely these will be important in gas
transfer, though there are no direct measurements showing this. In practice leak-
age of O
2
from the cracks and axial gradients of O
2
within laterals will lead to
gradients of O
2
release along laterals. However, for the intended purpose of the
model an elaborate treatment of these effects is not necessary; it is sufficient that
the loss of O
2
increases with the density of laterals and a constant leakage along
the length of laterals is assumed.
Figure 6.5 defines for the purposes of the model the distances at which the
porosity and coverage with laterals change. It is assumed that, because of the
changes in wall permeability along the root, nutrients are only absorbed by the
primary root in the zones beyond the laterals
(r
lat
<r<r
max
)
and by the laterals.
This is also the surface across which O
2
leaks.
