Agriculture Reference
In-Depth Information
(a)
(c)
30
120
28
100
26
80
24
22
60
Porosity
of cortex
20
40
0.6
0.7
0.8
18
20
16
14
0
0.0
0.2
0.4
0.6
0.8
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
Fraction of r
max
with laterals
O
2
respired and lost (nmol s
−
1
)
(b)
(d)
280
0.6
0.5
240
0.4
200
0.3
160
0.2
120
0.1
80
0.0
0.0
0.2
0.4
0.6
0.8
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
Net root respiration (nmol s
−
1
)
Fraction of r
max
with laterals
Figure 6.6
Effect of cortical porosity of primary root and fraction of root covered with
laterals on (a) maximum primary root length, (b) absorbing root surface per unit root mass,
and (c) absorbing root surface per primary root as a function of net O
2
consumption,
and (d) O
2
consumed in root respiration and loss to the soil. Numbers on curves are
porosities; other parameters have standard values (Kirk, 2003). Reproduced by permission
of Blackwell Publishing
laterals increases from
<
5 to 80%. The net O
2
consumption in root respiration
and loss to the soil decreases as the coverage with laterals increases above about
50%, in spite of the larger surface releasing oxygen.
Figure 6.6(d) shows that root respiration is the main sink for O
2
, accounting
for more than 30 times the O
2
loss to the soil at the minimum coverage with
laterals, though less than five times the loss to the soil at the maximum coverage
with laterals. Respiration in the lateral roots exceeds that in the primary root
by four-fold at the maximum coverage with laterals. These values compare with
ratios of respiration to loss of 13:1 in adventitious roots of
Phragmites australis
and 0.15:1 in laterals estimated by Armstrong
et al
. (1990) with a somewhat
larger
F
O
2
than here.
The results broadly tally with experimental findings for rice. The maximum
length of primary root required to sustain a plant depends on soil conditions and
planting density. Typically the depth to the plough pan in a puddled ricefield
is less than 2 dm, and a typical spacing between plant hills is 25 cm
×
25 cm
