Agriculture Reference
In-Depth Information
7
6
5
4
3
2
1
0
01234567
Duration of hypoxia (days)
Figure 6.12
Effect of hypoxia on influx of NH
4
+
into rice roots. Seedlings were culti-
vated in nutrient solutions containing 100
µ
MNH
4
, aerated for 3 weeks then at 15% O
2
for indicated times (Kronzucker
et al
., 1998a). Reproduced by permission of the American
Society of Plant Biologists
Table 6.2
Effect of hypoxia on Michaelis-Menten parame-
ters for NH
4
+
absorption by rice. Plants were grown in nutrient
solutions containing 100
µ
MNH
4
, aerated for 21 days and
then exposed to hypoxia for 7 days
O
2
pressure
(% of air-saturated)
V
max
(
µ
mol g
−
1
fresh wt h
−
1
)
K
m
(
µ
M
)
100
5.22
31.8
50
8.21
38.9
35
5.41
32.6
15
4.69
46.9
Source
: Kronzucker
et al
. (1998a).
compounds to neutralize acidity, and the subsequent down-regulation of influx
is as a result of restrictions in ATP supply. Though they did not measure the
changes in root morphology in response to hypoxia, other work shows that
within 7 days aerenchyma formation and other changes would have occurred
(Kronzucker, unpublished). But since influx was at steady state after 4 days at
about 50% of the pre-hypoxia level this had no very dramatic effect on the
capacity for NH
4
+
absorption.
Ammonium versus Nitrate Absorption
Kronzucker
et al
. (1999, 2000) have found that lowland rice (cv IR72) grown
hydroponically is exceptionally efficient in absorbing NO
3
−
, raising the possi-
bility that rice growing in flooded soil may absorb significant amounts of NO
3
−
formed by nitrification of NH
4
+
in the rhizosphere. This is important because
(a) this NO
3
−
is otherwise lost through denitrification in the soil bulk (Reddy
