Agriculture Reference
In-Depth Information
9
0.83
NO
3
−
=
0.32
assimilation/vacuole
xylem
efflux
8
0.09
NH
4
+
7
=
0.51
0.07
6
0.52
4.19
NO
3
−
=
1.37
5
0.19
2.28
NH
4
+
0.99
=
2.82
4
0.17
1.05
3
2.86
NO
3
−
2
1.12
=
3.18
0.08
2.04
1
NH
4
+
=
1.74
0.11
0
NO
3
−
NH
4
+
NO
3
−
+
NH
4
+
Figure 6.14
Fluxes of NO
3
−
and NH
4
+
within rice roots measured by analysing
13
N
efflux kinetics. Plants were grown on 100
µ
MNO
3
,NH
4
+
or NH
4
NO
3
(i.e. [NO
3
−
]
=
[NH
4
+
]
=
50
µ
M) for 3 weeks. Efflux kinetics were measured following 60min expo-
sure of roots to
13
N-labelled solutions of the same composition (data from Kronzucker
et al
., 1999)
When NO
3
−
and NH
4
+
were provided together in the nutrient solution at the
same total N concentration (100
µ
M, i.e. [NO
3
−
]
=
[NH
4
+
]
=
50
µ
M), NO
3
−
influx, accumulation and metabolism were repressed (Figure 6.14). However,
plasma membrane fluxes of NH
4
+
,NH
4
+
accumulation in the cytosol and NH
4
+
assimilation were larger than with solely NH
4
+
at 100
M, and NH
4
+
efflux
was smaller. Because very little free NH
4
+
is translocated to the shoot, enhanced
translocation of
13
N derived from
13
NH
4
+
in the presence of NO
3
−
indicates
that NH
4
+
assimilation was stimulated by NO
3
−
. As a result, net N acquisition
and translocation to the shoot were much larger than when NO
3
−
or NH
4
+
was
provided alone.
The extent of NO
3
−
absorption by soil-grown plants will depend on its rate of
formation and loss in the rhizosphere (this is considered in Section 6.5). Trans-
porters for amino acids have also been found in plant roots, and concentrations
of amino acids in the soil solution in flooded soils can be appreciable. Therefore
it seems likely that some N is also absorbed as amino acids, but as yet we do
not have the necessary data to quantify this.
µ
Prospects for Improving the Efficiency of Absorption
These results suggest various possibilities for increasing the efficiency of N
absorption and assimilation. The fact that the NH
4
+
and NO
3
−
transport systems
