Agriculture Reference
In-Depth Information
plants. In other experiments the kinetics of
13
N efflux out of previously labelled
roots were followed, and from the results the partitioning of the
13
N between
different subcellular compartments was inferred.
Effects of External and Internal Nutrient Concentrations
Wang
et al
. (1993a, b) studied the kinetics and regulation of NH
4
+
absorption by
rice roots using
13
N. In common with other plants and ions, this revealed at least
two transport systems for NH
4
+
influx: one active and operating at low external
concentrations of NH
4
+
(
<
1000
µ
M); the other passive and operating at higher
concentrations and associated with a significant efflux of NH
4
+
into the external
solution. Figure 6.11 shows results for the high affinity concentration range with
plants grown for 3 weeks at three different NH
4
+
concentrations. The data for
the different concentrations fitted Michaelis-Menten-type equations:
V
max
C
La
K
m
+
C
La
V
=
(
6
.
11
)
where
V
max
is the maximum influx in moles per unit root fresh weight per
unit time,
K
m
a constant and
C
La
the concentration in solution at the root sur-
face. Table 6.1 gives the values of
V
max
and
K
m
;
V
max
was four-fold larger and
K
m
six-fold smaller for plants grown in 2
µ
MNH
4
solutions than for those in
1000
µ
M solutions.
It is apparent that the roots have considerable flexibility in their response to
the external N concentration. Influx of NH
4
+
is 'up-regulated' as the plant's
internal N status decreases, but suppressed as the N status increases. Hence there
16
2
µ
M
12
100
µ
M
8
4
1000
µ
M
0
0
200
400
600
800
1000
[NH
4
+
] in external solution (
µ
M)
Figure 6.11
Concentration dependence of steady-state NH
4
+
influx into rice roots grown
at a range of external NH
4
+
concentrations. Prior to the influx measurements, the plants
were grown in solutions at the concentrations indicated on the curves (Wang
et al
., 1993b).
Reproduced by permission of the American Society of Plant Biologists
