Agriculture Reference
In-Depth Information
et al
., 1989), and (b) plant growth and yield are generally improved when plants
absorb their nitrogen as a mixture of NO
3
−
and NH
4
+
compared with either on
its own (Layzell and Turpin, 1990; Taiz and Zeiger, 2002).
Previous field research has shown the large potential for losses of NO
3
−
but not the potential advantages of NO
3
−
nutrition. Because NO
3
−
is rapidly
reduced in the plant, and there are no simple methods for measuring NO
3
−
fluxes into the plant, it is difficult to quantify the extent of NO
3
−
absorption
under field conditions.
Three lines of evidence from Kronzucker
et al
., suggest unusually efficient
NO
3
−
absorption. First, steady-state influx of NO
3
−
and NH
4
+
followed
Michaelis-Menten kinetics over the relevant concentration range (Figure 6.13a),
and
V
max
for NO
3
−
was some 40% larger than that for NH
4
+
and
K
m
50%
smaller. Second, induction of the root NO
3
−
transporters following its re-supply
to plants deprived of NO
3
−
for 24 h was exceptionally rapid, peaking within
2 h (Figure 6.13a). For comparison, in white spruce, which is not well adapted
to using NO
3
−
, full induction takes several days, and in barley, which is
considered one of the most efficient NO
3
−
users, full induction takes up to
24 h (references in Kronzucker
et al
., 2000). Third, subcellular pool sizes and
fluxes, estimated from the kinetics of
13
N efflux out of labelled roots indicated
highly efficient NO
3
−
use: while similar proportions of incoming NH
4
+
and
NO
3
−
were channelled into assimilation and to the vacuole, the proportion of
NO
3
−
translocated to the shoot was larger and that lost through efflux out of the
roots smaller (Figure 6.14).
(a)
(b)
9
NO
3
−
8
7
4 wk
6
5
4
NH
4
+
3
3 wk
2
1
0
0
100
200
300
400
500
0
5
10
15
20
25
[N] in external solution (
µ
M)
Time after exposure to NO
3
−
(h)
Figure 6.13
Influx of N into roots of intact rice plants grown on 100
µ
M N as either
NO
3
−
or NH
4
+
: (a) concentration dependence of NO
3
−
and NH
4
+
influx in 4-week-old
plants; (b) induction of NO
3
−
uptake in 3- or 4-week-old plants deprived of N for 24 h
before re-supply at 100
µ
M for the indicated periods (Kronzucker
et al
., 2000). Repro-
duced by permission of Blackwell Publishing
