Agriculture Reference
In-Depth Information
are down-regulated for the most part indicates that there is no need to incor-
porate additional transporters. Rather, efforts should focus on manipulating the
regulation of influx or decreasing efflux or both. Depending on the specific N
compounds that trigger down-regulation, and on their subcellular location, it
might be possible to dampen down-regulation by directing N to different bio-
chemical pathways or subcellular compartments or both, and, as necessary, to
store the additional N taken up in cell vacuoles. The rate of assimilation of
NH
4
+
, which must to a large extent occur in the root, may be limited by the
supply of carbon skeletons. The extent to which carbon skeletons are limiting
will depend on the C:N ratio of the amino acid or amide used to transport N
to the shoot, and this may be manipulable. In addition any process that led to a
lowering of cytoplasmic NH
4
+
concentrations would presumably lower efflux.
6.4 ROOT-INDUCED CHANGES IN THE SOIL
The following root-induced changes in the soil occur as an inevitable consequence
of the nature of submerged soils and plant adaptations to them.
(1) Organic compounds are released from the root into the soil. As for dryland
plants, this may account for up to 10-15% of the photosynthate, depending
on the plant's nutritional status and other factors. It includes microbial sub-
strate, as exudate or sloughed-off material; extracellular enzymes, actively or
passively released; and complexing agents, also actively or passively released.
(2) Oxygen diffusing down through the root's internal aerenchyma leaks out into
the soil, which has a lower O
2
concentration.
(3) Mobile inorganic reductants in the soil are oxidized, particularly Fe
2
+
which
is precipitated as Fe(OH)
3
on or near the root. As a result the concentration
of Fe
2
+
near the root falls and more Fe
2
+
diffuses in from the bulk soil. This
is then oxidized resulting in a zone of Fe(OH)
3
accumulation near the root.
(4) The oxidation of inorganic reductants generates H
+
:
4Fe
2
+
+
O
2
+
10H
2
O
=
4Fe
(
OH
)
3
+
8H
+
so the pH in the oxidation zone tends to fall.
(5) Because the main form of plant-available N in anaerobic soil is NH
4
+
,the
root absorbs an excess of cations
(
NH
4
+
,
K
+
,
Na
+
,
Ca
2
+
,
Mg
2
+
)
over anions
(
H
2
PO
4
−
,
Cl
−
,
SO
4
2
−
)
. Consequently H
+
is released by the root to maintain
electrical neutrality, tending to further decrease the soil pH. Note that if any
N is taken up as NO
3
−
as a result of nitrification of NH
4
+
in the rhizosphere,
the net acid-base change is the same because, although the root exports 2mol
less H
+
for each mol of NO
3
−
replacing a mol of NH
4
+
,2molofH
+
are
formed in the nitrification of each mol of NH
4
+
. Note also that Si, which is
taken up in large quantities by rice plants, crosses the root as the uncharged
H
4
SiO
4
molecule (p
K
1
=
9
.
46 at 25
◦
C).
