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Table 6.1 Parameters for Michaelis-Menten equations
fittedtoNH 4 + absorption data in Figure 6.11
Initial [NH 4 + ]
( µ M )
V max
( µ mol g 1 fresh wt h 1 )
K m
( µ M)
2
12.8
32.2
100
8.2
90.2
1000
3.4
122.1
Source :Wang et al . (1993b). Reproduced by permission of the American
Society of Plant Biologists.
is unused capacity in the root transporters in plants that are not very low in
N. Furthermore there is considerable efflux of absorbed NH 4 + back out of the
roots, implying a futile cycling of N across the root membrane. From studies
of the kinetics of efflux of 13 NH 4 + out of the roots, during a 30 min exposure
to 13 NH 4 + of plants grown in 100 µ MNH 4 + ,20%ofthe 13 N was assimilated,
20% sequestered in the vacuole, 40% retained in the cytoplasm and 20% lost
through efflux (Wang, 1993b). Concentrations of NH 4 + in the cytoplasm were
large—15 to 20 mM—with no sign of toxicity.
These results indicate it may be possible to improve the efficiency of absorption
and assimilation by altering the process of regulation. However the mechanisms
governing regulation are poorly understood. It is not known whether the regula-
tion is linked to the concentration of NH 4 + or NO 3 itself or to the concentrations
of products of N assimilation 'downstream' from NH 4 + or NO 3 ,suchaspar-
ticular amino acids. Nor is it known what the targets of the resulting feedback
mechanisms are.
Effects of Anoxia
The above studies were made in aerated growth media. To simulate anoxic con-
ditions in submerged soil, Kronzucker et al . (1998a) grew plants for 3 weeks in
aerated nutrient solutions and then transferred the plants to solutions bubbled with
N 2 -O 2 mixtures to give O 2 concentrations from 100 to 15% of air-saturation.
They found that the capacity for NH 4 + absorption remained large, even at very
small external O 2 concentrations. In the early stages of exposure to hypoxia,
NH 4 + absorption actually increased, but subsequently it declined reaching a
steady state after a few days (Figure 6.12). Thus as the plants adapted to hypoxia
influx of NH 4 + into the roots was both up-regulated and down-regulated. At
steady state, the maximum influx (V max ) varied with the degree of hypoxia but
the affinity for NH 4 + (K m ) was constant (Table 6.2).
The rate and extent of these changes are consistent with metabolic adaptations
to hypoxia rather than impairment of uptake due the changes in root morphol-
ogy. Thus Kronzucker et al . (1998a) argue that the initial up-regulation of NH 4 +
influx was a response to cytoplasmic acidosis involving decarboxylation of N
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