Although the process of NH 4 uptake is not completely understood, it is clear that passive and active
uptake must occur by different mechanisms. For passive uptake, the positively charged NH 4 ion may be
absorbed by a uniport following the electrochemical gradient across the plasmalemma [52,53]. Con-
versely, since membrane permeability to NH 3 is greater than that of NH 4 , passive uptake could also oc-
cur by nonspecific diffusion of NH 3 gas [54,55]. In the soil solution, the distribution of NH 4 and NH 3 is
a function of an equilibrium relationship driven by pH. At neutral (or lower) soil pH values, more than
99% of the total ammoniacal N is in the protonated (NH 4 ) form, which would result in limited absorption
of gaseous NH 3 by the root [54]. While aboveground plant parts can also absorb gaseous NH 3 through
stomata, the amounts acquired are limited in unpolluted air [56,57]. In addition, because high concentra-
tions of NH 3 are toxic to plant growth, especially roots [58], it seems unlikely that passive NH 3 absorp-
tion is a major source of N for plant growth. Therefore, the bulk of ammoniacal N absorbed by plants is
likely the result of active uptake of NH 4. The mechanism of active NH 4 uptake, which has not been clearly
established, appears to be carrier regulated, as indicated by saturation kinetics and the depression of up-
take by factors that limit energy metabolism [39,52].
- Factors Affecting Nitrogen Uptake
The uptake of NO 3 and NH 4 can be affected by internal factors, such as N and carbohydrate status, and
by external factors, such as temperature, O 2 level, and rhizosphere pH. Plant species and stage of plant
development can also influence N uptake. When the uptake of a specific N form is affected differentially
by these factors, contrasting patterns of N uptake and growth can result, depending on the form of N avail-
able to the plant.
Although NH 4 uptake does not appear to be affected by the presence of NO 3 [59,60], there are many
reports of NH 4 -induced inhibition of NO 3 uptake [40,43,61–63]. However, there are also cases in which
NH 4 appeared to have little or no effect on NO 3 uptake [64,65] or even resulted in a stimulation in uptake
[66]. Although the precise manner by which NH 4 inhibits NO 3 uptake is not clear, possibilities include
(1) a decrease in NO 3 reduction, resulting in feedback inhibition of NO 3 uptake, (2) an alteration in the
rate of activation or synthesis of the NO 3 uptake system, thereby restricting influx, and/or (3) an acceler-
ation in NO 3 efflux. For a description of NO 3 reduction and nitrate reductase, see Section III.B.3. Vari-
ous lines of evidence support each of these mechanisms.
Although some researchers have shown a decrease in the level of extractable nitrate reductase by
NH 4 treatment [67–70], others have shown that NH 4 or products of NH 4 assimilation do not interfere with
nitrate reductase [71–73]. In addition, the ability of NH 4 to inhibit NO 3 uptake in plants without detectable
nitrate reductase activity [74] and the lack of proportional changes in activity and NO 3 uptake in response
to NH 4 [63,75,76] further indicate that a change in nitrate reductase is not the main mechanism responsi-
ble for NH 4 -induced inhibition in NO 3 uptake.
Alternatively, NH 4 or one of its assimilation products may interact with NO 3 transporters at either
the external or internal surfaces of the plasmalemma and inhibit the activation or synthesis of the NO 3 ab-
sorption system [43,53,62]. One possibility is that NH 4 or the high acidity adjacent to the plasmalemma
resulting from NH 4 uptake in excess of NO 3 uptake causes an alteration in membrane permeability,
thereby restricting the capacity for NO 3 absorption [77]. Another possibility is that NH 4 may inhibit net
NO 3 uptake by increasing NO 3 efflux [78]; yet others suggest that NO 3 influx, not efflux, is inhibited by
NH 4 [79–81]. Although additional research is needed to elucidate the exact mechanism(s) involved in
NH 4 -induced inhibition of NO 3 uptake, the identification of genotypic variation for the extent of this in-
hibition [82–84] indicates that the process is under genetic control.
Many studies have shown that NO 3 uptake is more sensitive to low temperatures than is the uptake
of NH 4 [85–90]. For example, at temperatures below 9°C, perennial ryegrass plants absorbed more than
85% of their total N as NH 4 , while the proportion decreased to only 60% absorption as NH 4 at tempera-
tures of 17°C or above [86]. Although the reason for the preferential uptake of NH 4 over NO 3 at low tem-
peratures is unclear, physical changes in the membrane may be responsible, rather than differences in tem-
perature sensitivity of the two transport systems [85]. Alternatively, because temperature has a strong
influence on the rate of nitrification, it is reasonable to assume that the largest amounts of NH 4 will occur
in cool soils. Thus, the greater uptake of NH 4 at low soil temperatures may be partly the result of more
NH 4 -induced inhibition of NO 3 uptake.
Another important difference between NO 3 and NH 4 uptake lies in the sensitivity to pH of these two
N forms. The maximal uptake of NH 4 occurs at neutral pH values, and uptake is depressed as the pH falls
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