[74,91,92]. The limitation in NH 4 uptake at low pH can lead to N stress and a decrease in growth when
NH 4 is the only form of N supplied to the plant [92,93]. This problem is further exacerbated by the de-
crease in rhizosphere pH associated with the uptake of NH 4. Compared with NH 4 uptake, the opposite pH
optimum occurs for NO 3 , where more rapid uptake occurs at pH values of around 4–5 and uptake is de-
pressed at higher pH values [9,74]. The reduction in NO 3 uptake at high pH may be related to a compet-
itive effect of OHions on the NO 3 uptake system [74]. Similar to NH 4 , the alkalinity generated from
NO 3 uptake could further restrict NO 3 uptake. Thus, the consequences of absorbing NO 3 or NH 4 can have
rather detrimental effects on the subsequent uptake as a result of differences in the optimum pH for up-
take of the ion absorbed.
In addition to environmental and soil factors, the stage of plant development may influence the rel-
ative proportions of uptake between NO 3 and NH 4. Some evidence suggests that plants absorb NH 4 more
rapidly than NO 3 during early vegetative growth, and the reverse situation occurs and more NO 3 is ab-
sorbed than NH 4 as growth progresses [9,94,95]. Possibly, young plants may lack a completely functional
systems for NO 3 uptake and assimilation [96]. Alternatively, changes in the carbohydrate status of the
root during plant development could alter the N form that is preferentially absorbed [97,98].
- Nitrogen Assimilation
Regardless of the form absorbed, the inorganic N must be assimilated into organic forms, typically amino
acids, to be of use to the plant. Because NH 4 is toxic to plant tissues at relatively low levels, it is rapidly
assimilated in the roots and the N translocated as organic compounds. In contrast, NO 3 can be assimilated
in the root, stored in the vacuoles of root cells, or transported to the shoot, where it can also be stored or
assimilated. Nitrate storage and translocation play important roles in N metabolism inasmuch as NO 3 in
the vacuole can be made available for assimilation when external sources of N are depleted. However,
relatively little is known about factors that regulate the entry and exit of NO 3 in the vacuole.
Whereas NH 4 can be used directly for amino acid synthesis, NO 3 must first be reduced to NH 4. The
reduction of NO 3 to NH 4 is an energy-requiring process occurring by two main partial reactions. The first
step involves a two-electron reduction of NO 3 to NO 2 and is catalyzed by the enzyme nitrate reductase,
while the second step involves a six-electron reduction of NO 2 to NH 4 catalyzed by nitrite reductase. Of
these two enzymes, nitrate reductase is considered to be the rate-limiting step in the assimilation of NO 3
because it initiates the reaction and is the logical point of control when NO 3 is available. Nitrate reduc-
tase is also induced by its substrate NO 3 ; it has a short half-life, and its activity varies diurnally and with
environmental factors that affect the flux of NO 3 to the sites of induction and assimilation [61,99,100].
The reduction of NO 3 by nitrate reductase can occur in either the root or the shoot, and in both cases,
the energy is derived from the oxidation of carbohydrates [61]. The extent to which NO 3 is reduced in
roots and shoots varies widely with plant species and environmental conditions [101,102]. Based on the
contribution of total NO 3 reduction by the roots, plants can be classified into three main groups:
Species in which the root is the major site for reduction
Species exhibiting NO 3 reduction in both the root and the shoot
Species in which the shoot is the primary site for reductionThese three classifications are roughly typified by woody plants, perennial herbs, and fast-growing an-
nuals, respectively [101,102]. Although many studies have indicated a cytosolic location for nitrate re-
ductase [61,100,103], others have suggested that nitrate reductase is associated with chloroplasts, micro-
bodies, or the plasmalemma [100,104–106].
Two main types of nitrate reductase, which differ in the electron donor, have been identified in higher
plants [61,99,100]. One nitrate reductase uses NADH (reduced nicotinamide dinucleotide), while another
nitrate reductase uses NADH or NADPH (reduced nicotinamide dinucleotide phosphate). Essentially, all
higher plants contain the NADH-specific nitrate reductase, and it is the only form of nitrate reductase in
some species [99]. In contrast, other plant species contain both an NADH-specific and an NADPH-
bispecific nitrate reductase [107]. In some plant species, the NADH-specific nitrate reductase is found in
both leaves and root and constitutes the majority of the total nitrate reductase activity, while the NADPH-
bispecific form is found only in the roots [99].
Like NO 3 reduction, the reduction of NO 2 to NH 4 can occur in either the root or the shoot; the cel-
lular location and the electron donor, however, vary depending on the site of reduction. In the shoot, NO 2
reduction occurs in the chloroplast and is coupled to the light reaction of photosynthesis by the use of re-
NITROGEN METABOLISM AND CROP PRODUCTIVITY 389