duced ferredoxin as the electron donor [61]. Nitrite reduction in the root occurs in a plastid, which is anal-
ogous to a chloroplast of the leaf, but the reaction differs from shoot reduction in the following respects:
(1) the nitrite reductase of the root is similar but not identical to the leaf enzyme, (2) the electron donor is
a ferredoxin-like protein that is not identical to the leaf protein, and (3) the root ferredoxin is reduced by
NADPH and a corresponding enzyme, with the energy supplied from the oxidation of carbohydrates
[108].
The NH 4 that results from both NO 3 assimilation and the NH 4 absorbed directly by the roots is as-
similated by the glutamate synthase cycle, which involves two reactions operating in succession and cat-
alyzed by the enzymes glutamine synthetase and glutamate synthase [109]. A characteristic of this path-
way is the cyclic manner in which the amino acid glutamate acts as both acceptor and product of ammonia
assimilation. In this cycle, NH 4 is incorporated into glutamine by glutamine synthetase, which attaches
NH 3 to the carboxy group of glutamate, using energy supplied by ATP. In leaf cells, this reaction occurs
in chloroplasts, and in roots it most likely occurs in plastids [110]. In the chloroplast, the light-trapping
system provides the energy to regenerate ATP, while in root cells, other enzyme systems oxidize carbo-
hydrates to provide the energy for ATP regeneration.
Another isoform of glutamine synthetase is found in the cytoplasm of both leaf and root cells and is
not identical to the plastid enzyme [111,112]. The cytoplasmic enzyme can assimilate any free NH 3 or
NH 4 regardless of its origin (from either deamination of amino acids or absorption from the soil). Thus,
in addition to producing the key intermediate, glutamine, the glutamine synthetase reaction is a detoxifi-
cation process that avoids injury from the accumulation of NH 4 or NH 3.
Following the formation of glutamine, the amino group (MNH 2 ) is transferred to -oxo glutarate via
glutamate synthase to form two molecules of glutamate. This reaction can occur in shoots or roots, and in
both cases the enzyme is located in plastids [109,111,112]. There are three isoforms of glutamate synthase
in plant cells, which utilize different electron donors [108]. In leaf chloroplasts, the electron donor is re-
duced ferredoxin derived directly from the trapping of light energy. Conversely, the electron donor in root
cells is NADH or NADPH, where the energy to reduce the oxidized form of the pyridine-linked nu-
cleotides is derived from oxidation of carbohydrates [109,111,112].
A further series of related reactions mediated by specific transaminases transfers the amino group
from glutamate to the 2-oxy group (BO) of a 2-oxoacid. Biochemical modification of glutamate, glu-
tamine, and the array of amino acids produced by the transaminase reactions generates the 20 amino acids
required for protein synthesis. These amino acids can also be anabolized into a variety of complex ni-
trogenous compounds (e.g., chlorophyll, growth regulators, alkaloids, nucleic acids) that are involved in
plant growth and metabolism.
C. Timing of Nitrogen Accumulation
Like dry matter production, the seasonal accumulation of N by crop plants can be divided into three main
phases:
An initially slow accumulation due to limited crop biomass
A period of rapid, nearly linear accumulation that coincides with the onset of rapid plant growth
A cessation of N accumulation with advancing maturityExamination on a daily rate basis generally reveals two periods of rapid N accumulation, corresponding
to late vegetative growth and the onset of linear seed fill [113,114]. Although the maximum accumula-
tion rate usually occurs during linear vegetative growth, it can be delayed by a delay in the availability of
N [115]. The period of maximum N accumulation can also be affected by such other factors as planting
date, irrigation, and climate [114,115].
Under most conditions, the majority of plant N accumulated by cereal plants is acquired during veg-
etative growth. Numerous reports in the literature for maize and wheat show cases of 75% or more of the
total plant N accumulation having occured by anthesis [116–120]. However, there is some indication that
continued accumulation of N during grain fill can be a beneficial trait, especially for high-yielding geno-
types in good growing environments [121,122]. For example, the hybrid FS854, which holds the world
record yield for maize, 23.2 Mg ha^1 [123], has been shown to accumulate a substantial proportion of its
N during grain fill [119,124,125]. The proportion of plant N accumulated after anthesis, however, is
highly influenced by growing season [117], soil N level [126], and cultivar [120,126,127].
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