high-affinity NO 3 transport system where NO 3 acts as a signal for these events [67]. The uptake system
is inducible with NO 3 and can be blocked with inhibitors of RNA and protein synthesis [17] as well as
amino acid–modifying reagents [68]. This suggests that plasma membrane proteins are involved in the
transport process.
According to Redinbaugh and Campbell [16], when plant roots sense exogenous NO 3 , the primary
response involves the transcription of genes encoding NO 3 transport proteins followed by the synthesis
of these proteins. A model proposed by these investigators suggests that a constitutive NO 3 sensor pro-
tein system is the first component that detects and senses NO 3 in the environment. Binding of the NO 3
with sensor induces certain regulatory proteins, which in turn initiate the transcription of primary re-
sponse genes by RNA polymerase II. The resulting transcripts are further translated into proteins such as
NO 3 transporters, NO 3 translocators, and NO 3 assimilatory enzymes. Biochemical events leading to
secondary responses such as root proliferation enhanced respiration in response to environmental NO 3
are not yet clearly understood [16]. Because environmental NO 3 is the only inducer for the synthesis of
NR, NIR, and NO 3 transport proteins, it appears that NO 3 induces these processes at the plasma mem-
brane level before entering the cell [17]. However, in plants, a specific receptor for NO 3 has not yet been
identified.
Certain investigators suggest that the prime enzyme of NO 3 reduction, nitrate reductase, plays a sig-
nificant role in the uptake of nitrate [69]. A plasma membrane–bound NR has been detected in the roots
of barley (Hordeum vulgareL.) seedlings and it is observed that NO 3 transport is inhibited by anti-NR
immunoglobulin G (IgG) fragments [70]. Uptake of NO 3 by plant roots is also dependent on the NO 3 re-
duction process or reduced NO 3 products in the shoot [71]. At a high intracellular concentration of NO 3
or in the presence of NH 4 in the growth medium, NO 3 uptake tends to decline. Production of malate in
the shoot also influences NO 3 uptake. In order to neutralize the alkaline conditions due to NO 3 reduc-
tion in shoots, malate is produced. This is further transported to roots with Kas a counter ion, and in
turn, as a result of oxidation, bicarbonate ions are formed in roots. These are exchanged for NO 3 in the
external environment.
Nitrate uptake and its reduction activities in the plant tissues are coordinately regulated. In plant tis-
sues, NO 3 induces increase in NR activity. The activity of NR increases in root cells in response to ex-
ogenous NO 3 . Besides the NO 3 reduction process, which is the primary response to NO 3 uptake, plants
have systems for translocation of NO 3 within and between the cells [69]. After uptake, NO 3 may be
translocated to the vacuole in the cells, where it can be accumulated and serve as an NO 3 reserve [72].
The intracellular NO 3 translocation process possibly requires a tonoplast NO 3 translocator that is differ-
ent from the membrane NO 3 transporter [16]. The distinct NO 3 translocators present at the symplasm-
xylem interface control the translocation of NO 3 from root to xylem and then to different organs of the
plant [16]. Although the translocator proteins appear to be different from transport proteins and are en-
coded by different genes, all three processes—NO 3 transport, translocation, and reduction—are coordi-
nately regulated.
Although both forms of N (NO 3 and NH 4 ) are taken up by plants, only NH 4 is incorporated into or-
ganic molecules in the plant tissues by an enzymatic process. The primary step in the reduction process
involves the reduction of NO 3 to NO 2 catalyzed by the enzyme NR. Ammonium, either directly absorbed
by plant roots or as a result of NO 3 reduction, is further assimilated and incorporated into the amide
amino group of glutamine by the action of glutamine synthetase and subsequently into glutamic acid by
glutamate synthase. These two enzymes are responsible for the assimilation of most of the NH 4 derived
from NO 3 reduction under normal growth conditions. An alternative route of NH 4 assimilation into glu-
tamate involves the reductive amination of -ketoglutarate catalyzed by a mitochondrial enzyme gluta-
mate dehydrogenase. Other amino acids, such as alanine and aspartic acid, are further synthesized from
glutamic acid by transamination reactions.
III. NITROGEN ABSORPTION AND ASSIMILATION UNDER
DIFFERENT STRESSESCrops growing in adverse environmental conditions of salinity, drought, high or low temperature, low
light, and heavy metal–containing soils suffer severe losses in yield [3,5,6,8,12,73–76]. Harsh environ-
mental conditions interfere with normal growth, metabolism [15], and protein synthesis of plants [77],
NITROGEN ABSORPTION UNDER STRESS 639