crop yields. The processes of NO 3 uptake, its translocation inside and between the cells, and its reduc-
tion are coordinately regulated. The pronounced effects of most of the stresses include decreased NO 3
uptake and inhibition of the activity of the key enzyme of nitrate assimilation, NR. The NR is NO 3 in-
ducible and its activity is subject to regulation by a variety of environmental parameters that are readily
influenced under stresses. Levels of NO 3 and NR inside the tissues are directly related to plant health and
yield. Genotypes of plants differing in stress tolerance show different behaviors of NR and other N as-
similatory enzymes. For instance, a salt-tolerant variety shows increased NO 3 uptake and a high level of
NR that is further stimulated by salinization, whereas a salt-sensitive variety shows decreased NO 3 up-
take and decreased activity of NR with salinization. This is suggestive of stress tolerance and sensitivity
as complex phenomena, depending on the genetic and biochemical makeup of the species.
In spite of the extensive studies that have been performed, our knowledge of the biochemical mech-
anisms underlying the uptake of NO 3 by plants, the process of its assimilation, and the regulation of en-
zymes of NO 3 assimilation is still incomplete. Little information is available regarding molecular events
of NO 3 uptake, the NO 3 sensor protein system, signal transduction of environmental NO 3 , NO 3 induc-
tion regulatory proteins, primary responsive genes that are transcribed and translated as a result of NO 3
induction, etc. Besides this, the nature of the NO 3 transporters, NO 3 translocaters, events involving over-
all induction of NR by NO 3 and regulation of NR and other enzymes of NO 3 and NH 4 assimilation un-
der various environmental stress conditions such as light, water, temperature, and salinity stresses needs
to be examined in detail.
Although all the adverse environmental conditions discussed so far reduce NO 3 uptake and inhibit
NR activity, the precise biochemical mechanisms involved in these events remain to be investigated.
More extensive investigations are required to unveil the role of light and other factors in regulation of the
NR level in plants. Environmental stresses adversely affect the behavior of enzymes of NO 3 and NH 4
assimilation. In certain cases, such as salinity and water stresses, suppression of the GS/GOGAT pathway
and a sustained level of induction of the GDH pathway of ammonium assimilation are observed. Uptake
and metabolism of N are triggered in stressed environments so that specific soluble nitrogenous com-
pounds accumulate and provide adaptive value to the plants. Further information is required regarding the
molecular structures and catalytic properties of enzymes involved in NO 3 and NH 4 assimilation. The
precise effects of various environmental conditions in vivo as well as in vitro on behaviors of these en-
zymes, the sequences of events leading to accumulation of nitrogenous compounds, and the functional
roles of these compounds in stressed plants need to be studied in greater detail.
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NITROGEN ABSORPTION UNDER STRESS 651