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Physiological Mechanisms of Nitrogen Absorption
and Assimilation in Plants Under Stressful
Conditions
R. S. Dubey
Banaras Hindu University, Varanasi, India
Mohammed Pessarakli
The University of Arizona, Tucson, Arizona
637
I. INTRODUCTION
Nitrogen is one of the most essential elements for plant growth and development. It is a constituent of
many biomolecules such as proteins, nucleic acids, amino acids, coenzymes, vitamins, and pigments. Be-
cause of its high requirement by plants and its complete absence in the bedrock, it has a special place in
plant nutrition. Nitrogen supply in the soil is often the most important factor limiting plant growth and
yield. In the soil, N availability is due to the application of N fertilizers, biological action of N 2 fixing or-
ganisms, or natural fertilization. With the advent of modern agricultural practices, inorganic N fertilizers
have become the major input to the soil. In our quest to achieve sustainable food production, to meet the
increasing food requirements for global population, excessive uses of various forms of N fertilizers are
still likely in the near future.
Ammonium (NH 4 ) and nitrate (NO 3 ) are available forms of N that can be absorbed by plants [1,2].
However, NO 3 is the predominant form of N available to the most cultivated plants grown under normal
field conditions. Availability of nitrogenous nutrients, especially NO 3 , is considered rate limiting for
plant growth and crop production. Application of NO 3 in the soil medium induces NO 3 uptake and its
assimilation to ammonium by assimilatory enzymes.
Plants are often exposed to various kinds of harsh environmental conditions that adversely affect
their growth and metabolism. Adverse environmental conditions, such as soil salinity, drought, heat, cold,
and excessive heavy metal content in the soil, create considerable stress in growing plants [3] and severely
affect N absorption by the roots and its assimilation in the plant [1,2,4–15]. In order to adapt to changing
environments, higher plants show well-defined metabolic alterations in response to nutrient availability
in the environment [16]. For instance, NO 3 in the soil induces the system of its uptake, assimilation, trans-
port, etc. [16]. The biochemical events leading to the uptake of NO 3 by plants are not well defined [17].
However, the process of NO 3 reduction involving the enzymes nitrate reductase (NR) and nitrite (NO 2 )
reductase (NIR) has been extensively studied in the diverse plant species, and these enzymes have been
well characterized regarding their physicochemical properties and their subcellular localizations [18,19].
Environmental stresses influence N nutrition in plants by inhibiting N uptake [1–6,12–15,20–36] as well
as its assimilation [18,27,31,33–35,38–62].