This review chapter focuses on various N sources and their mode of absorption and assimilation by
plants and also presents information on the effects of different conditions on the N uptake and metabolism
processes.
II. NITROGEN SOURCES, THEIR ABSORPTION AND ASSIMILATION
A. Sources of Nitrogen
Different forms of N that are absorbed by plants from the soil are NO 3 , NH 4 , and organic compounds
such as amino acids and urea. The two major forms of soil N are NO 3 and NH 4 . Of these two forms, NO 3
is the more abundant and under normal conditions most of the N absorbed by plant roots from the soil is
in the form of NO 3 , which is further reduced to NH 4 in the plant tissue. Frota and Tucker [30], Saad [31],
and Pessarakli et al. [45] found that beans (Phaseolus vulgarisL.), C 3 plants, under either normal or stress
conditions absorbed more NO 3 than NH 4 . Sometimes, in the soil, the NH 4 form of N is abundant be-
cause of biological N 2 fixation by symbiotic association of N 2 -fixing organisms, free-living soil bacteria,
and blue-green algae. Although N fertilizers in ammoniacal forms are widely used in agricultural fields,
ammonium in the soil is readily oxidized to NO 3 by nitrifying bacteria present in the soil. Certain plant
species such as those inhabiting acidic soils and all C 4 plants (i.e., grasses) show a preference for the NH 4
form of N. Because of the deficiency of NO 3 in acid soils and the specific physiological, metabolic, and
photosynthetic pathways of C 4 plants, such plants prefer NH 4 over NO 3 . Plants that have low intrinsic
NO 3 reductase activity also absorb NH 4 in preference to NO 3 .
Because ammonia is toxic to the plants and interferes with various metabolic processes inside the
cell, after absorption NH 4 ions are rapidly assimilated into amino acids, amides, etc. in the roots. Plant
species that have an efficient ammonia-detoxifying system grow well on the NH 4 form of N [63]. These
plants can detoxify NH 3 by forming NH 4 salts of organic acids. The majority of the plant species grow
better when N is supplied as a mixture of both NO 3 and NH 4 forms in the soil [1]. Botella et al. [1] stud-
ied the uptake of NO 3 and NH 4 by wheat plants grown in nutrient solutions containing NO 3 , NH 4 or
NO 3 NH 4 , with 1 mM (control) and 60 mM (saline) NaCl each. These investigators found that under
saline conditions, the addition of both nitrogen forms was beneficial because higher nitrogen uptake rates
resulted in better growth and development of the plant. Botella et al. [2] in another N uptake study, using
NO 3 , NH 4 , and NO 3 NH 4 , reported that the best N source for wheat growth was a mixture of NO 3
and NH 4 , especially under saline conditions. Certain plants can absorb either NH 4 or NO 3 ions depend-
ing on the pH of the nutrient medium.
It has been suggested by certain groups of investigators that nutrient media containing both NO 3 and
NH 4 forms of N in a proper combination are more suitable for the growth of the cells as well as the plants
compared with the either form alone [1,2,64]. In many vegetable crops, NH 4 is taken up in preference to
NO 3 when its concentration is above 10% of the total N in the nutrient solution [65]. Especially at a low
root temperature, NH 4 is regarded as a safe source of N [65]. Genotypic diversity occurs in plants for N
use efficiency. Kafkafi [65], when evaluating 12 genotypes of pearl millet (Pennisetum glaucum) plants,
observed that genotypes varied greatly for uptake, translocation, and nitrogen use efficiency (NUE).
Genotypes with lower NUE and translocation indices for N showed lower grain yields.
In plant roots, the initial product of NH 4 assimilation is glutamine whether NH 4 or NO 3 is absorbed
by the roots. Other products of assimilation are asparagine, citrulline, amino acid, allantoin, and certain
other soluble nitrogenous compounds [66]. The assimilation products are then translocated to various or-
gans of the plants through xylem and phloem vessels. In some plants (i.e., tomatoes), NO 3 after absorp-
tion through the roots is reduced to NH 4 in the root itself, whereas in others (i.e., grasses), it may be trans-
ported as NO 3 to different organs.
B. Absorption and Assimilation of Nitrogen
Nitrate is the predominant form of N available in the soil, regardless of the NO 3 or NH 4 forms of fertil-
izers used. Availability of NO 3 in the soil is considered rate limiting for plant growth [16]. Systems re-
lated to the uptake, intracellular transport, and translocation of NO 3 are directly affected by the soil NO 3
level [17]. In response to environmental NO 3 , root tissues proliferate and a general increase in root
growth and metabolism occurs [16]. The uptake of NO 3 by plant roots is an active process involving a
638 DUBEY AND PESSARAKLI