From Agronomy to Molecular Genetics and Proteomics in an Effort to Improve Nitrogen Use Efficiency in Crops - Crop Improvement: New Approaches and Modern Techniques - page 349

et al. ( 1979 ) showed in sorghum, a decrease in the height of the plant with the
enhancement of NR activity while such relationship with tall and dwarf cultivars of
wheat were not observed (Abdin and Abrol 1997 ). Abdin and Abrol 1997 revealed
in Wheat genotypes more than two-fold variation in NR activity was observed, that
might be due to genetic levels of NR enzyme which is heritable. These genetic dif-
ferences in the NR activity are also reflected in N harvest and it may be associated
with improvement in growth and yield in some genotypes. In some of the high NR
genotypes, the grain N concentration was significantly higher. It was observed that
high NR (HNR) genotypes maintained higher levels of NR activity even under low
N levels (Abdin et al. 1992 , 1996 ) this may be because of high levels of NADH that
might enhance NR activity in high NR genotypes (Bauwe and Kolukisaoglu 2003 ).
The activity was especially maintained at the later stages of growth i.e. at the time
of flag leaf emergence and anthesis (Jain and Abrol 2005 ). Studies also indicated
that the activity of the NR was regulated at the level of gene expression (Jain and
Abrol 2005 ; Skiba et al. 2011 ). Recent studies of the genotypes that differed in the
levels of their NR activity have revealed that not only the single enzyme NR but the
whole N metabolism pathway operates at the elevated level viz. all the enzymes of
the pathway nitrite reductase, glutamine synthetase and glutamate synthase func-
tion at significantly higher levels in the high NR genotypes as compared with the
LNR genotypes, leading to higher accumulation of grain N (Xu et al. 2012 ). The se-
lection of genotypes with a more efficient mechanism of N uptake and metabolism
is a strategy aimed at increasing N utilization efficiency of the maize crop. Several
trials for efficient use of N under conditions of low N availability have been carried
out with maize (Machado et al. 1992 ). In order to characterize and select genotypes
for efficient use of N, several authors have used physiological and biochemical
parameters, such as high nitrate in leaves (Mollaretti et al. 1987 ), enlarged nitrate
reductase activity (Feil et al. 1993 ), glutamine synthetase activities (Machado and
Magalhães 1995 ), or increased enlistment of N from leaves and stems to the ker-
nels (Machado et al. 1992 ). Genetic diversity has several 'indicators', which are
measured using various tools such as Mendelian genetic analysis that are employed
to assess disparity in single known gene (qualitative traits), such as resistance to
disease (Smale and McBride 1996 ) or multivariate traits/quantitative traits. Also,
pair-wise coefficients of parentage are calculated from pedigree information that
serves as genetic diversity indicators of (Cox et al. 1996 ). Nitrogen use efficiency
is a complex quantitative trait which is governed by many genes depending on the
number of internal and external factors like nutritional and environmental that lim-
its the nitrogen availability.Quantitative genetic studies are associated with molecu-
lar markers provides insight to the identification of Quantitative Trait Loci (QTL)
that is known to be part of the genetic variation of a complex character such as NUE
(Harrison et al. 2004 ; Hirel et al. 2007 ) and gives a new turning point in identifica-
tion of agronomic traits.