1.
Introduction
Maize
(Zea mays L.) is originated in the Mexican highlands and diversified in
a wide range of agro ecologies in the world. The USA, China, and Brazil being
the top three maize producing countries globally while famous in Africa started
about 500 years ago and in the late 16th and 17th in
Ethiopia (Tsedeke et al., 2015).
Food
security is a problem and international distress even today which has a direct
effect to the people worldwide, especially in Sub Saharan Africa (FOA, 2017).
Maize is globally serves as a source of diet, silage and industrial raw
material in the manufacturing (Tahiru et al., 2015). In Ethiopia, it is
the second largest food security crop after tef
(Eragrostis tef) in area coverage and the first in its productivity
(Abera et al., 2013). Maize provides proteins, carbohydrates, and
minerals in the diet. However, now the production of this crop is insufficient
globally due to its extensive demand for food, feed and bio fuel production in
the industry (Banziger and Diallo, 2004; Ranum et al., 2014).
The other studies also revealed that, its supply is also deficient as a result
of most maize producing countries like conducted the production under low N
availability, due to low natural soil fertility, low N fertilizers asset and
production instability (Monneveux et al., 2005).
Currently the biotic and abiotic challenges
are faced worldwide at maize production. Among the abiotic factors drought and
soil acidity play a detrimental role in the deliberation of nutrients in maize
seed significantly affects the ability of a seedling to tolerate various
stresses (Chen et al., 2016). The rapid residents increase in synergy
with factors of poor soil fertility and climate variability brings enormous
compression on nutrient deficiencies like, P, boron (B), zinc
(Zn), molybdenum (Mo) and also poor microbiological activity leading low
nitrogen (N) and sulphur (S) availability (Rathod et al, 2017). Since
maize is highly alimentary dependence stated by Cancellier (2011), critically
it is more dependent on N nutrient and highly affected the yields when the N
amount below the optimum level (Demari et al., 2016 and Noor, 2017). The other
findings also conveyed that, it also contributed 1 to 4 % of dry matter and
used for lateral root elongation in crops, but inhibits the lateral root growth
if the N supply is extremely low (Sawyer and Mallarino, 2007). Eivazi and Habibi (2013) also reported
that N increases the leaf area, cells and protoplasm content which facilitate
the photo synthesis efficiency. Maize also takes half of its N supply at
the growth period between 8th leaf collars to silking stage (Prester
et al., 2003). Different agronomic studies showed that N emission is
high in maize at the 4th and 6th leaves, and at the end
of leaf differentiation stage (Pereira et
al., 2013). The negative effect of urea application only at a time of eight
leaves stage for maize was studied by Schiavinatti et al. (2011); hence
split application was recommended to increase the production.
As several investigations indicated that in
order to meet the increasing desires of the world population, food security in
the starvation areas and improving the use of non-renewable resources could be proficient through the development
and use of productive varieties (Tahiru et
al., 2015). The agronomic
solution as well provides to increase the productivity such as urea [CO (NH2)2]
and ammonium sulfate [(NH4)2 SO4] are the two
main N bases exploited for maize (Alva et
al. 2006). Sawyer and Mallarino, 2007 also reported that the adoption of
high yielding maize varieties increase with the demand of additional fertilizer
for enhancing the yield. Continuous N-endorsement
from soil during the dough stage reduces the remobilization of N from
vegetative to procreative tissues. Thus the crop does not have to cannibalize
the leaves to provide N for grain expansion allows the plant to retain more
green leaf and increases the duration of photosynthesis, carbohydrate
production and grain yield (Muhammad et al., 2017).
2.
The concept of nitrogen use efficiency (NUE)
and its role for the crop physiology
Critically
the NUE in maize has been studied seriously by various scientists. The N-use
efficiency is defined as the ability of a genotype to produce higher grain
yields under low soil N conditions in comparison with other genotypes (Kamprath
et al., 1982; Prester et al., 2003). Nitrogen uptake
efficiency (NUpE) obtained by calculating the total amount of N in the
developed plant, shared by the amount of N applied to soil whereas N
utilization efficiency (NUtE) is the ratio between grain weight and the total
amount of N in the full-fledged plant (Moose
and Below, 2009). The NUtE is influenced by
different physiological factors like N acceptance, metabolism, distribution and
remobilization (Gironde et al., 2015).
According to Hawkesford (2014), to minimize the N losses and the crop N option,
it is better growing the exploitation efficiency of applied nutrient by
selecting traits contributing for high yield with less input. The amount of NUpE,
NUtE, and NUE were quantified based on the formula developed by Moll et al. (1982).
NUpE =
, NUtE=
and
NUE= [
]
].
The existence of N from 95
to 99 % in a field is not readily available to crops because found unavoidable form as soil
organic matter
(Rangel and Silva, 2007). The organic matter and nutrients also currently
decreased due to the land use intensification and fast population increments (Kamara, et al., 2014). Soil N is
available to monocot crops in the form of nitrate (NO3) and ammonium
(NH4) ions (Araujo et al.
2012), but as Sharma and Bali, 2017,
reported that ammonia is the cheapest form
of nitrogen fertilizer comprehensively used with simply submission and complete
accessibility. Mineral N in the soil combined over the season and the root
layer, governs the potential of N availability to the crop critically in maize (Mastrodomenico et al., 2018). Hence N is
a vital nutrient for maize growth and development. This element is not only provides N source for amino acids, nucleic acids,
chlorophyll and adenosine tri phosphate (ATP), but also arbitrates the intake
of phosphorus, potassium and other nutrients in maize (Lv et al., 2016).
According to Nunes et al. (2013), nitrogen also enhanced the grain
quality by increasing the protein content and significantly related with the
number of ears per plant and biomass for maize.
3.
Breeding
mechanisms to improve NUE of maize
The
environmental heterogeneity that exists for nutrients in the soil is feasibly
not remarkable that there is important extent of genetic variation and
phenotypic flexibility for NUE. The strategy of improving the NUE is the
genetic modification make to improve the take up more organ N or mineral N from
the soil and utilize the engaged N proficiently. The research findings revealed
that obviously crop genetic improvement has been responsible for 50 % to 60 %
of increasing the crop yields (Han et al., 2015).
Different studies also showed that breeding technique highly contributed
to enhance the NUE in maize (Hirel et al.,
2011; Shaibu et al., 2016). The study
on the physiology and genetics of N uptake and utilization is crucial to the
development of N-efficient cultivar (Perchlik and Tegeder, 2017).
One operative strategy is reducing fertilizer necessities by
improving maize genotypes with high N use efficiency and yield potential
(Haegele et al. 2013). Genotypes with
high yield potential are also needed to support the rapidly growing population
and provide incentives to farmers who are annoying to make modest increase in N
application in their maize fields (Noelle
et al., 2017). Generally there are two key meanings improving the
NUE in the crop, one is the economic gain of the yield and the second advantage
is the reduction of N pollution to the environment
(Lv et al., 2017). The primary
bio chemical reactions in the crop occur due to the involvement of N. Since N
has several physiological role having different organic compositions in the
cell including amino acids, proteins, enzymes, and chlorophyll. Nitrogen is
mainly plentiful in leaves, chiefly in photosynthetic enzymes. Hence, biomass
production, and grain yield are strongly correlated with N in maize (Kappes et al.
2013; Otie et al.,
2016).
Nitrogen use efficiency is an important movement to be genetically
improved for this trait through breeding for developing more productive maize
varieties (Moose and Below, 2009). As the studies reported by Haegele et al. (2013) and Mastrodomenico et al.
(2018), indicated that the existence of genetic variation for NUE in maize
contributed to synthetic varieties conveying N uptake and utilization
efficiency at broad-sense
heritability. However, the molecular breeding has not yet further practiced to
improve the NUE of maize before globally, the two breading strategies
conventional and molecular approaches are important to develop stress tolerant
varieties.
Various studies indicated that great
variations have shown among the evaluated maize genotypes in NUE and directs
traits genetically determined and improved through breeding. Genetic improvement mechanisms for
agricultural crops in NUE have initially studied using conventional breeding
systems (Cassman et al., 2002). Hereditary studies with small grains
have been mainly concerned with enlightening NUE as a means of enhancing the
grain protein and yield response in the application of N fertilizer (Dawson et
al., 2008).
The science of genetics was used in the art
of breeding to improve the NUE and determine the level of genetic variation
existing in different landraces and hybrids of maize. Nitrogen uptake
efficiency has been widely studied about its trait components in maize and
assessed by growing inbreed and hybrids at a series of high and low N
conditions (Bubert, 2014). Most maize breeders used selection at the screening
phase to identify high yield potential,
resistance/tolerance to stresses based on the desirable grain traits and plant
type through successive selection and back crossing recurrently until the
desired trait acquired (Brusamarello Santos et al.,
2017). Varieties which exhibit stress tolerance or stress avoidance
through acclimation and adaptation mechanisms evolved through natural selection
(Mickelbart et al., 2015). Various studies showed that direct selection
for yield improvement under stressed conditions has been successful in breeding
(Sadras and Lawson, 2013). Selection for prospective yield in combination of
genotypes and environments favor stress variation, strategy to be effective
improving stress adaptation (Sadras and Richards, 2014).
Biotechnology applied to enhance the
detection and validation of genes controlling NUE and its constituent traits,
to develop molecular markers introducing transgenes that modify key
physiological processes contributing NUE (Moose and Below, 2009). The
studies reported before indicated that, NUE is a multifarious trait for which
substantial heritable dissimilarity happens in maize germplasm and identified
QTL controlling the NUE (Agrama et al.,
1999).The modern detailed molecular characterization of QTLs and associated
genes aid stress survival has enabled the expedient transfer of genes into the
crop by molecular markers related with the key genes in breeding (Mickelbart et
al., 2015). The studies used in molecular breeding indicated that root
traits are controlled by multi genes. Some quantitative trait loci (QTLs)
governing maize root growths to nutrient uptake have been identified, the
extent of genetic variation of root traits affecting nutrient acquisition in
maize is also unclear (Mu et al., 2015). Near isogenic inbred lines
(NILs) are a powerful tool for reviewing the function of plant traits. For
instance different researchers identified a number of genes controlling N
utilization using the hybrid mapping population of Inter mated B73 x Mo17 RILs
(IBM) by Illinois high protein (Liu 2014).
Crops with higher yields in good environmental conditions are more likely to
have higher yields in stressed conditions. This approach also increased the
yield in high crop environments. But it is increasingly apparent that specific
selection strategies are needed to enhance yield in low-yield under stressed environments. In molecular breeding there are
different selection techniques for the desired gene in maize for NUE
improvement. Marker assisted selection
(MAS) used a marker such as a specific phenotype, chromosomal banding, a
particular DNA (RNA motif), or a chemical label that associates with the
desired trait. For instance DNA marker closely linked to a stress resistance
locus used to predict whether a crop is likely to be resistant to that stress
or not. Hirel et al. (2005) highlighted those QTLs for the leaf
enzyme activity showed to coincide with QTLs for yield and the putative role of
glutamine synthetase (GS) enzyme in grain efficiency of maize. Similarly
candidate genes were obtained as reported by Hirel et al. (2007) using quantitative trait mapping, RNA communication
and metabolite describing and transgene testing. Li et al. (1993) also
investigated that two cytosolic GS enzymes (GS1) in maize, Gln1-3 and Gln1-4 genes by studying the molecular and physiological properties of
Mutator insertion mutants. The impact of knock out mutations on grain yield and
its components was studied grown under suboptimal N feeding (Martin et al.,
2006). Marker-assisted recurrent
selection (MARS) also involves crossing in selected individuals at each cycle and selection. Genome-wide (GW) or genomic selection
(GS)-relies on MAS for the feasibility of incorporating desirable alleles at
many loci that have slight genetic outcome when used independently. This is
applied to progeny in a breeding based on marker data only without the need of
phenotypic evaluation (Tester and Langridge, 2010). For instance Ms44 is an
example of specific gene which was done for NUE (Lv et al., 2016; Afsar et
al., 2017).
4.
Challenges improving the NUE in maize and
constraints of N fertilizer in the environment
One
of the challenges to improve a trait, since it is determined by multiple genes
is the collection of high-quality
phenotypic data. Economically important traits are frequently polygenic;
therefore, it is unlikely that a single genotype or recombinant inbred line
would contain the necessary contrast in the relevant genes behind the trait(s)
and a resource intensive. The absence of genetic variability under N stressed
condition, the requirement of long breeding cycle and selection intensity,
Nature of the trait to be transferred (polygenic traits), investment (initial
cost of research investment (for molecular breeding), lack of integration
across professions and the skill and facility of stress management are the key
factors. To achieve this principle requires understanding the nature of the
crop and how to use stress management, a suite of useful secondary traits that
relate to yield under stress, improved statistical designs for use during
selection, and appropriate choice of germplasm and breeding schemes are
crucial.
The use of natural variation for crop
improvement has generally been without the knowledge of specific causal genes
and associated biological mechanisms. Currently, genetic determinants can be
identified through quantitative trait locus (QTL) mapping, association mapping
and screening by recurrent selection.
Studies
on target genes and specific path ways for the development of NUE is static in
progress but the expression of these inheritable factors is very multifaceted
under diverse settings and also due to the related genetic material (Shrawat et al., 2008). As different research
findings indicated that operations can be done transgenically for various
assimilatory pathway paces and revealed satisfactory results under precise in
pots and field conditions (McAllister et
al., 2012).
Maize hybrids are
commonly evaluated under high N, but N deficiency has a big influence on canopy
variables, such as green leaf number at the grain filling period, leaf area
index (LAI) and specific leaf nitrogen (SLN), and cause reduction in radiation
use efficiency and light interception ultimately lower kernel number (KN) and
kernel weight (KW) due to yield loss under N deficient in Kernel number per
plant affected by plant growth rate (PGR) during the silking period as
indicated in figure one (Chen et al, 2016).
Reducing N
fertilizer use efficiency or retrieval competence by crops is the most problem
associated with the use of chemical fertilizers (Lv et al., 2016). Nitrogen fertilizers also account about 25 % of the total
input costs (seeds and pesticides) in maize production (Afsar, et al., 2017). The other studies also
confirmed that N has a (Hirel et al.,
2007), N fertilizer contribute annual energy supply and significant greenhouse
gas emissions (Li et al., 2015), due to its volatile nature, it
was estimated that 35% to 65% of applied N lost from soil (Koocheki et al.,
2013). The application of extreme N fertilizer inhibits root growth and
development influencing the ability of roots to absorb nutrients efficiency
(Liu et al., 2017). In the contrast
the use of excessive N does also bring environmental problems and agricultural
soils that pollute the aquatic systems through runoff (Guo et al., 2010; Santos et al.,
2010; Zhu et al., 2016). For instance
nitrous oxide (N2O) is a strong glasshouse gas; and nitric oxide
(NO) also plays a role in tropospheric ozone depletion (Sharma and Bali, 2017). As so many investigators argued that the
best solution to elucidate N pollution is the development and production of crop
varieties which have the high NUE ability and produce high grain yield
(McAllister et al., 2012; Garnett et al., 2015). Figure 1 specifies the influence of sensible
N stress on every of these kernels digit apparatuses characters for a
distinctive maize crosses. The ear size and the total amount of ovules are
alike, but may reveal the variances due to N stress. As the figures showed that
both ears also designed a trivial amount of ovules at their tip that did not be
fertilized, because of dawdling development of silks comparative to pollen
accessibility.
The gap
between pollen shed and silk appearance or anthesis silking interval (ASI)
usually turn into more prominent under sever N stress.
5.
Conclusions
Food
security is an international concern today which has a direct effect on the
human population, especially in Sub Saharan Africa.
But the deliberation of nutrients in maize seed greatly affected by the
ability of a seedling to tolerate various biotic and abiotic stresses. It is
strong-minded by the examination that both conventional and molecular breeding
techniques contributed for improvement of NUE, but difficult separately,
especially using only conventional technique because of extended time
obligation. In long term prospectus
molecular breeding (genetic engineering) is the most important methods to
improve the NUE of the crop combined with stepwise recurrent selection and back
crossing. In general developing N stress tolerant variety is still in infant
stage and it is better to strengthen the advanced breeding strategies like
genetic engineering. Since the use of stress tolerant variety is cost
effective, sustainable solution and environmental friendly.
6.
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