Agriculture Reference
In-Depth Information
commercial fertilizers constitute a major production cost. This overdose of N clashes with one of
the key aims of today's agriculture, that is, sustainability.
5.2.8 m
eChanIsms
r
esponsIBle
for
v
arIatIon
In
nue
In
C
rop
p
lants
It is widely reported in the literature that crop species and genotypes within species differ signifi-
cantly in NUE (Fageria and Baligar, 2003, 2005; Fageria et al., 2003, 2008, 2011; Fageria, 2013,
2014). Genetic differences in N uptake and/or grain per unit of N applied has also been reported
in different crops, including wheat, rice, corn, sorghum, and barley (Ortiz-Monasterio et al., 1997;
Muchow, 1998; Presterl et al., 2003; Anbessa et al., 2009). Several reasons have been cited why
some genotypes are more efficient in N utilization as compared to others (Thomason et al., 2002).
Moll et al. (1982) reported that NUE differences among corn hybrids were due to differing utiliza-
tion of N already accumulated in the plant prior to anthesis, especially at low N levels. Eghball and
Maranville (1991) reported that NUE generally parallels water use efficiency in corn. Hence, both
NUE and water use efficiency traits might be selected simultaneously where such parallels exist.
Kanampiu et al. (1997) reported that wheat cultivars with a higher grain harvest indexes had
higher NUE. Cox et al. (1985) reported that wheat cultivars that accumulate large amounts of N
early in the growing season do not necessarily have high NUE. Plants must convert this accumu-
lated N into grain N and must assimilate N after anthesis to produce high NUE. Forms of N uptake
(NHvsNO
4
+
. )
may also have effects on NUE (Thomason et al., 2002). Plants with a preferential
uptake of
NH
4
+
during grain fill may provide increased NUE over plants without this preference
(Tsai et al., 1992). Ammonium-N supplied to high-yielding corn genotypes increased yield over
plants supplied with
NO
3
−
during critical ear development (Pan et al., 1984). Salsac et al. (1987)
reported that
NH
4
+
assimilation processes require 5 ATP (adenosine triphosphate) mol
−1
of
NH
4
+
,
whereas
NO
3
−
assimilation processes require 20 ATP mol
−1
NO
3
−
.
This energy-saving mechanism
may be responsible for higher NUE in
NH
3
+
-.
In addition to the above-mentioned reasons, Table 5.12 summarizes various soil and plant
mechanisms and processes and other factors that influence genotypic differences in plant nutrient
efficiency. No attempt has been made to discuss these mechanisms or processes in details. For
extensive reviews related to nutrient flux and mechanisms of uptake and utilization in soil-plant
systems, see Mengel et al. (2001), Barber (1995), Marschner (1995), Fageria et al. (2011), and
Baligar et al. (2001).
Regarding genotypic variability for NUE, Rosielle and Hamblin (1981) reported that the herita-
bility for grain yield is usually lower for plants grown under low N versus high N. Thus, potential
progress would be lower for plants grown with low N as compared to high N target environments.
Banziger and Lafitte (1997) reported that the heritability of grain yield usually decreases for plants
grown under low N. Banziger et al. (1997) reported that secondary traits (ears per plant, leaf senes-
cence, and leaf chlorophyll concentration) are valuable for increasing the efficiency of selection for
grain yield when the broad-sense heritability of grain yield is low under low-N environments.
N
4
5.2.8.1 Physiological and Molecular Mechanisms Associated with NUE
There are two components of NUE in the plants. These are known as uptake and assimilation effi-
ciency (N is stored and assimilated into amino acids and other important nitrogenous compounds in
the leaves, roots, and young organs) and retranslocation or distribution and utilization in the grain
formation. During the growth and development process, the stored amino acids are further utilized
in the synthesis of proteins and enzymes involved in different biochemical pathways and the pho-
tosynthetic machinery governing plant growth, architecture, and development (Kant et al., 2010).
Both the components of NUE are important in increasing NUE in crop plants. Both the components
of NUE are influenced by environmental factors. In addition, several physiological and biochemical
changes occur in plants as adaptive responses to N limitations, including an increase in N uptake
by high-affinity transporters, remobilization of N from older to younger leaves and reproductive
