Agriculture Reference
In-Depth Information
in carbon and N metabolism and other metabolic pathways (Kant et al., 2010). In addition, nitrate is
reported to be responsible for the growth of root system and also root-to-shoot ratio. The function of
several structural genes involved in N uptake and assimilation have been studied extensively in the
past decade (Kant et al., 2010). In
Arabidopsis
, there are three families of nitrate transporters, that
is, NRT1, NRT2, and CLC with 53 NRT1, 7 NRT2, and 7 CLC genes identified (Kant et al., 2010).
The NRT2 are high-affinity nitrate transporters while most of the NRT1 family members char-
acterized so far are low-affinity nitrate transporters, except NRT1.1, which is a dual-affinity nitrate
transporter. The NRT1.1, NRT1.2, NRT2.1, and NRT2.2 are involved primarily in nitrate uptake
from the external environment (Miller et al., 2007; Tsay et al., 2007; Ho et al., 2009). Among the
CLC family members, CLCa is known to mediate nitrate accumulation in the plant vacuole (De
Angeli et al., 2006). Nitrate, after entering the plant cell, is reduced to nitrite by NR and further to
ammonium by nitrite reductase. The ammonium derived from nitrate or from direct ammonium
uptake by AMT transporters is further assimilated into amino acids via the glutamine synthetase
(GS)/glutamate synthase (GOGAT) cycle (Kant et al., 2010).
5.2.8.3 Genetic Mechanisms for Nitrogen Remobilization
The N accumulated in the leaves is the main source for retranslocation to the seeds during the
ripening growth stage of the plants. The majority of this remobilization occurs during senescence
and N is mainly transported via amino acids. Up to 80% of the grain N contents are derived from
leaves in rice and wheat (Tabuchi et al., 2005; Kichey et al., 2007). Plants have developed efficient
methods and mechanisms that release tied-up N from source tissues via protease activities during
leaf senescence (Kant et al., 2010). Approximately 80% of total N is located in the chloroplasts
mainly in the form of proteins and this is an important N pool for remobilization (Adam et al.,
2001). Among chloroplastic proteins, Rubisco (about 50% of total cellular proteins in C
3
and about
20% in C
4
plants) seems to serve as the major protein subjected to proteolysis and is responsible for
most N remobilization during leaf senescence for grain filling (Mae et al., 1993). All plant species
harbor ATG (AuTophaGy) genes designed to carry out this important recycling process and several
ATG genes have been identified in plants. Among these, ATG8s participate in tagging proteins for
degradation (Slavikova et al., 2005). A detailed discussion of genetic mechanism for N remobiliza-
tion is given by Kant et al. (2010) and readers may consult this article for additional information.
5.3 CONCLUSIONS
Fertilizer use efficiency is an important factor that needs to be taken into consideration. In agricul-
tural production systems, inefficient use of fertilizer inputs represents not only an environmental
hazard but also a substantial economic loss. In general, efficiency is output divided by input. The
higher the means value, the higher the efficiency or better use of applied resource. NUE in crop
plants are classified as AE, PE, APE, ARE, and UE. AE is defined as the economic production
obtained per unit of N applied. PE is defined as the biological yield obtained per unit of N uptake.
APE is defined as the economic production (grain yield in case of annual crops) obtained per unit
of N uptake. ARE is defined as the quantity of N uptake per unit of N applied. Nitrogen utiliza-
tion efficiency is the product of PE and ARE. In addition, nutrient use efficiency or NUE can also
be expressed in NER. NER is defined as the yield produced per unit element in the plant tissue.
Nitrogen recovery efficiency is less than 50% in most crop plants, indicating large loss of this ele-
ment in soil-plant systems. The losses of N may be related to leaching, denitrification, volatilization,
and surface runoff. Hence, there is significant opportunity and challenge to agricultural scientists to
improve NUE and reduce the cost of crop production as well as environmental pollution. Generally,
NUE has positive quadratic association with grain yield in crop pants. Hence, improving NUE
improves grain yield in crops. Climatic, soil, and plant factors are principal determinants of NUE.
A detailed knowledge of these factors is fundamental in improving NUE in crop plants. In addition,
the components of NUE interact in multiple and complex ways with other metabolic pathways.
