Molecular Soil Biology 2015, Vol.6, No.4, 1-12
6
inadequate mineral nutrition and mineral toxicities
(Zahran, 1999; Abdel-Latef and Ahmed, 2015)
and biotic factors such as competition of ineffective
indigenous rhizobia, insect pest and diseases
(Serrage and Adu-Gyamfi, 2004; Gommaa et al
.,
2006; Sofy et al
.,
2014). These factors can interpose
survival of rhizobia in the soil, the infection
process, nodule growth and nodule functioning
and SNF (Serrage and Adu-Gyamfi, 2004).
It is estimated that crops grown on 90% of arable
lands experience one or more environmental stress
(Abdel-Latef and Ahmed, 2015). All are interconnected in
the control of N
2
fixation, and hence none of them
could be considered in isolation. It is also difficult to
isolate the effects of such factors on the success of
inoculation from their effects on symbiosis
functioni ng and nitrogen- fixa tion. Rhizobia l
populations vary in their tolerance ability to
major environmental factors (Biswas et al
.,
2008). The
existence of genetic variability in tolerance to most
stress factors has been shown in both legume host
plants and their respective rhizobial strain (Hungria
and Vargas, 2000). This suggests the possibility
of overcoming the environmental constraints
limiting SNF potential. Optimal performance of
the N-fixing symbiosis depends upon selection of
both symbiotic partners for adaptation to the target
environment (Sessitsch et al
.,
2002).
The SNF process also depends on the occurrence and
survival of
Rhizobium
in soils and their efficiency
(Adamovich and Klasens, 2001). One of the most
common factors limiting a legume’s ability to fix N
2
is
the absence of sufficient numbers of efficient and
effective rhizobia in the soil (Unkovich et al
.,
2008).
Fortunately, strains of rhizobia can be introduced into
soil relatively simply by inoculation. Inoculation can
lead to the establishment of large rhizobial population
in the rhizosphere and improved nodulation (Boahen,
2008). An emerging paradigm suggests that agronomists
must factor in natural selection on both crop plants and
their symbionts to optimize crop production. Santos
et
al
. (1999) explained that when rhizobia are applied in
agricultural settings, the inoculum strains must be able
to thrive in soil under the varied conditions of the field.
This means that the inoculum strains must be able to
compete with indigenous rhizobia for nodulation,
efficiently escape from senescent nodules, and persist
in the soil in order to infect the next season of
cultivated hosts.
Inoculation with highly effective N-fixing rhizobial
strain requires survival and establishment in the soil
environment. An effective and persistent rhizobial
strain has several advantages, and is preferable to the
repeated inoculation in the subsequent season. To be
established in the field, introduced rhizobial strain
must coexist with competitors and predators, and
maintain itself during period of low nutrient
availability. It is possible to change the established
rhizobial strain by new rhizobial strain when the
inculated rhizobial strain loss its nitrogen fixing
capacity suffering genetic change, or when the host
legume cultivar is changed. During the introduction of
new rhizobial strains it may create competition barrier.
However, competition from indigenous rhizobia is not
necessarily the major determining factor for lack of
response to inoculation; rather the presence of an
adequate soil population to meet the N
2
fixation
requirements of the host is the primary reason for
failure of crops to respond inoculation (Thies
et al.,
1991). Osunde et al
.
(2003) stated that the presence of
a large indigenous population of compatible rhizobia
does not necessarily preclude response to inoculation,
provided the inoculant rhizobial strains a re
competitive and highly effective.
Inoculation Response and Promiscuity of
Nodulation
Rhizobia inoculants have been used to address the
problems of soil fertility and inadequate fertilizer
application in grain legume production. Application of
effective rhizobial strains as biofertilizers to improve
legume production is an important approach in
sustainable agriculture (Saharan and Nehra, 2011).
Researches on use of rhizobia inoculants for
production of grain legume showed it is a cheaper and
usually more effective agronomic practice for
ensuring adequate N nutrition of legumes, compared
with the application of N fertilizer (Chianu et al
.,
2008). Inoculation with compatible and appropriate
rhizobia is critical for sustained yield in farmlands
where N supply limits production of grain legume
(Abd-Alla
et al.,
2014). According to Odame (1997),
inoculation is needed in all agricultural lands deficient
in N and where N supply is a key limiting factor in
crop production. For legume crops to effectively add
