Agriculture Reference
In-Depth Information
Biological nitrogen fixation in legumes is often used to improve infertile agricultural soils
(Rehman and Nautiyal, 2002). The input of mineral nitrogen in the soil mainly consists of
symbiotic nitrogen fixation, transformation of organic matter and fertilization. Biological
activity is of great importance for the nitrogen balance in the soil because nitrogen fixation
and organic matter transformation both depend on biological activity. Fixation of atmospheric
nitrogen results from symbiosis between leguminous crops and rhizobia (van Hoorn et al.,
2001). This symbiotic association is by far the most important contributor to the world's
supply of biologically fixed N
2
. Rhizobial symbioses with more than 100 agriculturally
important legumes contribute nearly half the annual quantity of biological nitrogen fixation
entering soil ecosystems (Graham and Vance, 2000; Somasegaran and Hoben, 1994). For this
reason, taking into consideration the importance of legumes in animal and human
consumption, some attention has been given to the effects that environmental stresses exert on
Rhizobium
populations (Ibekwe et al., 1995).
Leguminous plants growing in saline environments require both free-living rhizobia and
host salt tolerant phenotypes. Therefore, it becomes important to find symbiotic partners that
can fix nitrogen under stress conditions (Abdelmoumen et al., 1999; Singleton et al., 1982).
The ability of rhizobia to establish nitrogen-fixing nodules on leguminous hosts enables plant
growth in soils with low nitrogen levels and consequently decreases in the contamination of
water reservoirs by inorganic nitrogen compounds (Hynes and O' Connell, 1990).
Many biotic and abiotic factors affect the growth and survival of rhizobia in soil.
Rhizobium
spp. strains are very sensitive to soil environment abiotic factors such as high salt,
pH and temperature stresses that affect their dinitrogen fixation and hence the productivity of
legumes (AbdelGadir and Alexander, 1997; Athar and Johnson, 1997).
Most rhizobial strains, which nodulate important crops, are also sensitive to soil
desiccation. Therefore, for the good growth of legumes in arid and semiarid regions of the
world where fertilizers are unavailable or expensive, it seems deemed necessary to plants,
being nodulated by an effective strain of
Rhizobium
that tolerate these adverse environmental
conditions (Rehman and Nautiyal, 2002).
Salinity not only affects free-living rhizobia but also considerably restrains the nodulation
process and symbiotic nitrogen fixation (Abd-Alla et al., 1998; Elsheikh and Wood, 1995).
Salt may affect symbiosis by its effects on the growth and survival of rhizobia in soil,
restrictions on root colonization, inhibition of processes of infection and nodule development,
or impairment of active nodule functioning (Rehman and Nautiyal, 2002). Some authors
reported
Rhizobium
and
Bradyrhizobium
strains that can survive and grow as free living
organisms under salt concentrations that inhibit the growth of most legumes (Bordeleau and
Prévost, 1994; Figueira 2000; Zahran et al., 1994). According to Brewing et al. (1993), the
genetic variability in
Rhizobium
is wide enough to ensure the survival of strains selected
under extreme conditions. Zahran et al. (1994) also obtained different salt responses among
several rhizobia isolates. Thus, host and symbiotic processes are generally thought to be the
factors limiting N
2
fixation under salt conditions (Abd-Alla et al., 1998; Cordovilla et al.,
1999; Delgado et al., 1993).
Genes conferring specific adaptation to adverse conditions, including salt stress are often
plasmid-borne (Silver and Misra, 1988). Plasmids are important genetic components for the
divergence and adaptation of microbial populations because they contribute to genomic
plasticity (Zhang, 2001). These plasmids might be lost or regained in populations, rapidly
change copy number and undergo higher mutation rates because of the common occurrence
