Agriculture Reference
In-Depth Information
soil stresses on the process of N
2
-fixation. In the case of soybean symbiotic bacteria,
Bradyrhizobium
japonicum
, geneistin is among the dominant signal molecules, pro-
duced by the host plant root. Pretreatment of
Bradyrhizobium
japonicum
with ge-
nistein can partially or completely alleviate the stress on the process of N-fixation.
In this case, the bacterial genes become activated under the stress and proceed with
the next stages of N-fixation process (Zhang and Smith
1995
). Accordingly, some
of the most recent findings regarding the methods used to alleviate soil stresses on
soybean growth and yield production with some prospects for future research are
presented.
Significance of Biological Nitrogen Fixation
Biological nitrogen fixation and specifically the symbiosis between legume plants
and rhizobium, is one of the most important biological activities, globally. In the
process of symbiotic association between legume plants and rhizobium, the atmo-
spheric N is fixed by the bacteria and turned into available N (NH
3
) by nitrogenase
enzyme. Depending on the conditions, most part of the N necessary for legume
growth and yield production is supplied by the process of N-fixation. However, it
is likely to enhance the efficiency of N-fixation, especially under stress (Unkovich
and Pate
2000
; Yasmin et al.
2006
).
With respect to the economical and environmental importance of biological ni-
trogen fixation, it can significantly contribute to the enhanced efficiency of eco-
system including crop production. Although N-chemical fertilization can rapidly
provide the necessary N for plant growth, it is subjected to leaching and hence can
adversely affect the water sources. N-fixation can also be economically consider-
able as there is a high annual rate of expenses, spent for the production and use
of synthetic N (Miransari and Smith
2007
,
2008
,
2009
; Miransari and Mackenzie
2010
,
2011a
,
b
).
There are different parameters affecting the process of N-fixation between le-
gume and rhizobium including plant species, bacterial strains, N-chemical fertiliza-
tion and soil and plant properties. There are some legume plant species and rhizo-
bium strains, with specific genotypic properties, which can perform more effec-
tively. Accordingly, some plant-bacterium combinations may be more efficient. Al-
though N-fertilization at seeding can enhance the growth of legume seedlings, high
amounts of N-fertilization can adversely affect the process of N-fixation. Under
optimal soil conditions, the efficiency of N-fixation process can be at the highest,
however, soil stresses can decrease the rate of N-fixation by negatively influencing
both the host plant and the bacteria (Valliyodan and Nguyen
2008
; Hamilton et al.
2011
; Salah et al.
2011
).
Climate properties including light, temperature, precipitation, and concentra-
tion of carbon dioxide can affect both plant and the symbiotic rhizobium. Light is
necessary for the process of photosynthesis and at optimal rates can increase the
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