Handling Soybean ( Glycine max L.) Under Stress - Crop Improvement: New Approaches and Modern Techniques - page 418

of N fixed by soybean is annually equal to 16.4 Tg, accounting for 77 % of the total
N fixed by leguminous plants. With respect to the importance of soybean as a source
of food for human and because N-fixation can contribute to a large part of neces-
sary N for plant use, there has been extensive research work regarding the process
of N-fixation in leguminous plants. Enhancing the efficiency of N-fixation can sig-
nificantly increase crop yield and is of environmental and economical significance
(Herridge et al. 2008 ).
The process of symbiotic N-fixation is between some soil bacteria, collectively
called rhizobium, and the leguminous plants in which the atmospheric N 2 is fixed
by the bacteria and reduced to ammonia. The ammonia is then assimilated by the
host plant into the structure of amino acids and proteins. The process of N-fixation
between rhizobium and leguminous plants is specific, indicating that only one strain
of rhizobium is able to colonize the host plant roots and form nodules. Nodules are
the place of rhizobium residence and hence N fixation (Hungria et al. 2005 ; Wang
et al. 2011 ).
Between a hundred to a few hundred kg ha −1 atmospheric N 2 is fixed by rhizo-
bium, providing a major part of necessary N for plant use. For example, in soy-
bean, 50-60 % of necessary N is supplied by biological N fixation (Salvagiotti et al.
2008 ). Although chemical fertilization can quickly supply the necessary nutrients
for plant use, it has also some disadvantages including: (1) adversely affecting soil
structure, and (2) being subjected to leaching and hence resulting in the pollution
of water resources. This is why biological nitrogen fixation is important, as it can
inhibit such un-favorable effects of chemical fertilization on the soil properties, and
hence on the environment (Evans 1993 ; Salvagiotti et al. 2008 ).
Usually rhizobium can be found in the soil, especially under soil optimal condi-
tions; however, its population may not be adequate to efficiently inoculate the host
plant. For this reason, use of bacterial inoculum can be a useful method to inoculate
the host plant with appropriate bacterial population. Inoculums have a carrier with
a high bacterial population, used to inoculate seeds before planting or at the time
of planting. The bacterial inoculum must have the ability to compete with the soil
bacteria, adapted to the soil conditions (Miransari 2010 , 2011 ).
Exploiting the potential genetic of plant and climate properties are among the
most efficient methods, resulting in the enhancement of soybean yield (Salvagiotti
et al. 2008 ). Although soybean is not a tolerant crop plant under stress, different
methods have been tested to increase its tolerance under different stresses includ-
ing salinity and drought, acidity, high amount of mineral N and sub-optimal root
temperature. Among the most important hypotheses that have been successfully
tested and approved by researchers is that soil stresses disrupt the process of mo-
lecular communications between rhizobium and the host plant. In the initial stages
of N-fixation the two symbionts, rhizobium and host plant must exchange signal
molecules to realize their presence and start the process of N-fixation. The disrup-
tion of such signaling exchange between the two symbionts can inhibit N-fixation
by rhizobium and the host plant (Miransari et al. 2007 , 2008 , 2009 ).
Preincubation of Bradyrhizobium japonicum with the signal molecules, from the
flavonoids biochemical group, have been shown to be a useful method to alleviate