Handling Soybean ( Glycine max L.) Under Stress - Crop Improvement: New Approaches and Modern Techniques - page 427

sitivity of Badyrhizobium japonicum to the signal molecule is decreased resulting
in the inhibition of N-fixation process. (Miransari and Smith 2007 , 2008 , 2009 ).
Soybean and Suboptimal Root Zone Temperature
There are areas in the world, which are subjected to sub optimal root zone tempera-
ture, most of the year. Under such conditions plant growth as well as microbial ac-
tivities is adversely affected. The symbiotic association between the host plant and
the N-fixing bacteria is also influenced by suboptimal root temperature, resulting in
the decline of N-fixation between the two symbionts. Similar to other stresses, sub-
optimal root temperature can also disrupt the initials stages of N-fixation, most im-
portantly the exchange of the signal molecules between the two symbionts (Smith
and Lynch 1993 ; Miransari 2012b ).
The optimum temperature for soybean growth is between 25 and 30 °C (Lynch
and Smith 1993 ). Falvonoids can act as Nod gene inducers and also enhance plant
resistance to pathogens in soil. Higher rates of flavonoids are produced at higher
temperature by plant roots. Accordingly, under lower temperature higher concen-
tration of signal molecule may be necessary to induce the molecular changes in
rhizobium as the effectiveness of the signal molecule is temperature dependent. The
signal molecule by itself and its concentration can also affect the communications
between the two symbionts (Miransari and Smith 2008 ; Miransari 2012b ).
The activity of nitrogenase enzyme is oxygen dependent and under stress where
plant growth is adversely affected the nodules permeability to oxygen may also
change. This may influence nodule functionality by affecting the activity of nitroge-
nase enzyme (Wei and Layzell 2006 ). Temperature fluctuations can alter root respi-
ratory demand and its permeability to oxygen. Hence, plant can use such a mecha-
nism to regulate root permeability to oxygen under decreased temperature, which
reduces root permeability to oxygen (Kuzma and Layzell 1994 ; Wang et al. 2012 ).
Using intact soil samples in cylinders, collected from the field soil, Miransari and
Smith ( 2008 ) stimulated some of the field conditions under greenhouse conditions.
They found that soil texture may also affect the signal communications between the
host plant and rhizobium. This has been attributed mostly to the physical properties
of the different soil textures. According to their results genistein addition was more
effective under loamy and clay textures. Textures with finer particles have a higher
rate of microporosity. Finer soil textures are higher in soil nutrients, because of their
chemical structure and hence can provide more nutrients to the soil microbes as well
as plant roots affecting their activities.
Different microbial activities are affected by soil texture including mineralization
of organic matter, microbial biomass, respiration, nitrification and denitrification
(Hassink 1992 ). Soils, with a more improved structure, result in the higher produc-
tion of roots exudates influencing microbial activities more effectively. Van Gestel
et al. ( 1996 ) indicated that relative to the bacterial population in sandy soils (7 %),
significantly higher bacterial population was found in clay soils (More than 50 %).