Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

264j 14 Salt-Tolerant Rhizobacteria: Plant Growth Promoting Traits
Figure 14.1 Growth of salt-tolerant B. cereus 80 strain at different
concentration (%) of NaCl.
Many species are able to survive at temperatures higher than those that allow growth,
by adopting metabolic shutdown either as vegetative cells or heat-resistant spores
and cysts [83]. Salt-tolerant rhizobacteria (e.g. Arthrobacter, Bacillus, Mycobacterium,
Pseudomonas, etc.) isolated from arid soils were able to survive at 45 C. In earlier
studies, two species of M. phlei and M. thermoresistibile were found to be able to grow
at 52 C [87]. However, bacteria isolated from alkaline soils in India during summer
months are subjected to high salt concentration, pH and temperature stress. In
alkaline soils, salt concentrations and pH may be as high as 2 and 10.2%, respectively,
and temperatures may range between 35 and 45 C [88]. Thus, bacterial strains
that are capable of tolerating high temperatures and salt concentrations have competitive
advantages over others to survive and proliferate in arid and saline regions of
the world.
14.5
Physiological Characterization of Rhizobacteria
Soil conditions stressful to bacteria include nutrient starvation, salinity, temperature
and low water activity. The availability of nutrients from organic matter decomposition
largely depends on the sites occupied by bacterial cells or microcolonies. Oligotrophs
are probably locally dispersed through most soils including arid soils.
Therefore, soils in general can be regarded as a grossly oligotrophic environment
[89]. Oligotrophs have been described as bacteria that are able to multiply in habitats
of low nutrient flux [90]. They show relatively high growth rates in nitrogen-free
medium culture and have the ability to survive in nitrogen-poor soils. In general,
both starvation and low-water activity represent typical stress conditions in most
soils. Other stress factors such as extreme temperatures, very high or low pH and