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

Table 14.10 Effects of PSB R. meliloti URM1 combined with
phosphate on nitrogen and phosphorus uptake (%) of cotton
grown in arid saline soils of Uzbekistan.
Other studies also reported that PSB increased phosphorus uptake with an increase
in the yield of peas and barley [135]. The bacterial strains used were able to
dissolve organophosphates. A number of studies reported that phosphorus uptake
efficiency and crop yield increased with both phosphorus application and with
inoculation [128,133,134]. Accordingly, B. megaterium is considered to be the most
effective among the phosphorus-solubilizing bacteria. In short, phosphate-solubilizing
Pseudomonas strains can enhance plant growth by producing plant growth
regulators and vitamins, enhancing plant nutrient uptake and suppressing pathogenic
or deleterious organisms [139].
14.7
Biomechanisms to Enhance Plant Growth
14.7 Biomechanisms to Enhance Plant Growthj273
Leaves Stem Bud case Cotton fibers
Treatments N P N P N P N P
NoPoKo 1.45b 0.51b 0.68a 0.21a 0.78a 0.19a 1.78ab 0.81a
NP superpK 1.55a 0.75a 0.75a 0.24a 0.83a 0.22a 1.87a 0.84a
NPphosphoritK 1.2c 0.2c 0.3b 0.1b 0.5b 0.1b 1.6b 0.4b
NP PSBK 1.62a 0.8a 0.75a 0.24a 0.83a 0.25a 1.9a 0.89a
Mean values followed by the different lower case letters in the same column were significantly
different at P 0.05.
Plant growth promoting rhizobacteria may induce plant growth directly or indirectly.
Direct influences include synthesis of phytohormones, siderophore production,
nonsymbiotic nitrogen fixation and stimulation of disease resistance mechanisms
[120,140,141]. Indirect effects include allelopathy, antibiotic production and competition
with deleterious agents [142,143]. Some help increase the availability of essential
nutrients through solubilization and chelation [144]. In addition, many
PGPR produce ACC deaminase and metabolize ACC (precursors to plant ethylene)
and thus reduce the inhibitive effects of stress-induced ethylene on plant root growth
[21]. It is reported that a bacterium containing ACC deaminase can enhance the
resistance of tomato seedlings to salt stress. However, the resistance mechanisms in
plants are based on active and mutual association between bacteria and plant root
surface [145].
One of the most commonly reported direct effects on plant growth by rhizobacteria
is the production of plant growth substances. B. amyloliquefaciens and B. subtilis
have been shown to synthesize plant growth promoting substances such as gibberellins
and indole-acetic acid (IAA) in nature [146,147]. It is reported that Azotobacter
paspali secreted IAA into culture medium and significantly increased the dry weight
of leaves and roots of several plants after root treatment [148]. As mentioned earlier,