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4.1.2 Non-symbiotic Nitrogen Fixation
Among the heterotrophic free-living N 2 -fixing bacteria, Azotobacter is the most-
intensively investigated genera. Azotobacter is present in neutral and alkaline soils.
A. chroococcum and A. lipoferum are the most commonly-occurring Azotobacter in
arable soils. Apart from fixing atmospheric nitrogen, they produce biologically-
active growth promoting substances such as IAA, gibberellins and B vitamins in
culture media. The beneficial effects of Azotobacter inoculation have been reported
on many crops including sugarcane (Saccharum officinarum L.), rice, tomato (Lens
esculentum L.), onion (Alium cepa L.), mustard (Brassica juncea ), sorghum, pearl
millet, cotton and safflower (Carthamus tinctorius L.). Bacteria belonging to the
genus Azospirillum form an associative symbiosis with many plants especially C 4
types. Apart from providing associative N fixation, several other plant-growth-
promoting mechanisms have been described for Azospirillum, resulting in better
nutrient and water use in inoculated crops. Azospirillum inoculants are recom-
mended for sorghum, millets, maize, sugarcane, wheat, paddy rice, cotton, oilseeds,
fruits and vegetables, flowers, spices and condiments, herbs, lawns and ornaments
and trees. Multi-location trials in India have shown yield increases of 10–17 % in
pearl millet and 7–31 % in sorghum after inoculation with A. brasilense (Subba Rao
1986 ). Azospirillum increased straw and grain yields of pearlmillet by 20 % and
15 %, respectively, on arid soils in Jodhpur (Venkateswarlu 1985 ).
The benefits of Azotobacter and Azospirillum inoculation on plant growth, nutri-
ent uptake and yield parameters in dryland crops are summarized in Table 3. Many
strains of plant-growth-promoting rhizobacteria (PGPR) including Pseudomonas,
Bacillus, Burkholderia and Serratia can also fix atmospheric nitrogen and have
potential application in dryland crops.
4.2 Plant-Growth-Promoting Rhizobacteria
PGPR inhabit the soil ecosystem (Kloepper et al. 1980 ) and are found in association
with the roots of many plants, presumably due to the presence of high levels of
exudates rich in nutrients. The effects of PGPR on plant growth can be mediated
through different mechanisms including the production of plant hormones such as
auxins, gibberellins and cytokinins, nutrient acquisition, suppression of phytopatho-
gens by the production of siderophores, HCN, ammonia, antibiotics, volatile metab-
olites, etc. and the induction of systemic resistance and systemic tolerance (Glick
1995 ; Yang et al. 2009 ). A particular PGPR may affect plant growth through one or
more of these mechanisms.
All nitrogen-fixing and phosphate-solubilizing bacteria are categorized as PGPR
as well as other genera including bacteria such as Arthrobacter, Bacillus,
Burkholderia, Enterobacter, Klebsiella, Pseudomonas, Xanthomonas, Serratia.
These PGPR can be used as single inoculants, mixed inoculants or as coinoculants
with other beneficial microorganisms (Saxena et al. 2006 ). Further, PGPR are not
M. Grover et al.