adequate supply of reduced C compounds such as sugars for energy (Kennedy et al.
2004). When wheat was inoculated with Azotobacter under aseptic (gnotobiotic)
conditions in the greenhouse, 50% customary requirement for urea-N was replaced
(Soliman et al., 1995; Hegazi et al. 1998). Kennedy et al. (1998) reported that strains
of this bacterial genus was epiphytes of wheat rhizoplane but not the endophytic root
invaders. It was reported that cotton yield could increase by 15–28% following
inoculation with Azotobacter (Iruthayaraj 1981), possibly due to biological nitrogen
fixation (BNF), production of antibacterial and antifungal compounds, growth
regulators, and siderophores (Pandey and Kumar 1989). Also, Azotobacter has
been reported to produce plant growth hormones, improved nutrient uptake, and
display antagonistic effects against plant pathogens (Parmar and Dadarwal 1997).
20.3.3.2 Azospirillum
Azospirillum is a Gram-negative, microaerophilic heterotroph, non-fermentative,
and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae (Roper
and Ladha 1995), and it is often associated with roots of cereals and grasses (Grifoni
et al. 1995).
Azospirillum grow robustly in the rhizosphere of gramineous plants such as rice
and wheats (Kennedy and Tchan 1992; Ladha et al. 1982; James et al. 2000). They
can also penetrate the root to grow endophytically in intercellular crevices (Sumner
1990), although they are usually considered as epiphytes growing close to or on root
surfaces (Kennedy et al. 2004). Both Azospirillum lipoferum and Azospirillum
brasilense have been isolated from roots and stems of rice plants (Ladha et al.
1982; James et al. 2000) while Azospirillum amazonese has been isolated from the
roots of wheat (Pereira et al. 1988).
In addition to biological nitrogen fixation (BNF) capability, Azospirillum also
produce plant growth factors (auxins) that cause the plant to produce more roots
(Khan et al. 2010). Azospirillum was also associated with the rhizosphere of ginger
followed by the treatment with that bacterium (Govindan et al. 2009). They also
reported the stimulation of root growth for inoculated as well as uninoculated but
with a great difference as evident from the fact that the mean root length for
inoculated plants was 16.9 cm while the corresponding figures for non-inoculated
plants was only 2.1 cm (Govindan et al. 2009).
Azospirillum diazotrophs could not induce nodulation in their associated plants
unlike Rhizobium-like symbiotic associations. However, BNF can occur inside
epidermal tissue (apoplast) as endophytes or in the rhizosphere as free-living bacteria
(Tejera et al. 2004). There have been many reports that tropical soils of Brazil often
contain approximately 10
4 to 10
6 cells of Azospirillum per g of soil. Yield of wheat
inoculated with two A. brasilense strains 245 and 107 that were isolated from surface
sterilized wheat roots resulted in repeatable yield increases (Baldani et al. 1983;
Baldani et al. 1987 and Boddey et al. 1986). The favorable effect of strains Sp
245 and Sp 107 on wheat, in contrast to a lesser reaction achieved by strain Sp
7, could be due to the more successful establishment of the Sp 245 and Sp
408
S.-C. Chun
2004). When wheat was inoculated with Azotobacter under aseptic (gnotobiotic)
conditions in the greenhouse, 50% customary requirement for urea-N was replaced
(Soliman et al., 1995; Hegazi et al. 1998). Kennedy et al. (1998) reported that strains
of this bacterial genus was epiphytes of wheat rhizoplane but not the endophytic root
invaders. It was reported that cotton yield could increase by 15–28% following
inoculation with Azotobacter (Iruthayaraj 1981), possibly due to biological nitrogen
fixation (BNF), production of antibacterial and antifungal compounds, growth
regulators, and siderophores (Pandey and Kumar 1989). Also, Azotobacter has
been reported to produce plant growth hormones, improved nutrient uptake, and
display antagonistic effects against plant pathogens (Parmar and Dadarwal 1997).
20.3.3.2 Azospirillum
Azospirillum is a Gram-negative, microaerophilic heterotroph, non-fermentative,
and nitrogen-fixing bacterial genus from the family of Rhodospirillaceae (Roper
and Ladha 1995), and it is often associated with roots of cereals and grasses (Grifoni
et al. 1995).
Azospirillum grow robustly in the rhizosphere of gramineous plants such as rice
and wheats (Kennedy and Tchan 1992; Ladha et al. 1982; James et al. 2000). They
can also penetrate the root to grow endophytically in intercellular crevices (Sumner
1990), although they are usually considered as epiphytes growing close to or on root
surfaces (Kennedy et al. 2004). Both Azospirillum lipoferum and Azospirillum
brasilense have been isolated from roots and stems of rice plants (Ladha et al.
1982; James et al. 2000) while Azospirillum amazonese has been isolated from the
roots of wheat (Pereira et al. 1988).
In addition to biological nitrogen fixation (BNF) capability, Azospirillum also
produce plant growth factors (auxins) that cause the plant to produce more roots
(Khan et al. 2010). Azospirillum was also associated with the rhizosphere of ginger
followed by the treatment with that bacterium (Govindan et al. 2009). They also
reported the stimulation of root growth for inoculated as well as uninoculated but
with a great difference as evident from the fact that the mean root length for
inoculated plants was 16.9 cm while the corresponding figures for non-inoculated
plants was only 2.1 cm (Govindan et al. 2009).
Azospirillum diazotrophs could not induce nodulation in their associated plants
unlike Rhizobium-like symbiotic associations. However, BNF can occur inside
epidermal tissue (apoplast) as endophytes or in the rhizosphere as free-living bacteria
(Tejera et al. 2004). There have been many reports that tropical soils of Brazil often
contain approximately 10
4 to 10
6 cells of Azospirillum per g of soil. Yield of wheat
inoculated with two A. brasilense strains 245 and 107 that were isolated from surface
sterilized wheat roots resulted in repeatable yield increases (Baldani et al. 1983;
Baldani et al. 1987 and Boddey et al. 1986). The favorable effect of strains Sp
245 and Sp 107 on wheat, in contrast to a lesser reaction achieved by strain Sp
7, could be due to the more successful establishment of the Sp 245 and Sp
408
S.-C. Chun