Role of Bio-fertilizers in Crop Improvement - Crop Improvement: New Approaches and Modern Techniques - page 195

and Bhaskara Rao 2012 ). Rice plant inoculated with A. brasilense at population
rate of 8 × 10 -7 /g of dry weight under field conditions showed a yield of 1.6-10.5 g
plant-1 (Mirza et al. 2000 ; Malik et al. 2002 ). Azospirillum population enhances the
uptake of P and NH 4 compounds in rice plants (Murty and Ladha 1988 ).
Azotobactor
Azotobactor is a gram-negative, aerobic, heterotrophic, rod shaped nitrogen fix-
ing bacteria present in alkaline and neutral soils (Lakshmi-narayana 1993). They
are free living organism present in soil, water and also in association with some
plants (Gandora et al. 1998 ; Martyniuk and Martyniuk 2003 ). Various species of
Azotobacter are A. agilis, A. chrococcum, A. beijerinckii, A. vinelandii, A. ingrinis.
Among these Azotobacter , Azotobactor chrococcum is the most commonly found
in arable soils of India. In addition to its capability to fix atmospheric nitrogen
(20-40 Kg N/ha) for different crops, it can also produce various growth promot-
ing substances viz., auxins, and gibberellins cytokinins, indole acetic acid includ-
ing vitamins and antibiotics, which control plant pathogens and help to maintain
soil fertility. Azotobacter produces slime like substances which help in aggregation
of soil particles. Many strains of Azotobactor exhibit fungicidal properties against
certain species of fungus. Various crop plants like rice, maize, cotton, sugarcane,
pearl millet, vegetable and some plantation crops show response to Azotobacter .
Occurrence of organic matter in uncultivated soil promotes its multiplication and
nitrogen fixing capacity. Field experiments carried out on Azotobacter under differ-
ent agro-climatic conditions pointed out that Azotobacter is suitable when inocu-
lated with seed or seedling of crop plants like onion, brinjal, tomato and cabbage.
Azotobacter being heaviest among breathing organism and requires a large amount
of organic carbon for its growth. Although it is poor competitor for nutrients in soil
but it enhances plant growth through nitrogen fixation, release of growth promot-
ing substances, and fungicidal substances. It improves seed germination and plant
growth. N fixation process which is highly sensitive to O 2 , Azotobacter have special
mechanism against O 2 it reduces the concentration of O 2 in the cells (Shank Yu
et al. 2005). Nitrogenase enzyme is also sensitive to O 2 , but is supposed that the
extreme respiration role of Azotobacter utilizes free O 2 within the cells and pro-
tects the nitrogenase (Kumar and Bhaskara Rao 2012 ). Azotobacter species have
various types of nitrogenases viz., molybdenum-iron nitrogenase, vanadium-iron
nitrogenase (Robson et al. 1986 ; Narula et al. 2000 ). Azotobacter requires carbon
source for their energy (Kanungo et al. 1997 ) and is capable of fixing 10 mg N/g of
carbohydrates in field conditions. Azotobacter is believed to be one of the signifi-
cant bio-fertilizer for rice and other cereals, it can be applied by seed dipping and
seedling root dipping methods (Kannaiyan et al. 1980 ; Kannaiyan 1999 ; Ruttimann
et al. 2003 ; Singh et al. 1999 ). Azotobacter can also able to enhance the growth in
wheat crop (Kader et al. 2002 ).