Physiological properties of Azotobacter paspali in culture and the rhizosphere

Changes in cell morphology of Azotobacler paspali were dependent on culture growth conditions. They were influenced by the C-source, sucrose content of the medium, presence or absence of N, PO₂ and the shaking rate. The bacteria produced different cell forms, including rods, filaments, pairs of cocci and cysts. The proportions between the morphological types changed according to the culture composition. Aggregation of cells in the culture increased under high C-to-N ratio and with culture age. The optimal PO₂ for nitrogenase activity by batch cultures of 10⁸ colony forming units (cfu) ml^-1 was 0.025-0.05 atm. A. paspali colonized the roots of several plants. Bacterial numbers counted on wheat and sunflower root surface, following inoculation with 5 × 10⁷cfu ml^-1 were between 10⁵ and 10⁶ cfu g^-1 root dry weight after 1 week, 10^-4-l0^-5 after 2 weeks and 10³cfu ml^-1 after 3 weeks. Bacterial numbers in the soil in the first week after inoculation were about l0⁵cfu g^-1 soil and 10³ after 3 weeks. Inoculation with A. paspali, 5 × 10⁸ cfu ml^-1 under gnotobiotic conditions in Petri dishes increased root hair formation in canola roots 24 h after inoculation.