Nitrogen fixation in the rhizosphere and rhizoplane of barley

Summary Aerobic and anaerobic N₂-fixing bacteria developed in the rhizosphere of barley seedlings and exhibited N₂ase activity when seedlings were grown in sterilized sand-nutrient cultures containing low levels of combined nitrogen. The source of the N₂-fixing bacteria appeared to be the seed. Average daily rates up to 0.9 μmoles C₂H₄ h⁻¹ g⁻¹ dry root tissue were measured, but the intensity of the activity was affected by moisture levels and concentration of combined N in the rhizosphere. Removal and washing of the roots did not remove the activity, and roots remained active even after surface-sterilization. An unidentified aerobic N₂-fixing bacterium was isolated from the rhizoplane of active barley roots. Inoculation of barley seedlings with the aerobic N₂-fixing bacterium enhanced N₂ase activity of excised roots 10-fold, with average rates of 0.9, 1.1 and 1.3 μmoles h⁻¹ g⁻¹ dry root assayed under pO₂ of 0.01, 0.02 and 0.04 atm respectively. The aerobic N₂-fixing bacterium also exhibited N₂ase activity when inoculated into the rhizosphere of oat, rice and wheat seedlings. Microscopic observations of sterilized live and stained barley roots suggest that the aerobic N₂-fixing bacterium is an endophyte which infects root tissue and metamorphoses into vesicle-like structures.