Studies on Nitrogen Fixation by Association of Bacteria with Rice Root

It is known that nitrogen fixation in the paddy fields is more effective than that in the upland soils, and is therefore of great significance to an intensive study on the associated function of rice and bacteria on the field of biological fixation (1–3). Since 1975 we have been engaged in the study of the bacteria associated with rice root system. 627 strains of bacteria were isolated from the roots of 45 rice varieties. According to the measurement by acetylene reduction method, 31 strains out of 435 exhibit the nitrogenase activity, suggesting that N-fixing bacteria do exist in the rice root system. Two of them, m-sm-1612 and st-sm-9021, were most active. They are both rod in shape (Pis. 1 and 2), 0.55 x 1.73–.3 μ in size, Gram negative, with peritrichous flagella (Pis. 3 and 4), of non-diffusing pigment, no capsule: on broth-agar media their colonies were round, slightly convex and yellow in colour. The physiological properties of the both are alike (Table 1).