NITROGEN FIXATION BY OLIGOTROPHIC BACTERIA ON THE FLOOD RICE ROOTS

概要

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Morphological and biochemical characteristics of 40 oligotrophic organisms isolated from the rice roots grown in a paddy soil were investigated. Almost all the organisms resembled each other in the morphological characteristics : They produced punctiform, convex, entire-edged colourless-colonies on 100-fold diluted nutrient broth agar under 20% O_2. They were gram-negative, motile and irregular rod-shaped. On the biochemical characteristics, the organisms were divided into 3 groups. Thirty-two organisms grew in the absence of fixed nitrogen source and were identical in utilization of the carbon compounds tested (Group A). The other organisms failed to grow on nitrogen-free media (Group B, C). All of the seven organisms arbitrarily selected out of Group A showed acetylene-reducing (nitrogen-fixing) activity under an atmosphere of 89% Ar, 10% C_2H_2 and 1% O_2. The number of bacteria belonging to Group A on the rice root samples was estimated to be 10^7 to 10^9 cells/g dry matter.
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