Abstract: TWO strains of nifogen fixing bacteria, Klebsiella pneumoniae K-12 and Enterobscter sp. E-25, isolated from the rhizosphere of local paddy fields were examined for their associative nitrogen fixation with rice seedlings cultured on C and N free media, in comparison with two known strains Azos-Pirillum lipoferum FS and Klebsiella oxytaca NG-13 introduced from abroad. All of the above strains could grow and possess associative nitrogen fixation activity under dark incubation conditions at 28. The nitrogen fixing activities of these associations were as follows. 10.5 nmol C₂H₄/seedling/hr. for Azospirillum Hpoferum FS,4.9 nmol C₂H₄/seedling/hr.for Klebsiella pneumon-iae K-12, 3.9 nmol C₂H₄/seedling/hr. for Klebsiella oxytoca NG-13, and 2.9 nmol C₂H₄/seedling/hr. for Enterobacter sp. E-25 respectively. With the exception of the aerobic Azospirillvm lipofervm FS, the other 3 strains grew vigerously around the endosperm of the seedling and formed a bacteria ring. By using DNA specific flourescenCe dye DAPI staining and scanning electron microscopic observations revealed that these 4 strains of the nitrogen fixing bacteria adhered to the root surface and built up a dense Population, Particularly at the root hair region. Scanning electron microscopic observation of the transverse section of the root and the re-isolation experiments of the endophytes indicated that the bacteria could invade the Parenchyma of the root cortex and propagate within the cell.

Key words: Nitrogen fixation, Rhizosphere of rice