Diazotroph endophytic bacteria in cultivated and wild rice in Thailand

Abstract

The population size of endophytic nitrogen fixing or diazotrophic bacteria was determined for three varieties of cultivated rice and four populations of wild rice. The dynamics of the bacterial populations from both types of rice was estimated at different stages of plant growth. The number of diazotrophic bacteria in roots, stems and leaves of all varieties of cultivated rice increased with ageing of the plants to a maximum at the heading stage (60 days after transplanting). However, nitrogenase activity could not be detected in bacterial isolates from cultivated rice. The highest bacterial population (5.25 × 106 per gram fresh material) was found in the roots of Oryza rufipogon, and this population showed the highest nitrogenase activity. In wild rice genotypes the populations of endophytic diazotrophs were stable, with the higher population in O. rufipogon as compared to O. rufipogon (18883) and O. nivara (18852). Cultures of diazotrophic bacteria were isolated and characterized as species of the genera Azosp̧irillum, Herbaspirillum, Beijerinckia and Pseudomonas. All isolates were Gram negative and motile, and produced both pectinase and cellulase. Optimum growth and nitrogen fixation activity for Azospirillum sp. was recorded at 30-35°C and pH 6.0-7.0, for Beijerinckia sp. at 25-30°C and pH 6.0-7.5, and for Herbarspirillum sp. at 30-35°C and pH 6.0-7.5.