Inoculation of rice plants with the endophytic diazotrophs Herbaspirillum seropedicae and Burkholderia spp.
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Four experiments were performed under gnotobiotic conditions to select strains of the endophytic diazotrophs Herbaspirillumseropedicae and Burkholderia spp. as inocula of rice plants. Eighty strains of H. seropedicae originally isolated from rice, sorghum and maize plants, were tested in test tube cultures with N-free agar as the substrate. Rice plants showed medium and high increases in their fresh weight in response to inoculation with nineteen strains. These strains were tested again, and six strains were then selected to evaluate their contribution to the N of the plant via biological N2 fixation (BNF) using an agar growth medium containing 5 mg N l–1of 15N-labelled (NH4)2SO4. The contribution of the strains to plant N via BNF varied from 54% when rice plants were inoculated with strain ZAE94, to 31% when strain ZAE67 was used. These results were confirmed in the fourth gnotobiotic experiment, which also included strains of the new N-fixing bacteria belonging to the genus Burkholderia, isolated from rice, as well as a strain of Burkholderia vietnamiensis, isolated from rice rhizosphere. Burkholderia spp. strains showed similar effects to those observed for H. seropedicae strains, while B. vietnamiensis fixed only 19% of plant total N. The best four strains were tested in a pot experiment where pre-germinated, inoculated rice seedlings were grown in soil labelled with 15N. The results confirmed the gnotobiotic experiments, although the levels of N in the rice plants derived from BNF of the selected H. seropedicae and Burkholderia spp. strains were lower. Nevertheless, there was an increase in N content in grains of inoculated plants, and the results showed that the method used for strain selection is very useful and can be applied to other strains of N2-fixing bacteria and plants.
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