Rhizobia inoculation improves nutrient uptake and growth of lowland rice

Growth-promoting diazotrophs can enhance the grow-th and development of ass ociated crops by transferring fixed N or by improving nutrient uptake throu gh modulation of hormone-linked phenomena in inoculated plants. Six rhizobi al diazotrophs isolated from a wide range of legume hosts were investigated to determine their growth-promoting activities in lowland rice (Oryza sati va L.) during 1997. Seeds and seedlings of rice Pankaj were inoculated with different rhizobia and grown in potted soil supplemented with varied amoun ts of mineral N. Inoculation with Rhizobium leguminosarum bv. trifolii E11, Rhizobium sp. IRBG74. and Bradyrhizobium sp. IRBG271 in creased rice grain and straw yields by 8 to 22 and 4 to 19%, respectively, at different N rat es. Nitrogen, P, and It uptake were increased by 10 to 28% due to rhizobial inoculation. Nitrogen-15-based studies indicated that the increased N upta ke was not due to biological N-2 fixation (BNF). Inoculation also increased Fe uptake in rice by 15 to 64%. Indole-3-acetic acid (IAA) accumulated in the external root environment of rice plants when grown gnotobiotically wit h rhizobia. The results indicate that certain strains of rhizobia can promo te rice growth and yield, most likely through mechanisms that involve chang es in growth physiology or root morphology rather than BNF.