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DIVISION S-3-SOIL BIOLOGY & BIOCHEMISTRY

Rhizobia Inoculation Improves Nutrient Uptake and Growth of Lowland Rice

^a Soil Microbiology Lab., Soil and Water Sciences Division, IRRI, P.O. Box 3127, Makati Central Post Office, 1271 Makati City, Philippines
^b Dep. of Microbiology, Michigan State Univ., East Lansing, MI USA

j.k.ladha{at}cgiar.org

Growth-promoting diazotrophs can enhance the growth and development of associated crops by transferring fixed N or by improving nutrient uptake through modulation of hormone-linked phenomena in inoculated plants. Six rhizobial diazotrophs isolated from a wide range of legume hosts were investigated to determine their growth-promoting activities in lowland rice (Oryza sativa L.) during 1997. Seeds and seedlings of rice Pankaj were inoculated with different rhizobia and grown in potted soil supplemented with varied amounts of mineral N. Inoculation with Rhizobium leguminosarum bv. trifolii E11, Rhizobium sp. IRBG74, and Bradyrhizobium sp. IRBG271 increased rice grain and straw yields by 8 to 22 and 4 to 19%, respectively, at different N rates. Nitrogen, P, and K uptake were increased by 10 to 28% due to rhizobial inoculation. Nitrogen-15-based studies indicated that the increased N uptake was not due to biological N₂ 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 with rhizobia. The results indicate that certain strains of rhizobia can promote rice growth and yield, most likely through mechanisms that involve changes in growth physiology or root morphology rather than BNF.

Abbreviations: ANUE, agronomic N-use efficiency • BNF, biological N₂ fixation • CFU, colony-forming units • DAI, days after inoculation • DAT, days after transplanting • IAA, indole-3-acetic acid • PGPR, plant growth-promoting rhizobacteria • RP-HPLC, reverse-phase high performance liquid chromatography • YM, yeast mannitol

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