Rhizobial inoculation influences seedling vigor and yield of rice

Rice (Oryza sativa L.) is one of the world's most important crops. The pres ent investigation was designed to assess the range of growth-promoting acti vities of various diazotrophic bacteria on rice seedling vigor, its carryov er effect on straw and grain yield, and the persistence of an inoculant str ain on rice roots under greenhouse conditions. Growth responses to inoculat ion exhibited bacterial strain-rice variety specificity that were either st imulatory or inhibitory. Growth responses included changes in rates of seed ling emergence, radical elongation, height and dry matter, plumule length, cumulative leaf and root areas, and grain and straw yields. Most notable we re the inoculation responses to Rhizobium leguminosarum by. trifolii Ell an d Rhizobium sp. IRBG74, which stimulated early rice growth resulting in a c arryover effect of significantly (P = 0.05) increased grain and straw yield s at maturity, even though their culturable populations on roots diminished to below detectable values at 60 d after planting. The test strains were p ositive for indole-3-acetic acid production in vitro, but only some reduced acetylene to ethylene in association with rice under laboratory growth con ditions. These studies indicate that certain strains of nonphotosynthetic d iazotrophs, including rhizobia, can promote growth and vigor of rice seedli ngs, and this benefit of early seedling development can carryover to signif icantly increased grain yield at maturity.