Title______________________________________________________________________ Influence of Rhizobium on the growth and yield of Rice
Abstract___________________________________________________________________ Rice (Oryza sativa L.) is one of the world's most important crops. The present investigation was designed to assess the range of growth-promoting activities of various diazotrophic bacteria on rice seedling vigor, its carryover effect on straw and grain yield, and the persistence of an inoculant strain on rice roots under greenhouse conditions. Growth responses to inoculation exhibited bacterial strain–rice variety specificity that were either stimulatory or inhibitory. Growth responses included changes in rates of seedling emergence, radical elongation, height and dry matter, plumule length, cumulative leaf and root areas, and grain and straw yields. Most notable were the inoculation responses toRhizobium leguminosarum bv. trifolii E11 and Rhizobium sp. IRBG74, which stimulated early rice growth resulting in a carryover effect of significantly ( P = 0.05) increased grain and straw yields at maturity, even though their culturable populations on roots diminished to below detectable values at 60 d after planting. The test strains were positive for indole-3-acetic acid production in vitro, but only some reduced acetylene to ethylene in association with rice under laboratory growth conditions. These studies indicate that certain strains of nonphotosynthetic diazotrophs, including rhizobia, can promote growth and vigor of rice seedlings, and this benefit of early seedling development can carryover to significantly increased grain yield at maturity. Rhizobia, the root-nodule endosymbionts of leguminous plants, also form natural endophytic associations with roots of important cereal plants. Despite its widespread occurrence, much remains unknown about colonization of cereals by rhizobia. We examined the infection, dissemination, and colonization of healthy rice plant tissues by four species of gfp-tagged rhizobia and their influence on the growth physiology of rice. The results indicated a dynamic infection process beginning with surface colonization of the rhizoplane (especially at lateral root emergence), followed by endophytic colonization within roots, and then ascending endophytic migration into the stem base, leaf sheath, and leaves where they developed high populations. In situ CMEIAS image analysis indicated local endophytic population densities reaching as high as 9 × 1010 rhizobia per cm3 of infected host tissues, whereas plating experiments indicated rapid, transient or persistent growth depending on the rhizobial strain and rice tissue examined. Rice plants inoculated with certain test strains of gfp-tagged rhizobia produced significantly higher root and shoot biomass; increased their photosynthetic rate, stomatal conductance, transpiration velocity, water utilization efficiency, and flag leaf area (considered to possess the highest photosynthetic activity); and accumulated higher levels of indoleacetic acid and gibberellin growth-regulating phytohormones. Considered collectively, the results indicate that this endophytic plant-bacterium association is far more inclusive, invasive, and dynamic than previously thought, including dissemination in both below-ground and above-ground tissues and enhancement of growth physiology by several rhizobial species, therefore heightening its interest and potential value as a biofertilizer strategy for sustainable agriculture to produce the world's most important cereal crops. Introdution_____________________________________________________________ Greater production of cereals brings forth higher production cost and pollutes the soil environment due to excessive use of chemical fertilizers. Therefore, crop scientists are exploring an alternative source namely biofertilizers which are cost effective and environment friendly. In the biofertilizer technology, Rhizobium-legume...
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