The effect of rhizosphere of various plants on the growth of Rhizobium

As elucidated in the previous paper¹⁾, the rhizosphere of the host plant maintains more abundant root nodule bacteria than non-rhizosphere soil. The success of seed inoculation of the root nodule bacteria is supported by the multiplication in the vicinity of the host plant.     

The growth of Rhizobium in the Rhizosphere of the host plant

Rhizobia pass through the host root surroundings before they infect the root tissue. Therefore, the environmental condition of rhizosphere is one of the greatest factors in the life of Rhizobia in the soil. It was observed that the nodule number produced on the root of the legume is not proportional to the density of Rhizobia inoculated, but nearly constant within some range. The experience presented in the previous paper showed that one or ten cells of Rhizobia inoculated in a test tube were sufficient to produce a number of nodules on the host plants grown in the tube. Such facts strongly suggest the multiplication of Rhizobia in the soil before infection into tissue.     

Estimation of number of root-nodule bacteria by a nodulation-dilution frequency method

Rhizobia spend most of their life in the soil outside the host plants. Soil conditions, therefore, may exert influence on the survival and variation of rhizobia so as to contribute to the nitrogen fixation of the legumes. The knowledge on the ecology of Rhizobium in soils may expected to give basis for the success of seed inoculation and establishment of effective strains in the field. In addition, it will render service for the study of the ecological soil microbiology, for example, ecology of pathogens of soil-borne plant diseases.