Studies on the Physiology of Nodule Formation

Abstract

With an adequate inoculum the number of infected root hairs in three species of clover (Trifolium parviflorum, T. patens and T. glomeratum) increased exponentially with time in two phases; the increase was rapid during the first 8-10 days before nodulation begins, but slower afterwards. T. parviflorum had most infections and T. glomeratum the fewest. Experiments on varying inoculum size, using an avirulent mutan^† strain of Rhizobhtm trifolii as diluent, showed that root-hair infection was differentially limited by inoculum size during the two phases. Infection in all three species was about doubled by doubling the density of the virulent bacteria in the rhizosphere before nodulation begins. After nodulation bacterial density had to be increased much more than twice to double the number of infections. This increase in the infecting population was inversely related to the numbers of infections formed on the three host species. Early infection and nodulation were promoted by high bacterial density in the rhizosphere.