DIFFERENCES AMONG RHIZOBIUM-MELILOTI AND BRADYRHIZOBIUM-JAPONICUM STRAINS IN TOLERANCE TO DESICCATION AND STORAGE AT DIFFERENT RELATIVE HUMIDITIES

The behaviour of Rhizobium meliloti and Bradyrhizobium japonicum (three strains of each) during storage in atmospheres of controlled relative humidities (r.h.) of from 3 to 83.5% at 30-degrees-C in the presence of air were investigated. Whatever the r.h. values imposed, the desiccation step did not significantly affect the survival rates except for B. japonicum G49. However, differences in behaviours and tolerances against protracted storage at the five r.h. values tested were noted between rhizobia and bradyrhizobia. Rhizobium meliloti strains were found more susceptible to storage at 3 or 83.5% r.h. than B. japonicum strains. Whatever the r.h. values, cell populations of B. japonicum declined at a rather constant rate and no surviving cells were observed after 100 days of storage. On the contrary, two step decreases were noted for R. meliloti stored at less-than-or-equal-to 67.8% r.h. and appreciable survival rates were still obtained after 100 days of storage at 22, 43.6 or 67.8% r.h. Based on scanning electron microscope examinations, it was found that drying by itself did not morphologically affect brady(rhizobia) cells. The combined processes of drying and rehydration led to minor increases in cell volumes (for cells stored at 83.5% r.h.) and appearance of numerous granules and strands of exopolymers (for cells stored at 3% r.h.). After storage in their dried states, loss of infectiveness was observed for B. japonicum cells whereas R. meliloti cells mmaintained the same degree of symbiotic infectivity.