Tanzania Journal of Agricultural Sciences 2002, Vol. 5(2) :71-79
Soil kept in a desiccated condition (6% moisture content, equivalent to 38.6 Mpa tension) for 2 years was found to contain 300. viable cells of cowpea bradyrhizobia, per gram of soil, contrary to the general consensus that rhizobia are highly sensitive to desiccation. Two strains of the native bradyrhizobial population were isolated via host legume infection. From each of the strains, doubly-labelled, antibiotic resistant mutants were developed in order to follow-up their survival when re-introduced into natural (non-sterile) soil. The double labeling was achieved by selection for sequential resistance to high concentrations (500 mg l) of the antibiotics streptomycin and spectinomycin. The antibiotic resistant strains were then stored in yeast-extract mannitol agar for 3 months at 4 oC. The survival ability of the antibiotic-resistant strains was then studied over a 6-week period in the same soil of their origin under conditions of favourable moisture (field capacity) or under slaw desiccation to the mosture level at which the same strains had previously survived for 2 years. This evaluation was undertaken both in previously sterilized soil and in natural (non-sterile) soil. Results of this study indicated that both strains generally survived poorly under conditions of soil desiccation. In fact the population of one of the strains declined to extinction within 6 weeks even in previously sterilized soil in which all probable antagonistic biotic factors were precluded. It was concluded from the findings of this study that the common practice of artificial cultivation and storage rhizobia in agar culture media and other forms of laboratory manipulations of the micro-organisms was partially implicated for their loss in survival ability under desiccation. It is therefore suggested that superior strains of rhizobia or bradyrhizobia intended for longer term storage for commercial inoculant production, be maintained in sterile carriers other than in agar media.