Measuring the spermosphere colonizing capacity (spermosphere competence) of bacterial inoculants.

Spermosphere establishment by bacteria which were coated onto seeds was studied using soybean seeds treated with four bacterial strains at levels of log10 1 to 4 colony-forming units (cfu) per seed planted in a field soil mix, and incubated 48 h. Each strain at every inoculum level developed spermosphere population densities of log10 4 to 8 cfu/seed, demonstrating an average multiplication of log10 3 cfu/seed. An alternative method was developed to differentially rank bacteria for spermosphere colonizing capacity, based upon incorporation of bacteria into a soil and monitoring the resulting spermosphere population densities around noninoculated seeds after 4 days at 14 degrees C. Fifty-seven bacterial strains which were isolated from soybean roots or from water samples, including Pseudomonas putida, P. putida biovar B, P. fluorescens, Serratia liquefaciens, Enterobacter aerogenes, and Bacillus spp. were tested in the spermosphere colonization assay. Average spermosphere population densities for the 57 strains ranged from 0 to log10 7.0 cfu/seed. Strains of a given taxon demonstrated marked diversity with ranges from 0 to log10 6.0 cfu/seed for Bacillus spp. and from log10 1.4 to 7.0 cfu/seed for Pseudomonas putida. The relative ranking of representative strains was consistent in repeating experiments. The potential usefulness of the assay for efforts to develop competitive bacterial inoculants for crop seeds is discussed.