Effect of pH and soybean cultivars on the quantitative analyses of soybeanrhizobia populations

Quantitative analyses of fast- and slow-growing soybean rhizobia population s in soils of four different provinces of China (Hubei, Shan Dong, Henan, a nd Xinjiang) have been carried out using the most probable number technique (MPN). All soils contained fast- (FSR) and slow-growing (SSR) soybean rhiz obia. Asiatic and American soybean cultivars grown at acid, neutral and alk aline pH were used as trapping hosts for FSR and SSR strains. The estimated total indigenous soybean-rhizobia populations of the Xinjiang and Shan Don g soil samples greatly varied with the different soybean cultivars used. Th e soybean cultivar and the pH at which plants were grown also showed clear effects on the FSR/SSR rations isolated from nodules. Results of competitio n experiments between FSR and SSR strains supported the importance of the s oybean cultivar and the pH on the outcome of competition for nodulation bet ween FSR and SSR strains. In general, nodule occupancy by FSRs significantl y increased at alkaline pH. Bacterial isolates from soybean cultivar Jing D ou 19 inoculated with Xinjiang soil nodulate cultivars Heinong 33 and Willi ams very poorly. Plasmid and lipopolysaccharide (LPS) profiles and PCR-RAPD analyses showed that cultivar Jing Dou 19 had trapped a diversity of FSR s trains. Most of the isolates from soybean cultivar Heinong 33 inoculated wi th Xinjiang soil were able to nodulate Heinong 33 and Williams showed very similar, or identical, plasmid, LPS and PCR-RAPD profiles. All the strains isolated from Xinjiang province, regardless of the soybean cultivar used fo r trapping, showed similar nodulation factor (LCO) profiles as judged by th in layer chromatographic analyses. These results indicate that the existenc e of soybean rhizobia sub-populations showing marked cultivar specificity, can affect the estimation of total soybean rhizobia populations indigenous to the soil, and can also affect the diversity of soybean rhizobial strains isolated from soybean nodules. (C) 2001 Elsevier Science B.V. All rights r eserved.