TEMPERATURE-SENSITIVE NODULATION AND N2 FIXATION OF Rhizobium leguminosarum BIOVAR Phaseoli STRAINS

Two strains of Rhizobium leguminosarum biovar phaseoli, both previously Nod+Fix+ and efficient in N2 fixation, now have altered symbiotic properties presumably through spontaneous mutation. Rhizobium leguminosarum biovar phaseoli strain RCR 3605 was Nod−Fix− in a cool growth temperature regime (14–23 °C) and Nod+Fix− in a warm regime (17–23 °C) on seven of nine bean cultivars studied. Rhizobium leguminosarum biovar phaseoli strain CIAT 57 was Nod+Fix− in both temperature regimes. CIAT 57 and RCR 3605 are synonyms for strains derived from the same parent strain but housed in different culture collections. The nodulation deficiency in CIAT 57 is presumably genetically predetermined in the bacterium rather than in the plant because no Nod+Fix+ phenotype was identified on nine bean cultivars tested. A control R. leguminosarum biovar phaseoli strain, RCR 3644, was efficient on all cultivars. In the warm temperature regime, the Nod+Fix− phenotype was observed with strain RCR 3605 only on some cultivars, suggesting that the host plant genome played a role in this ineffective symbiosis. It is likely that deleterious genetic mutations during prolonged storage may be altering the N2-fixing ability of these rhizobial strains and that commercial inoculants must be continually monitored for symbiotic performance of strains with a variety of host plant cultivars. Key words: Field bean, Rhizobium leguminosarum biovar phaseoli, N2 fixation