Competition between fast- and slow-growing tropical legume rhizobia for nodulation ofVigna unguiculata

Summary

The competition between slow- and fast-growing types of rhizobia for nodulation of cowpeas was studied using modified Leonard-jars. Rhizosphere populations of Rhizobium, mixed and pure populations, were examined on cowpea plants grown in large glass tubes. At 25°C and at a low level of inoculation, the fast-growing strain, NGR 234, dominated the rhizosphere but at 30°C, the slow-growing strain CB 756 dominated or was equally represented depending on its initial level in the mixed inoculum. At 25°C/23°C (day/night), the fast-growing strain from Lablab (NGR 234) was a superior competitor for nodule sites compared with three slow-growing strains of Rhizobium. When plants were grown at 30°/26°C, the slow-growing rhizobia were the better competitors for host nodulation. Fast-growing strains from Mimosa and Leucaena were poor competiors at both temperatures. The results were influenced by the proportions of the competing strains, as well as the total numbers of rhizobia, in the mixed inoculum. Other factors, including ‘host preferences’, host compatibility and time taken for nodule initiation, also contributed to the relative success of competing strains. The results reflect the persistent isolation in tropical regions of slow-growing cowpea-type of rhizobia from most tropical legumes.

Dual occupancy of nodules by both fast- and slow-growing rhizobia occurred in 0–15% of the nodules formed for most paired inocula. However, the fast-growing Leucaena isolate NGR 14/1 was present in 73 to 83% of the nodules formed by the slow-grower CB 756.