The tripartite association between rhizobia, vesicular arbuscular mycorrhizal (Vam) fungi and kidney bean (Phaseolus vulgaris, L.): Studies on the growth response

The effect of dual inoculation on three local red kidney bean (Phaseolus vulgaris, L.) cultivars with four strains of Rhizobium phaseoli and three species of vesicular arbuscular mycorrhizal (VAM) fungi was examined in sterilized and non-sterilized clay loam soil in a greenhouse. Sterilization of soil influenced the selection of effective rhizobia as those selected were not effective in non-sterilized soil. Symbiotic efficiency (promotion of plant growth, N and P nutrition) was found to be dependent on the particular combination of Rhizobium strain, VAM fungus and cultivar of kidney bean. The response of the three cultivars of kidney bean to inoculation with compatible pairings of VAM fungus and R. phaseoli selected from greenhouse experiments were field tested. All pairings of VAM fungi and rhizobia inoculated on the three cultivars significantly increased the pod yield, mycorrhizal colonization and nodulation over the uninoculated control in the field. The survival of four R. phaseoli strains were compared in both the clay loam soil and bauxitic soil under sterile conditions and of four strains of cowpea rhizobia. While cowpea rhizobia survived well in both soil types there was a rapid decline in R. phaseoli cell numbers within five days in the bauxitic silt loam soil. The influence of two VAM fungi on the competitive ability of introduced and indigenous R. phaseoli strains for nodulation of kidney bean cvs was examined under non-sterilized soil conditions. R. phaseoli strains became more competitive against native rhizobia in the presence of either VAM fungus and the VAM fungus selectively influenced the nodulation ability of competitive strains. The effect of five flavonoid compounds on the germination of G. aggregatum spores at varying concentration of flavonoid were examined. Flavonoid compounds have a significant effect on the germination of VAM spores and on the growth of kidney bean. Root exudate of kidney bean inoculated with VAM fungus and rhizobia singly or dually was examined for the presence of the five flavonoid compounds using HPLC analysis. Four of the flavonoid compounds were present when kidney bean was dually inoculated in comparison to two flavonoid compounds detected when inoculated with rhizobia alone or one flavonoid detected when kidney bean was inoculated with VAM fungus alone indicating that flavonoid compounds are important in the tripartite association. A preliminary study on the effect of VAM fungi on growth and development of citrus and coffee seedlings demonstrate that VAM fungi are able to increase the vigor of these plants at the nursery stage.