Microbial interactions between rhizobia and VAM fungi for nodulation and growth of cowpea (Vigna unguiculata)

Vesicular-arbuscular mycorrhizal (VAM) fungi from various parts of Jamaica have been isolated and identified based on their spore morphology. They were grouped into 9 species belonging to 3 genera including Glomus, Sclerocystis, and Scutellispora. Three were purified and multiplied on corn (Zea mays) roots. Ten native Bradyrhizobium strains were isolated and tested for their effectiveness on cowpea (Vigna unguiculata (L) Walp.) in sterilized and non-sterilized soil under greenhouse conditions. Two rhizobial strains were selected from sterilized soil and two other strains were selected from non-sterilized soil. These four Bradyrhizobium strains were then paired individually with either G. pallidum, G. aggregatum or S. microcarpa in separate sterilized and non-sterilized soil experiments under greenhouse conditions designed to determine the effect of sterilization of soil on the selection of effective cowpea rhizobia; and to see whether these rhizobia differed in their effect on cowpea growth when paired with various VAM fungi. The rhizobia selected in sterilized soil produced few growth responses in cowpea compared to the other introduced rhizobia irrespective of pairing with VAM fungi in sterilized or non-sterilized soil. The two rhizobia selected in non-sterilized soil significantly improved cowpea growth in non-sterilized soil, especially when paired with G. pallidum and S. microcarpa. These two VAM fungi and two rhizobial strains were co-selected and the response of cowpea to inoculation with the co-selected pairing of VAM fungus and Bradyrhizobium strains was field tested in two different locations in two soil types. Each of the four pairings increased pod yield, mycorrhizal infection, nodule formation and shoot P and N content over uninoculated control or single inoculation of either of the microsymbionts. The influence of VAM fungus G. pallidum on the competitive ability of introduced and native Bradyrhizobium strains for nodulation of cowpea was examined under non-sterilized soil conditions. In the presence of VAM fungus, introduced Bradyrhizobium strains became more competitive than native rhizobia. VAM fungus selectively influenced the nodulation ability of competitive strains. The effect of G. pallidum on the phosphatase and cellulase activities and cytokinin content in cowpea roots was also examined. Both acid and alkaline phosphatase activities were significantly increased in mycorrhizal compared with non-mycorrhizal roots. Cytokinin content was significantly increased in mycorrhizal roots than non-mycorrhizal roots. An increased level of cellulase activity was also noted in mycorrhizal cowpea roots with maximum activity around the 21st day after inoculation.