Growth responses of Calopogonium caeruleum to dual inoculation with vesicular-arbuscular mycorrhizal fungi and bradyrhizobia

Abstract

The growth response of the legume Calopogonium caeruleum, a ground cover used in rubber plantations, to dual inoculation with vesicular-arbuscular mycorrhizal (VAM) fungi and bradyrhizobia, was examined in both pot and field experiments. Legumes benefit from simultaneous infection with VAM fungi and Bradyrhizobium species, but nothing is known on the response of C. caeruleum to dual inoculation with both symbionts. In the pot experiment, C. caeruleum was inoculated with nine VAM-fungal species and three Bradyrhizobium strains. After 8 weeks, inoculation with VAM increased nodulation, shoot dry weight (DW) and phosphorus (P) and nitrogen (N) contents. Inoculation with Glomus sp. E 3 and Entrophospora colombiana more than doubled shoot weights. Bradyrhizobium strain RRIM77 produced significantly higher shoot DW than NC83, RRIM968 or the non- Bradyrhizobium treatments. In the field experiment, the two efficient VAM-fungal species and one Bradyrhizobium strain selected from the pot experiment were further tested on silty-clay and sandy-clay loam soils. Plants were harvested at 4, 8, 12, 24, 48 and 96 weeks for shoot DW, P and N contents. In general, VAM inoculation did not affect shoot weights at all samplings. Bradyrhizobium inoculation improved early nodulation at both sites, and shoot yields and nutrient contents at 8 weeks for one site. Significant interaction between VAM fungi and applied P occurred, where shoot weights (48 weeks at one site) or P contents (96 weeks at the other site) of plants inoculated with Glomus sp. E 3 were improved only in the presence of applied P. Similarly, interaction between VAM fungi and bradyrhizobia affected shoot DW and P contents at one site and shoot N contents at the other. It is concluded that VAM inoculation in field soils is unlikely to be worthwhile if the indigenous VAM endophyte population is effective.