Differential response in growth and mycorrhizal colonisation of soybean to inoculation with two isolates ofGlomus clarum in soils of different P availability

Soybean (Glycine max cv. Mikiwashima) seedlings were inoculated with two tropical isolates of the vesicular-arbuscular mycorrhizal fungusGlomus clarum (Gc); isolateGcA was isolated from soils of low phosphate (P) and isolateGcB from soils of high P availability. In soil with low P,GcA was more efficient in increasing growth, nodulation and nitrogenase activity ofG. max thanGcB. Upon contact with the root surface, pre-infection hyphae ofGcA penetrated the root directly and rapidly colonised the cortical cells, while those ofGcB grew extensively on theroot surface with little host penetration. Mycorrhizal colonisation was higher in roots inoculated withGcA. Dual inoculation with the two isolates did not increase the effect of the single inoculation withGcA. In soils of high P status, both isolates formed pre-infection hyphae with few entry ooints and percentage mycorrhizal root colonisation was consequently low. The variation in efficacy of the isolates emphasizes the significance of evaluating host specificity when selecting efficient VA mycorrhizae strains for field studies.