Interactive effect of suspension or encapsulated inoculum of Bacillus thuringiensis associated with arbuscular mycorrhizal fungus on plant growth responses and mycorrhizal inoculum potential

The inoculation of Bacillus thuringiensis applied as cells in suspension or encapsulated in combination or not with an arbuscular mycorrhizal (AM) fungus Glomus occultum was examined in a microcosm system on Trifolium repens plants. Succinate dehydrogenase (SDH) and alkaline phosphatase (ALP-ase) fungal activities, as index of active fungal metabolism, were determined in single and dual inoculated plants. The efficiency of G. occultum in increasing plant growth, nutrient content and nodulation was maximum when co-inoculated with the bacterium particularly in the encapsulated form. The percentage increases in growth and nutrition in AM plants co-inoculated with the encapsulated bacterium were 199 (shoot), 170 (N) and 199 (P) compared with plants inoculated with G. occultum only. Dual inoculation with these locally isolated microorganisms enhanced plant capacity for nutrient uptake. The mycorrhizal responses were correlated with the number of nodules and the amount of root cortex that became mycorrhizal. The bacterium, independent of the form of application, increased the extent of fungal mycelium showing ALP-ase activity. The impact of B. thuringiensis was greater on the ALP-ase activity in the mycelium than in the arbuscules.