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 myceliurn showing ALP-ase activity. The impact of B. thuringiensis was greater on the ALP-ase activity in the mycelium than in the arbuscules. This led to the hypothesis that the highest nodulation induced by the bacterium would interact with ALP-ase arbuscular activity. The increased metabolic mycelial ALP-ase activity as affected by B. thuringiensis, improved N and P content in shoots of co-inoculated plants. Furthermore, B. thuringiensis increased the inoculum potential of G. occultum from 129% (suspension) to ·147% (encapsulated). Cell encapsulation was the treatment that particularly improved the efficiency of the bacterial inoculum when associated with the mycorrhizal fungus. Thus, encapsulation can be considered as a good form for bacterial inoculum application.Adriana Marulanda and Astrid Vivas are grateful to ICI (Spanish CICYT) and Fundacion Gran Mariscal de Ayacucho (Venezuela) for the grants. Financial support for this study was obtained from FEDER-CICYT (REN 2000-1724-C03-02).Peer reviewe