Enhancement of drought tolerance on guinea grass by dry alginate macrobeads as inoculant of Bacillus strains

Drought is responsible for almost 70% of total crop damage in tropical and subtropical countries. In the Colombian Caribbean, drought has caused low availability of Guinea grass (Megathyrsus maximus) in forage amount and quality, generating increase in production costs in livestock systems. In this study, we aimed at designing and evaluating dry macro-polymeric inoculants of Bacillus strains and used them successfully to mitigate drought effect on Guinea grass (M. maximus). We chose Bacillus sp XT13 and Bacillus megaterium XT14 strains, previously selected for their capacity to mitigate drought stress on Guinea grass. For the inoculant formulation process, a concentration of 3.2% alginate was used for both microorganisms, selecting the polymer-bacterium ratio of 70:30 for XT13 and 80:20 for XT14. The results of the experiments in greenhouse showed that drought had a negative effect on Guinea grass growth, but the PGPB application with or without macro-polymeric dry matrix improved plant growth significantly. We also observed that co-inoculation of the strains generated a greater beneficial effect than individual inoculation. Interestingly, co-inoculation of XT13 and XT14 under the dry macro-polymeric formulation increased biomass production of the aerial part (7.32%), root biomass, (25.3%), nutritional quality parameters, such as digestibility (3.32%), protein (20.3%); proline accumulation increased (21.06%) and both neutral digestible fiber (2.43%) and APX antioxidant activity (24.2%) decreased with respect to co-inoculation of both bacteria without formulation. These findings have shown that dry macrobeads immobilization has a significantly positive influence on the capacity of XT13 and XT14 bacterial strains to induce drought stress tolerance in Guinea grass.