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The aim of this research was to evaluate whether the application of two plant growth-promoting (rhizo)bacteria might reduce nitrogen fertilization doses in cotton. We used strains Azotobacter chroococcum AC1 and AC10 for their proven ability to promote seed germination and cotton growth. These microorganisms were characterized by their plant growth-promoting activities. Then, we conducted a glasshouse study to evaluate the plant growth promoting ability of these strains with reduced doses of urea fertilization in cotton. Results revealed that both strains are capable of fixing nitrogen, solubilizing phosphorus, synthesizing indole compounds and producing hydrolytic enzymes. After 12 weeks, the glasshouse experiment showed that cotton growth was positively influenced due to bacterial inoculation with respect to chemical fertilization. Notably, we observed that microbial inoculation further influenced plant biomass (p<0.05) than nitrogen content. Co-inoculation, interestingly, exhibited a greater beneficial effect on plant growth parameters compared to single inoculation. Moreover, similar results without significant statistical differences were observed among bacterial co-inoculation plus 50% urea and 100% fertilization. These findings suggest that co-inoculation of A. chroococcum strains allow to reduce nitrogen fertilization doses up to 50% on cotton growth. Our results showed that inoculation with AC1 and AC10 represents a viable alternative to improve cotton growth while decreasing the N fertilizer dose and allows to alleviate the environmental deterioration related to N pollution.

Drought is a global problem for crop productivity. Therefore, the objective of this research was to evaluate five dry-Caribbean Bacillus spp. strains in drought stress amelioration in maize plants. Maize seeds were single-strain inoculated and sown in pots under greenhouse conditions. After 12 days, plants were subjected to 33 days of drought conditions, i.e., 30% of soil field capacity, and then collected to measure leaf and root dry biomass, plant height, antioxidant enzymes, proline accumulation, and P+, Ca2+, and K+ uptake. Results correlated drought stress amelioration with the inoculation of Bacillus spp. strains XT13, XT38 and XT110. Inoculated plants showed increases in dry biomass, plant height, and K+ and P+ uptake. The overall maize antioxidant response to bacterial inoculation under drought stress showed dependence on proline accumulation and decreases in ascorbate peroxidase and glutathione reductase activities. Moreover, results suggest that this stress amelioration is driven by a specific plant-strain correlation observed in antioxidant response changes in inoculated plants under stress. Also, there is a complex integration of several mechanisms, including plant growth-promotion traits and nutrient uptake. Hence, the use of dry-Caribbean plant growth-promoting Bacillus strains represents an important biotechnological approach to enhance crop productivity in arid and semi-arid environments.

The indiscriminate use of chemical fertilizers has contributed to the deterioration of the biological, physical and chemical properties of the soil, resulting in the loss of its productive capacity. For this reason, the use of biofertilizers has emerged as a technological alternative. The objective of this research was to develop a suitable liquid fermentation system and encystment for the multiplication of Azotobacter chroococcum AC1 strain, a bacterium employed in a biofertilizer formulation produced at present by CARPOICA, Colombia. Sequential statistical designs were used to determine the conditions in the fermentation system. The interaction between agitation, aeration and pH was evaluated on the viable biomass (CFU/ml) of AC1. In addition, the encystment ability of the strain was evaluated using two encystment agents and the potential plant growth-promoting rhizobacteria (PGPR) activity was assessed by different techniques, such as nitrogen fixation by ARA, phosphate solubilization by the phospho-molybdenum-blue reaction and indolic compound production by colorimetric reaction using the Salkowski reagent. Results showed significant effects (p<0.05) on the viable biomass in the three conditions (pH, aeration and agitation) tested individually, in one dual interaction and one tripartite interaction, were demonstrated to have a positive effect on the response variable aeration and agitation. The addition of the two encystment agents evaluated, AE01 and AE02, demonstrated the ability of AC1 to form cysts under stress conditions. Likewise, fermentation and encystment conditions did not affect the biological activities tested.