Biochemical Characterization and Assessment of Polyethylene Glycol (PEG)-Induced Drought Tolerance Potential of Azospirillum Isolates

Abstract

Drought stress is a major constraint to crop productivity and the use of drought-tolerant plant growth-promoting bacteria such as Azospirillum offers a promising strategy for sustainable agriculture. The present study aimed to characterize twenty Azospirillum isolates collected from the Culture Collection Bank of the Department of Agricultural Microbiology and assess their drought tolerance potential under Polyethylene Glycol (PEG-6000)-induced osmotic stress. The isolates were subjected to a series of biochemical tests, including Gram staining, catalase activity, methyl red, Voges–Proskauer and citrate utilization. All isolates were Gram-negative and catalase-positive, while variability was observed in MR-VP and citrate utilization profiles, indicating metabolic diversity among the isolates. Drought tolerance was evaluated by measuring the growth (OD₆₀₀) of isolates cultured in nitrogen-free Okon’s medium supplemented with increasing PEG concentrations (0%, 10%, 20% and 30%). A progressive decline in bacterial growth was observed with higher PEG levels; however, the magnitude of reduction varied markedly among isolates. Strong osmotic stress tolerance was demonstrated by Azos-CH-48, which continuously showed the highest growth across all PEG concentrations, followed by Azos-RS-177 and Azos-12. Overall, the study revealed significant biochemical and physiological variability among Azospirillum isolates and identified a subset of strains with superior tolerance to PEG-induced water stress. These drought-resilient isolates hold potential as promising bio-inoculants for improving crop performance under moisture-deficit conditions. Further molecular characterization and field validation are recommended to support their application in stress-prone agroecosystems.