Biochemical and growth responses of mungbean to actinobacterial inoculation under water stress

Drought severely limits mungbean growth by disrupting osmotic balance, photosynthetic function, and oxidative homeostasis. This study evaluated five actinobacterial inoculants in mungbean seedlings under water deficit to clarify how microbial treatments influence biomass, proline, chlorophyll, and malondialdehyde. The results showed a clear treatment-dependent physiological divergence. Among the tested isolates, Streptomyces clavuligerus PQ120343 and Streptomyces clavuligerus PQ120390 consistently produced the highest seedling dry weight together with increased proline accumulation, greater chlorophyll retention, and lower malondialdehyde relative to the uninoculated drought control. Integrative analysis indicated that biomass maintenance under drought was most strongly associated with chlorophyll retention and, within drought-exposed treatments, was further supported by higher proline and lower malondialdehyde. These findings show that effective actinobacterial drought mitigation in mungbean is not merely the reduction of stress symptoms, but a coordinated acclimation response involving osmotic adjustment, preservation of the photosynthetic apparatus, and attenuation of oxidative membrane damage. The study thus refines earlier observations by identifying chlorophyll retention as the strongest physiological link to drought-maintained biomass, while confirming S. clavuligerus PQ120343 and PQ120390 as the most promising candidates for development as drought-mitigating bioinoculants in mungbean.