Effects of Streptomyces pactum Act12 broth on growth and physiological characteristics of Medicago sativa seedlings under petroleum stress

Abstract [Objective] This study aims to investigate the impact of Streptomyces pactum Act12 broth on growth and physiological characteristics of Medicago sativa seedlings under petroleum stress. [Methods] The effects of different concentrations (0, 1%, 10%, and 100%) of Act12 broth under a petroleum stress level of 3% (W/W ) were examined by determining seedling growth, root morphology, photosynthetic characteristics, osmoregulatory substance content, and antioxidant capacity in M . sativa. [Results] The 3% petroleum significantly hindered photosynthesis and biomass accumulation of M . sativa seedlings, compared to the control group (CK). Moreover, it resulted in elevation in malondialdehyde level, decrease in soluble sugar and soluble protein content, as well as alteration in the activities of superoxide dismutase (SOD) and peroxidase (POD). Compared to the absence of Act12 broth, solutions with low and medium concentrations (1% and 10%) effectively mitigated the toxic effects of petroleum, with 10% Act12 broth exhibiting the most significant effect. The seedlings treated with Act12 broth exhibited significant enhancements in plant height, above-ground dry mass, underground dry mass, stem tolerance index, root tolerance index, total root length, root surface area, root volume, and number of root tips. Additionally, the level of MDA was decreased by 42.49% and 56.45%, soluble sugar was increased by 79.25% and 89.83%, while soluble protein was increased by 167.63% and 256.15% in the above-ground and underground parts, respectively. Moreover, SOD and POD activity were increased by 35.81% and 57.33%, respectively. [Conclusion] The fermented liquid with low/medium concentration effectively modulates antioxidant enzyme activity and osmotic regulation ability of seedlings, resulting in reduced MDA content, enhanced photosynthesis ability, increased biomass, and eventually alleviates the inhibition effects of petroleum stress on seedling growth.