Effect of rhizosphere bacteria on antioxidant enzymes and some biochemical characteristics of Medicago sativa L. subjected to herbicide stress

Abiotic stresses such as herbicides can affect plant growth and yield. Using herbicide-resistant plant growth-promoting bacteria is a new approach to mitigate these side effects. This study was conducted to evaluate the effect of three native plant growth-promoting bacteria isolated from the Medicago sativa rhizosphere, including Serratia rubidaea (A) and Pseudomonas putida (B), Serratia sp. (C) plus Synorhizobium meliloti (R) and their combinations (AB, AC, BC, ABC, AR, BR, CR, ABR, ACR, BCR, and ABCR) on microbial population, plant biomass, antioxidant enzymes (CAT, APX, and GPX) activities, and hydrogen peroxide and malondialdehyde contents at the presence and absence of imazethapyr herbicide. The results indicated that herbicide application decreased plant biomass but increased microbial population, antioxidant enzymes activities, and the concentrations of hydrogen peroxide and malondialdehyde of all inoculated and non-inoculated plants. Bacterial inoculation in most cases increased microbial population, plant biomass, and antioxidant activities. These increases were more evident under herbicide application. The highest increase in these attributes was achieved by AB, AR, and ABR inoculums in the presence and absence of the herbicide. The microbial population, plant biomass and antioxidant activities were decreased under BC, CR, BCR, and ABCR inoculations. It can be concluded that in addition to growth promotion, these bacteria increase resistance against herbicide stress by controlling free-radical induced oxidative stress and lipid peroxidation through antioxidant enzymes. These findings create new visions in biofertilizer preparation for reducing environmental stresses.