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HIGHLIGHTS  Growth of weeds influenced by safflower residues depend on seed size and genotypes of safflower.  Weed plants with smaller seed size were more sensitive to allelopathic residues.  Radicle length was affected more by safflower residues in comparison to other growth traits.

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.

Forty safflower genotypes were grown under normal irrigation and drought stress. In the first experiment, the allelopathic potential of shoot residues was evaluated using the sandwich method. Each genotype residue (0.4 g) was placed in a sterile Petri dish and two layers of agar were poured on that. Radish seeds were placed on agar medium. The radish seeds were cultivated without safflower residues as the controls. The length of the radicle, hypocotyl, and fresh biomass weight and seed germination percentages were measured. A pot experiment was also done on two genotypes with the highest and two with the lowest allelopathic activity selected after screening genotypes in the first experiment. Before entering the reproductive phase, irrigation treatments (normal irrigation and drought stress) were applied. Shoots were harvested, dried, milled and mixed with the topsoil of new pots and then radish seeds were sown. The pots with safflower genotypes were used to evaluate the effect of root residue allelopathy. The shoot length, fresh biomass weight, and germination percentage were measured. Different safflower genotypes showed varied allelopathic potential. The results of the first experiment showed that Egypt and Iran-Khorasan genotypes caused maximum inhibitory responses and Australia and Iran-Kerman genotypes resulted in minimum inhibitory responses on radish seedling growth. Fresh biomass weight had the most sensitivity to safflower residues. The results of the pot experiment were consistent with the results of in vitro experiments. Residues produced under drought stress had more inhibitory effects on the measured traits. Safflower root residue may have a higher level of allelochemicals or different allelochemicals than shoot residue.

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