The phytoremediation of soil contaminated with petroleum oil products relies on co-operation between plants and rhizosphere bacteria, including the plant growth-promoting effect of the bacteria. We studied the capacity of strains of Pseudomonas, selected as oil degraders, to produce plant hormones and promote plant growth. Strains with intermediate auxin production were the most effective in stimulating the seedling growth of seven plant species under normal conditions. Bacterial seed treatment resulted in about a 1.6-fold increase in the weight of barley seedlings, with the increment being much lower in other plant species. The strains P. plecoglossicida 2.4-D and P. hunanensis IB C7, characterized by highly efficient oil degradation (about 70%) and stable intermediate in vitro auxin production in the presence of oil, were selected for further study with barley. These strains increased the seed germination percentage approximately two-fold under 5% oil concentration in the soil, while a positive effect on further seedling growth was significant when the oil concentration was raised to 8%. This resulted in a 1.3–1.7-fold increase in the seedling mass after 7 days of growth, depending on the bacterial strain. Thus, strains of oil-degrading bacteria selected for their intermediate and stable production of auxin were found to be effective ameliorators of plant growth inhibition resulting from petroleum stress.

Restoring forests in areas where they once stood is an important step towards increasing carbon sequestration. However, reforestation requires an increase in current levels of seedling production in the tree nurseries. The purpose of this work was to study the effectiveness of preparations based on bacteria and humic substances (HSs) to stimulate the growth of tree seedlings in a nursery. Two selected strains of Pseudomonas and humic substances were used to treat pine and poplar plants. The treatment of seedlings was carried out during their transplantation and after it, and the effects of treatment on shoot elongation, shoot and root mass were evaluated. Treatments with both bacterial strains enhanced the growth of poplar and pine shoots and roots, which was explained by their ability to synthesize auxins. P. protegens DA1.2 proved to be more effective than P. sp. 4CH. The treatment of plants with humic substances increased the nitrogen balance index and the content of chlorophyll in the leaves of poplar seedlings, which can elevate carbon storage due to the higher rate of photosynthesis. In addition, the combination of humic substances with P. protegens DA1.2 increased shoot biomass accumulation in newly transplanted pine plants, which indicates the possibility of using this combination in plant transplantation. The increase in length and weight of shoots and roots serves as an indicator of the improvement in the quality of planting material, which is necessary for successful reforestation to increase capture of carbon dioxide.

The reaction of plants to simultaneous stress action and treatment with biological stimulants still remains poorly studied. Laboratory and field experiments have been conducted to study the growth and yield of bread wheat (Triticum aestivum L.) of the variety Ekada 113; stress markers and quantitative ratios of phytohormones in plants under insufficient soil moisture; the effects of spraying with herbicide containing 2,4-D and dicamba and growth-stimulating bacterium Pseudomonas protegens DA1.2; and combinations of these factors. Under water shortage conditions, spraying plants with Chistalan reduced their growth compared to non-sprayed plants, which was associated with inhibition of root growth and a decrease in the content of endogenous auxins in the plants. Under conditions of combined stress, the treatment of plants with the strain P. protegens DA1.2 increased the IAA/ABA ratio and prevented inhibition of root growth by auxin-like herbicide, ensuring water absorption by the roots as well as increased transpiration. As a result, the content of malondialdehyde oxidative stress marker was reduced. Bacterization improved the water balance of wheat plants under arid field conditions. The addition of bacterium P. protegens DA1.2 to the herbicide Chistalan increased relative water content in wheat leaves by 11% compared to plants treated with herbicide alone. Application of the bacterial strain P. protegens DA1.2 increased the amount of harvested grain from 2.0–2.2 t/ha to 3.2–3.6 t/ha. Thus, auxin-like herbicide Chistalan and auxin-producing bacterium P. protegens DA1.2 may affect the balance of phytohormones in different ways. This could be the potential reason for the improvement in wheat plants’ growth during dry periods when the bacterium P. protegens DA1.2 is included in mixtures for weed control.

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