Both rhizosphere bacteria and humic substances (HSs) can promote plant growth when applied individually and even greater effects of their combination have been demonstrated. We aimed to elucidate the relative importance of the stimulating effects of HSs on bacterial growth and the effects of the combination of bacteria and HSs on plants themselves. The effects of humic (HA) and fulvic acids (FA) (components of humic substances) on the growth of Pseudomonas plecoglossicida 2,4-D in vitro were studied. We also studied the effects of this bacterial strain and HSs applied individually or in combination on the growth of wheat plants. Although the 2,4-D strain showed low ability to use HSs as the sole source of nutrition, the bacterial growth rate was increased by FA and HA, when other nutrients were available. HSs increased root colonization with bacteria, the effect being greater in the case of HA. The effects on roots and shoots increased when bacteria were associated with HSs. FA+ 2,4-D was more effective in stimulating shoot growth, while HA + 2,4-D was in the case of root growth. The latter effect is likely to be beneficial under edaphic stresses.

The search for ways to increase plant productivity in drought conditions is of fundamental importance, since soil moisture deficiency is widespread and leads to critical crop losses. The aim of this study was to identify the effects of plant growth-promoting bacteria and humic substances on the growth, chlorophyll, flavonoids, nitrogen balance index, and concentration of cytokinins and abscisic acids in wheat plants grown in the laboratory under conditions of water deficit. An increase in the accumulation of plant mass was shown during the treatment of wheat plants with Pseudomonas plecoglossicida 2,4-D and humic substances in these conditions. It has been shown that stimulating plant growth is associated with increased root growth, which leads to an increase in the nitrogen balance index, chlorophyll, and flavonoid concentrations in treated plants. The detected increase in the concentration of chlorophyll in plants treated with P. plecoglossicida 2,4-D correlated with a decrease in the concentration of abscisic acid in plant shoots and, in plants treated with humates, with an increase in the concentration of cytokinins in shoots. The higher efficiency of treating plants with a combination of bacteria and humic substances than with any of them individually may be associated with the additive effect of these treatments on the hormonal balance.

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.

HORMONE BALANCE AND SHOOT GROWTH IN WHEAT (Triticum durum Desf.) PLANTS AS INFLUENCED BY SODIUM HUMATES OF THE GRANULATED ORGANIC FERTILIZER A.M. Nazarov, I.N. Garankov, I.O. Tuktarova, E.R. Salmanova, T.N. Arkhipova, I.I. Ivanov, A.V. Feoktistova, G.R. Kudoyarova

Improving the quality of tree planting material for carbon sequestration through reforestation can help solve environmental problems, including the need to reduce the concentration of carbon dioxide in the atmosphere. The purpose of this study was to investigate the possibility of using humic substances in combination with rhizosphere microorganisms Pseudomonas protegens DA1.2 and Pseudomonas sp. 4CH as a means to stimulate the growth of seedlings of pine, poplar, large-leaved linden, red oak, horse chestnut, and rowan. Humic substances stimulated the growth of shoots and roots of pine, large-leaved linden, and horse chestnut seedlings. The effects of bacteria depended on both plant and bacteria species: Pseudomonas protegens DA1.2 showed a higher stimulatory effect than Pseudomonas sp. 4CH on pine and linden, and Pseudomonas sp. 4CH was more effective in the case of chestnut. An additive effect of humates and Pseudomonas protegens DA1.2 on the growth rate of pine and linden saplings was discovered. Poplar, red oak, and rowan seedlings were unresponsive to the treatments. The growth-stimulating effects of the treatments are discussed in connection with the changes in carbon, chlorophyll, and nitrogen contents in plants. The results show the need for further research in bacterial species capable of stimulating the growth of plant species that were unresponsive in the present experiments.

Plants of the genus Populus are among the most economically important woody plants and are an experimental model system used to study woody species. We studied the ability of plant growth-promoting bacteria (PGPR) and humic substances (HSs) to influence the rooting of cuttings, a process that plays an important role in vegetative propagation. We used strains of bacteria from the collection of microorganisms of the Ufa Institute of Biology: Pseudomonas protegens DA1.2 and Enterobacter ludwigii BLK. For the experiment, cuttings of black poplar (Pópulus nigra L.) were used. They were placed in aqueous solution with either the addition of bacteria, HSs, or their combination. After 15 days, the number and length of adventitious roots were measured, and they were sampled for hormone immunoassay analysis and the determination of gene expression. The contents of the auxin indole-3-acetic acid (IAA) and abscisic acid (ABA) were measured, since both substances have an antagonistic effect on rooting. It was shown that the activation of root formation was due, on the one hand, to increased auxin levels under the influence of IAA-producing bacteria, resulting in increased expression of the PnRGF9 gene that encodes the root meristem growth factor. On the other hand, the activation of rooting was due to a decrease in ABA content resulting from its breakdown by bacteria of the DA1.2 strain, which catabolize this hormone. HSs, which can inhibit ABA synthesis, also played a role. As a result, the preparation for the rooting of black poplar cuttings with the greatest effectiveness was observed in the combined use of DA1.2 bacteria and HSs.