Peculiarities of soybean-rhizobial systems subject to different levels of water supply fol-lowing treatment with succinic acid and epibrassinolide
Authors: Kots S. Y. , Rybachenko L. I. , Kukol K. P. , Pukhtaievych P. Р. , Khrapova A. V. , Rybachenko О. R. , Omelchuk S. V. 
Published: January 2024

All around the world, one of the leading – according to area of cultivated fields – oleic crops is soybean, which has a high demand for moisture. Given the significance of this crop and negative impact of drought on its yield, integrated research of the influence of insufficient water supply on the intensity of physiological-biochemical processes in those plants is necessary for identifying and understanding the drought-tolerance mechanisms of soybean, as well as symbiotic systems created with its participation, and also for search for ways to adapt it to this stressor. Therefore, our objective was determining the specifics of formation and functioning of the symbiotic systems of soybean and Bradyrhizobium japonicum, following treatment with succinic acid (0.01 g/L) and 24-epibrassinolide (0.00001 g/L), subject to different levels of watering. Our studies revealed that pre-sowing treatment of the seeds with a solution of 24-epibrassinolide with their subsequent inoculation with B. japonicum Т21-2 resulted in the most pronounced stimulation of formation and functioning of the symbiotic systems of soybean in the optimal growing conditions. At the same time, during water shortage, the intensity of nitrogen fixation was the highest in the plants grown from seeds that had been successively treated with the acid and the inoculant. We confirmed that water deficit led to significant increase in the overall content of phytohormones of cytokinin nature in the soybean root nodules, depending on the way the seeds were treated. However, the largest pool of cytokinins was seen in the plants that had been treated with succinic acid against the background of both optimal and insufficient water supply. Treatment of the seeds with 24-epibrassinolide caused significant excess of content of zeatin riboside over the content of zeatin during the flowering stage, whereas in the stage of pods formation it led to an opposite effect – excess of zeatin over zeatin riboside. Fourteen days-long water deficit decreased the content of chlorophylls in the leaves and grain productivity of the plants of all variants of the experiment. The use of growth regulators managed to alleviate the negative impact of stress and protect the pigment complex from ruination. Treatment of the seeds with solutions of succinic acid and 24-epibrassinolide provided the growth of soybean grain productivity regardless on water-supply level. The most efficient was 24-epibrassinolide. Therefore, use of 24-epibrassinolide for pre-sowing treatment of the soybean seeds provided formation of effective symbiotic systems with high nitrogen-fixing activity and caused a number of specific changes in the pattern of accumulation of free and complex forms of cytokinins in the root nodules of those plants. At the same time, the treatment provided the highest concentration of photosynthesis pigments in the soybean leaves, and as a result produced the greatest increase in grain productivity of plants of all the variants, regardless of levels of water supply. In turn, use of succinic acid produced the highest level of nitrogen-fixing activity in the case of the lowest number of root nodules in the conditions of insufficient water supply, and also caused significant accumulation of cytokinins in the nodules, compared with other studied variants against the background of both optimal and insufficient water supply. Therefore, it did result in increase in soybean grain productivity, but this was lower than in the plants treated with 24-epibrassinolide.