Preparations for protecting plants, particularly those with fungicidal activity, continue to be relevant in agricultural production. They are used to effectively combat phytopathogens and ensure high yield of cultivated plants. However, they are among the anthropogenic factors which impose a heavy chemical load on ecosystems. Data about the effects of pesticides on physiological parameters of plants are essential for understanding the main regulatory mechanisms as preconditions to the phytotoxic state of compounds, as well as greater substantial understanding of the functional condition and implementation of adaptive potential of plants during and post stress. An important and relevant task – both practically and theoretically in the conditions of action of fungicide on seeds - is studying the possibilities of application of phytolectines as biologically active compounds with broad spectrum of action, including fungicidal effect, for spraying legumes in order to stabilize their development, ensure effective functioning of legume-rhizobial symbioses and cause fuller realization of productive potential against the background of decrease in chemical pressure on agrocenoses. Therefore, we aimed to evaluate the biological activity of soybean seed lectin (according to the parameters of productivity and functional activity of soybean-rhizobial symbiosis) at the spraying of Glycine max (L.) Merr. plants against the background of seed treatment of pesticide, Standak Top with fungicidal and insecticidal actions on the day of sowing and inoculation with Bradyrhizobium japonicum 634b. We used physiological, biochemical, microbiological and statistical methods of studies. We determined that Standak Top, applied on soybeans that were afterwards inoculated with rhizobia, exerted negative tendency on formation of vegetative mass by plants at the beginning of vegetation, though in the following phases of ontogenesis, their development and productivity reached the level of the control. Nitrogenase activity of symbiosis and the condition of photosynthetic pigment complex (content of chlorophyll and carotenoids and their ratio) were at the level or significantly lower than in the control plants. After spraying soybeans with lectin (without use of the fungicide) in the phase of development of two true leaves, there occurred significant increase in functional activity of the symbiotic system (according to total nitrogenase activity of symbiosis, higher by 1.91 and 1.79 times compared with the controls with inoculation and inoculation + fungicide) and the content of photosynthetic pigments (chlorophylls were higher by 1.12–1.45 times, carotenoids by 1.14–1.39 times) and development of strong leaf apparatus (by 1.33–1.42 times). This caused highest level of realization (by 13.9% and 10.1% higher compared with the controls with inoculation and inoculation + fungicide) of productive potential of cultivated plants. After spraying plants with soybean lectin against the background of use of fungicide, notable and reliable increases occurred in the level of absorption of molecular nitrogen (by 1.72 and 1.52 times according to total activity of symbiosis, compared with the controls with inoculation and inoculation + fungicide), content of chlorophyll (1.25–1.64 times) and carotenoids (1.12–1.42 times) in leaves of soybean, and also plants were actively developing during vegetation (1.12–1.40 times), producing yield that exceeded by 12.8% and 9.1% the controls with inoculation and inoculation + fungicide. Therefore, use of soybean seed lectin for spraying plants against the background of seed treatment of pesticide Standak Top on day of sowing can stabilize and even increase the level of realization of symbiotic and productive potential of soybean-rhizobial symbiosis compared both with the control (inoculation with rhizobia) and the variant with treatment of seeds (rhizobia + fungicide). This indicates on the perspectives of further studies of biological activity of phytolectins aiming at decreasing chemical pressure on ecosystems by leveling out or decreasing the negative impact of chemical means of protection on the plants and symbiosis.
