Bilokonska O. Viability and functional activity of Аzotobacter bacteria under their introduction into cucumber (Сucumis sativus L.) agrocenosis

This thesis paper theoretically justifies and experimentally proves maintenance of viability and functional activity of Azotobacter bacteria exposed to environmental factors for their introduction into cucumber agrocenosis and regulation of crop production process. Nitrogen-fixing bacteria, in particular Azotobacter, contribute to improvement of nitrogen nutrition of agricultural crops due to colonization of plant rhizospheric zone. Azobacter produces biologically active substances significantly influencing growth and development of plants. After vegetable plant rhizosphere is enriched with Azobacter, their resistance against diseases improves. This is due to the fact that Azotobacter bacteria produce antibiotic substances characterized by antibacterial and antifungal action, as well as synthetize fungistatic — tetraenoic acid ester. During the study 83 bacterial isolates belonging to Azotobacter were identified. According to the parameters of nitrogen-fixing activity, these isolates were compared with the reference strains of Azotobacter vinelandii М-Х, Azotobacter chroococcum М-70 and consortium of Azotobacter vinelandii and Azotobacter chroococcum М-70/2 strains. Six most active isolates were selected among them. A. chroococcum 2.1 (ІМВ В-7836) has the highest activity among nitrogen-fixers tested. We have studied the survival of Azotobacter bacteria cells on the seeds exposed to abiotic factors (temperature, UV exposure, chemical substances). Experimental results obtained during research allowed to establish conditions (use of cells in the form of cysts, reduction of the temperature and intensity of UV exposure) under which Azotobacter bacteria survive on the seeds and the ways to prevent the reduction of viability of diazotrophs when PPC is applied. Temperature exposure to the microorganisms applied on cucumber seeds stored at +4 °С to +28 °С were studied. It was established that survivability of bacterial cells reduces with temperature increase. PPC contributes to improved viability of bacteria both in the form of cysts and vegetative cells. The number of viable cells in the form of cysts after addition of PPC exceeds viability value of the cells in the form of cysts without PPC. After a 3-month storage, the difference between the number of viable cells in the form of cysts after addition of PPC was 2.1-fold higher than without PPC. It was established that bacterization of cucumber seeds contributes to the improvement of nitrogen-fixing ability in the root zone of plants. The results suggest that activity of nitrogen fixation under bacterization with A. chroococcum 2.1 (ІМВ В-7836) cells in the form of cysts with addition of PPC contributes to 3-fold increase in nitrogenase activity relative to control. Treatment of seeds with the suspension of A. chroococcum 2.1 (ІМВ В-7836) vegetative cells with the addition of PPC contributed to increased nitrogen-fixing ability by 76.8 %. Application of the microbial inoculate based on A. chroococcum 2.1 (ІМВ В-7836) strain in the technology of cucumber cultivation contributes to the increased yield of the culture compared to the absolute control (without bacterization) in average up to 36.7 % throughout the and compared to the positive control (consortium of А. chroococcum and А. vinelandii М-70/2) — up to 18.7 %. It was found that pre-sowing inoculation of seeds with the cells of A. chroococcum 2.1 (ІМВ В-7836) in the form of cysts with the addition of PPC contributes to the reduction of cost per unit by 10.8 %, increase in profit margin by 353019 UAH/ha (49.4 %) and increase in production profitability predicted level for vegetable product by 34.9 percentage points, which provides rate of return of extra charges by the profit at the level of 10.49 UAH/UAH. Energy efficacy ratio of cucumber production by total energy content of the main and secondary product is 7.5 %. Whilst the energy content of the cucumber yield increases from 108375 to 149200 MJ/ha (37.7 %). Thus, we have selected a new strain of Azobacter characterized by high nitrogen-fixing and phytohormonal activity. Bacterization with A. chroococcum 2.1 (ІМВ В-7836) positively influences biometric parameters of plants (plant height, length of internodes) and dry weight of cucumber. During the thesis work it was established that reduction in the efficacy of inoculation after early application of Azobacter can be compensated due to polysaccharide-protein complex that improves viability of microbial cells at rest. Pre-sowing bacterization with A. chroococcum 2.1 (ІМВ В-7836) in the form of cysts with the addition of PPC was the most efficient. A. chroococcum 2.1 (ІМВ В-7836) contributes to the increased level of economic and energy efficacy of cucumber production.