The influence of seed-dressings on the formation and functioning of symbiotic soybean systems with fungicide-resistant rhizobia

Abstract

The soybean are one of the most valuable crops in the world’s agriculture. It has a unique chemical composition, combines high content of protein, fat, carbohydrates, enzymes, vitamins, minerals and has the ability to assimilate molecular nitrogen in symbiosis with nodule bacteria.

The application of bacterial fertilizers, prepared on the basis of active, competitive strains of nitrogen-fixing microorganisms and fungicidal seed-dressings for pre-sowing seed treatment in soybean cultivation technologies is characterized by high biological and economic efficiency. Therefore, it is important to find such combination of chemicals and microbial preparations, for the complex use of which will be maintained their main purpose. The aim of the work was to investigate the formation and functioning of symbiotic soybean systems with fungicide-resistant nodule bacteria Bradyrhizobium japonicum PC09 and B144 under the influence of Standak Top and Fever seed-dressings.

Vegetation experiment was performed with soybean plants (Glycine max (L.) Merr.) of Almaz variety, included in the State Register of Plant Varieties of Ukraine since 2007, is early ripening, has a neutral photoperiodic reaction and is recommended for cultivation in the Forest-Steppe of Ukraine. The study used seed-dressings Standak Top (BASF, Germany) with active ingredients: fipronil (250 g/l) + thiophanate-methyl (225 g/l) + pyraclostrobin (25 g/l) and Fever (Bayer CropScience AG, Germany) with active substance of prothioconazole (300 g/l). Prior to sowing, the seed was treatmented by seed-dressings and inoculated for 1 hour with a suspension of cells from an analytically selected strain of B. japonicum PC09 and a Tn5-mutant B144, obtained by transposon mutagenesis of B. japonicum 646 from Escherichia coli S17-1 containing plasmid pSUP5011::Tn5mob. The strains of nodule bacteria involved in the work according to the results of our laboratory experiments showed resistance to various rates of fungicides in pure culture.

Differences in the effect of chemical plant protection products on the formation of symbiotic systems of soybean variety Almaz and nitrogen-fixing microorganisms have been established. The number of nodules on soybean roots was lower compared to plants of other variants of the experiment throughout the growing season, regardless of the strain of B. japonicum used for inoculation, under the influence of prothioconazole, which is the active substance of Fever. A decrease by 9.2‒24.1 % in the number of nodules formed on the roots during the soybean growing season with the combined use of Fever with rhizobia bacterization of PC09 strain was revealed. At inoculation the seed with Tn5-mutant B144, use of the same preparation, the decrease of this indicator compared to control plants was 6.5‒32.8 %. As a result, the weight of root nodules formed with the participation of B. japonicum PC09 and B144 decreased in the stage of three true leaves by 13.8 and 26.3%, in the stage of budding-beginning of flowering - by 34.6% and 21.0%. The weight of root nodules in plants inoculated with PC09 and B144 cultures at treatment of Fever exceeded similar indicator of control plants in the stage of bean formation. An increase in the weight of root nodules by 10.3‒36 % was found under the effect of Standak Top and bacterization by both strains of rhizobia. There was a decrease in the intensity of N₂ assimilation compared to control plants by 24.2 and 42.3 % in the stage of three true leaves with the combined use of Fever with inoculation and a gradual decrease of the toxic effect of pesticides on the functioning of symbiotic systems in the budding-flowering stage. At shared treatment of seeds with rhizobia strains B144 and PC09 and Standak Top revealed an increase in nitrogen fixation intensity by 6.2‒25 % in the stage of three true leaves, 12.5‒58 % in the stage of budding-beginning of flowering and by 28.1‒42.3 % in the stage of bean formation.

The literature data and the results obtained by us confirm the opinion that there is a relationship between the intensity of the toxic effects of pesticides on plant-microbial interaction, the properties of biological objects and chemicals used, and point to the role of adaptation of legume-rhizobial symbiosis to the action of this anthropogenic factor. In the conditions of vegetation experiments it was found that to ensure high nitrogen-fixing activity of soybean plants it is advisable to carry out seed treatment with Standak Top and Fever in combination with bacterization of pesticide-resistant nodule bacteria B. japonicum PC09 or at the treatment of seed by Standak Top with inoculation of B. japonicum B144.

Maintaining high level of nitrogenase activity of legume-rhizobial symbiosis, formed with the participation of pesticide-resistant nodule bacteria with using of seed-dressings, will provide plants with available forms of nitrogen, protect sown seeds from soil and seed infections and create conditions for grain production.