Avaliação de métodos para controle de qualidade de inoculantes para soja

Abstract

The use of nodulating N2-fixing bacteria (NNFB) capable of establishing mutualistic symbiosis with legumes can meet plant N needs in an economic and environmentally sound way. In Brazil, the best example of biological nitrogen fixation (BNF) use is the NNFB inoculation process with the genus Bradyrhizobium in soybean (Glycine max (L.) Merrill) cultivation, where, chemical N fertilizer was completely replaced. The success of inoculation in legumes is associated with an effective quality control program. The first part of this study aimed to test the official methods established by Brazilian legislation and other techniques to evaluate the quality of commercial soybean inoculants that purport to reduce the time and cost of performing the analyses. We evaluated the media and techniques for counting viable cells and the error amplitude introduced by analysts of 11 inoculants with different substrates containing strains approved for soybeans registered with the Brazilian Ministry of Agriculture, Livestock, and Supply (Ministério da Agricultura, Pecuária e Abastecimento). All inoculants met the criteria for the minimum levels of colony-forming units (CFU) and the maximum number of contaminant CFU established by legislation. Our results indicate that the microdrop technique was superior to the spreading technique in operating procedure, experimental cost, and estimated CFU count. Culture medium 79 with Congo red or bromothymol blue was effective for counting bacterial colonies. However, bacterial contaminants remained uninhibited after vancomycin addition. There was no difference in the cell counts between analysts, but all materials and procedures were strictly standardized. In the second part, the objective was to evaluate the symbiotic effectiveness of isolates with distinct cultural characteristics of B. japonicum strains and B. elkanii present in commercial inoculants for soybean. Cultural characterization and partial sequencing of the 16S rRNA gene were used to identify the isolates. The isolates and strains recommended SEMIA 5079, SEMIA 5080, SEMIA SEMIA 587 and 5019 were tested separately for the same cultivar under controlled conditions in greenhouse. The isolates were characterized in different cultural groups. The identification by sequencing of the 16S rRNA gene showed that some isolates showed no similarity with the inoculants in product formulation. There was no loss in nodulation of the isolates. However, there was significant difference in symbiotic characteristics among isolates, the inoculants and controls without nodulation. However, the presence of morphological variants or exotic rhizobia did not influence the characteristics of the symbiotic inoculant product. The results of this study provide important information for methodological quality control programs.