Coinoculação com Bradyrhizobium e Azospirillum em sementes de feijão-caupi influencia na nodulação, teor de nitrogênio e isoflavonas

Abstract

Biological N fixation is a well-established practice in various legumes such as soybeans; however, it is still little explored in the case of cowpeas (Vigna unguiculata (L.) Walp). In this context, it is important to understand how the application of nitrogen-fixing bacteria (N), alone or in association, can bring benefits in terms of the demand for N in the crop and reduce the use of nitrogen fertilizers, which negatively affect sustainability in agriculture through the emission of greenhouse gases, as well as the contamination of groundwater. The aim of this study was to determine whether co-inoculation of Bradyrhizobium and Azospirillum favors nodulation, N content and isoflavone production in cowpea crops. The experiment was set up in a randomized block design, on two varieties of cowpea, with seven treatments consisting of the control, isolated application of Bradyrhizobium japonicum and Azospirillum brasilense, and different doses of co-inoculation (75, 150, 225 and 300 mL for 50 kg-1 of seed of each inoculant), with four replications. At 30 days after emergence, the number of nodules, dry mass of nodules, roots and aerial parts of the plants were assessed. N content and isoflavone content were also assessed in the fully developed third trefoil from the apex of the plants harvested 45 days after emergence. The use of inoculants alone was not beneficial, nor was co-inoculation in high doses, due to the effect of competition between the bacteria in the plant's rhizosphere. Co- inoculation of B. japonicum and A. brasilense in cowpea seeds for the BRS Tumucumaque and BRS Novaera varieties is a viable and efficient practice. The dose of 75 mL of each inoculant favors the formation of nodules, root development and N content, as well as contributing to the production of isoflavones in the cowpea crop for both varieties studied.