Estudos prévios para utilização de Azotobacter chroococcum como inoculante para cultura do algodão

Abstract

Cotton is the most significant fiber crop in the world, with an average of 32 million hectares cultivated globally, present in approximately 100 countries, particularly those with tropical and subtropical climates. In Brazil, the crop has been reaching new levels every year, reaching the milestone of the third largest global producer in the 2022/2023 harvest. To achieve a quality crop, the use of various field maintenance tools is necessary, such as chemical fertilizers, especially nitrogenous ones. Excessive or improper use of chemical fertilizers can result in a series of harmful effects, both for the environment and human health, as well as often experiencing significant price fluctuations and leading to high total production costs, as they represent a large portion of plantation expenditures. One alternative to replace or reduce their use is inoculation with rhizobacteria. Rhizobacteria are microorganisms that inhabit the rhizosphere and can provide various benefits to plants, such as biological nitrogen fixation, phosphorus and potassium solubilization, and hormone production. Among the highlights of these microorganisms, the genus Azotobacter is known to be a nitrogen-fixing bacterium capable of benefiting various plants. Thus, the objective of the present study is to investigate the potential of Azotobacter chroococcum as an inoculant for cotton. The bacterium was tested for the consumption of various carbon sources and potassium and phosphorus solubilization in vitro. In addition, the study of medium optimization was carried out in Erlenmeyer flasks and benchtop bioreactors, and from the same, the production capacity of indole-3-acetic (IAA) in the presence and absence of L-tryptophan was analyzed. Simultaneously, its erformance in a greenhouse was investigated compared to the treatment with chemical nitrogen. The bacterium was able to consume 7 out of 8 carbon sources and solubilize the potassium sources tested, however, the same effect was not observed in the analyzed phosphorus source. Regarding the optimization of the culture medium, the results found in this work, with a C:N ratio of 14 and using sucrose and yeast extract as carbon and nitrogen sources, respectively, had results 1.6 times higher than the standard medium in Erlenmeyer flasks and 2.7 times higher in benchtop bioreactors. The production of IAA was positive both in the absence and presence of L-tryptophan, reaching concentrations 210 times higher in the presence of 5 g.L-1 of the amino acid in the culture medium after 48 hours of fermentation. In greenhouse tests, the bacterium stood out in the vast majority of the analyses performed, with doses of 0.5 and 1.0 L.ha-1 being the most suitable, potentially providing a total or partial reduction in chemical nitrogen fertilizer. Thus, this study points to great potential for the use of A. chroococcum as an inoculant for cotton.