Respostas fisiológicas de plantas de soja coinoculadas com diferentes microorganismos e submetidas ao alagamento em condições de elevado CO2

Abstract

The increase in atmospheric CO₂ has prompted extensive research into its impacts on terrestrial life, with projections indicating that current levels (419 µmol mol⁻¹) may double by the end of the century. Simultaneously, the area affected by flooding due to irregular rainfall is expanding, with approximately 17 million km² considered vulnerable. Brazil, the world’s largest soybean producer, relies heavily on inoculation with diazotrophic bacteria to achieve high yields. In Rio Grande do Sul, the country’s fourth largest soybean-producing state, cultivation often occurs in rotation with irrigated rice, in areas with a history of flooding. Although elevated CO₂ benefits carbon and nitrogen metabolism in C₃ plants such as soybean, flooding impairs photosynthesis and biological nitrogen fixation by Bradyrhizobium. Coinoculation with Rhizophagus intraradices and Azospirillum brasilense has emerged as a promising strategy to mitigate abiotic stresses and enhance the metabolic performance of plants. In this context, it is essential to investigate the combined effects of elevated atmospheric CO₂ and root-zone flooding, also considering the influence of coinoculation on the physiological response of soybean under these conditions. Therefore, the objective of this study was to elucidate the carbon and nitrogen metabolic adaptations in soybean plants coinoculated with Bradyrhizobium elkanii, Azospirillum brasilense, and Rhizophagus intraradices, grown under elevated CO₂ and subjected to flooding. To this end, soybean plants were exposed to high CO₂ levels (750 µmol mol⁻¹) and flooding. In the first chapter, plants were grown in pots filled with soil, inside open-top chambers under ambient CO₂ (a[CO₂], 420 µmol mol⁻¹) and elevated CO₂ (e[CO₂], 750 µmol mol⁻¹). At the V5 stage, plants were subjected to seven days of flooding followed by four days of drainage. Fermentative (alcohol dehydrogenase and lactate dehydrogenase) and antioxidant (superoxide dismutase, catalase, and ascorbate peroxidase) enzyme activities, as well as biometric parameters such as shoot and root dry mass, were evaluated. Coinoculation under e[CO₂] attenuated the negative effects of flooding. Notably, plants inoculated only with B. elkanii under e[CO₂] showed similar performance to coinoculated plants under a[CO₂], indicating that coinoculation alleviates flood-induced stress, yielding physiological responses comparable to those of plants grown under e[CO₂]. In the second chapter, under similar conditions, foliar gas exchange, glycolate oxidase, glutamine synthetase, glutamate synthase, and biometric characteristics were assessed. Elevated CO₂ significantly enhanced gas exchange, especially in plants coinoculated with Azospirillum (CA). Overall, e[CO₂] improved photosynthesis and suppressed photorespiration, while CA mitigated photorespiratory stress caused by flooding and preserved nitrogen metabolism. The results demonstrate that coinoculation with Azospirillum under e[CO₂] synergistically enhances soybean resilience to flooding, offering a sustainable strategy for agriculture. Finally, in the third chapter, foliar coinoculation with Azospirillum was evaluated in soybean plants initially inoculated with Bradyrhizobium. Using the same variables as in Chapter 2, high efficiency of foliar coinoculation with Azospirillum was observed. Under normal CO₂ conditions, foliar coinoculation emerged as an alternative approach that effectively mitigates the harmful effects of flooding. In conclusion, coinoculation with Azospirillum or Rhizophagus is a promising strategy to increase soybean tolerance to flooding, particularly in the context of climate change and elevated CO₂. These findings contribute to the development of stress-tolerant cultivars and the advancement of coinoculation techniques in soybean cultivation.