To mitigate the deleterious effects of abiotic stress, the use of plant growth-promoting bacteria along with diazotrophic bacteria has been increasing. The objectives of this study were to investigate the key enzymes related to nitrogen and carbon metabolism in the biological nitrogen fixation process and to elucidate the activities of these enzymes by the synergistic interaction between Bradyrhizobium and plant growth-promoting bacteria in the absence and presence of salt stress. Cowpea plants were cultivated under axenic conditions, inoculated with Bradyrhizobium and co-inoculated with Bradyrhizobium sp. and Actinomadura sp., Bradyrhizobium sp. and Bacillus sp., Bradyrhizobium sp. and Paenibacillus graminis, and Bradyrhizobium sp. and Streptomycessp.; the plants were also maintained in the absence (control) and presence of salt stress (50 mmolL−1 NaCl). Salinity reduced the amino acids, free ammonia, ureides, proteins and total nitrogen content in nodules and increased the levels of sucrose and soluble sugars. The co-inoculations responded differently to the activity of glutamine synthetase enzymes under salt stress, as well as glutamate synthase, glutamate dehydrogenase aminating, and acid invertase in the control and salt stress. Considering the development conditions of this experiment, co-inoculation with Bradyrhizobium sp. and Bacillus sp. in cowpea provided better symbiotic performance, mitigating the deleterious effects of salt stress.
Resumo Problemas de salinidade são comuns em regiões áridas e semi-áridas como o Nordeste do Brasil, onde grandes segmentos populacionais têm, no caju, importante fonte de renda. Com o objetivo de avaliar a influência da salinidade no cultivo de cajueiro anão-precoce, plantas do clone CCP 76, com cerca de sete anos, foram irrigadas durante nove meses com água contendo diferentes níveis de sais (CE = 0,5; 1,5; 3,0; 6,0 e 9,0 dS m-1). Neste período, foram avaliados o crescimento vegetativo (altura da planta e diâmetro da copa), as trocas gasosas (fotossíntese líquida, condutância estomática e transpiração) e os teores foliares de K, P, Ca e Na. Os níveis de salinidade da água de irrigação não afetaram as trocas gasosas e favoreceram o crescimento vegetativo das plantas adultas. As trocas gasosas e os teores de minerais nas folhas variaram ao longo do período experimental, parecendo resultar de mudanças no ambiente e na idade fisiológica das folhas amostradas. O teor de Na nas folhas aumentou em função da salinidade da água de irrigação, porém não atingiu o limite de toxidez da espécie.
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