Three experiments were conducted. Two nitrogen-fixing endophytes from Cuban sugarcane, Gluconacetobacter diazotrophicus (strain T2) and isolate 9°C (an unidentified bacterium) were grown in liquid and/or semisolid LGI-P medium. Nitrogenase activity (ARA) and respiratory rate (RR) (CO2 evolution measured by IRGA, Qubit System Inc.) were determined at different O2 levels. Roots from tissue-cultured sugarcane plants and from plants growing in pots (10 months old), were introduced by different periods in liquid cultures of G. diazotrophicus. Respiratory rate, guiacol-dependent-peroxidase (GdP) activity and superoxide dismutase (SOD) (Misra and Fridovich 1977), were measured in the roots. Respiration rate was also measured in the cultures of T2 that were in contact with the roots.
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