Nitrogenase activity was inhibited significantly by NH4C1, KNO3 and Glutamine in Anabaena azollae and Anabaena variabilis. Methionine sulfoximine relieved the “inhibitory effect of NH4+ and NO3− but not of glutamine. Observations indicated that NH4+ is not the regulator for inactivation of nitrogenase and at least the conversion of NH4+ to glutamine is required to achieve the inhibition of nitrogenase activity.
