Chemical Insecticide Effects on Growth and Nitrogenase Activity of Azospirillum sp. OAD-2

The growth and nitrogenase activity of Azospirillum OAD-2 were examined in presence of five insecticides at recommended dose (RD), half dose (HD), and double recommended dose (DRD). The growth of the Azospirillum sp. OAD-2 in Luria agar plates was inhibited in descending order by chlorpyrifos, fenvalerate, quinalphos, monocrotophos, and endosulfan. The inhibition zone of Azospirillum sp. OAD-2 ranged from 2.6 to 13.2 mm and 5 to 20 mm at RD (2 mL L−1) and DRD (4 mL L−1) of insecticides, respectively. The growth of Azospirillum sp. OAD-2 in semisolid N free medium was not affected by endosulfan and monocrotophos, but inhibited by chlorpyrifos, fenvalerate, and quinalphos. However, Azospirillum sp. OAD-2 growth in malate liquid medium plus endosulfan or monocrotophos, was initially decreased (lag phase), increased after 24 h and cell growth attained a stationary phase after 48 h. Two organophosphorus insecticides, chlorpyrifos and quinalphos, and a pyrethroid, insecticides fenvalerate inhibited growth and nitrogenase activity of Azospirillum OAD-2 at all concentrations. Conversely, endosulfan (organoclorine) and monocrotophos (organophosphorus) had no detrimental effect on growth and nitrogenease activity of Azospirillum OAD-2 at RD (2 mL L−1).