Effect of nano-zinc oxide on nitrogenase activity in legumes: an interplay of concentration and exposure time

Experiments were carried out to study the effect of zinc oxide nanoparticles (nano-ZnO) on nitrogenase activity in legumes. In the first experiment, nodulated roots of cluster bean, moth bean, green gram and cowpea were dipped in Hoagland solution containing 1.5 and 10 lg mL-1 of nano-ZnO for 24 h. Nitrogenase activity in cluster bean, green gram and cowpea roots increased after dipping in solution containing 1.5 lg mL-1 nano-ZnO, but decreased in roots dipped in solution containing 10 lg mL-1 nano-ZnO. However, in moth bean roots, nitrogenase activity decreased after dipping in solution containing either concentration of nano-ZnO. In the second experiment, nodulated roots of green gram were dipped in Hoagland solution containing 1, 4, 6, 8 and 10 lg mL-1 nano-ZnO for 6–30 h before estimating nitrogenase activity. Results showed that an interactive effect of nanoZnO concentration and exposure time influenced nitrogenase activity. The possible reasons behind this effect have been discussed. A model [A = 3.44 ? 0.46t - 0.01t 2 - 0.002tc2 (R2 = 0.81)] involving linear and power components was developed to simulate the response of nitrogenase activity in green gram roots to the concentration and exposure time of nano-ZnO.