In the present study, the green synthesized Mo nanoparticles (NPs) were firstly prepared using vermicompost extracts and, then, two experiments were separately carried out in a completely randomized design. The first experiment was conducted to investigate the effects of ammonium nitrate (AN) on nitrate (NO3−) accumulation rates as well as some other vegetative traits in spinach in four treatments and three replicates and the second experiment was done to investigate the effects of elemental Mo and green synthesized Mo NPs on NO3− accumulation, nitrate reductase (NR) activity and some morphological parameters in seven treatments with three replicates. The results of the first experiment indicated that the greatest accumulation of NO3− in the aerial parts of the plants was observed in the 3 M AN treatment. That is why the same concentration was utilized in the second experiment to study the effects of elemental Mo and green synthesized Mo NPs on the NR activity, NO3− accumulation and the other traits. The results of the second experiment indicated that various concentrations of elemental Mo and green synthesized Mo NPs have significant effects on all measured traits including the fresh and dry weights of the plant, NO3− concentration, NR activity, chlorophyll a (Chl a) and chlorophyll b (Chl b) rates, total chlorophyll (Chl a+b) and the plant height. Moreover, it was found that the green synthesized Mo NPs, as compared to elemental Mo, have a greater effect on the increase of NR activity and, consequently, significant reduction of NO3− accumulation.
