Acute Effects of Engineered Nanoparticles on the Growth and Gas Exchange of Zea mays L.-What are the Underlying Causes?

The increasing use of nanoparticles (nps) in consumer and industrial applications raises concerns about their potential risks to ecosystems and biological systems. The nps can cause negative effects on bacteria, algae, and animals. However, only little is known about their effects on higher plants and the underlying mechanisms. Zea mays L. "Ayrro" was used to investigate effects of ZnO (30-40 nm), TiO2 (5-15 nm), and Ag (15 nm) nps, in comparison their corresponding bulk counterparts, on germination and early seedling growth. Treatment with nps affected growth positively (ZnO) or negatively (TiO2, Ag) in a dose-dependent manner. Effects of the corresponding bulk counterparts were either similar (TiO2) or opposite (ZnO), or even absent (Ag). To separate direct np effects (Bnano effects<^>) from effects of ions released from nps, roots of 5-week-old plants were either treated with Ag nps or Ag+ ions with the same effective concentrations of dissolved free Ag+ ions, each with or without CaCl2 to precipitate free Ag+ ions as AgCl. Both Ag treatments reduced transpiration and assimilation rate. After addition of CaCl2, these negative effects disappeared, indicating that acute negative effects can be largely attributed to free Ag+ ions, rather than to specific nano effects. Further research with longer exposure times and different growth media could provide further insights in the analysis of np effects on plants.