Toxicity of biosynthetic silver nanoparticles on the growth, cell ultrastructure and physiological activities of barley plant

Silver nanoparticles (AgNPs) were biosynthesized using the cell-free filtrate of bacterium Proteus mirabilis, reacted with 1 mM of AgNO3 solutions at 37 degrees C. The synthesis of AgNPs was monitored by UV-Vis spectroscopy and transmission electron microscopy (TEM) equipped with selected area electron diffraction (SAED). The results point to formation of spherical to cubical particles of AgNPs ranging in size from 5 to 35 nm with an average of 25 nm in diameter. The toxicity of Ag on barley (Hordeum vulgare L. cv. Gustoe) that was subjected to Ag+ as AgNO3 and AgNPs was explored. The grain germination and seedling growth of barley decreased in the presence of 0.1 mM Ag+ and was inhibited at 1 mM Ag+. In contrast, our results indicated that the AgNPs at low concentration (0.1 mM) could be useful for barley grain germination and seedling growth. However, the higher concentrations of AgNPs (0.5 and 1 mM) reduced grain germination and exhibited a stronger reduction in the root length. A decline in the photosynthetic pigments and disorganization of chloroplast grana thylakoids in Ag+ and AgNPs-treated plants confirmed the leaf chlorosis. An increase of plastoglobuli within chloroplasts was observed in Ag+ and AgNPs-treated leaves. Ag+ caused dense aggregation of nuclear chromatin materials and degeneration of mitochondria. Ag+ and AgNPs increased contents of malondialdehyde, soluble proteins, total phenolic compounds and activity of guaiacol peroxidase in barley leaves; these results point to activation of plant defence mechanisms against oxidative stress in barley.