013% to 0.066% (w/w) No effect on germination, improved Rapamycin in vitro shoot/root ratio [13] Beneficial and adverse effects of metal oxide nanoparticles Bulk and nanosized TiO2 particles have different impacts on plants and microorganisms. Concentrations of bulk and nanoparticles ranging from 1 to 500 ppm have been tried on wheat germination and seedling growth. The Ti compounds showed the following improvements after the crop or seedlings were treated with it [158]: (i) The enhancement of yield of various crops, 10% to 20%   (ii)

An improvement of some essential element contents in plants   (iii) An increase in enzyme activity like peroxide, catalase and nitrate reductase activity in plant tissue   (iv) Enhancement of chlorophyll pigment   TiO2 nanoparticles have also been demonstrated to increase the rate of germination and growth of spinach (Spinacia oleracea) [10]. It is believed that such nanoparticles influence the plant growth due to

their antimicrobial properties. However, it is one of the several factors but not the consequence of antimicrobial properties that is responsible for the growth of plants. Nanosized TiO2 particles can promote nitrogen metabolism in the plant leading to growth as a whole. On the other hand, alumina nanoparticles affected adversely VX809 the elongation of corn, cucumber, soybean,

cabbage and carrot [146]. Besides TiO2, other metal nanoparticles have also been shown to influence the crop production and their vegetative growth (Table 2). In almost all studies, the size of nanoparticles appears to be the critical factor. As the concentration of metal or metal oxide nanoparticles increases, the growth increases and reaches an optimum value after which either it becomes constant or retardation triclocarban in growth occurs. In such instances, the enzyme activity is either lost or the nanoparticles block the passage of other nutrients as a consequence of accumulation. The germination time of seed with TiO2 was reduced to 0.89 days; shoot and seedling length was also increased after treatment of wheat seeds with TiO2 nanoparticles at 2- and 10-ppm concentration. When the concentration was raised to 100 ppm, no improvement was observed [10]. The effect of TiO2 nanoparticles on seed growth and germination is size and concentration dependent, because the small particles can easily penetrate the cell wall of the plant and move to various other parts.