Seed germination is the beginning of a physiological process that needs water imbibitions [25]. However,
in this case, rice seed germination occurred normally but the toxic effect is more pronounced in the roots,
probably due to the rice seed coat, which can act as a protector for the embryo but cannot totally guard the
whole seed. This result related is similar to the report of Yang and Watts [14] who found that alumina
nanoparticles (nano-Al2O3) at 2000 mgL-1 could inhibit root elongation of five plant species. However, in our
case, nano-ZnO was found to be more toxic than nano-Al2O3 when considering on concentration.
This evidence supporting that some engineered nanoparticles could exert physical or chemical toxicity
on plant depending on their chemical composition, size, surface energy and importantly is the species of
plant which resulting in different ways. Therefore, the challenge for further studies is the uptake kinetics and
interaction mechanisms within cells, also the maximum amenable amount of these nanoparticles which
plants can take without showing any signs of stress.