Toxic effect of cadmium adsorbed by different sizes of nano-hydroxyapatite on the growth of rice seedlings

Information regarding the toxic effects of cadmium (Cd) adsorbed by nano-hydroxyapatite (NHAP-Cd) on the growth of crop plants remain limited. We investigated the mechanism of NHAP-Cd (diameters, 20 and 40 nm; NHAP20-Cd and NHAP40-Cd, respectively) phytotoxicity. Rice seedlings treated with Cd and NHAP20-Cd showed more severe growth retardation compared to those treated with NHAP40-Cd, for the same Cd concentration. Transmission electron microscopy revealed NHAP in the seedlings. The nanoparticles entered the rice seedlings with no Cd2+ signals in the NHAP treatments compared to −0.47 pmol cm−2 s−1 of Cd2+ fluxes in the Cd treatment. The higher Cd2+ content in the leaves and mesocotyl of NHAP20-Cd-treated rice seedlings suggested that smaller NHAP-Cd can translocate easily to the aboveground parts. Further, NHAP-Cd increased oxidative stress, which was determined as catalase activity changes in this study. Thus, NHAP-Cd particles in the growth medium can be transported to rice seedlings and cause toxicity.