Anatomical and ultrastructural alterations in Ceratopteris pteridoides under cadmium stress: A mechanism of cadmium tolerance

The present research is an appraisal of anatomical and ultrastructural alterations in aquatic fern, Ceratopteris pteridoides under cadmium (Cd) exposure. Plants were cultured hydroponically for 12 consecutive days in different Cd treatments: 10 mu M L-1 (CDT1), 20 mu M L-1 (CDT2), 40 mu M L-1 (CDT3) and 60 mu M L-1 (CDT4). Anatomical and ultrastructural changes of different vegetative tissues of C. pteridoides were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Cd stress significantly (P < 0.05) decreased water content percentage (WC%), relative growth rate (RGR) and root activity in C. pteridoides, especially at highest Cd concentration (treatment CDT4). Significant (P < 0.05) drop of stress tolerance indices (STI) was noticed in C. pteridoides under treatment CDT4. Anatomical study of the Cd-treated C. pteridoides showed stomatal closure of leaves, reduction of diameter in xylem tracheids of stem and root, and decrease of intercellular spaces. Furthermore, ultrastructural alterations of leaf, stem, and root cells were evident with a damaged membrane system of chloroplast and mitochondria, disorganization of chloroplastic components, accumulation of large starch grains and plastoglobules, and formation of multivesicular bodies. The deposition of electron-dense material in the cell wall of root cells can be regarded as an important tolerance mechanism of C. pteridoides under Cd stress. Fourier transform infrared (FTIR) spectroscopy analysis of Cd-treated C. pteridoides biomass illustrated Cd-binding interaction with O-H, N-H, C-H, C equivalent to C, C=O, P=O, -C-OH and C-S functional groups of different metabolites.