The Toxic Effects Of Hg～(2+), Cd～(2+) And Their Combined Pollution In Lemna Minor L. And Mechanism Of Apoptosis

In this paper, we studied the toxic effects of Hg2+,Cd2+ and their combined pollution on Lemna minor L., and mechanism of apoptosis as well, by means of measurement of physiological index , observation of electron microscopy , RAPD , DNA ladder and TUNEL. The experimental results showed that Hg2+, Cd2+might injure photosynthetic systems , protective enzyme systems, substance metabolism systems, and cell structure of Submerged macrophytes. And the toxic effect of Cd2+ was more obvious than that of Hg2+, the interaction of Hg2++Cd2+ showed synergism. Resistance ability from endogenous protective systems and stability of biological membrane were key to resist heavy metals in plants. The two species of ions increased reactive oxygen species(ROS) content of plants. The activities of superoxide dismutase (SOD),peroxidase (POD) and catalase (CAT) were induced in high level to eliminate and resolve ROS in order to stand up for stability of membrane systems. With more serious damages, the activities of protective enzymes decreased, the defense systems of cells could not protect cells, malondialdehyde(MDA) content of plant cellular increased, and integrality of cell membrane was destroyed. Ultrastructural observation showed plasmolysis, the polymerized chromatin ,dispersion of nucleolus disruption of nuclear membrane, swelling or vesiculation of thylakoids of chloroplast and cristae of mitochondria, and disintegration of mitochondria and chloroplast.The mechanism of apoptosis in Lemna minor L. cell induced by Hg2+, Cd2+ was also studied in this article, major in the damage of DNA primary structure, ultrastructure and changes in antioxidant enzyme activities. The results showed that obvious damage of DNA was found in the process of apoptosis induced by 5-10mg/LHg2+, and 20 mg/LHg2+ had already necrotic injury. RAPD method was more sensitive than DNA Ladder when they were compared . So the former could be considered as an important method for monitoring apoptosis. As far as Cd2+ was concerned, the condensation and margination of chromatin were observed when the Cd2+concentration from 5mg/L under the electron microscope. TdT-mediated dUTP nick end labeling (TUNEL) analysis of cells revealed that the nuclear DNA strand breaks could be identified by labeling free 3'-OH termini, which was the typical feature during the apoptosis process. And 20mg/L Cd2+ had alreadybeen seen necrotic injury. The results also indicated that the activities of antioxidant enzymes could be stimulated exposing to low Cd2+ concentration to cleanup the active oxygen, and the cell apoptosis happened when their activities declined rapidly and the accumulation ROS reached certain value.