| Heavy metal (Cd) pollution of water bodies has become an increasingly serious environmental problem. Cd can cause serious health hazards to animals and humans through the food chain due to easy accumulation toxicity, refractory and other characteristics. the present study investigated (1) toxic effects of various concentrations of Cd (0,0.05,0.1,0.2,0.4mmol/L) on subcellular distribution of Cd and mineral elements, antioxidant capacity, as well as the contents of chlorophyll, soluble protein and phytochelatins (PCs) were investigated in leaves of Alternanthera philoxeroides.(2) A. philoxeroides callus, a tissue culture material, exposed to various concentrations of cadmium (Cd)(0,0.1,0.2,0.4,0.8,1.6mmol/L)) for7d was investigated to determine the bioaccumulation of Cd and its effect on photo synthetic pigment, soluble protein and hydrogen peroxide (H2O2) content, as well as various antioxidant enzymes and non-enzymatic antioxidants. The results showed that:(1) The induction of A. philoxeroides aseptic seedlings should use MS medium; with appropriate6-BA and NAA, it can promote the growth of seedling; low concentrations of NAA and6-BA induce more easily aseptic seedling, but currently those induced in this laboratory have stems thin and delicate, significantly different with natural growned plants, so we should consider add some CCC in the medium to ease the stems thin phenomenon. In the callus culture process, the best sterilization of A. philoxeroides stems is70%Alcohol50s+5%NaClO10min+0.1%HgCl210min, the best selection time of A. philoxeroides’s aseptic seedlings and callus induction is October to November. In addtion, we found in this study callus induction rate of aseptic seedlings run up to100%, so we should cultivate aseptic seedlings as callus induction materials. But at this stage, technical difficulties on the A.philoxeroides callus induction reside in how to tissue culture aseptic seedlings well growned under laboratory conditions.(2) With the rise of Cd concentration in culture medium, a) Content of Cd increased significantly in all subcellular components of A. philoxeroides leaves, and mainly distributed in cell wall, less in soluble fraction and least in chloroplast and mitochondria. As Cd concentration exceeded0.2mmol/L, Cd distribution increased in soluble fraction and decreased in cell wall, indicating metal binding to cell walls was possibly less important than compartmentalization and metal chelation in A. philoxeroides leaves as Cd concerntration increased. b) Cd stress resulted in significant imbalance of mineral elements. With the increasing of Cd concentrations, increased concentration of Ca was observed in all subcellular fractions especially in cell wall. In addition, Cd treatment severely reduced P, K contents in cell wall and soluble fraction as well as Mg, Fe in chloroplast. c) Contents of soluble protein and chlorophyll showed a gradual decrease trend, d) Levels of glutathione (GSH) and ascorbic acid (AsA) increased progressively up to0.05mmol/L and0.2mmol·L-1, respectively, followed by a subsequent decline. Meanwhile, Cd stress dramatically induced the production of PCs. It also significantly increased the level of total antioxidant capacity (T-AOC) of the tested plants. These results indicated that A. philoxeroides was resistant to Cd stress. The correlation between toxic effects of Cd and its concentrations was significant. The accumulation of Cd in subcellular components of A. philoxeroides leaves caused obvious toxicity as confirmed by imbalance of nutrients and decline in soluble protein and chlorophyll contents. A massive accumualtion of Ca in cell wall as well as the increased levels of PCs and T-AOC enhanced the tolerance of A. philoxeroides against Cd stress. It was also obvious that there was significant positive correlation between PCs content and Cd toxicity in A. philoxeroides, indicating that PCs could be considered as a sensitive biomarker for estimating Cd phytotoxicity.(3) In A. philoxeroides callus, Cd bioaccumulation increased in a dose-dependent manner with increasing Cd supply. As concentration of Cd augmented, the photosynthetic pigment content declined significantly while soluble protein, glutathione (GSH) and ascorbic acid (AsA) contents firstly increased respectively and then followed by a subsequent decline. Moreover, AsA was more affected than GSH by Cd stress. With the increasing Cd dose, the activity of peroxidase (POD) increased immediately at0.1mM Cd concentration and kept high level all the time, while superoxide dismutase (SOD) and catalase (CAT) activities inhibited each other and increased alternately. Owing to the contribution of POD, CAT and SOD, an initial rise was recorded in H2O2content which was lower than the control finally. In addition, non-protein thiols (NP-SH) and phytochelatins (PCs) abundant biosynthesis improved the ability of A. philoxeroides callus to tolerate cellular metal load. Our present study concluded that antioxidant defense system in A. philoxeroides callus had strong capacity to scavenge reactive oxygen species (ROS). POD played a vital role in protection the plant from Cd damage. It was also obvious that there was significant positive correlation between PCs content and Cd toxicity in A. philoxeroides callus, indicating that PCs could be considered as a sensitive biomarker for estimating Cd phytotoxicity. |
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