| Ecotoxicology is a newsubjecl just founded in about 1980 and is developing rapidlyOver the past two decades, ecotoxicology has experienced a rapid progress,thus it has become to be very important in protecting ecological safety from toxic contaminants. In our country, however, ecotoxicology is in its babyhood, to which we do not know very clear in many aspects. Enhancing the research in this field not only can faditate the advancement of this new subject in our country, but also greatly hasten the conversion of the economic pattern of our country into sustainable development.Based on ecology, ecotoxicology is to study the hazardous influences of contaminants, especially, toxic chemicals on the structure and functions of ecosystem, the response of ecosystems to them, and the pathways of the toxicants moving, transforming and decomposing in the surrounding. Also, acotoxicology provides the technological base and tboretic steering for the ecological remediation of contaminated environment and ecological risk assessment. Of the toxic chemicals, heavy metals are given special attention by ecotocicological researchers because they are important inorganic poIlutantsEcotoxicology brought a profound revolution in toxicology, and led a change of the direction of the advance ofome environmentaltechniques.Heavy metals have lots of effects on the plants, not only on the individuals, and on the populations and the comnunities in higher levels, but also in physiological and biochemical aspects and in genetics and evolution, e.g. in physiology, depressing the activity of some enzymes, initiating a few free radicals, reducing the growth of plants, degrading chlorophyll, inhibiting the elongation of the root and so on. But, the plant also develops plenty of mechanisms to resist the toxicities of heavy metals, such as stimulating stress proteins and stress ethylene production, activating antioxidant system to eliminate the fe radicals, synthesizing metallothioneins-like and phytochelatins, excluding more metals and accumulating and compartmentalizing in some organs. To understand the toxicity and mechanisms underlying of the heavy metals can increase our knowledge about thai ecological effects, and facilitate us to know their hazardous ecological effects more clear, which would urge us to use them more availably and rationally so that their negative effects could be alleviated as possible.Various chemicals, especially toxc chemicals, such as heavy metals or heavy metals-contained, insecticide, herbicide and so on, have great toxic effects orthe structure and functions ofinosphere. To assess their toxicity, including phytotoxicity, is the basis of scientific management fa them, and is profound to protect environmental quality and ecological safety.Developed countries not only have weHestablished standard method to process the toxicity evaluation of toxicants, but also many related laws and rules for it. Our country drps greatly behind them in this aspect. However, the environmental status of our country has been so severe that we have to enhance the research in this field. It should be a short way to study the feasibility of using indigene organisms in assessing the taicity of pollutants based on using the established standard method. Management for reference, legislation and executing the law of toxic chemicals could be on the scientific basis when standard toxicity assessment method of our country is established. By far, the most commonly used vascular plants in toxicity tests are duckweeds, members of Lemanaceae.This thesis conducted a completely ecotoxicological studyon the effects of heavy metals on two duckweed species, most common in our country,recollected from natural populations instead of from special strains selected in laboratory with axentic conditionsThe objects are to provide an available autochthonous model testing material for phytotoxicity assessment ofiiic chemicals in our country, an excellent plant material for phytoremediation of cadmimpolluted water. In...
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