| Recently,global economy has developed rapidly, however the pollution of human living environment has become more and more serious. Human healthy is threatened heavily, and heavy metal pollution is the problem which the human have to face. Cadmium is one of the most toxic and migratory aptitude heavy metal. It is accumulated in human body through food chain and does harm to them. Cadmium in human body is cumulate; it will be difficult to be excluded once it is absorbed into the human body. The characteristics which do harm to human are low dose, long-term and chronic poisoning. In surgical repair field of contaminated soil, compared with traditional physical and chemistry repair, phytoremediation becomes a hot research focus in the field of heavy metal pollution control in all the world because of its no pollution and low cost green repair mechanism.A lot of scientific research achievements were got in study on the screening of hyper-accumulator, but most of the research fields aggregate at screening of hyper-accumulator. The study on resistance mechanism of plants to heavy metal is less, it is unknown to the mechanism so that it limits the applying.Prerequisite for phytoremediation technology is to find heavy metal hyper-accumulator, but there are only few species was found in the world. Through a lot of survey in heavy metal mining area on plants there in Fujian, studying team of my teacher found Conyza canadensis which had a strong ability to accumulate Cd. But the mechanism of Conyza canadensis on Cd is not yet clear that greatly restricted the wide application of cadmium enrichment plant.In view of this,this paper took Conyza canadensis which was a kind of cadmium hyper-accumulator selected by our group as experimental material, and made a pot experiment. Through determining its physiological and biochemical indexes such as antioxidant system, malondialdehyde, soluble protein, chlorophyll, membrane permeability, cell ultrastructure, the cadmium's poisoning effect to Conyza canadensis could be studied. Through the research on organic acid, nonprotein sulfhydryl, phytochelatin and heavy metal contents in the plant, tolerance mechanism of Conyza canadensis could be studied. We wanted to provide scientific basis for its applying in surgical repair field of heavy metal pollution soil through the study of Conyza canadensis's accumulation and tolerance mechanism to cadmium. The main results are as follows:1. The fresh weight of Conyza canadensis's height, aerial and underground part appeared to have a decline trend with Cd treatment. It had little influence when the concentration was low, it showed a certain tolerance.While the decreasing range was increasing with the increase of the concentration of treatment.Conyza canadensis's height,aerial and underground part fresh weight showed a significant negative correlation with Cd concentrations(P<0.01).The correlation coefficient were -0.869,-0.864,-0.862. It showed that Cd treatment caused obvious inhibition to the growth of Conyza canadensis.2. The relative conductivity of Conyza canadensis's leaves increased gradually with Cd treatment, it showed that cadmium treatment destroyed cell membrane system and caused cell membrane permeability increase. Chlorophyll-a content,chlorophyll-b content and total chlorophyll in Conyza canadensis's leaves appeared to have a decline trend overall with the increase of Cd concentrations. When the Cd concentration was 400 mg·kg-1, they were significantly different between them and control (P<0.05). It showed that Cd treatment probably destroyed chloroplast structure and chlorophyll decomposes, presented as chlorophyll content decreased. SOD increased at first and then decreased with increase of Cd concentrations and the activity reached a maximum value when the treatment concentration was 100 mg·kg-1, POD and CAT followed the rule,too. While the activity reached a maximum value when the treatment concentration was 200 mg·kg-1.It showed that these three antioxidant enzymes'scavenging activity to reactive oxygen species was effected by Cd concentrations, it was effective in a certain range, however enzyme's scavenging activity was inhibited when Cd concentrations exceeded a certain limit. A great amount of reactive oxygen species were accumulated in cells and caused membrane lipid peroxidation. MDA content in Conyza canadensis's leaves increased at first and then decreased with increase of Cd concentrations, but the changing scale was small and had no significant change(P>0.05).There was little influence to Conyza canadensis with the increase of Cd concentrations and exhibited a strong antioxidative activity. Soluble protein in Conyza canadensis's leaves increased slowly at first and then decreased with increase of Cd concentrations and the content kept relatively stable level on the whole. Cd could induce the composition of new protein with low concentration and restrain the composition when it was high. It would cause some protein to decompose, but the degree of damage was slighter.3. The contents of nonprotein sulfhydryl, GSH and PCS in Conyza canadensis's leaves and roots showed a increased tendency gradually with the increase of Cd concentrations, but the contents reduced slightly when the concentration was 400 mg·kg-1 in the roots. The correlation coefficients in leaves of Cd concentrations and nonprotein sulfhydryl, GSH and PCS were 0.907,0.827,0.813 and it showed a significant negative correlation between them(P<0.01)and in roots were 0.945,0.923,0.813 which also showed a significant negative correlation(P<0.01). Synthesis mechanism of PCS in Conyza Canadensis was started, and PCS chelated with cadmium to create innocuous chelatecompound so that it could alleviate the injure of cadmium.We could consider that PCS was probably one of mechanisms which Conyza canadensis withstanded the injure of heavy metal.4. Change of organic acid in leaves with cadmium treatment showed that there was no obvious entrainment of Cd to malic acid, tartaric acid, oxalic acid, acetic acid and citric acid in leaves. The contents of malic acid and citric acid in roots increased with Cd treatment. The correlation coefficients of Cd concentrations and citric acid content in roots was 0.671, which was positively and significantly correlated with each other(P<0.01). Oxalic acid content with Cd treatment was smaller than the control, it was more than the control when concentration was the highest and there was no obvious entrainment. So, it remained a problem that organic acid might be not a tolerant mechanism to Cd in Conyza canadensis.5. Chloroplasts began to swell but the structure didn't change obviously when the Cd concentration was low. Some Chloroplasts swelled to sphere from oblong oval when it was high and thylakoid's arrangement was disorder.Cristaes of mitochondria's arrangement were disordered when the concentration was low and some cristaes are swelling, mitochondria exhibited vacuole degeneration when it was high and cristaes were swelling, but bilayers was still complete. Nucleoplasm of cell nuclear well distributes and nuclear membrane was clear and intact,there was no significantly harmed evidence.It showed that chloroplast and mitochondria were sensitive, however, cell nuclear didn't be harmed under the stress. This might be the reason why Conyza canadensis could continue to grow even chlorophyll content decreased and inhibited growth with cadmium treatment. 6. Cadmium accumulation in leaves and roots showed an increased tendency with the increase of Cd concentrations, and the increasing speed was a low to high trend. Cadmium content in leaves was 180.248 mg·kg-1 and in roots was 232.412 mg·kg-1 when cadmium accumulation was 400 mg·kg-1, translation coefficient of cadmium to Conyza canadensisis was less than one, which didn't reach the standard of hyper-accumulator. Above-ground biomass of Conyza canadensis was large and its requirement to living environment was low. So as a cadmium hyper-accumulator, it could be used in soil remediation field where cadmium pollution was very serious.
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