| In this paper, We selected Alternanthera philoxeroides callus and plant as experimental material, Hg2+ as the stress factor, studied the toxic effects of Hg2+ and resistance mechanism of plants from different aspects. Especially, the structural and functional responses of the whole photosynthesis were dicussed systemically. The results are as follow:Ⅰ. Study on tissue culture of Alternanthera philoxeroidesIn this paper, Micropropagation of A philoxeroides is established through stem organogenesis. Internode segments were cultivated in MS medium supplemented with 6-BA:NAA=3:0.0mg. L-1, and solidified with 0.65% agar. Buds were regenerated after incubation and developed seedings. After 2-4 weeks cultivation the sprouts were transferred into proliferation medium containing 6-BA:NAA=0.1:2 mg. L-1 for root induction; Besides, young stems or aseptic seedings were suitable organs for tissue culture, the best culture medium was 1/2 MS+3% sucrose+0.65% agar+6-BA:NAA (6-BA:NAA=3:0.2 or 3:0.1); The best pretreatment of explants were surface-sterilized with 84 (1:100) for lOmin and then flushing for 6 times with sterile water, cultured in tissue-box, after three days, continued abovementioned treatment until three times. And treated explants were surface-sterilized with 70% C2H5OH for 1min,0.1% HgCL2 (Tween-20) lOmin, and then 5%NaC10 for 5min.Ⅱ. Physiological and ultrastructural responses of A philoxeroides callus to Hg2+The results showed that, with the increase of Hg2+ concentration, contents of total chlorophyll, chlorophyll a, chlorophyll b and soluble protein increased first and then decreased, chlorophyll a/b decreased gradually; Superoxide dismutase(SOD), peroxidase (POD) activities enhanced and followed by a decline, catalase(CAT) activity decreased drastically; Meanwhile, levels of O2-, MDA and H2O2 showed the trend of escalation. Transmission electron microscope observation indicated that Hg2+ caused irreversible damage to cell membrane system, especially chloroplast, mitochondrion and nucleus. It can be observed that nuclear membrance disrupted on account of secreting excessive membrane vesicle. The lethal concentration of Hg2+to A philoxeroides was 20-40μmol-L-1 based on the experiment results.Ⅲ. The effect of Hg2+ on photochemical activity of thylakoid membrane from A philoxeroides callus The results showed that, with the increase of Hg2+ concentration, contents of total chlorophyll, chlorophyll a, membrane protein decreased gradually; Chlorophyll a/b, chlorophyll b, and whole electron transport activity poincreased first and then decreased; It was shown by fluorescence emission spectrum at room temperature that toxic effects of Hg2+ on PSII was significant than PSI; With increasing the concentration of Hg2+, Through the antenna pigment content showed an overall rising trend, light-harvesting system of PSII reaction center damaged seriously, the antenna pigment content increasing stressly can not alleviate the poisoning of Hg2+ Transmission electron microscope observation indicated that Hg2+ caused irreversible damage to thylakoid membrane system.Ⅳ,This paper studied the toxic effect of Hg2+ gradient concentrations(0,5,10, 20,40) on the chlorophyⅡfluorescence parameters and chlorophy content of A philoxeroidesThe results showed that with the increase of Hg2+ concentration, total chlorophyll enhanced and followed by a decline, while chlorophyll a/b decreased gradually; the maximum quantum yield (Fv/Fm),effective quantum yield (ΦPSII),photochemical quenching (qP) fistly decline, then increased but lastly decline rapidly, the non-photochemical quenching (NPQ) under stress increased at first and then decreased; there are stonger resistance of A philoxeroides than callus under low Hg2+ concentration (0μmol·L-1-20μmol·L-1); but with the sequential increase of Hg2+ stress, all photosynthetic activities of PSII system injures, lastly lead to die. |
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