|其他摘要||The model plant Arabidopsis thaliana was used in the present study to investigate toxic effects of heavy metals, like cadmium (Cd), copper (Cu), mercury (Hg), nickel (Ni), lead (Pb), zinc (Zn), on the growth, appearance symptom, and especially photosynthetic performance of plants. The toxic effects of heavy metals on chlorophyll content, photosynthetic oxygen evolution, the activities of photosystems were tested. The difference between their toxic effects was compared to each other. The different effects of these heavy metals on photosystem I (PSI) and II (PSII) were analyzed. Electron transfer inhibitor was also used to investigate the response of cyclic electron flow (CEF) around PSI to different heavy metals treatments. Two strains of cyclic electron transport mutan were studied together with the wild type.We tested the binding ability of heavy metals and photosystems to detect the cause of the different toxic effects of these heavy metals.
The main conclusions were as follows:
(1) Among these heavy metals, Hg and Cu showed most obvious inhibition on growth of the plants. And their toxic effects were strongest. Cd showed less toxic effect than Hg and Cu. Ni, Pb and Zn showed less toxic effects on the whole plant than above heavy metals.The toxic effect of Ni was the weakest.
(2) The toxic effects of heavy metals on A. thaliana were also tested by photosynthetic oxygen evolution rate and maximal photochemical efficiency (Fv/Fm). Similar to the inhibition on growth, Hg, Cu and Cd seriously decreased oxygen evolution rate and Fv/Fm. The toxic effect of Zn was less than above heavy metals. Ni and Pb almost caused no significant inhibition on oxygen evolution and Fv/Fm.
(3) Hg was the most toxic heavy metal which showed stongest inhibition on the electron transport rates (ETRs) of both PSI and PSII. Cu caused strong inhibition on ETR of PSII as same as Hg treatment. However, Cu showed no inhibition on electron transport rate of PSI [ETR(I)]. Cd showed grester impact on light response curve of ETR(II) than that of ETR(I). Pb at high concentratin had toxic effect on electron transport of both PSII and PSI. The toxic effect of Pb on PSI electron transport was comparable with Hg. Zn could inhibit the electron transfer of PSII, but had no inhibitory effect on that of PSI. Among these heavy metals, the inhibition effect of Ni was minimal. Ni could enhanced ETR(I) at some concentrations. For the light response curve of ETR(II), the order of inhibitory effect of heavy metals from strong to weak was: Hg > Cu > Cd~Zn > Pb > Ni. As for PSI, Hg had the highest toxicity. Pb also caused inhibitory effect. Cd, Zn and Ni showed no significant effects, while Cu had a promoting effect.
(4) After exposure to various heavy metals, CEF was excited in wild type A. thaliana to play a protective role. GPR5 pathway played more important role than NDH pathway as found in the CEF mutants. Under Cd treatment, CEF was sufficiently excitated to pretect PSI. This was more evident in Cu treated samples. Hg decrease the activities of PSI and PSII, and the whole electron transport chain. Ni showed less inhibition on the electron transport activity. Relatively lower excitation of CEF is enough to protect photosynthetic apparatus under Ni treatment. High concentration of Pb caused significant inhibition of electron transport in PSI with CEF not excitated. Zn and Hg could inhibit NDH pathway, which led the abolishment of CEF in the pgr5 mutant.
(5) Cd, Cu and Zn mainly showed inhibitory effect on PSII derived from low temperature fluorescence spectra of extracted thylakoid membranes. Their toxicity to PSII were less than Hg. Hg showed high toxicity to both PSI and PSII at high concentration. PSII was more susceptible to Pb treatment than PSI. But the effect of Pb treatment was less than Cd, Cu, Zn and Hg. Ni caused no significant inhibition on two photosyntems. Hg showed strongest binding with photosystems, these leading to its strongest toxicity to photosynthesis and the activities of photosystems. Cd and Zn were more easier to bind with PSII. These results explained that why Cd and Zn showed stronger toxicity to PSII than PSI. Pb showed weaker binding with photosystems compared to other heavy metals, causing its weakest toxicity.|