| Heavy metal Contamination in soil is one of the most serious global environment problems, which brings great harm to the health of people and ecosystem. One of the other most serious environmental problems was the ever increasing atmosphere CO2, leading to rising temperature and other environment problems. Elevated CO2 alters the growth and development of plants in short and long terms. Lots of researches elucidate the mechanisms that involved response of plants to elevated CO2 in normal environment, but little has done on plants response to elevated CO2 in the heavy metal contaminated soil. As elevated CO2 may change the uptake of the heavy metals, influence the toxic effect of heavy metals on plants, and enhance the ability of plants that tolerance to heavy metals, the study of plants response to heavy metals under elevated CO2 was a needed. Open top chambers (OTC) and environment-controlled chamber was used to investigated the effects of elevated CO2 on the uptake of Cd, toxic of Cd, and the antioxidant system response in L. mutiforum and L. perenne. In this study, we investigated the growth indexes, like tiller number, biomass, root/shoot ratio, root development; the physiological indexes, like gas exchange, chlorophyll content; Cd uptake and transport; the anti-stress system, like SOD, GSH, PCs of L. mutiforum and L. perenne.These results systematical elucidated the physiological and physiochemical response to Cd under elevated CO2.1.Elevated CO2 significantly improved the growth of L. mutiforum and L. perenne, showing by the increased tiller number, root biomass and shoot biomass, and the elevated CO2 induced increase percentage was higher under Cd stress.The elevated CO2 induced biomass increase of L. mutiforum was 67%,77% and 81% under 0,4,16 mg/kg Cd treatment respectively, and 62%,65% and 68% of L. perenne in the hydroponic culture experiment. The increase in shoot biomass was 29.3% and 34.0% for L. mutiforum and L. perenne under non-Cd treatment, and 43.4% and 45.2% under 100 mg/kg Cd treatment; 29.0% and 34.4% for roots under non-Cd treatment,54.2% and 44.6% under 100 mg/kg Cd treatment. The root/shoot ratio was decreased under elevated CO2. The increase of tiller number was higher for L. perenne than L. mutiforum.2.Root volume, length,surface area, tip numbers were significantly reduced by the addition of Cd for L. mutiforum and L. perenne under both ambient and elevated CO2.L. perenne was more sensitive to Cd than L. mutiforum when taken these parameters into consideration. These parameters were all significantly enhanced by elevated CO2, and the increases were much higher under Cd stress than non-Cd treatment. The average diameter of L. mutiforum was higher under elevated CO2 than under ambient CO2, but decrease was seen of L. perenne.Cd treatment decreased the number and percentage of fine roots both under ambient and elevated CO2, but elevated CO2 enhanced the fine roots under 4, 16 mg/kg Cd treatment.3.Cd affects gas exchange under both ambient and elevated CO2. We observed a decrease in net assimilation rate of CO2 (Pn) and stomatal conductance (Gs) in both species with increasing Cd concentration in soils. There was also a numerical, but not statistical decrease in the transpiration (E) and WUE. The Pn and Ci increased under elevated CO2.Under elevated CO2 Gs and E were lower than under the ambient CO2 level for the same Cd exposure. Increasing Cd concentrations in soil caused a continual decrease in the contents of chlorophyll a, chlorophyll b, and carotenoids, irrespective of CO2 treatment. With increasing Cd concentration in soils, the chlorophyll a/b ratio decreased. There were no differences in all three pigments for plants grown on soils amended with 0 and 25 mg/kg Cd under either elevated or ambient CO2, but plants grown on soils amended with of 100 mg/kg Cd had higher contents of chlorophyll a and b under elevated CO2 than ambient CO2.The chlorophyll a/b ratio was not influenced by elevated CO2.4. Sulfur concentration in the roots and shoots increased with increasing Cd concentration in soils irrespective of plant species and CO2 concentration. For the same Cd treatments, sulfur concentration in the plant organs was lower under elevated CO2 than under ambient CO2.Cd spiking reduced the C content of both species, increased the N content, and reduced the C/N ratio. And elevated CO2 otherwise significantly increased the C content, reduced N content, and increased the C/N ratio.5.Cd spiking significantly increased the Cd concentration in roots and shoots of both species, while the highest concentration observed for L. mutiforum and L. perenne was 3469 and 4460 mg/kg in roots and 417 and 466 in shoots, showing a strong ability of Cd uptake. The Cd concentration in shoots and roots of both species decreased under elevated CO2, although, the total Cd uptake were enhanced by 42.2%-73.4% because of the significant biomass increase. The transport index (Ti) was increased under elevated CO2.6. MDA content was increased in both species under Cd stress, but it was lower under elevated CO2 compared to their respective control, showing a decreased oxide stress under elevated CO2. Several reasons could be the explanation:(1)SOD activity and GSH content was higher under elevated CO2 for the Cd treatment; (2) large amount of PCs was synthesized under Cd stress, and it well related to the Cd concentration, the calculated chelate ratio of shoots was mostly above 100%(except for 80 and 160μM Cd treatment), and lower than 100% of roots. The PCs/Cd ratio was basically not influenced by elevated CO2, but the larger PCs molecular was higher in amount; (3)chlorophyll content was higher under elevated CO2 in 100 mg/kg Cd treatment.7. An efficient method was established for the determination of thiols using HPLC coupled with FLD. The most advancement of this method was the shorter determination time, while the separation and determination of Cys, GSH, PC2-6 was finished in 19 min; and we also have some breakthrough in regression equation, accuracy, precision, limits of determination.The conclusion can be made due to the results above, that elevated CO2 enhanced the tolerance of L. mutiforum and L.perenne to Cd, reduced the Cd concentration, but enhanced the total Cd uptake due to the high biomass increases. This means that Cd stress will be ameliorated under the further elevated atmosphere CO2, and this will benefit to the survival of them under Cd stress. On the other hand, this provided lots of useful information for using CO2 enrichment as a method for phytoremediation efficiency enhancement.
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