Effects And Mechanisms Of Elevated Carbon Dioxide Concentration On Growth And Cadmium Accumulation Of Sedum Alfredii

Soil pollution by heavy metals exacerbates increasingly and threatens seriously agricultural product security and human health, which has become one of the major environmental problems and current social issues. On the other hand, due to human activities such as excessive deforestation and combustion of fossil fuels, the atmospheric carbon dioxide concentration rises increasingly. In natural ecosystems, there co-exist two current environmental issues:elevated CO2and soil pollution of heavy metals, and their interaction will have a profound effect on plant growth and absorption and accumulation of heavy metals by plant. Sedum alfredii Hance is originated from China and it is a Zn/Cd hyperaccumulator with Crassulacean acid metabolism (CAM) and a good choice for phytoremediation of heavy metal from polluted soil. Both hydroponic experiments and pot experiments were carried out and several advanced techniques, such as root automatic scanning, IMAGING-PAM chlorophyll fluorescence, PCR-DGGE and ICP-MS were used in this paper to aim to examine the response characteristics of the hyperaccumulator S. alfredii to elevated CO2(800μL L-1), exploring the relationships among root morphology, photosynthesis, rhizospheric ecological characteristics and Cd accumulation and, offering scientific reference to forecasting potential impacts of elevated CO2on plant in ecosystems contaminated by heavy metals and offering theoretic support for phytoremediation enhancement using CO2fertilizer measure. The main conclusions are as follows:1. The effects of elevated CO2on biomass, root morphology, mineral nutrient absorption and Cd accumulation were studied in hydroponic experiments with Cd stress and pot experiments with Cd pollution. The results showed that compared with the ambient conditions (350μL L-1), shoot biomass and root biomass were significantly increased by24.6%-36.7%and35.0%-52.1%, separately, and root-to-shoot ratio was significantly increased by10.7-18.4%in all Cd treatments under elevated CO2conditions. In hydroponic experiment with50μM Cd, elevated CO2significantly increased the root length (SRL), root surface area (SRA), root volume (SRV) and the tip number of root hairs by31.5%,50.5%,35.0%and23.8%, separately. Elevated CO2significantly improved the fine root growth (diameters between0-0.1mm), and especially improved lateral root and root hair development to increase the surface area of the root system and to enhance absorption of mineral nutrient and cadmium by S. alfredii. Elevated CO2increased shoot biomass by24.6-36.7%and Cd concentration in shoot by2.7-17.1%and as a result, total Cd uptake and phytoextraction efficiency were enhanced by37.5-41.2%and34.2-48.3%separately in Cd polluted soil under elevated CO2condition. This research shows that it is feasible to improve phytoremediation of heavy-metal polluted soil by S. alfredii using carbon dioxide fertilizer.2. The effects of elevated CO2on photosynthetic pigment contents, net photosynthetic rates (Pn), transpiration rate (Tr), respiration, quantum efficiency (AQY), carboxylation efficiency (Vcmax) and chlorophyll fluorescence parameters were studied in hydroponic experiments with Cd stress and pot experiments with Cd pollution. The results showed that for one thing, elevated CO2significantly increased carbon absorption by increasing Pn (105-149%) and maximum net photosynthetic rates (Pnmax)(38.8%～63.0%) in all Cd treatments, and for another, elevated CO2significantly reduced carbon consumption by reducing dark respiration (Rd,17.4%～67.3%) and photosynthetic respiration (Lcp,38.0%～75.8%) in all Cd treatments. In all Cd treatments, Vcmax was decreased by11.1-22.7%and AQY was increased by20.0-34.8%under elevated CO2conditions. In hydroponic experiment with50μM Cd, elevated CO2significantly increased the apparent quantum yield of CO2fixation (AQY), the effective quantum yield of PSII (Φ(Ⅱ)), the maximum quantum yield of PSII (Fv/Fm) and the photosynthetic capacity at saturating light (Pm). This research shows that the control of photosynthesis shifts from carboxylation controlled by Rubisco to light quantum controlled by RuBP regeneration under elevated CO2conditions and it is the increased photosynthetic electron transport activities that enhance carbon absorption and then improve growth of S. alfredii; elevated CO2increases PSII activities under Cd pollution and assists the plant to adapt to Cd polluted environment, which offers a good physiological foundation for accumulating more Cd in polluted soil.3. The effects of elevated CO2on root fresh weight, carbon and nitrogen contents, microbic biomass and diversity and available cadmium and zinc concentrations in rhizosphere of S. alfredii were studied in pot experiments. Rhizospheric dissolved organic carbon (DOC) contents were higher than those in soil without plant, but rhizospheric DOC under ambient conditions were not significantly different from those under elevated CO2conditions in the Cd/Zn polluted soil. However, there was a reduction of13%in total soluble nitrogen (TSN) content in rhizosphere of the hyperaccumulating ecotype of S. alfredii (HE) after atmospheric CO2concentration rose. There were decreasing trends of pH values in rhizosphere of the two ecotypes of5. alfredii after atmospheric CO2concentration rose. The results also showed that there were increasing trends of bacterial diversity by PCR-DGGE analysis in rhizosphere of the two ecotypes of S. alfredii when the growth conditions changed from ambient to elevated CO2, but there were no significant differences in microbial biomass between the two CO2conditions. Elevated CO2significantly increased Cd and Zn concentrations by16.6%and5.9%in rhizospheric soil of hyperaccumulor S. alfredii (HE) but had insignificant influence on NH4NO3-extract Cd and Zn concentrations in rhizospheric soil of nonhyperaccumulor S, alfredii (NHE). This research shows that it is likely that the increased CO2has more contribution to immobile carbon such as root biomass than to mobile carbon such as DOC and elevated CO2may help to increase the bioavailability of Cd and Zn by promoting rhizopheric microbial diversity and rhizospheric acidification and therefore help to increase the absorption and accumulation of Cd and Zn by S. alfredii.