The Effects Of Elevated CO₂ On The Growth And Quality Of Rice Under Cu And Cd Stress

The global climate change with the main features of CO₂ concentration increase in the atmosphere and the consequent warming will influence the ecological environment and agriculture development. Research in this area has been subject to wide attention of scientists. The current study about elevated CO₂ impact on crops focused on physiology and ecology, growth and yield, transpiration and water use efficiency, food quality. Heavy metals were concealed, hysteretic, cumulative, irreversible, nonbiodegradable, hazardous in the soil. Once the soil polluted by heavy metals, it will not only harm plant growth and development, but also threaten the ecological security, affecting the quality of agricultural products, jeopardize human health through the food chain. With the situation of heavy metal pollution in our country and even around the world become more and more worse, and rice production in China and Asia's importance, it is necessary to carry out the research work about the effects of elevated CO₂ on the growth and health quality of rice under heavy metal stress, and to discuss the elevated CO₂ as an aid to improve the phytoremediation efficiency for heavy metal polluted farmland.A pot experiment in six open-top chambers with two levels of CO₂ and two multi-metal contaminated soils was conducted to investigate combined effects of elevated CO₂ levels and metals (Cu and Cd) on rice. Based on the effects of elevated CO₂ on the uptake of Cu and Cd by rice, we assess their potential value for phytoextraction of contaminated paddy soils, and the mechanism was discussed using hydroponic culture. The main conclusions drawn in this dissertation are as following:(1) Elevated CO₂ reduced the growth period of rice, increased the plant height and biomass significantly, the increase in proportion in high contaminated soil is larger than low contaminated soil. Elevated CO₂ increased the panicle and setting rate of rice, grain weight was also showed an increasing trend, but the effects of elevated CO₂ on rice spikelets was not significant.(2) We observed dilution/little varied phenomena in grain Cu concentration in six rice varieties grown on both contaminated soils under elevated CO₂. We found significantly higher Cd concentrations in the parts of three rice varieties under elevated CO₂, but lower levels for the others. CO₂-inducedaccumulation of metals in rice might be a component contributing to the potential health risk in thefuture, with Cd being a more important threat to human health than Cu.In the absorption of mineral elements, in addition to N content generally decreased in the six rice varieties, the other mineral elements showed a greater difference between varieties.(3) The total dry weight biomass of the six rice varieties was significantly higher when grown with elevated CO₂. CO₂ elevation increases the biomass of all six different rice varieties grown on contaminated soils has favorable implications for using CO₂-induced rice varieties for phytoextraction purposes.(4) The pH value in the rhizosphere soils of the six rice varieties exhibited a slightly decreasing trend under elevated CO₂ compared to the ambient CO₂ control, but significant differences were found. Elevated CO₂ increased exchangeable fraction of Cu and Cd, but decreased carbonate-bound fractions of Cu and Cd significantly. (5) Lower Cd treatment benefit of rice tillering and root growth, high concentration of Cd significantly inhibited the growth of rice roots. Elevated CO₂ concentratio stimulated the growth of rice roots, total root length, root surface area, root volume, and root tips increased significantly, root average diameter changed no more. The effects of elevated CO₂ on root development showed a greater difference between varieties.(6) Cysteine (Cys) concentration decreased and glutathione (GSH) and phytochelatins (PCs) of root increased under Elevated CO₂. Elevated CO₂ decreased the concentration of shoot Cys, GSH, and PCs. There are also exist differences among the different rice varieties.We proposed that some of rice varieties have greatest potential for phytoextraction as indicated by their metal BFs if they are grown for phytoextraction purposes under assistance of elevated CO₂. Under the global climate change in the future, elevated CO₂ may affect the food security. So, screening of appropriate rice varieties for the region that have weak metal translocation from soil to rice grains is a needed response to increases in global CO₂ concentration.