| Rice (Oryza sativa L.) is one of the most important staple cereal crops in a large part of the world, especially in China. However, anthropogenic activities, such as agricultural practices, transportation, industrial activities and waste disposal have increased the concentrations of toxic elements such as cadmium (Cd) in agricultural soils. Cadmium has no metabolic role in living organisms, and is highly toxic and non-biodegradable. However it can be effectively absorbed and transported within rice plants. Thus, it can readily enter into the food chain, resulting in phytotoxicity and consequently causes serious threats to human health.This dissertation aimed to understand the behavior of Cd speciation in rhizosphere microzone, determining the genetic difference between two rice cultivars in its growth, physiological-biochemical characteristics, absorption and accumulation regulation in the different rice tissues. The main experiments and conclusions are as follow:1. To investigate rhizosphere effects on the chemical behavior of Cd, a glasshouse experiment was conducted in rhizobox, where two rice cultivars were grown in a soil spiked with cadmium at concentrations of3.9±0.5and8.3±0.5mg per kg soil until ripening stage. Chemical forms of cadmium with respect to root surface were then assessed using sequential extraction procedure (SEP). The results showed significant difference of Cd species, especially EXC-Cd (exchangeable-Cd) between two rice cultivars as affected by rice roots. The soil lowest EXC-Cd of Zhenong54appeared at near-rhizosphere area with a little difference under two Cd levels (lower Cd level showed the lowest EXC-Cd in1mm as higher Cd level showed in2mm) while Sixizhan had its lowest EXC-Cd at the root compartment. Both cultivars presented slight changes of FMO-Cd (Fe/Mn oxide-bound fraction) at ripening stage while the control treatments without plants had a significant increasing of FMO-Cd at ripening stage, indicating a transformation from less bioavailable form (FMO-Cd) to more bioavailable forms (EXC-Cd). Soil total microbial biomass carbon in the vicinity of the root surface had the opposite trends to some extent with EXC-Cd. Thus, the change of Cd speciation in the rhizosphere was mainly because of the suitable root exudates level, the microbial activities and the rice ability to absorb EXC-Cd in rhizosphere. 2. Plant Growth and physiological-biochemical characteristics under Cd pressure were investigated and the absorption regulation in the different rice tissues was studied. The results indicated that in tillering stage Sixizhan transferred more Cd from the root to the shoot, the height, dry weight and activities of Antioxidant enzyme system of Sixizhan were all decreased at some extent. Thus Cd caused more severe damage than Zhenong54in tillering stage. While in the harvesting stage, Sixizhan had less Cd accumulation in the shoots. However, Zhenong54had the just opposite trend Sixizhan had. Zhenong54had less Cd in the shoot and suffered less severe damage in the tillering stage, while Cd accumulation obviously increased in the harvesting stage, which finally caused larger grain Cd content in Zhenong54than in Sixizhan. Therefore, it is not appropriate to evaluate the effect of Cd pressure on rice just in one growth period because the Cd effects were different among different cultivars and among different growth periods.3. In order to evaluate the root exudates difference between rice cultivars under different iron conditions and the exudates effect on Cd absorption by rice, a Hydroponic experiment was conducted under different Cd and Fe concentrations. The results showed that genetic-induced root exudates under different Fe conditions can finally affect the rice Cd absorption. The two rice cultivars indicated almost the same trend with or without Fe added:rice root exudates content (low molecular weight organic acid) was enlarged when Fe was not added, and the rice Cd accumulation was also increased whereas the dry weight of rice shoot and root were decreased. Despite those some trend, the two rice cultivars had their difference in different situations:with no Fe added, Sixizhan had larger root exudates amount than Zhenong54did, the Cd accumulation in its shoot and root was also obviously larger than Zhenong54s’; when Fe was added, the shoot and root dry weight of Zhenong54was larger than Sixizhan.4. A green house experiment was conducted to study the different rice Cd absorption between different rice cultivars when exogenous Zn, Ca and S added. It was found that rice growth and plant Cd absorption difference between two rice cultivars were affected by exogenous Zn, Ca and S, and was changed with the rice growth period. Different added Cd levels obviously affected the rice Cd absorption and Cd distribution in different rice shoots (p<0.001). At mean time, Cd accumulation in rice shoots was varied between rice cultivars and rice growth period. Overall, the Zn and S contents in two rice cultivars decreased in harvesting stage, whereas the Ca contents increased; the Zn and Ca treatment increased rice shoot dry weight, especially in Teqing cultivar. |
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