| Since the twentieth century, in China and world wide, soil cadmium(Cd) pollution has become more and more common in farmland, causing a series of ecological and environment problems. Soil Cd pollution not only affects crop yield and quality, but can also be harmful to human beings and animal health through Cd accumulation in food chains. Therefore, keeping agricultural production safe from Cd polluted soils, or removing Cd from soils by the use of heavy metal hyperaccumulator has a practical significance. Oilseed rape(Brassica napus L.), as an important oil crop, has a long history of cultivation and planting experience, play a important role in the development of our national economy. Based on numerous seed resources of oilseed rape cultivars in China, we used solution and pot culture studies to screen a high-Cd accumulator and a low-Cd accumulator, and analysised mechanisms of their differences in rhizosphere processes affecting soil Cd bioavailability, Cd uptake, translocation and accumulation, the relationship between Cd accumulation and Cd tolerance, and the remediation potential for soil Cd pollution. The main results are as follows:1. Screening for high or low Cd accumulation cultivarsThe biomass and Cd concentrations in shoots of 49 oilseed rape cultivars have been studied through screening experiments in hydroponic culture and pot culture. Under a treatment of 0.2 mg/L Cd in hydroponic culture, the shoot biomass of the tested 49 cultivars was in the range of 2.56-7.15 g/plant(DW) with a mean value of4.71 g/plant(DW) and a variation coefficient of 28.1%, and shoot Cd concentrations were 7.10-23.14 mg/kg(DW) with a mean value of 14.31 mg/kg(DW) and a variation coefficient of 8.5%. Under a treatment of 0.5 mg/L Cd, the shoot biomass of the 49 tested cultivars was in the range of 1.33-5.21 g/plant(DW) with a mean value of 3.36 g/plant(DW) and a variation coefficient of 28.8%, and shoot Cd concentrations were 19.05-55.22 mg/kg(DW) with a mean value of 31.10 mg/kg(DW) and a variation coefficient of 23.1%. Under pot cultivation conditions(0.64 mg/kg Cd), the shoot biomass of the tested 49 cultivars was in the range of 3.17-7.85 g/plant(DW) with a mean value of 5.32 g/plant(DW) and a variation coefficient of 20.9%, and shoot Cd concentrations were 2.24-6.82 mg/kg(DW) with a mean value of 4.04 mg/kg(DW) and a variation coefficient of 25.6%. These results indicated that there were significant differences in the biomass and Cd concentrations among different cultivars shoots. Analysing the biomass and Cd concentrations of shoots, as the screening indeces from hydroponic culture and pot culture experiments, we defined cultivar 6(L351) as a high-Cd accumulator and cultivar 12(L338) as a low-Cd accumulator, the shoots cadmium concentrations of L351 and L338 were 23.14 and 7.10, 44.62 and 19.05, and 5.78 and 3.63(mg/kg DW) under 0.2 and 0.5 mg/L Cd treatments for hydroponic culture, and 0.64 mg/kg Cd for soil pot culture respectively.2. The molecular mechanisms of differences in Cd uptake, transport and accumulation between the high(L351) and the low Cd accumulator(L338)We investigated the physiological and genetic processes involved in Cd uptake and transport of two oilseed rape cultivars(Brassica napus L.) by hydroponic culture. Under a treatment of 0.03 mg/L Cd, the shoot and root Cd concentrations of the low Cd accumulator(L338) were 4.29 and 32.52 mg/kg(DW), while for the high Cd accumulator(L351) were 7.06 and 15.98 mg/kg(DW). Under a treatment of 0.3 mg/L Cd, the shoot and root Cd concentrations of the low Cd accumulator(L338) were 23.76 and 179.12 mg/kg(DW), while for the high Cd accumulator(L351) were 49.80 and 128.83 mg/kg(DW), indicating that the high Cd accumulator(L351) accumulated more Cd in shoot but less in root than the low Cd accumulator(L338). A scanning ion-selective electrode technique(SIET) and uptake kinetics of Cd showed that roots were not responsible for the shoot Cd accumulation since the high Cd accumulator(L351) showed a lower Cd uptake ability. However, the concentration-dependent and time-dependent dynamics of Cd transport by xylem showed that the high Cd accumulator(L351) exhibited superior capacity of translocating Cd to shoots. Additionally, the Cd concentrations of shoots and xylem sap showed significant correlation for each cultivar respectively(R2=0.92 and 0.95). Furthermore, gene expression levels related to Cd uptake by roots(IRT1) and Cd transport by xylem(HMA2 and HMA4) were consistent with the trends of Cd uptake and transport at physiologocal level. In other words, the high Cd accumulator(L351) had stronger gene expression for Cd transport but lower for Cd uptake. We concluded that the process of Cd translocation to shoots was the determinative factor for accumulating Cd in shoots of the high Cd accumulator(L351).3. Differences in response of the antioxidant systems to Cd stress between the high Cd accumulator(L351) and the low Cd accumulator(L338)We investigated the physiological and molecular processes involved in Cd tolerance and Cd resistance of the two oilseed rape cultivars. The total Cd accumulation in the high Cd accumulator(L351) was higher than the low Cd accumulator(L338), particularly with increasing concentrations of Cd exposure, and reached a maximum of 1.83 times. Higher activities of antioxidant enzymes(CAT, APX, GR, DHAR) as well as GSH and As A contents were all observed in the high Cd accumulator(L351) under Cd treatments, especially at high Cd levels. The activities in the high Cd accumulator(L351) were 50.77%、44.65%、47.57%、14.11%、58.52%、20.54% and 11.36%、18.81%、47.65%、16.67%、45.68、14.82% higher in shoots and roots than in the low Cd accumulator(L338) respectively. Furthermore, gene expression levels of Bn CAT, Bn APX, Bc GR and Bo DHAR in the low Cd accumulator(L338) roots decreased significantly under Cd application, which suggested that roots were less efficient than shoots at the molecular level, while no marked changes were found in the high Cd accumulator(L351). It was concluded that antioxidant enzymes and the ascorbate-glutathione cycle play important roles in Cd accumulation and tolerance of oilseed rape cultivars.4. Differences in the rhizosphere processes affecting soil Cd bioavailability between the high(L351) and the low Cd accumulator(L338)We also investigated difference in rhizosphere processes affecting availability of soil Cd between the high Cd accumulator(L351) and the low Cd accumulator(L338) using a root-box experiment.Under two levels of Cd treatment, non-rhizosphere soil p H of the two cultivars showed slight but non significant decline with growth stages, and no significant differences were found between the two cultivars. Under the low Cd treatment(1.2 mg/kg), the rhizosphere soil p H of the low Cd accumulator(L338) was lower than the high Cd accumulator(L351), particularly and significantly at the seedling stage. In contrast, under the high Cd(12 mg/kg) treatment, the rhizosphere soil p H of the high Cd accumulator(L351) was significantly lower than that of the low Cd accumulator(L338) at all four growth stages. Under both levels of Cd treatment, the soil available Cd concentrations of non-rhizosphere and rhizosphere soil showed a tendency to decrease with the prolonged growth period, and were markedly higher for the high Cd accumulator(L351) than for the low Cd accumulator(L338). Overall, with the low Cd treatment, the roots of the low Cd accumulator(L338) showed more powerful for Cd activation and Cd uptake, leading to the lower soil available Cd content. While under high Cd treatment, the ability of rhizosphere acidification for the high Cd accumulator(L351) was significantly higher than for the low Cd accumulator(L338), leading to higher rhizosphere soil Cd bioavailablity. These results suggested that rhizosphere acidification is also a mechanism of soil Cd activation and accumulation of Cd by the high Cd accumulator rape cultivars(L351).5. Additional screening for high or low Cd accumulation rapeseed cultivars and their remediation abilities for Cd polluted soilWe finally analyzed the shoots biomass and Cd concentrations of 165 oilseed rape cultivars grown in hydroponic cultivation, and we identified 14 high-Cd accumulation cultivars and 8 low-Cd accumulation cultivars. Soil pot culture experiment was conducted to evaluate the remediation efficiency of 22 varieties for Cd polluted soil. Under a 0.3 mg/L Cd treatment, the shoot biomass of the tested 165 cultivars was 0.59-1.40 g/plant(DW) with a mean value of 0.99 g/plant(DW) and a variation coefficient of 17.0%, and shoot Cd concentrations were 19.57-57.51 mg/kg(DW) with a mean value of 37.99 mg/kg(DW) and a variation coefficient of 21.9%, significant differences were found in shoot biomass and Cd concentrations among the varieties. Under the conditions of a pot experiment with1.2 mg/kg Cd treatment,the tissues biomass of 22 oilseed rape cultivars was in the order of stem>seed> glume>root, and the tissue Cd concentrations were in the order of stem>root>glume>seed. The seed Cd concentrations of the 22 rape cultivars were 0.09-1.06 mg/kg, with 36.36% of the 22 cultivars exceeding the Cd safety threshold(0.5 mg/kg) as the standard prescribed by the state in the peanut crop. The soil Cd remediation efficiencies were 1.80-6.88% with a mean value of 3.50%. Except for cultivar 112, the Cd uptake capacities of 22 cultivars were consistent with the soil Cd remediation efficiency. Considersing safe production and high soil Cd remediation efficiency, we selected the cultivar102 as a ideal cultivar. |
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