| To explore the behavior of Pb and Cd in soil-plant(Brassica chinensis L. cv. Suzhouqing) and their polluting effect, distribution of Pb and Cd in soil different organic fractionsand in the different tissues of plant (shoots and roots) were studied by a in-situ field experiment. Subcellular distributions of Pb and Cd in roots and shoots of B. Chinensis L. pakchoi from sand culture experiment were studied by differential centrifugation and inductively coupled plasma mass spectrometry (ICP-MS). Synchrotron radiation X-ray fluorescence spectroscopy (SRXRF) microprobe. a sensitive technique for trace element analysis, was used to study the distribution of Pb and other elements (Ca, Mn, Fe, Cu and Zn) in cross-sections of the root and stem of B. chinensis pakchoi. Besides, growth parameters, physiological and biochemical responses of B. Chinensis at32d expoused to Pb or Cd soil were analysed. The results were shown below.i) After140days, GRSP bound Pb accounted for0.21-1.78%of the total Pb, and GRSP bound Cd accounted for0.38-0.98%of the total Cd content in the soil. However when compared on a soil organic matter (SOM) basis, around only4%of the Pb or Cd was bound to the GRSP fraction of the SOM compared with40-54%of the Pb or Cd bound to the hum in (HM) and fulvic acids (FA) in the SOM fraction. The mass ratio between Pb and GRSP-carbon changed from2.3to271.4mg (100g)-1in all Pb levels soil, while the high-Cd treatment group the mass ratio between Cd and GRSP-carbon (0.36mg (100g)-1) was higher than the mass ratio seen with Cd-bound humic acid (HA) fractions fractions. In the highest Pb or Cd treatment, the capacity of binding to heavy metals followed the order (FA> HA> SOM> HM> TG) for Pb. whereas following the order (FA> SOM> HM> TG> HA) for Cd. During the cultivated time, in the high Pb or Cd treatment groups, the available Pb (DTPA-Pb) proportion in the total Pb decreased and SOM-Pb proportion showed a significant increase, while the share of available Cd (DPTA-Cd) on the total Cd was significantly increased and SOM-Cd proportion of the total Cd was significantly declined during the same time. Additionally, we found that the fraction of metal bound to GRSP increased even though the total GRSP content declined over time, and the change of SOM-metal concentrations were directly correlated with the change of HA-and FA-metal fractions.ii) At80d after metals exposure, our study found that Pb and Cd mainly accumulated in the root. The Pb and Cd maximum concentrations of0.277and0.099μg g-1, respectively, were detected in the root. The Pb and Cd maximum concentrations in the shoots were0.215and0.066μg g-1, respectively. The Cd bioaccumulation factors in shoots (6.12-10.7%) and roots (0.083-0.307%) were higher that of Pb. The Pb translocation factors from the root to the shoot decreased with increasing Pb levels, while Cd transfer factors increased with increasing Cd levels. Subcellular fractionation of different tissues from the sand culture by differential centrifugation showed that Pb predominantly accumulated in the cell wall and vacuoles of the root and the shoot. While Cd mainly accumulated in cell wall of the root, and in the vacuoles and cell wall of the shoot. SRXRF analysis was able to show that when Pakchoi seedlings were placed under excess Pb stress the Pb, Ca, Cu, and Zn were concentrated in the cortex and vascular bundle of the root, while mixed Fe-Mn plaques covered the surface of the pakchoi root. Higher levels of most elements (except for Fe) were found in the cortex and vascular bundle in the stem. iii) At32d of metals exposure, we found that plant growth parameters (fresh biomass, root length and plant height) had a clear decrease upon addition of Pb levels. A clear decline in root length and plant height of pakchoi was observed upon increasing of Cd levels, while fresh biomass was not affected significantly. Leaf chlorophyll content (chl. a, b and a+b) in leaves of B. chinensis was decreased with the increasing Pb or Cd levels, whereas the ratio of chl.a/chl.b was increased. The uptake of Ca and Fe in the roots and Ca in the shoots were increased upon the addition of Pb levels, whereas had an antagonism effect on the absorption of Mg and P in the roots. The root K and Fe and shoot P, Mg, Zn and Fe contents were less affected by the concentration of Pb. Cd promoted root’s the absorption of Ca and P, whereas restrainted the absorption of root K and the transfer of shoot Ca, Mg and Zn contents were reduced. Root Mg, Fe and Zn and shoot Fe, K and P showed no significant alteration. SOD activity in the roots and leaves of B. chinensis were decreased with the addition of Pb or Cd contents in soil, while POD and CAT activities were increased. MDA content in the roots and leaves was increased with the increasing of Pb levels, while MDA content was not affected at different Cd levels. Under the Pb or Cd stress, acid soluble SH, GSH and PCs concentration in the roots and leaves of B. chinensis were significantly increased with the rising of heavy metal levels.In conclusions, our study provides new evidences on the behavior of Pb and Cd in soil-plant (B. chinensis L.) by the in situ field and sand culture experiment. The in situ field study provides new evidence on the role of GRSP takes in heavy metal sequestration. Further studies that characterize the heavy metal binding kinetics of GRSP would further enhance the understanding and potential use of this glycoprotein or the fungi for remediating heavy metal contaminated soils. In response to Pb and Cd stress, Suzhouquing pakchoi uses roots, cell walls and vacuoles to reduce the transport of these heavy metals through the plant. Pakchoi show significant variation in growth physiological and biochemical response to Pb and Cd. |
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