Physiological Mechanism Of Cadmium Absorption And Translocation In Different Amaranth(Amaranthus Mangostanus L.)

Heavy metal cadmium（Cd） as the primary pollutants in soil pollution of our country, not only causes soil degradation, soil dysfunction, but also affects the growth and metabolism of plants. Cadmium is absorbed and transferred into the food chain to endanger human health by plants. Phytoremediation technology as a kind of low cost, small environment disturbance in situ repair technique becomes the hot spot in the worldwide. In this study, compare the characteristics of Cd uptake, translocation and cellular distribution in two cultivars of Amaranth（Amaranthus mangostanus L.）. By several hydroponic experiments, physiological mechanisms of Cd uptake and translocation in two cultivars were investigated by a combination of several techniques including metabolic inhibitor, ICP-OES and non-invasive micro-test technique（NMT）. The characteristics of Cd distribution in two cultivars of amaranth（Amaranthus mangostanus L.） were investigated at tissue and cellular levels by several techniques including scanning electron microscope combined with energy dispersive spectrometry（SEM-EDS） and transmission electron microscope（TEM）. Cadmium accumulation and distribution of Zibeixian, a low Cd accumulator, and Tianxingmi, a high Cd accumulator were compared, and strong tolerance and hyperaccumulation mechanism of Tianxingmi were revealed. The results are summarized as follows:1. Effect of Cd treatment time on the Cd uptake and translocation of two amaranth cultivars. After 1d exposure to 30μmol/L CdCl₂, the total biomass of Tianxingmi is twice of Zibeixian, maximizing 5.90g/plant. The cadmium concentration in the roots, stems and leaves of Tianxingmi reached 609 mg/kg, 254mg/kg and 62.3mg/kg, is 1.4 times, 1.9 times and 1.6 times than Zibeixian. The cadmium accumulation in the shoot and whole plant of Tianxingmi reached 602.0μg/plant and 1308μg/plant. Bioaccumulation factor（BCF） and translocation factor（TF） of Tianxingmi are 2.1 times and 1.5 times than Zibeixian. All of these results showed significant difference between the two cultivars of amaranth（P<0.05）.2. Effect of Cd treatment concentration on the Cd uptake and translocation of two amaranth cultivars. Except 60μmol/L CdCl₂ treatment, there was no phytotoxicity to Tianxingmi and plants grew healthy. The total biomass of Tianxingmi increased by 42%（P<0.05） compared with the control group（CK）, maximizing 3.69g/plant of 30μmol/L Cd treatment. Due to the phytotoxicity of Cd, leaf biomass of Tianxingmi significantly reduced at 60μmol/L Cd treatment（P<0.05）. The total biomass of Zibeixian was maximum 2.35g/plant at 5μmol/L Cd treatment, and significantly reduced exceeding 5μmol/L Cd treatment（P<0.05）. Cd concentration in plant organs increased steadily with the increasing addition of Cd in solution. At the maximum biomass, the Cd concentration in the roots, stems and leaves of Tianxingmi reached 950mg/kg, 305mg/kg and 205mg/kg contrasted with 269mg/kg, 62.9mg/kg and 74.8mg/kg of Zibeixian. The Cd accumulation in the shoot and whole plant of Tianxingmi reached 736.3μg/plant and 1344μg/plant contrasted with 134.3μg/plant and 260.7μg/plant of Zibeixian. The Cd concentration in the roots, stems and leaves of Tianxingmi increased significantly（P<0.01, P<0.05） contrasted with the Cd concentration only in the roots of Zibeixian increased significantly（P<0.05） following the increasing addition of Cd from 10μmol/L, and the translocation factors of Tianxingmi（TF or S/R, the ratio of shoot concentration to root concentration） were above one.3. The net Cd²⁺ fluxes in the roots of the two cultivars of amaranth by non-invasive micro-test technique（NMT）. There was the biggest difference by the screening test that the net Cd²⁺ influx in the roots of the two cultivars of amaranth within 0-300μm from the tips. The net Cd²⁺ influx in the roots of Tianxingmi is 3.75 times than Zibeixian, and the enrichment characteristic is consistent with the results of NMT.4. Effect of metabolic inhibitor（carbonylcyanide-m-chlorophenylhydrazone, CCCP） on the Cd uptake and translocation of two amaranth cultivars. The treatments of metabolic inhibitors significantly reduced the Cd indexes of Tianxingmi（P<0.05）, but little effect on Zibeixian.5. Cellular distribution of cadmium in two amaranth cultivars differing in cadmium accumulation. In roots, SEM-EDS and TEM confirmed that the Cd distribution pattern of Tianxingmi was the whole cross-section from epidermal cells to xylem vessel, but the Cd distribution pattern of Zibeixian was mostly in epidermal cells and exodermis. The main Cd storage sites of Tianxingmi are apoplast, especially in cell walls and intercellular space, but for Zibeixian that Cd mainly accumulated in epidermal cells and exodermis. In leaves of Tianxingmi, Cd was preferentially localized in vacuoles of leaf epidermal cells, only exceeding the Cd tolerant threshold accumulated into mesophyll cells.In conclusion, the characteristics of Cd uptake and translocation in two cultivars of amaranth showed significant difference. These results illustrate that stronger abilities of Cd uptake and root-to-shoot translocation, an active process in plants of Tianxingmi, and symplastic pathway rather than apoplastic bypass contributes greatly to these in comparison with Zibeixian. These results illustrate that Tianxingmi has stronger abilities of Cd tolerance and accumulation. There are different xylem loading control systems in roots of two cultivars. The predominant sequestration of Cd in apoplast of roots and in vacuoles of epidermis of leaves may play a major role in strong tolerance, hyperaccumulation and detoxification mechanisms of Cd in Tianxingmi.