Study On Mechanism Of Cr Accumulation And Detoxification In Leersia Hexandra Swartz

As a hygrophyte with chromium hyper-accumulative species, Leersia Hexandra Swartz was found for the first time in china. The L. Hexandra is easy to cultivate due to its perennial gramineae characteristic, and which made it a useful material for studying. The distribution patterns and combination forms of Cr at organ and cellular levels were investigated by using sequential chemical extraction method, differential centrifugation technology and atomic absorption spectrometry (AAS). By biological microscope and tranmmission electron microscope (TEM), the effects of chromium on the organ structure and the ultrastructure of Leersia Hexandra Swartz were observed. Absorption inhibitor experiments were conducted in this study for exploring the mechanism of Cr3+ uptake by L. Hexandra. The results throughout the paper were summarized as follows:1. L. Hexandra had a great accumulation capacity for Cr. A high Cr bioaccumulation was observed in the leaves, stems and roots of L. Hexandra. The maximum Cr concentrations in the leaves, stems and roots were 5430mg·kg-1, 1956mg·kg-1and 40599mg·kg-1 respectively. When the Cr concentration in nutrient solution was high(≥30mg·L-1), Cr concentrations in leaves were higher than 1000mg·kg^-1, the minimum Cr concentration for a Cr-hyperaccumulator.2. L.Hexandra had a great resistant capacity to Cr toxicity. In hydroponics experiment, the biomass of leaves, stems and roots were not significantly declined in the plants that exposed to the highest Cr concentration (60mg·L^-1, as CrCl₃). In addition, during the investigation of chromium different levels distribution in L. Hexandra, it showed that: (1)as far as the accumulate ability of different organ in L.Hexandra were concerned, root>leaf>stem. (2)among the chromium content of cellwall, cellorgan, and vacuole, vacuole>cellwall>cellorgan in leaf cells; cellwall>cellorgan>vacuole in root cells; Through the observation of organ structure and ultrastructure, we found that the stem fibro-vascular bundle in the Cr-treated plant was littler than thant in the control plant(no Cr). It was chromium poisoning slightly. In comparasion with the control plant, the organ structure of roots and leaf in the Cr-treated plant were not significantly changed. Moreover, there were a lot of high-density electronic deposition in the vacuole inside leaf cell and the cellwall of root cell, which might be Cr accumulation in vacuole and cellwall. It indicated that vacuole and cellwall played important roles in the major mechanism for L. Hexandra to hyperaccumulate Cr and resistant to Cr toxicity.3. Studies on the chemical forms of chromium inside plant tissues showed that a high proportion of the oxalic integrated Cr was found in the leaves of L. Hexandra with Cr treatment. Most accumulations of Cr in root were observed in residue fraction, which indicated that the high Cr concentration accumulated in roots might be bound to the low bioavailability forms.4. Inhibitor studies indicated the P-type ATP enzyme inhibitor (Na₃VO₄) is an inhibition for uptake of Cr in L. Hexandra, which suggested that the Cr³⁺ uptake in L.Hexandra may be an active process. Moreover, the Cr³⁺ uptake in L.Hexandra is related to Ca²⁺ channel due to the inhibition of Cr³⁺ uptake from LaCl3. But the uptake of Cr in L.Hexandra is unrelated to K+ channel inhibited by TEA.