| Summary: | 碩士 === 國立臺灣大學 === 農業化學研究所 === 90 === When treated with heavy metal (Cd), Plant produces phytochelatin (PC) for the sequestration of the toxic metals. PC has the general structure of (g-Glu-Cys)n-Gly (n = 2~11, PCn), and forms a lower molecular weight complex (LMW) with Cd first in the cytosol. The LMW complexes are then transferred into the vacuole, incorporated with acid-labile sulfide, and produces the higher molecular weight complex (HMW) as the stable storage form. In 1985, Fujita identified the Cd-binding substances from Cd treated water hyacinth (Fujita 1985). We further examined these Cd-binding substances, and found that water hyacinth also contained LMW and HMW complexes, which was composed of PC3 and PC4 essentially.
This study explored the transport of the Cd-binding complexes between organs in water hyacinth. The plant was treated with Cd for 5 d in 0.01 mM CdCl2 (pulse), and then transferred to the Cd-free medium (chase) for several days. The LMW and HMW complexes in roots, stem, thick stem, and leaves were isolated and analyzed by gel filtration chromatography (Sephadex G-50). During the pulse, roots accumulated both LMW and HMW complexes promptly, but in the chase period the LMW complex decreased gradually, which might transform into the HMW complex. In the late stage of chase, leave started to accumulate minor amount of the complexes, indicating that the Cd might be transferred upward to the leaf. All these observations were confirmed by measuring the Cd content in the ash of the samples by the atomic absorption spectrometry. These results imply that water hyacinth might serve as a tool in the phytoremediation for removing the heavy metals in the environment.
Since the water hyacinth accumulated significant amounts of PC molecules in the roots, the presence of PC-synthesizing enzyme (PCS) was expected. Preliminary trial has detected the PC-forming activity in the crude extract of the roots from water hyacinth pulsed with Cd. This catalytic activity reached its maximum at the early stage of Cd treatment.
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