The Relation Of Organic Acids And Cd Accumulation In Sedum Alfredii Hance

The contamination of heavy metals in soils is a serious problem that all of the countries throughout the world are facing. Some plants growing in the environment of high dense heavy metal ions for a long time have developed a variety of biochemical defense strategies to prevent heavy metal poisoning. These plants may also accumulate high level of the heavy metals, and they are known as hyperaccumulator. Organic acids such as citric, oxalic and/or malic acid, which is a significant kind of metal chelators in hyperaccumulator, have shown to enhance the absorption, transport and accumulation the heavy metals by the formation of chelate compounds and play a considerable role in metal tolerance in hyperaccumulator.There are varieties of chelators in plants, including organic acid, amino acid, polypeptide, phytochelatind (PC) and metallothionein (MT) and so on . Binding of the potentially poisonous metal ions by organic acids or other kind of chelators, followed by transport to the vacuole may reduce the activity of metal ions in the metabolically active cell compartments . Organic acid, as an important kind of chelator, involved in the uptake, transport and accumulation of metals in plant, extensively exists in the body and rhizosphere of hyperaccumulator . Organic acids found to chelate metals in the body and rhizosphere soil include oxalic acid, malate and citrate and so on . Therefore, identifying the function of organic acids in heavy metal tolerance mechanism in hyperaccumulator will benefit further understanding of the biological and molecular mechanisms that hyperaccumulator tolerate and hyperaccumulate heavy metals and is necessary for successful phytoremediation. This paper summarizes the function of organic acids that have shown to increase the uptake, transportation and accumulation in plants.1. In solution culture, the non-mined ecotype plants of Sedum alfredii H (NME) grew naturally at the level of 10μmol L~-1 cadmium ions and the chlorophyll concentration was not lowed greatly. Cadmium concentration in varied parts increased with increasing externalCd supply levels either for NME or for ME. Cd concentration in roots were higher largely than that in stems and leaves, and leaf Cd concentration were far lower than that in stems. Compared Cd concentration in various parts of ME, the outcome is that Cd concentration in leaves were higher greatly that in stems and that in stems rather higher than that in roots. From these, we maybe make a conclusion that ME can uptake, transport and accumulate Cd ions more effectively than NME.2. The sorts of the organic acids were very similar in various parts of both for ME and for NME. There were glutarate, oxalate, tartrate, malate, lactate and acetate, and so on. Affected by Cd, oxalate and tartrate concentration in roots of NME increased with increasing external Cd supply levels, while that in stems and leaves of NME decreased greatly. On the contrary, oxalate and tartrate concentration in roots of ME increased enormously with increasing external Cd supply levels. For ME, oxalate and tartrate maybe, as a kind of complexing agent, bind the potentially poisonous Cd ions, transport them to the vacuole and reduce the activity of metal Cd ions in the metabolically active cell compartments (Ernst et al., 1992;Verkleij et al., 1990). Probably, this is a very important heavy metal tolerance mechanics in ME.3. In solution culture, two ecotype plants of Sedum alfredii H (NME) grew naturally at the level of cadmium/znic ions complex pollution in this experiment and the chlorophyll concentration was not lowed greatly. Cadmium concentration in varied parts increased with increasing external Cd supply levels either for NME or for ME but was not affected by Zn remarkably. And Zn concentration in roots, stems and leaves enhanced largely with Zn supply levels, but was nor affected by Zn evidently.4. The sorts of the organic acids were very similar in various parts of both for ME and for NME. There were glutarate, oxalate, tartrate, malate, lactate and acetate, and so on. Affected by Cd, oxalate and tartrate concentration in roots of NME increased with increasing external Cd supply levels, while oxalate, tartrate and malate in stems and leaves of NME decreased greatly. On the contrary, oxalate and tartrate concentration in roots ofME increased enormously with increasing external Cd supply levels. While Zn supply restrained oxalate and tartrate in various parts of ME, but was benefit for malate. For ME, oxalate and tartrate maybe, as a kind of complexing agent, bind the potentially poisonous Cd ions, transport them to the vacuole and reduce the activity of metal Cd ions in the metabolically active cell compartments (Ernst et al., 1992;Verkleij et al., 1990). Probably, this is a very important heavy metal tolerance mechanics in ME.5. Besides, we can find that the plants of NME grew better than ME in this carried experiment when Zn and Cd were not aupplied. Without Zn or Cd supply, the dry weights of roots, stems and leaves of NME were a litter larger than that of ME similarly. Before treatment, the color of roots of NME was white. After treatment, the color grew brown. On the contrary, by Zn or Cd treatment, the color of roots of ME grew white from brown.