Mechanism Of Chromium Tolerance In Typha Angustifolia And Its Possible Role In Chelate-assisted Phytoextraction Of Metal Polluted Soils

Heavy metal pollution in soils poses a major environmental hazard to terrestrial plants.It is crucially important to identify appropriate plant species/genotypes which may resist/tolerate heavy metals or other contaminants to identify/study the related genes in order to remediate the contaminated environment.While,understanding physiological and biochemical mechanisms of heavy metal tolerance/resistance is fundamental for discovering or creating new metal resisting plant species.We found a kind of gramineous plant species,Typha angustifolia,with high resistance to Cr stress growing on a Cr-contaminated area.The present research was conducted for two successive growth seasons to evaluate its ability to tolerant Cr stress and the relevant physiological responses and the possible role in chelate-assisted phytoextraction of metal polluted soils. The results were as follows:Pot experiments were conducted to study the potential of T.angustifolia plants for Cr toxicity tolerance and subsequent physiological response involved in this phenomenon. Plants exposed to 50μM Cr(Ⅵ)showed a slight increase in plant height and biomass, compared with control,and there was no difference in these growth parameters between the plants exposed to 100μM Cr(Ⅵ)and control(0μM),while increasing Cr levels to 200-800μM induced a significant decrease in plant height and biomass,but without significant injured leaf symptoms of Cr toxicity(necrotic patches on leaf blade)even in 800μM treatment.Thus,it can be concluded that the plant can survive well in 800μM Cr stress.However,T.angustifolia plants demonstrated a relatively low Cr concentration in comparison with the plants classified as hyper-accumulators which require a concentration of at least 1000 mg Cr kg~-1 DW.with a translocation factor(TF)of 0.02 to 0.10 among different Cr treatments of this plant.Therefore,it can be concluded that the plant tends to restrict soil-plant and root-leaf transfer,and consequently has a lower Cr concentration in its tissues.Since the extraction of metal from Cr-amended medium depends on plant species,this plant,like many other plants,may have evolved Cr exclusion strate(?). Chromium induced significant increase in SOD and POD activities.Meanwhile. significantly positive correlation was detected between Cr and Mn or Cu in leaves,and roots,respectively,and slender positive correlation also occurred between Cr and the other three microelements in leaves and roots.The Cr tolerance of the plants appeared to be associated with the enhancement of SOD and POD activities and the improvement of the essential microelements uptake and translocation.Improvement of the capacity of plants to accumulate metals by the application of chelating agents provides a new possibility for phytoremediation using high biomass species.Pot experiment was conducted to study the performance of EDTA and citric acid (CA)addition in enhancing phytoextraction of Cd,Cu,Pb and Cr from artificially contaminated soil by T.angustifolia.The results demonstrated the remarkable resistance to heavy metal toxicity of T.angustifolia with no visual toxic symptoms such as chlorosis and necrosis on shoots of the plants exposed to metal stress.EDTA-addition significantly reduced plant height,biomass production and stunted plant growth compared with the control,while 2.5 and 5 mM CA addition induced significant increases in root dry weight. Both of EDTA and CA significantly increased shoot Cd.Pb and Cr concentrations compared with the control,with EDTA being more effective than CA.At final harvest,the highest shoot Cd,Cr,and Pb concentrations were recorded in 5 mM EDTA,while maximal root Pb concentration found in 2.5 mM CA treatment.However,shoot Cd accumulation of 10 mM CA treatment was 36.9%higher than that of 2.5 mM EDTA,and similar with 10 mM EDTA,but lower in shoot Pb accumulation relative to EDTA treatments.Meanwhile, root Cd,Cu and Pb accumulation of CA treatments and shoot Cr accumulation in 5 or 10 mM CA treatments was markedly higher than that of control and EDTA treatments.Both of EDTA and CA have no significant effects on shoot Cu accumulation.The results also showed that EDTA dramatically increased the dissolution of Cu,Cr.Pb and Cd in soil. while CA addition has much less effect on water-soluble Cu,Cr,and Cd,and no effect on Pb level.It may be suggested that CA can be regarded as a good chelate candidate for T. angustifolia as environmentally safe phytoextraction of Cd and Cr in soils.