Hg-Induced Oxidative Stressful Response In Fern And Indian Mustard And Phytoremediation In HG Contamination

Mercury (Hg) contamination has evocked great concerns worldwide because of its harmful effect on agricultural ecosystem and environment. Phytoremediation is a nearly perfect strategy for controlling soil heavy metal contamination. However, no Hg hyperaccumulator has been found thus far. Despite of this, great efforts have been made to explore physicological response of selected plant speices to Hg stress as well as their ability to accumulate Hg from Hg-contaminated soils, air and waters.Pteris vittata, a fern, is a well-known arsinic-hyperaccumulating plant, while its counterpart Nephrolepis exaltata is arsinic-sensitive. We made comparative investigation on the Hg accumulation and its phytotoxicity in the test plants. The roots of both ferns accumulated a large amount of Hg, but exhibited limited Hg translocation to shoots. Pteris vittata accumulated more Hg in shoots and roots than Nephrolepis exaltata. Exposure to high level of Hg induced visible toxic symptoms in the shoots of Pteris vittata, e.g. withering and chlorosis. Electronic microscopy analysis revealed the shrinkage of vascular cells and breakdown of chloroplasts in Pteris vittata leaves under Hg exposure. In contrast, no visible toxicity and foliar structural changes were observed in Nephrolepis exaltata under Hg stress. Hg exposure induced substaintial accumulation of H2O2 and TBARS in the shoots of Pteris vittata. But the activities of anti-oxidative enzymes such as SOD, CAT, and GR remained relatively unchanged. For Nephrolepis exaltata, Hg exposure significantly incresed the activities of these anti-oxidative enzymes, which play critical roles in removing excess H2O2 and consequently suppressed TBARS production under Hg stress. Additionally, Nephrolepis exaltata had a higher level of ASA/(ASA+DHA) ratio than Pteris vittata. Overall, the results indicated that Nephrolepis exaltata appeared to be more tolerant to Hg than Pteris vittata with regard to their response to Hg-induced oxidative stress. Indian mustard is believed to be a potential candidate plant for phtoremediation of heavy metal-contaminated soils. Two major commercial cultivars of Indian mustard (Brassica juncea), Broad leaf and Long-stading, were used in the study. Mature plants accumulated more Hg than seedlings. The roots of the two cultivars accumulated high level of Hg from root-bathing solutions. Shoots accumulated higher amounts of Hg than other plants as previously reported, indicating the significant translocation of Hg from roots to shoots. Content of Hg in Broad leaf was higher than that of Long-standing at the external concentrations of Hg within 12.2-16.7 mg/L. However, at such levels, both cultivars showed chlorosis and stunted growth of roots. In gerenal, Hg exposure significantly increased the activities of CAT, POD, and SOD, but decreased the concentratios of H2O2 in the shoots of Indian mustard. Liner analysis showed negative correlations between H2O2 concentrations and the activities of CAT and POD, which implyed that CAT and POD probably played important roles in protecting the plants from H2O2 toxicity under Hg stress.We screened and investigated the potential of three plants (Pteris vittata, Brassica juncea, and Polypogon monspeliensis) in phytoextraction of Hg-contaminated soil. Three independent experiments were performed in turn. (1) Pot study with potting mix and HgCl2 solution. (2) Pot study with freshly HgCl2-spiked soil. (3) Experiment using aged soil contaminated with different Hg sources. Both shoots and roots of Pteris vittata could accumulate large amount of Hg from many kinds of Hg-contaminated soils (such as HgCl2, Hg(NO3)2, or HgS) without any visible stress symptoms. Therefore, Pteris vittata was a possible candidate for Hg phytoextraction and phytostabilization. Elemental Hg can evaporate into air because of its special physical properties. So we designed a chamber experiment to study the foliar Hg uptake by plants. Shoots of both Pteris vittata and Brassica juncea accumulated Hg at the concentratios of 63-105 mg/kg from Hg-contaminated air. So both Pteris vittata and Brassica juncea probably have potential in remediating Hg-contaminated air.The application of remote sensing in monitoring plant physiological status has been growing concerns. We found that relative spectral reflectances in visible region (390-780 nm) and near infrared region (NIR,800-1300 nm) were sensitive to leaf structure and physiological status in Pteris vittata during the phytoremediation of Hg-contaminated soil. We selected several high spectral resolution reflectance vegetation indices to evaluate the change of spectral reflectance in Pteris vittata. We also estabilished the correlations between NDVI and biomass, R1110/R810 and leaf structure, WI and relative water content, respectively, in the shoots of Pteris vittata grown in Hg-contaminated soil. Hg stress induced significant increase in relative reflectance around 680 nm corresponding to the breakdown of chloroplast observed in TEM photographs in Pteris vittata. These results provided some theoretical evidences for the application of remote sensing in monitoring plant physiological status during the phytoremediation of Hg-contaminated soil.