Physiological Mechanism Of Cadmium Tolerance In Athyrium Wardii (Hook.)

Phytostabilization primarily focuses on sequestration of the metals within the roots and rhizosphere but not into aboveground plant tissues. This technique creates a vegetative cap for the long-term stabilization and containment of tailings. Further, plant canopies serve to reduce aeolian dispersion, while plant roots prevent water erosion, immobilize heavy metals by adsorption or accumulation, and provide a rhizosphere wherein metals precipitate and stabilize. In addition, plant roots reduce the metal fluxes in the soil due to the effect of transpiration by rendering them harmless. These processes decrease metal mobility and reduce the likelihood of metals entering the ecosystem. However, potential use of metallophytes (metal tolerate plants) in phytostabilization is limited by a lack of knowledge of many basic plant processes. Athyrium wardii (Hook.) is a common perennial plant; specifically, a type of fern that grows in fascicles. It was found to have a high potential in accumulating high concentrations of Pb and Cd in roots at the contaminated sites in China, which may be useful for the phytostabilization of soils contaminated by these two metals. However, to the best of our knowledge, there is little information available regarding the process by which A. wardii stabilize and take up Cd in soils. The differences of accumulative ability and effective of physical and chemical characteristics of mining ecotypes (ME) and non-mining ecotypes (NME) were analyzed under pot experiment. The objectives of the present study were to investigate the effects of Cd on the growth and uptake of A. wardii within the two ecotypes; characteristics of antioxidant enzyme, subcellular distribution and chemical forms of Cd in the two ecotypes of A. wardii and their implication in Cd tolerance; and compare the rhizosphere characteristics of the two ecotypes A. wardii to enable a better understanding of the mechanisms by which A. wardii stabilize and tolerate Cd from Cd pollution soils. The main results are as follows:(1) A significant decrease (p<0.05) in shoot biomass was observed as the soil Cd concentration increasing, whereas the decrease in ME was lower than NME and the average biomass in ME was up to1.8times higher than that of NME. However, increasing Cd concentrations in A. wardii were observed with increasing soil Cd levels, whereas the increase in ME was greater than NME. In general, the MDA content in the fronds of the two ecotypes A. wardii increased significantly as the soil Cd concentration increased. There was no significant difference between MDA content in the ME and NME at lower Cd treatment. However, a significant difference was observed at higher Cd treatment (50mg/kg), and the highest MDA content in both ME and NME were observed and the MDA of ME A. wardii was1.22times higher than that of NME. SOD, POD and CAT activities of the ME were significant greater than the NME at higher Cd levels, however, there was no significant difference at lower Cd levels.(2) Based on the results, Cd concentrations in tissues of the two ecotypes decreased following the order of roots>petioles>fronds due to its long-distance translocation from roots to petioles and fronds. Cd analysis at the subcellular level of tissue in plants demonstrated that large proportion of Cd (24.02%-53.54%) was bound to the cell wall fraction which seems to function as the first barrier protecting the protoplast from Cd toxicity. Besides,19.61%-43.29%of the total Cd was stored in the soluble fraction which consists mostly of vacuoles and acts as the subdominant site of preferential sink for Cd in all test tissues. Additionally, Cd associated with organelles was lowest in the tested plant. For both ME and NME A. wardii, the amount of Cd forms extracted by1M NaCl and2%HAC were predominant for all treatment levels, representing41.52%-71.76%of the total Cd in different tissues, whereas only minority of total Cd in roots was in inorganic form (extracted by80%ethanol and deionized water). Therefore, A. wardii was suggested low capacity to be transported to aboveground tissues. Moreover, it could be suggested that Cd integrated with pectates and proteins in cell wall or compartmentalization with organo-ligands in vacuole might be responsible for the adaptation of the mining ecotypes A. wardii to Cd stress.(3) Cd stress significantly reduced the plant height and biomass of the two ecotypes A. wardii, but significantly increased the Cd concentration in plant tissues. In general, soil solution pH and dissolved organic carbon (DOC) concentration in the rhizosphere of the two ecotypes A. wardii were higher than the bulk soil and initial values (before planting). Moreover, the increments of the ME A. wardii were greater than NME. Soil solution pH increased by0.3-0.5and0.1-0.5units in the rhizosphere soil of the ME and NME, respectively. At10and200mg/kg Cd of soil, the maximum DOC concentrations in the rhizosphere of the ME A. wardii were observed and were83.6%,100.2%times higher than the soil before planting. However, there were a slightly change in the rhizosphere soil of NME A. wardii, ranging from96.2to119.8mg/kg. Generally, exchangeable Cd in rhizosphere soil of the two ecotypes A. wardii were lower than the bulk soil and/or soil before planting when exposed to the same Cd levels. However, carbonate combination state, ferric-manganese oxidation state, organic state and residue state of Cd were greater than the bulk soil and/or soil before planting. Moreover, the rangeability in the ME A. wardii was higher than NME. It is suggested that rhizosphere alkalinisation and the exudation of high amounts of DOM to reduce heavy metal mobility might be the two important mechanisms involved in the metal tolerance/accumulation of mining ecotype A. wardii.