Cadmium Uptake,Accumulate And Phytoremediation Potential Of Alien Invasive Plant Petunia Hybrida Vilm.

Soil is the most basic component of ecosystem and the basic materials for human survival.With the development of industry and agriculture,all kinds of pollutants,especially all kinds of heavy metals,are discharged into the soil,resulting in soil pollution.Phytoremediation for heavy metal pollution in soil is in the spotlight for its green,pollution-free,low-cost remediation.The key of phytoremediation of heavy metal-contaminated soil is to find excellent metal-accumulative plants and then to deeply understand the mechanism of heavy metal uptake and accumulate in them,and explore ways to promote the remediation process.However,some of the known restorative plants can not adapt to the harsh environment of the restored area,coupled with slow growth and small biomass,thus limiting the application of this technology.Alien invasive plants have the advantages of strong vitality,strong adaptability,wide distribution range and large biomass.If they are applied to phytoremediation of heavy metal contaminated soil,they can just make up for the defects of many remediation plants.Therefore,this paper firstly selects Petunia hybrida Vilm,which has the ability to accumulate cadmium（Cd）,from 55 kinds of alien invasive plants through field sampling and laboratory planting,then combines with indoor hydroponics and soil culture experiments,and conducts dynamic sampling at different time under different concentrations of Cd treatment.To verify the Cd accumulation ability of this plant.Secondly,the mechanism of Cd accumulation in this plant was systematically studied by measuring related indexes such as antioxidant enzymes and osmotic regulatory substances,root secretions and organic acids in various parts of the plant,chemical morphology of heavy metals before planting,and in rhizosphere and non-rhizosphere soil after planting,chemical morphology and subcellular distribution of heavy metals in leaves.Finally,P.hybrida were planted in soils contaminated by heavy metals from different sources,and compared with known Cd hyperaccumulator Solanum nigrum L.,Bidens pilosa L.to study the remediation potential of P.hybrida on soils contaminated by heavy metals.Moreover,the root exudates were added according to a certain concentration,and the optimal concentration was obtained to improve the Cd accumulation ability of P.hybrida.The main research results are as follows:1.All the 55 alien invasive plants tested could grow normally in contaminated soil without showing obvious toxic effects,indicating that these plants had good tolerance to heavy metals.All plants had good accumulation of Cd,and 13 plants had bioconcentration factor and translocation factor greater than 1,but the accumulation ability of other heavy metals was weak,and the bioconcentration factor was less than1.The Cd content in the roots,stems and leaves of P.hybrida were 4.18,3.63 and 16.21mg·kg^-1,respectively,after growing in the polluted soil for 90 days.The bioconcentration factor and translocation factor of leaves were 5.81 and 3.88respectively.2.Under the conditions of hydroponics and pot experiment,P.hybrida could grow normally at low and medium concentration of Cd,and their biomass did not decrease significantly except 100 mg·kg^-1 Cd treatment.In the dynamic study of Cd accumulation in P.hybrida,the Cd content in all parts of P.hybrida increased steadily with the increase of Cd concentration and culture time.When the concentration of Cd in solution was 25 mg·L^-1 for 7 d,the Cd content in the leaves reached 275 mg·kg^-1.After soil culture for 60 d and soil Cd concentration of 50 mg·kg^-1 and 100 mg·kg^-1,the Cd content of leaves reached 160 mg·kg^-1 and 188 mg·kg^-1,respectively,and the bioconcentration factor was greater than 1.P.hybrida is a potential Cd hyperaccumulator with strong Cd tolerance and accumulation ability,and is a good candidate for phytoremediation of Cd contaminated soil.3.Cadmium treatment promoted the activities of superoxide dismutase（SOD）,peroxidase（POD）,catalase（CAT）and the contents of glutathione（GSH）,ascorbic acid（ASA）,proline and soluble protein.Oxalic acid and citric acid were detected in root exudates under hydroponic conditions,and oxalic acid was also found under soil culture condition.Both organic acids existed in plants,and Cd treatment also promoted their contents.Compared with the soil before plant cultivation,the exchangeable fraction of Cd in the rhizosphere soil after P.hybrida planting decreased significantly.In leaves of P.hybrida Cd mainly existed as F₄（cell soluble fraction）and F₁（cell wall and unbroken residual fraction）,and the main distribution forms were the water-soluble（F_W）and pectin and protein-bound(F_{Na Cl})fractions.Therefore,the activities of SOD,POD,and CAT,as well as the contents of ASA,proline and soluble protein were significantly increased to eliminate the reactive oxygen species（ROS）produced by Cd accumulation,ensuring the normal survival and healthy growth of P.hybrida.In addition,exchangeable Cd was absorbed by roots with the help of oxalic acid and citric acid exudation by roots.Cadmium in plant leaves was chelated by oxalic acid,citric acid,and GSH,which migrated Cd toxicity and stored it in cell walls and vacuoles to achieve Cd accumulation.Result from stepwise regression analysis showed that leaf Cd concentration in P.hybrida under soil culture condition was significantly and positively correlated with soluble protein content and the proportion of Cd in F₄（cell soluble fraction）in the leaves.4.In four contaminated soils from different sources,P.hybrida could grow normally,and its biomass,aboveground Cd content and removal rate were significantly higher than those of Solanum nigrum L.and Bidens pilosa L.Exogenous addition of oxalic acid with a certain concentration gradient could promote the increase of biomass and aboveground Cd content of P.hybrida,and the optimal addition concentration was0.01 mg·L^-1.In conclusion,under different culture conditions,P.hybrida has a good ability to uptake,accumulate Cd.The toxic effects caused by Cd accumulation can be eliminated by increasing the activity of antioxidant enzymes and the content of osmotic regulatory substances.Moreover,Cd in plants can be stored in cell walls and vacuoles after reducing toxicity through chelation of oxalic acid,citric acid,GSH and other substances.so as to realize the enrichment of Cd.And 10μg·L^-1 oxalic acid was added exotically to promote Cd accumulation in P.hybrida.Therefore,P.hybrida is a kind of alien invasive plant with both ornamental and Cd pollution remediation ability.