Unraveling The Mechanisms Of Cadmium Tolerance And Detoxification In Populus X Canescens

With the massive exploitation of mineral resources and wide application ofchemical product,pesticide and fertilizer,heavy metal contamination has been takenas a major threat to our living environment. It has been reported thatphytoremediation is a brand new technology to repair the soil contaminated by heavymetal. In this study, Populus×canescens was used as the experimental material andhydroponic experiment was conducted under four Cd²⁺concentrations (10,30,50,and70μM·L-1) for7,14,21and28d by using non-Cd as the control, in order toinvestigate the effects of Cd²⁺on poplar photosynthesis, antioxidantive ability andthe relation between Cd²⁺chelation and ability of Cd²⁺tolerance in plant. Thus, ourstudy is dedicated to elucidate the mechanism of Cd²⁺accumulation and toleance inPopulus×canescens and provide evidence for its application in phytoremediation.Our results are as follows:(1)It was indicated grey poplar had a strong tolerance and enrichment ability toCd²⁺. The indicating low concentration Cd²⁺(less than10μM·L-1) had a positiveimpact on grey poplar growth while higher concentration (more than30μM·L-1)hindered its growth. These findings further revealed that the bioaccumulationcapacity in grey poplar root were higher than that of leaves, bark and stem. Thebioaccumulation factors of root, leaves and bark exceeded1.0, but thebioaccumulation factors of wood lower1.0, the tolerance index of exceeded0.5.(2)With the increase of Cd²⁺concentration and extension of stress time, Chl a,Chl b, Chl a＋Chl b and Car showed a decline trend while Chl a/b ratio presented aupward tendency when stressed by Cd²⁺for7d and28d. Under Cd²⁺stress, Chl b ofgrey poplar was more vulnerable to injury compared with Chl a, and the decline ofCar content in leaves of grey poplar affected ROS scavenging.(3)Low concentration Cd²⁺(less than10μmol·L-1) was able to enhance the netphotosynthesis rate in leaf, the stomata conductance and transpiration rate while reducing the intercellular CO2concentration. Low concentration Cd²⁺(10μM·L-1) canstimulate the elevation of Fv/Fm, Fv/Fo, ΦPSâ…¡,qNand qPin grey poplar while highconcentration can reverse the effect. However, higher concentration Cd²⁺(more than30μM L-1) decreased net photosynthesis rate, stomata conductance and transpirationrate, and increased intercellular CO2concentration. Higher concentration Cd²⁺coulddecrease Fv/Fm, Fv/Fo, qNand qPin grey poplar and render the surplus of ROS. Itcould also aggravate lipid peroxidation and weaken the net photosynthesis ratefollowed by the emergence of photo inhibition. This study revealed that the declineof PSâ…¡ activity was one of the non-stomatal limiting factors leading to Pndecrease.(4)With the increase of Cd²⁺concentration and extension of stress time, thecontents of MDA, O2, H2O2among different tissues in Populus×canescenssignificantly increased, which indiated that the lipid peroxidation was aggravated.Grey poplar could reduce the lipid peroxidation by increasing the contents of solublesugar and soluble protein, and the SOD, CAT, APX, GR, DHAR and MDHAR activityand total AsA, AsA, AsA/DHA contents. While, under high concentration Cd²⁺stress, theactivities of CAT, SOD, APX and the contents of DHA, GSH, GSH/GSSG,GSH+GSSG both decreased gradually; GPX activity in grey poplar was decreasedwith the increased of Cd²⁺concentration, this results indicated an crucial role playedby GPX in Cd²⁺tolerance and detoxification in grey poplar.(5)With the increase of Cd²⁺concentration and extension of stress time, redundantCd²⁺were transfered to protoplasms when cell wall precipitation were saturated, theCd concentration were mainly accumulated in vacuole.