Physiological Changes And Tolerance Of Quercus Variabilis Under Lead And Cadmium Stress

The increasing sreious situation of heavy metal pollution in soil had attracted many experts to pay attention to phytoremediation.Lead（Pb）and cadmium（Cd）were two of the five toxic elements and did great harm to human body.Quercus variabilis Blume was widely distributed in East Asia.Previous fieid experiment of our group had found the species featured with hight tolerance to heavy metal,however,the physiological responses and resistent mechanism were still not clear.In the present study,three provenances of Q.variabilis（Baotianman BTM,Pinggu PG,Jiaozuo JZ）（one year old,1a）seedlings and seedlings of different ages in PG（1a,2a,3a）were selected for hydroponic and soil culture respectively.The treatment concentration were as followed:Pb-1:100μmol·L^-1,Pb-2:200μmol·L^-1,Pb-3:400μmol·L^-1,CK:0μmol·L^-1,Cd-1:100μmol·L^-1,Cd-2:200μmol·L^-1.Hydroponics period spent 37 days（37d）.Soil culture period extended 123 days（123d）.The differences of provenances（provenance effect）and age of the same provenances（age effect）under different concentrations of lead and cadmium were compared.The results showed that:No obvious age effect was observed in RGR of basal diameter and plant height under stress treatments.Under soil culture,heavy metal stress did not significantly affect the biomass allocation,but the responses of different provenances to heavy metal stress were different.The differences in the photosynthesis and fluorescence between heavy metal treatment were significant.The MDA and PRO were significantly different between provenance.The observed index fluctuation in hydroponics was greater than soil culture with many extreme values.From the point of view of different provenances of the same age,we found that the root activity of JZ provenances was the highest.And JZ were not sensitive to stress,indicating that it may be more suitable for the restoration of mining areas.BTM showed strong sensitivity to heavy metal.The 2a seedlings had stronger adaptability through the analysis of different age groups of the same provenance（PG）.Root system was the first protective barrier for heavy metals entering plant organism,so the cell wall of Q.variabilis roots from JZ provenance was extracted to examine its adsorption capacity.The adsorption quantity of Pb and Cd was increased significantly with the increase of p H.When the adsorption equilibrium was reached（p H=4.5）,the amount of Pb adsorbed increased rapidly with the increase of the solution concentration,especially in the lower Pb concentration region（0～0.5mg/L）.With the increase of Cd concentration（p H=5.5）,the Cd adsorptive capacity increased linearly.Using Langmuir isothermal adsorption model,the Qmax of Pb and Cd were respectively calculated to be 36.24 mg·g^-1and 45.03mg·g^-1.From the adsorption kinetics curve,the adsorption dynamics of Pb and Cd appeared the similar trends.There was a fast adsorption period between 200 and 300 minutes,and then slowed down.In particular,the adsorption of Cd tended to be nearly saturated in 300 minutes,while the adsorption of Pb has not yet been saturated.From the desorption kinetics curve,it could be seen that the desorption amount of Pb reached 70%of the equilibrium desorption amount at100 min,while that of Cd reached 95%of the equilibrium desorption amount at the same time.The Pb distribution of root,petiole,main and secondary veins tissues were scanned by SRXRF.It showed that the soluble lead around the rhizosphere was transported to the endothelium through vascular bundles,and was intercepted by the casparian strip.Therefore,the depositional sites were distributed along the cortex and most of Pb could not go deep into the stele.This proved that the casparian strip had a strong interception effect on Pb,so as to inhibit the toxic components transfer from roots to above-ground.So Q.variabilis was well tolerated to Pb.Secondly,because most of the lead was deposited in the root,the rest lead was transported to the petiole,the main and secondary veins.There were more high-energy areas in the secondary veins,however,the distribution regularity was not obvious.In terms of elemental relationship,the relationship between Pb deposition sites and K and Mn was not special in this experiment,but occasionally showed antagonistic relationship with Fe and Zn under different concentrations and time treatments,and basically antagonistic relationship with Cu,and had nothing to do with plant sampling sites.To sum up,different provenances of Q.variabilis have different sensitivity to Pb and Cd,and JZ had the best comprehensive ability.The responses of seedlings from the same provenance at different ages were also different,the 2a seedlings had higher resistance to heavy metals,which was beneficial to the restoration of mining areas.