| Lead(Pb)is one of the most widespread heavy metal contaminants in soils worldwide.Pb causes toxic effects in plants,including disruption water balance,mineral nutrition,photosynthesis,enzyme activity and membrane organization.Pb also may enter in human body through food chain and causes serious diseases.Therefore,the remediation of Pb-polluted soils has drawn much attention.Phytoremediation is a relatively cheap and environmentally friendly technique.Rhus chinensis Mill,the Chinese sumac or nutgall tree,is a plant species in the genus R.chinensis is the pioneer plant species because of the strong adaptability,fast growth,drought and barren stress tolerance.The previous study suggested that R.chinensis was implied to have Pb tolerance and phytoextraction potential in moderate Pb-contaminated areas.The objectives of this study were to elucidate the adaptive capabilities of R.chinensis for growth under Pb stress condition and explore the uptake and translocation of Pb in plants.Results of this study will help to understand their potential use for phytoremediation of Pb contaminated soils and/or water.In this study,we made a probe into the mechanisms of Pb uptake and tolerance in R.chinensis,with the aim to reveal the Pb chemieal forms and subcellular distribution in R.chinensis,the mechanism of Pb absorption,distribution and accumulation in R.chinensis organs,and the role of root cell walls in Pb accumulation.The main results are as follows:(1)High Pb concentrations inhibited plant growth,increased membrane permeability,elevated antioxidant enzyme activity in roots and leave,and caused a significant increase in root and leave H2O2 and malondialdehyde content.R.chinensis could be to maintain its normal growth and to tolerate Pb at low concentration treatments might be linked to responding to the antioxidant system.Under the hydroponic or soil culture condition,most Pb accumulated in plant roots,especially in lateral root.Analysis of Pb content at the subcellular level showed that most Pb was associated with the cell wall fraction,followed by the soluble fractions,and with a minority present in the mitochondrial and nucleus-rich fraction.And most of the Pb in the R.chinensis leaves was integrated with oxalic acid,pectates and protein,and it had low bioavailability.In this study,the ratio of Pb in trophoplast increased and the ratio of Pb in cell wall decreased.And the same time the ratios of NaCl-extractable form of the leaves,stems,and roots were increased.We also found that the biomass of plant positively correlated with the ratio of Pb in cell wall,and negatively correlated with the ratio of Pb in trophoplast.It signified that the cell wall and chemical forms might be playing a key role for the detoxification of Pb.Under these treatments,R.chinensis transported metal to the shoots better,and the translocation factor values were 0.21-0.55 and 0.19-0.44,respectively.LMWOAs secreted by R.Chinensis roots showed a strong positive correlation with Pb concentrations in plant tissues.These results suggested that R.Chinensis plants might have the potential for effective phytoextraction of Pb,and LMWOAs might play an important role for phytoextraction process.(2)Rhus chinensi root cell walls(CW)and their three further isolated fractions(CW1,CW2,CW3)were used to adsorb Pb from aqueous solution.Results showed that cellulose,lignin and pectin played a greater role in binding Pb.At the same time,the protein might also be involved in absorbing Pb.Adsorption dynamics analysis indicates that pseudo-second-order model could be described very well to the adsorption of Pb on R.chinensis all root materials.Intra-particle diffusion process is not the unique rate-limiting step.The adsorption process of all the root materials were well described by the Freundlich,and the parameters pointed out a favorable adsorption for all root materals.The thermodynamic analysis suggested that the adsorption process was endothermic.(3)By several hydroponic experiments,passway of Pb uptake and translocation were investigated in the R.chinensis.The results showed that low temperature(4℃)treatment significantly inhibited Pb uptake in R.chinensis.And the transloeation rate was notablely decreased by low temperature treatment.Pb concentration in roots was significantly reduced by addition of metabolic inhibitors CCCP and DNP.However,the inhibition of transpiration rate in the R.chinensis has no effect on Pb accumulation in roots.These results suggest that Pb uptake and translocation is an active process in R.chinensis.Pb concentration of R.chinensis roots increased with the increase of Pb concentration in solution.A modified Michaelis-Menten equation could to describe the concentration-dependent kinetics of Pb uptake through the roots,stems,and leave of R.chinensis,which indicated that Pb uptake across the plasma membrance by transporter-mediated.The uptake capacity(Vmax/Km)value was 0.972.It indicated the that R.Chinensis plants might have the potential for effective phytoextraction of Pb.(4)The results showed that a signifcantion inhibition of Pb concentration of root was induced by lower Ca treatment,whereas Pb concentration of root was increased at higher Ca treatment.LaCl3 and CHD could be inhibited the Pb uptake by R.chinensis,which may be the result of the transport channels with Ca ions might be participate in transport of Pb ion.The scanning ion-selective electrode technique data showed that the outward Ca2+ and K+ were lower than that in cntrol in the root hair cells,due to the Ca and K concentration of root was decreased under Pb stress.In this study,Pb might to impair the uptake of Ca+ and K+,subsequently disturbing ion homeostasis in R.chinensis.(5)Compared to the various changes of control treatment,two levels of citric and oxalic acid treatment alleviated the Pb toxicity and damage.The low level of citric acid(0.5 mM)and high level of oxalic acid(1.0 mM)were more sensitive than the other treatments,which caused a series of physiological changes in plants under lead toxicity and played an important role.The effect of low concentration of citric acid(0.5 mM)exerted better effect on Pb accumulation and transport for R.chinensis.While the Pb concentration of root in lower oxalic acid treatment was lower than that in control.But exogenous citric and oxalic acid promoted the transport of Pb by plants.The translocation efficiency value was increased from 0.23 to 0.32 or 0.54 in lower citric acid and oxalic acid treatment,respectivly.In conclusion,exogenous citric and oxalic acid alleviated the oxidative damage by forming chelate with Pb in R.chinensis plants. |
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