| Cadmium（Cd）and arsenic（As）are two common elements among many heavy metals which result in soil pollution.Many researches have documented that exogenous application of plant growth regulators（PGRs）can effectively alleviate the damage caused by heavy metals to plants.Many achievements have been made regarding the growth promotion of heavy metal hyperaccumulators and elevation of soil remediation efficiency by PGRs.However,many results were obtained from pot experiment,and there are few studies on the combined application of different PGRs to increase the remediation efficiency of hyperaccumulators under the field conditions.Therefore,based on the optimal ratio of indoleacetic acid（IAA）and kinetin（KT）(IAA:KT=25mg·L-1:20mg·L-1)to significantly increase the remediation efficiency of Pteris vittata,which was found in a previous pot experiment of As hyperaccumulator P.vittata by our research group,we further used this optimal ratio in the field scale to deal with its effect and physiological and biochemical mechanism of the combined application of IAA and KT on P.vittataand Solanum nigrum.The farm land was firstly divided 45 plots（5m×4m）,and the heavy metal pollution in soil and its physico-chemical properties were investigated.Three phytohormone treatments were designed as IAA application alone,KT application alone and the combined application of IAA and KT.Each phytohormone treatment was divided five sub-treatments:soil control（without plant）,phytohormone control（the phytohormone was replaced with distilled water）,S.nigrum alone,P.vittata alone and intercropping of two plants.Taking a months as the time dynamic,the plants were sprayed with hormones twice and the plant and soil samples were collected three times.The heavy metal accumulation and main physiological and biochemical indexes,including root activities,antioxidant enzyme activities and malonaldehyde content in leaves of plants were measured.Moreover,the heavy metal extraction efficiency by plants was calculated.Finally,the cost of three phytohormone treatments was comparatively analyzed and an ideal scheme was obtained to remediate the cropland soil polluted by combined heavy metals.The main results showed as follows:1.The concentrations of Cd,Pb and As in the farmland soil exceeded the screening values in"Soil environmental quality Risk control standards for soil contamination of agricultual land（tentative standard,GB15618-2018）.The multiple factor potential ecological risk indexes（RI）of Cd,Pb and As in each polluted area were over 200 and belonged to medium risk.The degree of risk followed the order of Cd>As>Pb.The bioavailable concentrations of Cd and Pb were high,accounting for48%and 51%of their total concentrations,respectively.The soil p H was weakly acidic,the contents of total nitrogen and available nitrogen were low,and the overall soil fertility was inferior.2.IAA and KT combined application could promote the rapid growth of two hyperaccumulators.Under the condition of field soil containing 3.1 mg·kg-1Cd,the plant height,fresh weight,Cd concentrations in shoots and roots,Cd translocation factor and bioconcentration factor of S.nigrum were significantly increased with the combined application of 25 mg·L-1IAA and 20 mg·L-1compared to those of the control.Furthermore,the Cd extraction efficiency was the highest,up to 16.02%.Under the condition of field soil containing 78.7 mg·kg-1As,the plant height,fresh weight,As concentrations in shoots and roots,As translocation factor and bioconcentration factor of P.vittata were significantly increased with the combined application of 25 mg·L-1IAA and 20 mg·L-1compared to those of the control.Furthermore,the As extraction efficiency was the highest,up to 7.89%.After combined treatment with 25 mg·L-1IAA and 20 mg·L-1KT,the root activity and leaf peroxidase（POD）activity was significantly increased,and the content of malondialdehyde in plant leaves was significantly decreased.The results from stepwise regression analysis showed that the Cd extraction efficiency of S.nigrum was significantly positive correlated with leaf POD activity（P<0.01）,and the As extraction efficiency of P.vittata was significantly positive correlated with plant fresh weight and leaf POD activity（P<0.01）.Therefore,a higher POD activity in leaves contributed to the Cd and As phytoextraction by two plants.3.The remediation cost was divided to labor cost,material cost and maintenance cost,and the total cost of there phytohormone treatments was calculated respectively.The result showed that the total cost of IAA alone,KT alone and combined applicaiton of IAA and KT was 2484,2472 and 2507 yuan/mu.At the treatment of combined application of IAA and KT,the extraction efficiency of Cd by S.nigrum and As by P.vittata was the highest,but the total remediation cost was only 23 and23.9 yuan/mu higher than that in single phytohormone treatments.Therefore,the combined application of IAA and KT was feasible to remediate the combined heavy metal pollution in soil with high metal bioavailability. |
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