Effects Of Exogenous Organic Acids On Cadmium Accumulation Efficiency Of Salix Variegata Franch

Phytoremediation efficiency is usually restricted by the low bioavailability of heavy metals in the contaminated soil.Application of chelator is an important technology to improve phytoremediation efficiency,since it can activate the heavy metals in the soil.Salix variegata Franch.is a perennial woody plant with a large biomass and wide distribution.Studies on the accumulation and tolerance of cadmium in S.variegata have shown that this species has potential to restore the cadmium-contaminated soil.How to further improve the efficiency of S.variegata on cadmium-contaminated soil is the key to better application and promotion.In order to investigate the characteristics of organic acids in the root exudates of S.variegata under different cadmium concentrations(0,2,5,10,20,40 mg·L^-1 Cd²⁺),a hydroponic experiment with different cadmium treatments was conducted over 30 days.The organic acids that may help improve cadmium accumulation efficiency of S.variegata were selected from root exudates.A pot-cultured experiment was conducted to detect the effects of exogenous organic acid on the growth and cadmium accumulation of S.variegata,and the bioavailability of heavy metals in soil.The soil cadmium concentration was set to 10 mg·kg^-1,and there were three exogenous organic acids:oxalic acid,tartaric acid and EDTA?as positive control?.Each acid treatment included two concentrations:5 and 10 mmol·kg^-1.The results are as follows:?1?Four kinds of organic acids in the root exudates of S.variegata had been detected under different cadmium stresses,that were oxalic acid,tartaric acid,succinic acid and acetic acid.Among them,oxalic acid was the main organic acid.The content of oxalic acid had similar change which increased first then declined with the concentration of cadmium,and it increased up to the maximum under treatment with 5 mg·L^-1 Cd²⁺.The contents of tartaric acid and succinic acid were the secondarily,while acetic acid was detected only in two cadmium treatment groups of 20 and 40mg·L^-1 Cd²⁺,and the content was only 0.4%-0.9%of total organic acids.?2?After adding EDTA to the cadmium-contaminated soil,the height,base stem number and biomass of each organ of S.variegata were significantly decreased when compared to control.This result indicated that the growth of S.variegata had been significantly inhibited by cadmium-contaminated soil with EDTA.The photosynthesis ability of S.variegata decreased,and the leaves of plants were dry and curled.On the contrary,the growth of S.variegata was not affected by cadmium-contaminated soil with added oxalic acid and tartaric acid.The S.variegata could maintain a higher net photosynthetic rate by increasing the proportion of chlorophyll in photosynthetic pigments to ensure sufficient reaction center pigment and maintain high levels.The high photosynthetic ability showed strong growth adaptability of S.variegata.?3?The cadmium contents in the root and stem of S.variegata increased under low concentration of EDTA treatment.However,the inhibitory effect of EDTA on the growth of S.variegata had increased with the increase of EDTA concentration due to the toxicity of EDTA and EDTA-cadmium complexes.This process resulted in the ineffectiveness of exogenous addition of EDTA on the cadmium accumulation efficiency of S.variegata.There were different effects of oxalic acid and tartaric acid on the cadmium accumulation of S.variegata.Exogenous tartaric acid had a significant positive effect on the cadmium accumulation of S.variegata,the effect of low concentration is better than that of high concentration.The addition of oxalic acid did not significant effect on cadmium accumulation of S.variegata.?4?The addition of exogenous organic acids reduced the residual cadmium content in the soil,and increased the exchangeable cadmium contents in the soil.The addition of EDTA and tartaric acid showed better activation of heavy metal cadmium,and the activation effect of low concentration was better than that of high concentration.But the addition of oxalic acid had insignificant activation of cadmium in soil.In summary,the S.variegata showed high cadmium tolerance.Under different cadmium concentration stress,the S.variegata will mainly secreted oxalic acid,and supplemented by tartaric acid,succinic acid and acetic acid when responding to environmental changes.In order to investigate the effects of exogenous organic acid on the growth and cadmium accumulation of S.variegata,oxalic acid,tartaric acid and EDTA were selected as exogenous acids,and then added to cadmium-contaminated soil,The results showed that the low concentration of EDTA could activate the cadmium in the soil and increase the cadmium contents in the root and stem of S.variegata.But the toxicity of EDTA and EDTA-cadmium complexes directly affected the normal growth of plants and reduced the biomass of the plants,which resulted in the ineffectiveness of exogenous addition of EDTA on the cadmium accumulation efficiency of S.variegata.Oxalic acid and tartaric acid,two kinds of natural organic acids,did not show inhibitory effect on plant growth.There were different effects of oxalic acid and tartaric acid on the cadmium accumulation of S.variegata.The addition of oxalic acid had no significant effect on cadmium accumulation of S.variegata.Exogenous low concentration tartaric acid could activate the cadmium in the soil and had a significant positive effect on the cadmium accumulation of S.variegata,the repair efficiency was2.06 times that of cadmium treatment,then it was a suitable method to accelerate the restoration of cadmium-contaminated soil by S.variegata.