| Copper?Cu?is an essential micronutrient for plants,participating in many physiological processes.However,Cu in excess is toxic to plants.The remediation and utilization of Cu contaminated soil have become a world-wide issue.In recent years,castor is considered as an ideal species for phytoremediation because of its fast-growing,high biomass,tolerance to heavy metals,and high economic value.Castor seeds collected from the Tonglushan mine area were used as the materials for this study.The hydroponic experiment was conducted to explore the sequestration of Cu in castor root and the effectiveness of sulfur to alleviate Cu toxicity to castor.In addition,pot experiment was carried out to study the effects of sulfur on Cu speciation,castor growth and the uptake of Cu by castor in two types of Cu-contaminated soil.16S rRNA gene sequencing was used to investigate the key microbe that controlled sulfur behavior in two Cu-contaminated soils.The research contents and relevant results are listed as follows:1.The hydroponic experiment was conducted to investigate the accumulation and distribution of Cu in castor and the sequestration of Cu in castor root cell wall.The results showed that castor roots accumulated the majority of Cu.At 12.5?mol L-1Cu treatment,most of the Cu in root cells was in the soluble fraction and cell wall fraction,accounting for 48.76%and 40.95%,respectively.However,when the Cu supply in the growth medium increased to 25 and 50?mol L-1,the proportion of Cu in cell wall fraction increased from 40.95%to 58.01%and 57.86%,respectively.Most of the Cu in castor root cell wall resided in the hemicellulose1 fraction,accounting for44.85-67.78%of the total cell wall Cu.Besides,a proportion of Cu resided in hemicellulose2 fraction and pectin fraction.Compared with hemicellulose1 and hemicellulose2,the ability of the pectin to bind Cu was easy to reach saturation.The results of two-dimensional fourier-transform infrared correlation spectroscopy analysis?2D-FTIR-COS?demonstrated that the hemicelluloses C-O-C and C-C or C-OH gave the fastest responses to Cu stress,followed by pectin C=O and syringyl or C=O in lignin and xylans,and protein C=O.2.Cu stress stimulated the production of phytochelatins?PCs?in castor root to bind with excessive Cu ions,leading to the depletion of glutathione?GSH?.Sulfur can alleviate the toxicity of Cu to castor root.But sulfur alleviates the Cu toxicity to castor root in different ways at varying degrees of Cu stress.At lower Cu levels(?25?mol L-1 Cu treatments),increasing sulfur supply could promote the synthesis of GSH to alleviate Cu-induced oxidative stress in castor root.At higher Cu levels(50 and 100?mol L-1 Cu treatments),increasing sulfur supply could alleviate Cu toxicity by promoting the synthesis of PCs in castor root.3.The pot experiment was carried out to explore the effect of sulfur on growth,antioxidant system and Cu accumulation of castor in two types of Cu-contaminated soil.In the soil with pH 7.90 and 1507 mg kg-1 Cu?collected from Tonglushan copper mine,MS-Soil?,the application of sulfur(0-4 g kg-1)reduced the accumulation of Cu in castor and promoted its growth.With the application of sulfur,the Cu content of castor roots and shoots was reduced by 12.96-45.35%and 12.73-23.85%,respectively,while the fresh weight of shoots increased by 10.48-29.14%,compared with the control plants.However,the addition of sulfur aggravated the toxicity of Cu to castor roots and inhibited the growth of castor in the soil with pH 6.46 and 790 mg kg-1 Cu?collected from a farmland near the Tonglushan copper mine,FS-Soil?.At 4 g kg-1sulfur treatment,the Cu content of castor roots was increased by 43.82%,while the fresh weight of roots was reduced by 45.98%,compared with the control plants.In the pot experiment,that the application of sulfur could influence the uptake of Cu by castor may be the primary cause of affecting castor growth.4.The application of sulfur had different effects on the speciation and bioavailability of Cu in two types of Cu-contaminated soil.In the MS-Soil,the addition of sulfur reduced the acid-soluble Cu fraction and reducible Cu fraction,while increased the oxidizable Cu fraction and residual Cu fraction.Compared with the control soil,the proportion of reducible Cu fraction in sulfur-treated soils decreased from 29.83%?CK?to 4.71%(4 g kg-1 sulfur treatment).Conversely,the proportion of oxidizable Cu fraction and residual Cu fraction in sulfur-treated soil increased from 12.41%and 56.82%?CK?to 30.83%and 60.52%(4 g kg-1 sulfur treatment),respectively.In the FS-Soil,the sulfur treatment decreased the soil pH and increased the bioavailability of Cu in soil.With sulfur application at 4 g kg-1,the DTPA-extractable Cu was increased by 4.45%,compared with the control soil.5.16S rRNA gene sequencing was used to investigate the effects of sulfur application on the microbe community structure in soil.The two types of Cu-contaminated soil used in this study were quite different in microbe community structure.Sulfur application decreased the microbe alpha diversity of both MS-Soil and FS-Soil.But the effect of sulfur addition on microbe community structure is different.In MS-Soil,both planting castor and sulfur application increased the relative abundance of Euryarchaeota and Proteobacteria.Thiobacillus was the key genus that controlled sulfur oxidation in MS-Soil.In FS-Soil,planting castor and sulfur application increased the relative abundance of Proteobacteria and Acidobacteria.The key genus that controlled sulfur oxidation in FS-Soil was Sulfurifustis.The difference in the effect of sulfur on Cu speciation in MS-Soil and FS-Soil might be caused by the distinctness of rhizosphere soil properties and microbe community structure. |
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