Accumulation Of Cadmium In Water By Landitla Punctata And Its Tolerance Mechanism

Cadmium(Cd)pollution in soil,rivers and lakes has been regarded as a serious problem with the development of current industrialization and urbanization in China.Aquatic plant restoration is a promising technology in treating water bodies contaminated by heavy metals.Unfortunately,information on the tolerance mechanisms of heavy metal hyperaccumulation by plants is not yet clear,which limits the application and improvement of this technology.Duckweed is an ideal species to restore Cd-polluted waters due to its fast growth,easy harvesting and Cd hyperaccumulation.The current research on duckweed mainly focuses on the restoration of heavy metal polluted water?ecological toxicity and energy utilization.However,those on Cd tolerance and detoxification mechanisms for duckweed remain largely unclear.This study conducted a comprehensive survey of duckweed species resources in Jiangxi Province,and screened out the dominant species(Landitla punctata)with strong Cd-enriched ability and high biomass.Then hydroponic experiments were carried out to explore the physiological and biochemical responses of Cd absorption and tolerance mechanisms by analyzing the Cd accumulation,Cd subcellular distribution,Cd chemical forms,physiological and biochemical indicators of Landitla punctata.The main findings were as follows:(1)There are 5 species of duckweed(Lemna minor,Lemna perpusilla,Spirodela polyrrhiza,Landoltia punctata,and Wolffia arrhiza)collected from Jiangxi Province,which belong to 4 generations.Lemna minor was the dominant species accounting for46%of the total collected duckweed samples,followed by Spirodela polyrrhiza,accounting for 19%,and the Lemna perpusilla and Landoltia punctata each accounting for 15%,while Wolffia arrhiza sporadically distributed.The concentration ranges of p H,NH₃-N,NO₃-N,TN,and COD in the water where Lemna minor grew were wider than those of the other four types.The TP range was the broadest in the water of Landoltia punctata.The concentration ranges of heavy metals including copper(Cu),lead(Pb),zinc(Zn)in the water where Lemna minor grew was the widest and Spirodela polyrrhiza showed well adaptability for manganese(Mn).Redundancy analysis(RDA)indicated that the distribution of duckweed was affected by the contents of TN?TP?NH₃-N?NO₃-N?Cd?Mn and Cr.The first and the second sort axis explained 22.05%and 13.82%of all information,respectively.(2)The preliminary screening results of Cd tolerance showed that Landoltia punctata has the strongest ability and mildest symptoms of poisoning.The re-screening results confirmed that Landoltia punctata(07SGZP01),a dominant species,had the highest Cd enrichment and the largest biomass.The dominant varieties treated with different Cd concentrations showed that the biomass of Landoltia punctata,significantly lower than the control group(CK),decreased(the lowest under the 5 mg L^-1 Cd treatment)and then increased with the increasing of Cd concentrations.The Cd contents in Landoltia punctata drastically increased from 2511.1 to30641.01 mg kg^-1 with an increase of Cd levels from 0.3 to 20 mg L^-1.The transport coefficient(TF)increased with the Cd levels from 0.3 to 2 mg L^-1,and the Cd content in leaves was higher than that in roots(TF>1),while the Cd content in roots was higher than that in leaves(TF<1)when the Cd treatment was above 5 mg L^-1.The bioaccumulation factor(BCF)decreased significantly with the increase of the Cd level(P<0.05).The removal rate of Cd in the solution gradually decreased with the increase of Cd concentration,and the removal rate was the highest(75%)when the Cd treatment was 0.5 mg L^-1.(3)Compared with CK,the contents of chlorophyll a(Ca),chlorophyll b(Cb),carotenoid(Cc)and total chlorophyll(Ct)in the Cd treatment group were significantly reduced(P<0.05),and their contents decreased with the increasing of Cd concentrations.The enrichment of Cd caused damage to the PS?photoreaction center.F_v/F_m,F_m,Y(II),and ETR of each treatment group were significantly lower than the CK(P<0.05),and F_v/F_m,F_o,F_m,Y(II),q P,and ETR all decreased with the increase of Cd concentrations;while q N,Y(NPQ),Y(NO)increased with the increase of Cd concentration,and both q N and Y(NPQ)in each treatment group were significantly higher than the CK(P<0.05).In addition,the Cd treatment(2 mg L^-1)not only damaged the ultrastructure of the Landoltia punctata,such as chloroplast deformation and cell vacuolation,but also caused most of the stomata to close,and the leaf epidermal cells to be damaged and ruptured.(4)The results of subcellular distribution showed that the majority of Cd was localized in the soluble fraction(23.4-55.2%)and the cell wall fraction(21.7-53.6%),and only 13.7-23.1%of Cd was located in the organelles.The proportion of Cd in the soluble fraction decreased,and the proportion of Cd in the cell wall fraction increased with the increasing of the Cd concentration.Analysis of the Cd chemical forms demonstrated that the largest portion of Cd was found in F_{Na Cl} and F_d-H2Oextracts.The content of F_{Na Cl} increased from 28.6 mg kg^-1(CK)to 10511.8 mg kg^-1(5 mg L^-1 Cd),while the content of F_d-H2O increased from 7.2 mg kg^-1(CK)to 1536.3 mg kg^-1(5 mg L^-1 Cd).On average,the Cd contents of the extracts were in the following order:F_{Na Cl}>Fd-H2O>FHAc>Fethanol>FHCl>Fresidue.The FNa Cl extracts were mainly protein or pectin-bound Cd.Further analysis showed that albumin-and globulin-bound Cd were predominant at each Cd level,accounting for over 80%of the total protein-bound Cd.The results of fourier infrared spectroscopy(FTIR)also showed that the absorbance value of the infrared spectrum absorption wavelength(1648 cm^-1)representing the protein had a peak in each treatment group,and this value was the lowest under the 0.5mg L^-1 Cd treatment(lower than the CK),and highest under 5 mg L^-1 Cd treatment(higher than the CK),indicating that high Cd treatment will stimulate the synthesis of protein and peptide substances to cope with Cd stress.Compared with the CK,Cd treatment can significantly reduce the activities of CAT and GR(P<0.05),and significantly increased the contents of MDA,SOD,APX and small molecule non-enzymatic substances(GSH,As A,NP-SH,PCs),soluble protein(P<0.05).MDA and APX increased with the increase of Cd concentration and had a significant positive correlation with Cd concentration(P<0.01).POD,SOD,GSH,As A,NP-SH,and PCs firstly increased and then decreased with the increase of Cd concentration.CAT and GR decreased significantly with the increase of Cd concentration(P<0.05),and soluble protein decreased with the increase of Cd concentration,and CAT and GR were extremely negatively correlated with Cd concentration(P<0.01).Cd treatment leads to membrane lipid peroxidation,and the antioxidant system produced a response to reduce adverse effects.(5)Compared with the CK,ammonium nitrogen(NH₄-N),nitrate nitrogen(NO₃-N),and ammonium nitrate treatments significantly reduced the Cd accumulation in Landoltia punctata(P<0.05),and increased the biomass.The proportions of the sum of cell wall components and soluble components increased,and the proportion of organelles decreased with increasing nitrogen treatment levels.The sum of the proportions of cell wall components and soluble components in the NH₄-N treatment group was greater than that of the NO₃-N treatment group.Compared with the CK,the proportion of F_{Na Cl} extracts reduced,and the proportion of F_d-H2O extracts increased.The proportion of F_{Na Cl} extracts and F_d-H2O extracts were predominant at each treatment,accounting for more than 75%of the total Cd extracts.The proportion of F_{Na Cl} extracts in the NH₄-N treatment group was greater than that in the NO₃-N treatment group.The application of different forms of nitrogen reduced the contents of Ca,Cb and Ct,and increased the content of Cc,which meant that the enrichment of N would increase the damage of photosynthetic pigments.Nitrogen treatments increased the F_v/F_mand Y(II)values.The application of three different forms of nitrogen can alleviate the Cd stress of Landoltia punctata,and the detoxification effect of NH₄-N treatment on Cd enrichment in Landoltia punctata was stronger than that of NO₃-N treatment.