| Mercury pollution in soil has always been an environmental problem that has attracted much attention from researchers and the public.Although scholars at home and abroad have made rapid and substantial progress in soil remediation technology of heavy metal pollution in recent years,it is still a difficult task to carry out large-scale remediation of moderately and slightly polluted areas,reduce the intake of heavy metals by crops,and avoid the risk of chronic poisoning of human beings.Foliar spraying technology has been widely used in agricultural production for nearly a century.It can effectively control plant nutrition status.In recent years,foliar spraying has also been used to prevent contamination of heavy metals in crops,thereby improving crop quality and yield.At present,studies on the regulation of heavy metal content in crops by foliar spraying mainly focus on heavy metals such as Cd,while there are only scattered reports on mercury.In addition to soil,mercury in plants mainly comes from atmospheric absorption,so the effect and mechanism of foliar spraying organics on mercury may be different from that of other heavy metals.Screening foliar spraying materials that can reduce plant mercury and preliminary discussion on the mechanism are of great significance for controlling moderate and mild mercury pollution in farmland and reducing the risk of mercury exposure in population.In this paper,polyaspartic acid,salicylic acid,malic acid,citric acid,fulvic acid,humic acid,benzoic acid,an organic acid,an organic fertilizer and other organic foliar spraying materials were selected.Chinese cabbage was selected as the main research object,and field pot experiments were carried out in Wanshan mercury-contaminated soil area to screen out organic foliar spraying materials that can effectively reduce the mercury content of Chinese cabbage.Then,the organic foliar mercury-blocking materials were selected for pot experiment to analyze the effects of foliar organics spraying on the distribution of mercury concentration and physiological and biochemical indexes of Chinese cabbage.Meanwhile,combined with the comparative experiment of organic acid soil addition,the barrier mechanism of foliar organics spraying on mercury content of Chinese cabbage was initially discussed.The main results of this paper are as follows:(1)In the preliminary screening experiment of organic spraying materials,it was found that humic acid,an organic acid and an organic fertilizer spraying could significantly reduce the total mercury content of Chinese cabbage leaves(p<0.05),and the reduction rates were 31.07%,46.27% and 40.44%,respectively.(2)The pot experiment was carried out based on the effective spraying materials selected in result(1).The results showed that the total mercury content in stems and leaves of Chinese cabbage was significantly decreased by spraying organic acid and organic fertilizer on leaves(p<0.05),and the reduction rates for stems and leaves were 59.08% and 60.60%,and 34.45% and 51.35%,respectively.However,humic acid spraying only significantly decreased the total mercury in leaves of Chinese cabbage(33.66%)(p<0.05),and increased the total mercury in stems of Chinese cabbage(22.32%).For the soil treatment group,the addition of organic acid and humic acid significantly increased the mercury content of cabbage leaves by 41.36% and 22.84%,and the addition of organic fertilizer and humic acid significantly decreased the mercury content of cabbage stems by42.40% and 17.36%,respectively.However,foliar spraying and soil addition of organic acid,organic fertilizer and humic acid could significantly reduce the methylmercury content in leaves of Chinese cabbage,and the reduction rate ranged from 45.47% to 69.04%.(3)The distribution of mercury content in sub-cellular structure of Chinese cabbage leaves showed that mercury content in cell wall(F1)accounted for the highest proportion in all treatment groups except for the addition of an organic acid in soil,so the cell wall was the first barrier against mercury entering the body of plants.Compared with the control group,the proportion of soluble components was significantly increased in foliar spraying of organic acid,humic acid and organic fertilizer treatment groups(p<0.05),and the proportion of organelles was relatively decreased,indicating that these organic acid spraying can induce the transfer of mercury binding to the soluble components of cells,thereby reducing the proportion of mercury in organelles and protecting the normal metabolism of plant cells.The proportion of cell wall components was also increased in the group treated with organic acids,but it was not significant.(4)Foliar spraying with organic acid,organic fertilizer and humic acid increased the contents of superoxide dismutase(SOD),peroxidase(POD)and catalase(CAT),and significantly decreased the content of malondialdehyde(MDA)(p<0.05),indicating that spraying with organic acid can improve the antioxidant capacity of plants.Reduce the peroxy membrane lipidation reaction of plants under mercury stress,so that the H2O2 in plants is maintained at a dynamic equilibrium level,so as to protect the integrity of the membrane structure of leaves,and then prevent mercury from entering the plant,so as to reduce the mercury content in plants.(5)Foliar spraying of organic fertilizer and humic acid could significantly increase the contents of non-protein sulfhydryl substances(NPT)in plant leaves by 41.60%and 47.93%,respectively.Spraying with an organic acid also showed a tendency to increase NPT in leaves,indicating that spraying organic matter on leaves can stimulate NPT synthesis in plants under mercury stress.The macromolecular mercury compounds formed by the chelation of NPT and mercury may promote the transfer of mercury to vacuoles,resulting in an increase in the proportion of mercury in soluble components of leaves and a decrease in the proportion of mercury in organelles,thus protecting plant cells and maintaining normal plant growth under mercury stress.At the same time,the NPT trapping effect in leaves may also reduce the transfer of atmospheric mercury absorbed by leaves to stems,so that the reduction rate of stem mercury by spraying organic acid is much higher than that of leaf mercury.(6)After spraying with different organic matter,the absorption of mineral elements in leaves of Chinese cabbage was different: after spraying with organic acid,the contents of zinc,potassium,calcium and copper in leaves were significantly lower than that of control(p<0.05);After spraying some organic fertilizer,only copper was significantly lower than control(p<0.05);Only potassium in the humic acid treatment group was significantly lower than that in the control group(p<0.05).In the organic acid spray group,there was a significant positive correlation between plant mercury and calcium(r=0.536,p<0.05,n=8).The results showed that there was a correlation between mercury and these mineral elements,which would be affected by some physiological mechanism of plants at the same time.Therefore,spraying organic acids could affect the enrichment of mercury in plant species by regulating the absorption difference of mineral elements in plants. |
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