Study On The Difference Of Cadmium Accumulation In Different Brassica Rapa Varieties And Its Regulation Mechanisms

With the development of economy,the exploitation of mineral resources and the extensive use of pesticides and fertilizers containing heavy metals,the pollution of heavy metals in soil is becoming more and more serious,which has become the focus of global attention.Cadmium?Cd?,one of the most serious heavy metals in farmland,has long-lasting retention,poor mobility,and is difficult to be decomposed by the soil itself.If it enters the human body through the food chain,it will cause great harm to human health.Brassica rapa is an indispensable food in people's daily diet.It is also a plant with high enrichment ability for Cd among Brassicaceae.In recent years,news reports about its Cd content exceeding the standard have occurred from time to time,which has attracted wide public attention.Therefore,how to effectively reduce the accumulation of Cd in Brassica rapa to achieve safe production has become an urgent issue to be solved.Firstly,the differences of Cd accumulation in different Brassica rapa genotypes were clarified.On this basis,the effects of high Cd accumulating plant Kochia scoparia and different soil amendments on soil remediation and Cd content in Brassica rapa were studied.Calcium magnesium phosphate fertilizer with significant remediation effects were selected to treat 2.96 mg·kg^-11 Cd polluted-soil.The effects of this treatment on genes related to cadmium transport,accumulation and antioxidant enzymes in Brassica rapa and their specific molecular regulatory mechanisms were studied.The main results are as follows:1.Differences in Cd accumulation in different varieties of Brassica rapaThe results of pot experiment under 2.96 mg·kg^-11 Cd stress showed that the above-ground Cd content of 25 Brassica rapa subsp.chinensis genotypes was8.75-43.33 mg·kg^-1DW,with an average of 21.23 mg·kg^-1.The above-ground dry weight Cd content of 19 Brassica rapa subsp.pekinensis genotypes ranged from16.16 mg·kg^-11 to 66.36 mg·kg^-1,with an average of 35.36 mg·kg^-1.However,the difference of Cd transport coefficient between the above-ground parts of Brassica rapa subsp.chinensis and that of Brassica rapa subsp.pekinensis was small,averaging 1.26 and 1.29,respectively.The correlation analysis showed that there was a significant correlation between the above-ground and underground Cd enrichment coefficients of Brassica rapa subsp.chinensis,but there was no significant correlation between the above-ground and underground Cd enrichment coefficients of Brassica rapa subsp.pekinensis.Combining the above-ground and below-ground Cd content and biomass of the plants,we screened of Brassica rapa subsp.chinensis variety with relatively low accumulation of Cd with high biomass-Shanghaiqing and Brassica rapa subsp.pekinensis variety-Sijikuaicai,and Cd relatively high accumulating variety of Brassica rapa subsp.chinensis-Xialv 2 and Brassica rapa subsp.pekinensis variety-Zaoshuwuhao.2.Effects of Kochia scoparia-Brassica rapa rotation system on Cd uptake,accumulation and translocation in Brassica rapaThe results showed that Kochia scoparia L.had strong Cd enrichment ability.The enrichment ability of Cd in different parts of Kochia scoparia L.was leaf>root>stem,and the corresponding enrichment coefficient were 15.07,5.44 and 2.97,respectively.The rotation of Kochia scoparia L.and Brassica rapa can significantly reduce the Cd content in Brassica rapa and reduce the Cd content in soil,but it has no significant effect on soil pH and available Cd content,and has little effect on the transport efficiency of Cd in Brassica rapa.Moreover,the rotation treatment does not affect the normal growth and development of Brassica rapa,and can also increase the activity of soil urease and acid phosphatase to restore the inhibition of the enzyme activity caused by Cd,and reduce the toxic effect of Cd on Brassica rapa.3.Effects of soil amendments on Cd uptake,accumulation and transport in Brassica rapaSix soil amendments including lime,sheep manure,sepiolite,medical stone,calcium magnesium phosphate fertilizer and charcoal,were used to treat the soil.The results showed that the available Cd content in the soil was reduced to varying degrees by all the amendments except medical stone.The best reduction effect was obtained by lime and calcium magnesium phosphate fertilizer?43.25%and 62.75%respectively in 90 days of treatment?,and there was a significant negative correlation between Cd content(Y_Cd=3.473-0.334X_pH)and the soil pH value within 90 days of treatment.Moreover,after 90 days of treatment,four different Brassica rapa varieties were planted in the soil to verify the restoration effect of various amendments.The results showed that the trend of Cd content of four kinds of Brassica rapa varieties under different amendments was basically the same,but the trend of Cd content was more obvious in the high accumulating varieties.Among them,calcium magnesium phosphate fertilizer had the most significant effect on the Cd content of the above-ground parts of Brassica rapa,with an average of 62.87%,without significant effect on the biomass.4.Effects of calcium magnesium phosphate fertilizer on the expression characteristics of genes related to cadmium transport,accumulation and antioxidant enzymes in Brassica rapaShanghaiqing and Zaoshuwuhao,two varieties with different accumulation characteristics of cadmium,were treated with calcium magnesium phosphate fertilizer under 2.96 mg·kg^-1Cd stress to determine its effects on the expression characteristics of genes related to cadmium transport,accumulation and antioxidant enzymes in the seedlings.The results showed that the calcium magnesium phosphate fertilizer treatment could effectively regulate the changes of NRAMP5,CAX4,APX and CAT genes under Cd stress,thus reducing the toxicity of Cd.Moreover,several genes in leaves of the two varieties had different response to the treatment of calcium magnesium phosphate fertilizer.