Selection For Low Cadmium Accumulative Cultivars Of Pakchoi And Research On The Biochemical Mechanisms

With the rapid developments of industrialization and urbanization,and the excessive dependence on fertilizer,sewage irrigation and mining activities,environment pollution in agricultural soil is becoming more and more serious.A survey by the Environmental Protection and Land and Resources ministries in April 2014,found that the farmland soil point exceeding the standard rate of 19.4%,while that of cadmium was 7.0%.Soil heavy metal pollution not only affects crop yield and quality,but also can enter human body through the food chain,posing a potential threat to human health.Therefore,it is important that reducing heavy metal pollution into crops to ensure food safety.In the present study,we screened two low-Cd-accumulating cultivars（LCACs）and two high-Cd-accumulating cultivars（HCACs）from 36 pakchoi cultivars,and investigated the influence of exogenous sulfur on Cd contents and nutritional quality in edible parts under Cd stress.The hydroponic experiment was conducted to investigate the differences of cadmium absorption,transportation and accumulation of two pakchoi cultivars in different cadmium treatment concentrations,which provides valuable guidance for pollution-free cultivars in heavy metal pollution area.The main results were listed as follow:1.The pot culture experiments were performed to investigate the differences of biomass and Cd content from 36 pakchoi cultivars,the results showed that the biomass and Cd contents of edible parts under the three Cd treatments significantly varied across cultivars.Two cultivars,ZLL and AJH,meet all the screening criteria of LCACs,and thus can be categorized as LCACs.By contrast,HPCGB and KR605 were selected as HCACs.In conclusion,cultivating LCACs（ZLL and AJH）with 1 mg·kg^-1 Cd concentration,Cd contents in edible parts of the two cultivars were lower than the National Food Safety Standards in China(0.2 mg·kg^-1),which meet food safety requirements.2.The soluble protein,soluble sugar and nitrate contents in edible parts of the four selected-cultivars were all higher than that of control exposed to Cd1,whereas the soluble protein and soluble sugar contents in two LCACs were significantly decreased exposed to control.Exogenous sulfur can effectively decrease Cd contents and nitrate contents,whereas increase the soluble protein,soluble sugar and vitamin C contents in edible parts of pakchoi,and improve vegetable quality.3.In hydroponic experiment,Cd stress resulted in a significant reduction in shoot fresh weight,length of roots and chlorophyll contents of both pachoi cultivars（KR605 and ZLL）compared to control.The results indicated that the high-Cd-accumulating cultivar（KR605）accumulated more Cd in shoot but less in root than low-Cd-accumulating cultivar（ZLL）.Furthermore,higher Cd concentration of xylem sap and gene expression levels related to Cd transport by xylem（HMA2 and HMA4）in KR605 showed that the transport capacity of xylem plays a determinative role for accumulating Cd in shoots of the high-Cd-accumulating cultivar（KR605）.The low-Cd-accumulating cultivar（ZLL）have lower translocation factor（TF）indicated that accumulate the cadmium into root and reduce cadmium into the shoot.Cadmium was largely found in the vacuole,and at a lesser extent in the cell wall of both cultivars（KR605 and ZLL）.The overall pattern of its distribution followed the order soluble fraction>cell wall>plastid>mitochondria>nucleus in two cultivars.The low-Cd-accumulating cultivar（ZLL）have higher proportions of Cd in soluble fraction of shoots compared to KR605.The results indicated that soluble fraction were identified as a major storage site for cadmium,and effectively alleviate Cd-induced toxicity in LCACs.