| Rare earth pollution has become a peculiar problem in China. After rare earth elements entering into soil, it can be accumulated by crops and eventually enters the human body through the food chain, which pose a threat to the human health. Vegetables are the daily food of rural residents, so a slight increase of rare earth contents will cause great enrichment in human body. Therefore, it has practical significance to study the migration accumulation rule of rare earth elements in soil-vegetable system for the safety of vegetables.In this study, 17 kinds of popular vegetables in Guangdong Province were selected for a cerium contaminated soil experiment. By analysis cerium content in edible parts of vegetables which cultured under the cerium condition of 50 mg/L, the cerium low accumulation vegetables were screened. The cerium high accumulation and cerium low accumulation vegetables were selected for further research by pot soil cultured.Experiments compared its accumulation characteristics between cerium species in high and low accumulation of vegetables, researched effects of cerium stress on chlorophyll,root activity, enzyme activity, proline and malondialdehyde(MDA) and the influence on the activity of soil enzyme. The results on which species uptook and accumulated high and low cerium as well as physiological mechanisms, and had practical significance for manage- ment, repair and scientific, safe use of rare earths polluted soil. The main results were shown as follows:(1) 17 kinds of vegetables grew in good condition under the same cerium concentration hydroponics. Results were found that cerium content in the edible part of Brassica juncea was the highest, up to 98.21 mg/kg, while cerium accumulation in the esculent part of Lactuca sativa L. was the lowest, only 6.64 mg/kg. From the results we can identified that Brassica juncea was cerium high accumulative vegetable, and Lactuca sativa L. belonged to the low accumulative vegetable.(2) Low concentration of cerium could promote the growth of plants, but once the concentration crossed a certain threshold would have inhibitory effects, and produced toxic effects on plants. In pot experiment, when cerium concentration was 600 mg/L, the biomass of Brassica juncea and Lactuca sativa L. reached the maximum, respectively,higher than that of the control group 79.98% and 19.74%. Brassica juncea showed stronger tolerance to cerium.(3) Total and available cerium in root rhizosphere soil of Brassica juncea and Lactuca sativa L. showed a highly significant positive correlation(P<0.01), and Brassica juncea revealed better correlation. cerium content in vegetables have a close relationship with the total cerium in soil, and cerium accumulation increased with the increase of cerium content in soil. The cerium contents in the edible part of Brassica juncea and Lactuca sativa L. were up to 35.66 mg/kg and 8.87 mg/kg, respectively, 17 and 28 times in the control group. The cerium content in edible part of Brassica juncea correlation between total cerium and available contents in soil was significant(P<0.05), and cerium content in edible parts of Brassica juncea and the available content of Ce in soil revealed better correlation. Cerium contents in edible parts of Lactuca sativa L. had significantly correlation between total cerium content in soil(P<0.05).(4) Research showed that, with the increase of cerium content in soil, the chlorophyll content, antioxidant enzymes(SOD, POD, CAT) activity, root vitality,proline content in plants increased at first and then decreased, and the content of MDA is a rising trend in general. On the whole, the toxic effects by cerium on Lactuca sativa L.than to Brassica juncea, Brassica juncea revealed stronger tolerance to cerium.(5) The activity of soil enzyme was affected by the rhizosphere soil environment,and different kinds of enzyme affected by the rhizosphere soil was also different. In the rhizosphere soil of Brassica juncea, the activity of urease, phosphatase, invertase,cellulase all showed activation in the experimental concentration range, and the activation effect increased first and then decreased, while catalase activity showed activated at first and then restrained, indicating catalase was more sensitive to cerium.For Lactuca sativa L., the different enzymes showed different response to cerium. The activity of catalase, phosphatase, invertase all showed activation, and activation was from strong to weak, urease activity was inhibited, and the activity of cellulase increased with the increasing cerium concentration and then tended to be stable line. In short,cerium is one of the main factor affecting the enzyme activity.
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