Mechanisms Of Translocation And Accumulation Of Cadmium In Typical Soil-Rice Systems

In recent years, increasingly outstanding problems of cadmium-contaminated rice have already been paid high attention by the countries all over the world. This investigation studied changes of soil pH values and changes of exchangeable and TCLP extractable contents of soil cadmium (Cd) at rice different growth stages, and studied dynamic characteristics of Cd translocation and accumulation in different rice organs and the adjustment or tolerance of rice by using a pot experiment with three rice varieties (Xiangwanxian 12, Weiyou 46 and Yuzhenxiang) at different Cd concentration soil. The main results were summarized as follows:(1) The contents of chlorophyll, SOD, CAT and MDA varied in three rice varieties during the whole growth period planted in soils with different Cd concentrations, but no significant differences were observed. It was difficult to reflect the level of Cd contamination in soil through the physiological and biochemical indexes of rice because of its resistance to Cd contamination.(2) The pH values in three soils with different Cd contamination levels did not vary much between the jointing stage and the mature stage of rice growth. Except for the pH values in Xiangwanxian 12 planted soil were in the order:soil M (middle Cd concentration soil)> soil H (high Cd concentration soil)> soil L (low Cd concentration soil), the pH values in the other two rices planted soils were generally in the order:soil H> soil M> soil L. The pH values in the soil planting Xiangwanxian 12 was significantly lower than that in the soil planting Weiyou 12 in the soil H; the range of pH values changes remained the same in the whole growth stage of three rice varieties.The changes in exchangeable contents of soil Cd remained the same trend during whole growth stage of the same rice variety in soils with different Cd concentrations. Except the exchangeable contents of soil Cd were in the order soil M> soil H for the filling stage and mature stage of Weiyou 46, the exchangeable contents of soil Cd for three rice varieties during the whole growth stage were generally in the order:soil H> soil M> soil L. Furthermore, the exchangeable contents of soil Cd showed significant difference in the same rice among soil M, soil H and soil L (P<0.05). There were significant negative correlations between soil pH values and exchangeable contents of soil Cd, but there were no significant correlations between soil pH values and TCLP extractable contents of soil Cd, which indicated that soil pH values could well reflect the changes of exchangeable contents of soil Cd.(3) The Cd contents in different rice organs planted in the three soils with different Cd levels were in the order:iron plaque> shirane> stem> brown rice> husk> ear> leaf. The Cd contents in the same rice organ generally followed the order: soil H> soil M> soil L. The Cd contents in brown rice of Xiangwanxian 12 planted in soil L and soil M were lower than the national standard (Cd≤ 0.2 mg-kg-1). The Cd contents in brown rice of Weiyou 46 and Yuzhenxiang were lower than the national standard (Cd≤ 0.2 mg-kg-1) planted in soil L, but were higher than 0.2 mg-kg-1 planted in soil M and soil H.(4) The biomass in different rice organs grown in the soils with three different Cd concentrations changed in a similar pattern. Total rice biomass reached the maximum at the ripening stage, and generally followed soil L> soil M> soil H. The Cd accumulation in brown rice gradually increased with rice growth and reached the maximum at the mature stage. The total Cd accumulation in rice plants were in the order:soil H> soil M> soil L, and the total Cd accumulation of rice plants in soil L showed a significant difference compared to those in soil M and soil H.When rice was planted in soil L, less resistence was observed to transfer soil Cd from rice roots to rice stems, and most Cd accumulated in the stems. With Cd contents increasing in soils (from soil L to soil H), rice roots gradually strengthened the resistance to Cd transportation, and as a result, Cd accumulated much more in the roots than that in the stems. At the mature stage, the distribution of Cd accumulation in the other rice organs followed in the order:brown rice> husk> leaf> ear.Similar to Xiangwanxian 12, Cd translocation between the stems and brown rice of Weiyou 46 planted in the three Cd contaminated soils mainly happened from husk to brown rice at the filling stage and the ripening stage and from ear to husk at the mature stage. For Yuzhenxiang planted in the three Cd contaminated soils. Cd translocation mainly happened from husk to brown rice at the filling stage and from ear to husk at the ripening stage and the mature stage.