The Accumulation Of Heavy Metal In Taro And The Response To Cd And Pb Stress

The accumulation pattern of four heavy metals As, Hg, Pb and Cd in taro tubers wasinvestigated and the factors affecting the accumulation of heavy metals were analysed throughrandom sampling of taro tubers and the growing environments. Parallel studies on the indicesof physiological and biochemical changes, as well as the variations in the absorption,distribution and accumulation of Pb and Cd in pot grown taro (variety8520) under simple Pband Cd pollutions were carried out, in order to clarify the physiological and biochemicalresponses of taro to Pb and Cd stresses, and to provide the theoretical bases for the safeproduction and assessment.1. Patterns of heavy metal accumulation and the influencing factors in taro.The analysis onthe four heavy metals As, Hg, Pb and Cd in taro tubers showed that the accumulation of Cdwas the highest with an average of0.06mg/kg, followed by that of Pb (0.0028mg/kg), whilethe accumulation of As and Hg was very low (0.00005mg/kg). Further studies revealed thatthe Pb and Cd content was very significantly, or significantly correlated with the Pb and Cdcontent in the soil,indicating that the Pb and Cd in tubers mainly come from the soil, and theCd content in the tubers had a very significant negative correlation with the pH of the soil,indicating that the lower the soil pH, the higher the Cd accumulation in taro tubers. Therefore,the Cd content in taro tubers can be lowered by regulating the pH of soil.In addition, the As,Pb and Cd content in soil was very significantly or significantly correlated with the organicsubstances; As and Pb content in soil was significantly correlated with soil pH and fastnitrogen, As was negatively correlated with fast phosphor and Pb was positively correlatedwith fast potassium, respectively, showing that the physical and chemical properties of soil areimportant factors that can affect the heavy metal accumulation in taro, especially the organicsubstances and the soil pH.2. The absorption pattern of Cd and the related physiological responses in taro.The applicationof Cd with the pot grown taro showed that the residual Cd in soil was significantly correlatedwith the Cd accumulation in taro tubers, leaf stalks and leaves, with the correlation coefficientbeing0.984,0.898,0.919and0.849, respectively. This is an indication that the Cd in taro wasmainly from the soil, and that Cd can reside in soil in large amount. The descending order of Cd content in different organs was as follows: stalk>leaf>tuber. The measurements ofSOD,POD, CAT, MDA and proline content showed that,along with the increase in the Cdcontent in the soil, the activity of the above three enzymes all increased together with theincrease in MDA and proline content. This indicates that under Cd stress, all the three organssuffered from oxidative damage. The fact that the activity of SOD, POD and CAT all peakedat a Cd concentration of20mg/kg in the soil indicated that this Cd concentration may be athreshold of resistance for taro. The higer proline content in taro leaves than tubers and leafstalks indicated that taro leaves may have a higher resistance to Cd stress.3. The absorption pattern of Pb and the related physiological responses in taro.The result ofPb stress on taro plants showed that the residual Pb in soil was linearly correlated with theamount of Pb applied to the soil, with a correlation coefficient of higher than0.99(p＜0.01)indicating that the soil had a high ability to retain Pb. The content of Pb in the threeorgans tubers, leaf stalks and leaves of taro increased with the increase in soil Pb content, andthe Pb content in leaf stalks and leaves was very significantly or closely correlated with thesoil Pb content, with correlation coefficients of0.967(p＜0.01)and0.902(p＜0.05), and adescending order of the Pb content in the organs is as follows: tuber>leaf>leaf stalk. Themeasurements of SOD, POD, CAT, MDA and proline content showed that, along with theincrease in the Pb content in the soil, the activity of the above three enzymes all increasedtogether with the increase in MDA and proline content, indicating that under Pb stress, taroleaves, tubers and leaf stalks all suffered from oxidative damage. The higer proline content intaro leaves than tubers and leaf stalks indicated that taro leaves may have a higher resistanceto Pb stress.