UPTAKE, DISTRIBUTION, AND BINDING OF CADMIUM AND NICKEL IN DIFFERENT PLANT-SPECIES

For better understanding of mechanisms responsible for differences in uptake and distribution of cadmium (Cd) and nickel (Ni) in different p lant species, nutrient solution experiments were conducted with four p lant species [bean (Phaseolus vulgaris L.), rice (Oryza saliva L.), cu rly kale (Brassica oleracea L.) and maize (Zea mays L.)]. The plants w ere grown in a complete nutrient solution with additional 0.125 and 0. 50 mu M Cd or 0.50 and 1.00 mu M Ni. Large differences in Cd and Ni co ncentrations in shoot dry matter were found between plant species. Mai ze had the highest Cd concentration in the shoots, and bean the lowest . Contrary to Cd, the Ni concentrations were highest in the shoots of bean and the lowest in maize. A gradient of Cd concentrations occurred in bean and rice plants with the order roots >> stalk base >> shoots (stems/sheaths > leaves). A similar gradient of Ni concentrations was also found in maize and rice plants. In the xylem sap, the Cd and Ni c oncentrations were positively correlated with Cd and Ni concentrations in the shoot dry matter. In the roots of maize, about 60% of Cd could be extracted with Tris-HCl buffer (pH 8.0), while in roots of other p lant species this proportion was much lower. This higher extractabilit y of Cd in the roots of maize is in accordance with the higher mobilit y as indicated by the higher translocation of Cd from roots to shoots and also the higher Cd concentrations in the xylem sap in maize than i n the other plant species. Similarly, a higher proportion of Ni in the soluble fraction was found in the roots of bean compared with maize w hich is in agreement with the higher Ni accumulation in the shoots of bean. The results of gel-filtration of the soluble extracts of the roo ts indicated that phytochelatins (PCs) were induced in the roots upon Cd but not Ni exposure. The higher Cd concentrations and proportions o f Cd bound to PC complexes in the roots of maize compared with the oth er plant species suggest that PCs may be involved in the Cd translocat ion from roots to shoots.