COMPARISON OF THE CHEMICAL-CHANGES IN THE RHIZOSPHERE OF THE NICKEL HYPERACCUMULATOR ALYSSUM-MURALE WITH THE NON-ACCUMULATOR RAPHANUS-SATIVUS

Changes in pH acid redox potential were studied in the rhizosphere soi l of a nickel hyperaccumulator plant (Alyssum murale) and of a crop pl ant, radish (Raphanus sativus). Differences in rhizosphere pH and redu cing activity were found between the lateral and the main roots of bot h species, but the pH changes in the rhizosphere were similar in both species. Changes in pH were associated with the relative uptakes of ca tions and anions; whether the concentrations of heavy metals in the gr owth medium did not have any effect on the rhizosphere pH. The source of nitrogen (ammonium or nitrate) was the major factor determining the pH of the rhizosphere of both species. The redox potential of the rhi zosphere was influenced by both the N-source and the concentrations of heavy metals. When heavy metals were not present in the growth medium , and nitrate was the N-source, the reducing capacity of A. murale roo ts was enhanced. However, the reducing activity of A. murale was alway s smaller than that of radish. Therefore, the mechanism of metal solub ilization by the hyperaccumulator plant does not involve either the re duction of pH in the rhizosphere or the release of reductants from roo ts. The acidification and reducing activity of the roots of A. murale was always smaller than that of R. sativus.