Copper tolerance of Acer pseudoplatanus L. (sycamore) in tissue culture

The growth of Acer pseudoplatanus L. (sycamore) callus tissue on solid and liquid growth media containing elevated copper concentrations was studied in an attempt to understand the responses of trees to metal contamination and to provide an index of tolerance which may show a parallel expression of tolerance to the whole plant. Callus growth on solid media was very variable, but was significantly reduced at 10 mg l^-1 , with necrosis occurring at 20 mg l^-1 ; copper was readily removed from the media in direct proportion to its concentration. Cell suspension cultures differed in their response to copper according to their site of origin. In cultures originating from uncontaminated sites, growth was inhibited at 12.5 and 15.0 mg l^-1 Cu, but cultures originating from mature trees at a metal-contaminated site were not affected by these concentrations. This metal tolerance trait was stable in cell culture through repeated sub-culturing on uncontaminated media for periods of more than 12 months. The trait was not present in a culture derived from a (non-tolerant) seedling at the metal-contaminated site, but it was induced in one non-tolerant culture by repeated exposure to high metal levels. The significance of these findings is discussed in the context of the possession of appropriate genetic variation in trees for surviving the impact of metal contamination.