The effect of copper treatments on the number of shoots regenerated per embryo without transformation. Each bar represents the average number of shoots per embryo (recorded after 10 weeks in tissue culture) from 20–30 individual embryos. 'Standard' tissue culture regime: weeks 1–4 callus induction (CI) medium with no additional copper. 'Additional copper' tissue culture regime: weeks 1–4 CI + 5 μM CuSO4. Callus from both treatments was transferred to standard transition media (which includes CuSO4) after 4 weeks and standard regeneration medium after a further 2 weeks.