A comparative study of metal levels in leaves of some Al-accumulating Rubiaceae

Concentrations of Al, Si, Fe, Mn, Cu and Ca were analysed in leaves of. ten Rubiacea. e species, most of which are At accumulators, and these were compared with concentrations in one species of Melastomataceae. Quantitative data confirmed the distribution of Al accumulation as previously determined by semi-quantitative tests, and suggest that there is an apparent congruency between the shoot Al concentration and the number of accumulators within a certain genus or tribe. Al accumulators within the Rubiaceae are most characteristic of the Rubioideae subfamily, although a second origin is likely in at least a few members of the tribes Vanguerieae and Alberteae. While the leaf Si concentration in Melastomata malabathricum L. (Melastomataceae) was negligible, all Rubiaceae studied showed relatively high Si levels (mostly >3000 mg kg(-1)). It is hypothesized that an Al-Si complex is formed in the shoot tissues of Al-accumulating Rubiaceae and that this may contribute to Al detoxification. However, the Si : Al mote ratio tended to differ widely among species. There was no significant correlation between Al and the other metals analysed. A remarkably high Mn concentration was found in Coptosapelta olaciformis Elm. (C) 2003 Annals of Botany Company.