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 Rubiaceae species, most of which are Al 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 mole 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.