Stimulatory effects of aluminum on in vitro growth ofFrankia

Abstract

To investigate the effects of aluminum (Al) on in vitro growth (total protein content) of Frankia, seven isolates were incubated for 25 days at five nominal Al concentrations (0-500 µM) at pH 4.8. The concentrations of monomeric Al ranged between 25.6 and 106.7 µM. The experimental medium was devoid of inorganic phosphate and Fe-EDTA to avoid Al precipitation. There was an Al-free control at pH 6.4 to assess the effects of low pH alone on Frankia growth. Growth estimated as total protein of all strains was notably increased at pH 6.4, but only Frankia CcI3 was able to grow in the Al-free medium at pH 4.8. Nominal Al concentrations from 125 to 500 µM enhanced the growth of all Frankia isolates. The Al response was strain dependent, but all strains were stimulated at the 500 µM (107 µM monomeric) Al concentration. The final pH of media decreased as Al concentrations increased. At the end of the experiment, the presence of citric, oxalic, malic, and lactic acids as possible Al binding agents in media at 500 µM Al was assayed by enzymatic techniques. However, none of them could be detected. These results suggest that the survival and growth of Frankia strains in acid soils that are characteristically high in Al are more likely than previously estimated from in vitro growth assays.Key words: acid soils, actinorhizal plants, aluminum, Frankia.