Specific secretion of citric acid induced by Al stress in Cassia tara L.

A rapid and sensitive assay method for Al-chelating activity was established to screen Al-chelating substances secreted from roots of Al-resistant species in response to Al stress. From one Al-resistant species, Cassia tora L., an Al-chelating substance was detected in the root exudates when they were exposed to 50 μM Al in 0.5 mM CaCl₂ solution at pH 4.5; the dominant component was identified as citric acid. The secretion of citric acid was very low during the first 4 h after initiation of Al treatment, but increased markedly thereafter. A 3-h pulse with 50 μM Al also induced significant secretion of citric acid after 6 h. The lag between Al addition and secretion of citric acid suggests that inducible processes are involved. A dose-response experiment showed that the amount of secreted citric acid increased with increasing external concentrations of Al. Eight-d treatment of P deficiency did not induce the secretion of citric acid. Exposure to 50 μM of either lanthanum (La³⁺) or ytterbium (Yb³⁺) did not induce the secretion of citric acid either. These findings indicate that the secretion of citric acid is a response specific to Al stress in C. tora and constitutes a mechanism of Al resistance.