Specific Secretion of Citric Acid Induced by Al Stress in Cassia tora L.

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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 CaCl_2 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 (La^<3+>) or ytterbium (Yb^<3+>) 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.
日本植物生理学会の論文