SPECIFIC SECRETION OF CITRIC-ACID INDUCED BY AL STRESS IN CASSIA-TORAL

A rapid and sensitive assay method for Al-chelating activity was estab lished to screen Al-chelating substances secreted from roots of Al-res istant species in response to Al stress. From one Al-resistant species , Cassia tora L., an Al-chelating substance was detected in the root e xudates when they were exposed to 50 mu M Al in 0.5 mM CaCl2 solution at pH 4.5; the dominant component was identified as citric acid. The s ecretion of citric acid was very low during the first 4 h after initia tion of Al treatment, but increased markedly thereafter. A 3-h pulse w ith 50 mu M Al also induced significant secretion of citric acid after 6 h. The lag between Al addition and secretion of citric acid suggest s that inducible processes are involved. A dose-response experiment sh owed 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 mu M of either lanthanum (La3+) or ytterbium (Yb3+) did not induce the secretion of citric acid either. These findings indicate that the secretion of citr ic acid is a response specific to Al stress in C. tora and constitutes a mechanism of Al resistance.