One proposed mechanism of aluminum (Al) tolerance in plants is the release of an Al-chelating compound into the rhizosphere. In this experiment, two cultivars of snapbeans (Phaseolus vulgaris L. "Romano" and "Dade") that differ in Al tolerance were grown hydroponically with and without Al under aseptic conditions. After growth in nutrient solutions for 8 days, aliphatic and phenolic organic acids were analyzed in the culture solutions with an ion chromatograph and a high pressure liquid chromatograph. The tolerant snapbean, "Dade", when exposed to Al, exuded citric acid into the rhizosphere in a concentration that was 70 times as great as that of "Dade" grown without Al, and 10 times as great as that of "Romano" grown with or without Al. The sensitive cultivar, "Romano", exuded only slightly more citric acid into the growing medium under Al-stress, compared to nonstressed conditions. Citric acid is known to chelate Al strongly and to reverse its phytotoxic effects. Also, citric acid has been shown previously to enhance the availability of phosphorus (P) from insoluble Al phosphates. Thus, one mechanism of Al-tolerance in snapbeans appears to be the exudation of citric acid into the rhizosphere, induced either by toxic levels of Al or by low P due to the precipitation of insoluble Al phosphates. Our experiment was not able to distinguish between these two factors; however, tolerance to both primary and secondary Al-stress injuries are important for plants growing in Al-toxic soils.Al toxicity is a major factor limiting plant growth in strongly acid soils (8,9). Liming is used to correct this problem in the plow layer; however, amendment of acid subsoils is not feasible economically (9). Moreover, acid soils are found often in the tropics and subtropics, where resource-poor farmers are not able to afford such a high-input solution (14).An alternative, low-input solution to this problem is to utilize the crop plant's genetic potential for tolerance to Al stress (9). Plant species and cultivars within species vary widely in their resistance to Al injury, and some of these differences are heritable (9 One hypothesized mechanism of Al tolerance is the chelation and detoxification of Al by organic acids, either within the plant (internal tolerance) or in the rhizosphere (exclusion) (8,9,30). Organic acids have been reported to chelate Al and to ameliorate its phytotoxic effects (8, 9, 30). Hue et al. (15) showed that the addition of citric, oxalic, and tartaric acids to the hydroponic solution alleviated the inhibitory effect of Al on root extension of cotton (Gossypium hirsutum L.). Similarly, Bartlett and Riego (3) found that Al complexed by citric acid or EDTA did not reduce root and shoot growth of corn plants (Zea mays L.), as did ionic Al. In these studies, the organic acids could have detoxified Al either externally in the rhizosphere or internally after absorption by plant roots.Suhayda and Haug (26)(27)(28) found in vitro that organic acids reversed the Al-induced conformational change in the regulato...
