The toxic effect of aluminum (Al) on the growth of Carrot cells (SO-l) decreased to a greater degree with addition of a medium conditioned by Al-tolerant carrot cells (TA-l) than with a medium conditioned by SO-l cells. The toxic effect of Al was reduced greatly by adding an acidic fraction of the conditioned media, but little or not at all by a neutral or basic fraction. Of four organic acids detected in the acidic fraction, the major one was citric acid which was present in a much greater amount in the conditioned medium of TA-l cells than in that of SO-l cells.The toxic effect of Al was reduced by adding citric or malic acid instead of the conditioned medium, but not by succinic or fumaric acid. Chelating abilities of the organic acids were evaluated by shifts in their titration curves, and were found to be closely correlated with the detoxification effects. Thus, the Al tolerance of TA-l cells may in fact be due to the chelating effect of citric acid which is abundantly released into the medium by the Al-tolerant carrot cells.
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