INTERNAL DETOXIFICATION MECHANISM OF AL IN HYDRANGEA - IDENTIFICATIONOF AL FORM IN THE LEAVES

An internal detoxification mechanism for Al was investigated in an Al- accumulating plant, hydrangea (Hydrangea macrophylla), focusing on Al forms present in the cells. The leaves of hydrangea contained as much as 15.7 mmol Al kg(-1) fresh weight, and more than two-thirds of the A l was found in the cell sap. Using Al-27-nuclear magnetic resonance, t he dominant peak of Al was observed at a chemical shift of 11 to 12 pa rts per million in both intact leaves and the extracted cell sap, whic h is in good accordance with the chemical shift for the 1:1 Al-citrate complex. Purification of cell sap by molecular sieve chromatography ( Sephadex G-10) combined with ion-exclusion chromatography indicated th at Al in fractions with the same retention time as citric acid contrib uted to the observed Al-27 peak in the intact leaves. The molar ratio of Al to citric acid in the crude and purified cell sap approximated 1 . The structure of the ligand chelated with Al was identified to be ci tric acid. Bioassay experiments showed that the purified Al complex fr om the cell sap did not inhibit root elongation of corn (Zea mays L.) and the viability of cells on the root tip surface was also not affect ed. These observations indicate that Al is bound to citric acid in the cells of hydrangea leaves.