Analysis of early response and tolerance mechanism to Al stress in plant.

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The internal detoxification mechanism of aluminum was investigated in an Al-accumulating plant, Hydrangea macrophylla, focusing on Al forms present in the cells. The leaves of Hydrangea macrophylla contained as high as 15.7 mmol Al kg ^<-1> in fresh weight, and more than two thirds of the Al was found in the cell sap. Using ^<27> Al-nuclear magnetic resonance (NMR), the dominant peak of Al was observed at a chemical shift of 11 to 12 ppm in both the intact leaves and extracted cell sap, which 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 that Al in fractions with the same retention time as citric acid contributed to the observed ^<27>Al 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 further examined by mass spectrometry and ^1 H-nuclear magnetic resonance after purification, and the molecular weight, chemical shifts and coupling constants were the same as those of citric acid. Bioassay experiments showed that the purified Al complex from 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 affected. These observations indicated that Al is bound to citric acid in the cell of hydrangea leaves. Therefore, formatin of strong Al-citrate complex, a non-toxic species of Al, is an internal detoxification mechanism of Al in hydrangea leaves.