Form of aluminium for uptake and translocation in buckwheat (Fagopyrum esculentum Moench)

The forms of Al for uptake by the roots and translocation from the root to the shoot were investigated in a buckwheat (Fagopyrum esculentum Moench, cv . Jianxi) that accumulates Al in its leaves. The Al concentration ill the x ylem sap was 15-fold higher in the plants exposed to AlCl3 than in those ex posed to an Al-oxalate (1:3) complex, suggesting that the roots take up Al in the ionic form. The Al concentration in the xylem sap was 4-fold higher than that in the external solution after a 1-h exposure to AlCl3 solution a nd 10-fold higher after a 2-h exposure. The Al concentration in the xylem s ap increased with increasing Al concentration in the external solution. The Al uptake was not affected by a respiratory inhibitor, hydroxylamine, but significantly inhibited by the addition of La. These results suggest that A l uptake by the root is a passive process, and La3+ competes for the bindin g sites for Al3+ on the plasma membrane. The form of Al in the xylem sap wa s identified by Al-27-nuclear magnetic resonance analysis. The chemical shi ft of Al-27 in the xylem sap was around 10.9 ppm, which is consistent with that of the Al-citrate complex. Furthermore, the dominant organic acid in t he xylem sag was citric acid, indicating that Al was translocated in the fo rm of Al-citrate complex. Because Al is present as Al-oxalate (1:3) in the root, the present data show that ligand exchange from oxalate to citrate oc curs before Al is released to xylem.