Form of Al changes with Al concentration in leaves of buckwheat

Buckwheat (Fagopyrum esculentum Moench. cv. Jianxi) is known as an Al-accumulating plant. The process leading to the accumulation of Al in the leaves was investigated, focusing on the chemical form of Al using ²⁷Al-nuclear magnetic resonance. Leaves with different Al concentrations were prepared by growing buckwheat on a very acidic soil (Andosol) amended with or without CaCO₃ (1 or 3 g kg^-1 soil). When the Al concentration of the leaves was lower, only one major signal was observed at a chemical shift of 16.1 ppm, which was assigned to an Al-oxalate complex at a 1:3 ratio. However, when the Al concentration of the leaves increased to a high level (e.g. 12 g Al kg^-1), an additional signal at a chemical shift of 11.2 ppm was observed. This signal was assigned to an Al-citrate complex at a 1:1 ratio. In the leaf with a high Al concentration, both Al-oxalate (1:3) and Al-citrate (1:1) were detected in marginal and middle parts, while only Al-oxalate was detected in the basal part. The oxalate concentration did not differ very much between leaves with low and high Al concentrations at the same position, while citrate concentration significantly increased with increasing Al concentration when the oxalate/Al ratio became lower than 3.0. As the Al-citrate complex has been demonstrated to be the form of transport in the xylem, the results suggest that when internal oxalate is enough to form a complex with Al at a 3:1 ratio in the leaves with a low Al concentration, Al-citrate converts to Al-oxalate. However, this conversion does not occur in the leaves with a very high Al concentration, resulting in the coexistence of both Al-oxalate and Al-citrate complexes.