Sensitive -Al

sensitive +AI

Tolerant -Al

Tolerant +AI

Malic acid

0.08 ±0.08

0.33 ±0.00


3.57 ±0.08

Succinic acid

0.08 ±0.08

0.08 ±0.08

0.08 ±0.08

0.58 ±0.08

Citric acid

0.17 ±0.08

0.08 ±0.08

0.08 ±0.00

0.17 ±0.00

0 100 200 300 400

Concentration of organic acid (pM)

0 100 200 300 400

Concentration of organic acid (pM)

■ Fig. 1.8.7. Chelation of Al3+ by organic acids. Citric, malic and succinic acids were incubated for 1 h with a nutrient solution containing 50 |.iM Al3+, 3.2 mM Na acetate buffer (pH 4.2) and 250 |.iM haematoxylin. The formation of Al-haematoxylin complexes was measured by absorption at 540 nm. The reduction of absorption in the presence of organic acids shows that organic acids chelate part of the aluminium which is then no longer available for the formation of a complex with haematoxylin. (After Delhaize et al. 1993 b)

that finally effects aluminium tolerance, but the complexing of the Al3+ ion. Similarly, some Al3+-insensitive species secrete phosphate by their roots, leading to the formation of insoluble A1P04.

Rapid, Al3+-triggered excretion of chelating acids leads to the question how aluminium ions are perceived. Excretion of malic or citric acid can be blocked by inhibitors of anion channels. This, and the fact that malate is a common metabolite of the cytosol, suggest that the primary effect of Al3+ is the opening or activation of a malate-transporting channel. But how Al3+ gates this channel is still unsolved.

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