100 200 300 400 500 Distance from root apex (pm)

100 200 300 400 500 600 Distance from root apex (pm)

rig. 1.8.9. A Diagram of the proton uptake by the tip of an Arabidopsis root. The length of the arrow indicates the extent of the proton influx. B Influence of aluminium ions on the pH in the rhizoplane ("root surface") of the Al-sensitive wild-type and the Al-resistant mutant alr-104. (After Degenhardt et al. 1998)

was therefore assumed that aluminium resistance, based on exclusion, could also be achieved by alkalinisation of the rhizosphere. Such a mechanism has been recently shown with Al-resistant Arabidopsis plantlets, using a pH microelectrode. This alkalinisation is achieved by proton uptake through the root tip leading to a small increase in alkalinity (ca. 0.1 pH units) of the medium at the root surface. Measurements of root growth confirm the significance of the seemingly small pH difference (Fig. 1.8.9). However, such an effect could also result from a reduced H+ release from the root. This would then mean that Al3+ inhibits the proton pump of the root cell plasma membrane. The slight inhibition of root growth by Al3+ at pH 4.5 compared with that at pH 4.4 could also be due to a decrease in the mole fraction of Al3+ in favour of Al(OH)2+ (see Fig. 1.8.1).

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