The aim of this study was to quantify the long-term effects of Al in solution on mineral ion uptake and associated H+ release by the roots of maize cultivated in conditions compatible with field data. Two maize (Zea mays L.) cvs of differing sensitivity to Al were compared. The culture was conducted in hydroponic conditions for culture periods of 2–5 d in the presence of 0–30 μM Al. Proton fluxes were localized along the roots using a videodensitometry method with bromocresol green on agarose gel.
Aluminium reduced the uptake of anions, particularly NO3−, whereas the uptake of K+ and NH4+ was unaffected. These effects were accompanied by an increase in net H+ release which was quantitatively comparable with the reduction in NO2− uptake. The reduction in NO3− uptake and the concomitant increase in H+ release increased with the quantity of Al accumulated in the roots, which was itself dependent on the concentration of Al in solution. The Al-sensitive cultivar bound more Al to its roots than the Al-tolerant one; the reduction in NO3− uptake and the increase in H+ release were also greater in the Al-sensitive cv. Dye indicator videodensitometry showed that the increased H+ release was general all along the roots.
These results led us to specify the relation between the Al deposition in roots, the reduction in NO3− uptake and the stimulation in rhizosphere acidification by the plants. They show that, with maize, Al induces an inhibition in NO3− uptake and an increase in net H+ release in a 1:1 ratio. These concomitant effects suggest that inhibition in NO3−/OH− exchange unmasks active H+ excretion which is otherwise insensitive to Al. Both these effects are closely related to the Al content in roots expressed per unit root length.